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  • 1.
    Alizad Banaei, Arash
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Rahmani, Mona
    Univ British Columbia, Dept Math, Vancouver, BC V6T 1Z2, Canada..
    Martinez, D. Mark
    Univ British Columbia, Dept Chem & Biol Engn, Vancouver, BC V6T 1Z3, Canada..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Inertial settling of flexible fiber suspensions2020In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, no 2, article id 024301Article in journal (Refereed)
    Abstract [en]

    We study the inertial settling of suspensions of flexible and rigid fibers using an immersed boundary method. The fibers considered are inextensible and slender, with an aspect ratio of 20. For a single Galileo number of Ga = 160, we examine a range of dimensionless bending rigidities 0.1 < gamma < 20 and fiber concentrations 0.5 < nL(3) < 25, with n being the fiber number density and L the fiber length, that spans dilute and semidilute regimes. The settling fibers form streamers, regions where the fibers are packed and settle faster than the average settling velocity of the suspension, for nL(3) > 10. In the low-concentration regions outside the streamers, the fibers either go upward or have low settling velocities. Flexible fibers exhibit higher packing inside the streamers and smaller streamers compared to the streamers formed by the rigid fibers. Due to this higher packing, the flexible fibers settle faster compared to the rigid fibers. The formation of the streamers counterbalances the hindering of the settling velocity at higher concentrations. At higher nL(3), however, the maximum local concentration of fibers relative to a uniform distribution diminishes for both flexible and rigid fibers as the mobility of the fibers becomes limited due to the presence of other fibers in their vicinity. Due to this limited mobility, the deformation of the fibers and their settling orientation become insensitive to nL(3) for nL(3) > 7. In both the dilute and semidilute regimes, flexible fibers are more aligned with the direction perpendicular to gravity compared to rigid fibers.

  • 2.
    Andersson, Evert
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group.
    Berg, Mats
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group.
    Nelldal, Bo-Lennart
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Transport planning. KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group.
    Stichel, Sebastian
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group.
    Varför behövs Nya Stambanor i Sverige?2020Report (Other academic)
    Abstract [sv]

    Stora investeringar och omdaningar planeras i vårt transportsystem. Transporterna väntas öka starkt i framtiden och mera kapacitet måste skapas på ett hållbart sätt. Diskussionerna om vilka transportmedel som ska prioriteras, såväl som vilka objekt som vi ska satsa på, är livliga.

    En viktig fråga är satsningen på Nya Stambanor avsedda för snabba persontransporter i de redan idag hårt belastade stråken Stockholm‒Göteborg och Stockholm‒Malmö, med ett stort antal mellanliggande orter. Denna typ av järnvägar finns redan eller planeras i de flesta av världens ledande ekonomier. Syftet med att bygga nya stambanor är att öka den totala kapaciteten för person- och godstrafik på järnväg, öka punktligheten och öka tillgängligheten genom korta restider. Det ger också förutsättningar för större regionala arbetsmarknader och ökat bostadsbyggande utanför storstäderna samt en bättre miljö. Nuvarande stambanor avlastas och lämnar plats för bl a effektivare godstransporter.

    Denna rapport behandlar först järnvägens egenskaper. Järnvägen är det energieffektivaste transportmedel vi känner till, den tar liten plats och är mycket trafiksäker. Moderna tåg på modern bana är vårt snabbaste transportmedel till lands. Tåg kan bereda plats och komfort för arbete och avkoppling under resan. Enligt författarnas uppfattning bör dessa egenskaper göra järnvägen till ett förstahandsalternativ för effektiva och hållbara transporter i de segment där järnvägen är eller kan bli konkurrenskraftig.

    Prognoser och analys, samt erfaren­heter från utlandet, visar att trafikunderlaget i Sverige är tillräckligt för nya stambanor. Med de förslagna banorna väntas järnvägens totala kapacitet öka till mer än det dubbla i de mest belastade stråken. En viktig faktor är att den snabba och långsamma tågtrafiken separeras. Denna åtgärd ger ökad kapacitet, utöver vad de dubblerade spåren ger, eftersom tågen kan köra tätare efter varandra och störningarna i tågtrafiken minskar.

    Restiderna för orterna längs de nya stambanorna minskar kraftigt, i regel mellan 30 och 65%. Tillsammans med ökad turtäthet och minskade störningar ger det stora ökningar av tågtrafiken. De officiella prognoserna lider dock av ett antal allvarliga brister, varför både trafikökningen och den samhällsekonomiska lönsamheten beräkningsmässigt framstår som mindre än vad den enligt KTH:s prognoser och internationell erfarenhet borde vara.

    Författarna anser att anläggningskostnaderna är rimliga i relation till nyttorna och jämfört med vad andra omställningar i samhällets transportsystem kostar. Detsamma gäller den engångs ”klimatskuld” som uppkommer vid de flesta satsningar för framtiden inom alla trafikslag. Nya transportslag i ett tidigt utvecklingsskede (elflyg, magnettåg, Hyperloop etc) är mycket osäkra beträffande när eller om de överhuvudtaget kommer att bli tillgängliga för användning i stor skala. I flera fall skulle krävas stora tekniska genombrott som vi idag inte känner till. Vi anser att man rimligen inte idag kan besluta att satsa på helt nya tekniska system för vilka framtiden är mycket osäker. Vi kan inte heller ”vänta och se”, eftersom ytterligare kapacitet behövs redan idag och ledtiderna är långa.

    Sammanfattningsvis är de nya stambanorna ett samhällsbyggnadsprojekt och en del i transportsektorns nödvändiga omställning. De ger korta restider och effektiva transporter mellan våra största städer, liksom till och från ett stort antal mellanliggande orter, med omnejd. Godstransporterna kan också få plats på spåren och de kan utvecklas och effektiviseras. Det handlar om hållbar mobilitet för människor och gods i framtiden.

  • 3.
    Banaei, Arash Alizad
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Rosti, Marco E.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Brandt, Luca
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Numerical study of filament suspensions at finite inertia2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 882, article id A5Article in journal (Refereed)
    Abstract [en]

    We present a numerical study on the rheology of semi-dilute and concentrated filament suspensions of different bending stiffness and Reynolds number, with the immersed boundary method used to couple the fluid and solid. The filaments are considered as one-dimensional inextensible slender bodies with fixed aspect ratio, obeying the Euler-Bernoulli beam equation. To understand the global suspension behaviour we relate it to the filament microstructure, deformation and elastic energy and examine the stress budget to quantify the effect of the elastic contribution. At fixed volume fraction, the viscosity of the suspension reduces when decreasing the bending rigidity and grows when increasing the Reynolds number. The change in the relative viscosity is stronger at finite inertia, although still in the laminar flow regime, as considered here. Moreover, we find the first normal stress difference to be positive as in polymeric fluids, and to increase with the Reynolds number; its value has a peak for an intermediate value of the filament bending stiffness. The peak value is found to be proportional to the Reynolds number, moving towards more rigid suspensions at larger inertia. Moreover, the viscosity increases when increasing the filament volume fraction, and the rate of increase of the filament stress with the bending rigidity is stronger at higher Reynolds numbers and reduces with the volume fraction. We show that this behaviour is associated with the formation of a more ordered structure in the flow, where filaments tend to be more aligned and move as a compact aggregate, thus reducing the filament-filament interactions despite their volume fraction increases.

  • 4. Benard, N.
    et al.
    Sujar Garrido, Patricia
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bonnet, J. -P
    Moreau, E.
    Shear Layer and Shedding Modes Excitations of a Backward-Facing Step Flow by Surface Plasma Discharge2020In: Advances in Effective Flow Separation Control for Aircraft Drag Reduction, Springer, 2020, p. 55-74Chapter in book (Refereed)
    Abstract [en]

    The present experimental study interests in determining the influence of a linear plasma actuator (dielectric barrier discharge) on the development of a separated turbulent shear layer. More specifically, the plasma actuator is used to impose periodic perturbations at the step corner of a backward-facing step. Two different modes of excitation are explored. One concerns the shear layer mode of instability, a mode whose amplification leads to a minimization of the recirculation bubble. The present investigation shows how a dielectric barrier discharge plasma actuator can impose periodic perturbations that excite the shear layer mode and result in a strong regularization of the vortex street. The case of excitation at the shedding mode is also experimentally investigated using a DBD actuator. The measurements show the increase in Reynolds stress caused by this excitation as well as the specific growing mechanism of the shear layer. Indeed, phase-averaged flow measurements highlights the difference in the mechanism of development of the shear layer regarding the type of excitation used, the shear layer mode promoting a growing mechanism by fluid entrainment while the shedding mode enhancing the pairing of successive vortical flow structures.

  • 5.
    Bottier, Mathieu
    et al.
    Eq. 13, Institut Mondor de Recherche Biomédicale, Inserm U955, Créteil, France.
    Peña Fernández, Marta
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Biomechanics. Zeiss Global Centre, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, PO1 3DJ, UK.
    Pelle, Gabriel
    Eq. 13, Institut Mondor de Recherche Biomédicale, Inserm U955, Créteil, France.
    Isabey, Daniel
    Eq. 13, Institut Mondor de Recherche Biomédicale, Inserm U955, Créteil, France.
    Louis, Bruno
    Eq. 13, Institut Mondor de Recherche Biomédicale, Inserm U955, Créteil, France.
    Grotberg, James B
    Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI, USA..
    Filoche, Marcel
    Eq. 13, Institut Mondor de Recherche Biomédicale, Inserm U955, Créteil, France.
    A new index for characterizing micro-bead motion in a flow induced by ciliary beating: Part II, modeling.2017In: PloS Computational Biology, ISSN 1553-734X, E-ISSN 1553-7358, Vol. 13, no 7, article id e1005552Article in journal (Refereed)
    Abstract [en]

    Mucociliary clearance is one of the major lines of defense of the human respiratory system. The mucus layer coating the airways is constantly moved along and out of the lung by the activity of motile cilia, expelling at the same time particles trapped in it. The efficiency of the cilia motion can experimentally be assessed by measuring the velocity of micro-beads traveling through the fluid surrounding the cilia. Here we present a mathematical model of the fluid flow and of the micro-beads motion. The coordinated movement of the ciliated edge is represented as a continuous envelope imposing a periodic moving velocity boundary condition on the surrounding fluid. Vanishing velocity and vanishing shear stress boundary conditions are applied to the fluid at a finite distance above the ciliated edge. The flow field is expanded in powers of the amplitude of the individual cilium movement. It is found that the continuous component of the horizontal velocity at the ciliated edge generates a 2D fluid velocity field with a parabolic profile in the vertical direction, in agreement with the experimental measurements. Conversely, we show than this model can be used to extract microscopic properties of the cilia motion by extrapolating the micro-bead velocity measurement at the ciliated edge. Finally, we derive from these measurements a scalar index providing a direct assessment of the cilia beating efficiency. This index can easily be measured in patients without any modification of the current clinical procedures.

  • 6.
    Bouchouireb, Hamza
    KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics.
    Advancing the life cycle energy optimisation methodology2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The Life Cycle Energy Optimisation (LCEO) methodology aims at finding a design solution that uses a minimum amount of cumulative energy demand over the different phases of the vehicle's life cycle, while complying with a set of functional constraints. This effectively balances trade-offs, and therewith avoids sub-optimal shifting between the energy demand for the cradle-to-production of materials, operation of the vehicle, and end-of-life phases. This work further develops the LCEO methodology and expands its scope through three main methodological contributions which, for illustrative purposes, were applied to a vehicle sub-system design case study.

    An End-Of-Life (EOL) model, based on the substitution with a correction factor method, is included to estimate the energy credits and burdens that originate from EOL vehicle processing. Multiple recycling scenarios with different levels of assumed induced recyclate material property degradation were built, and their impact on the LCEO methodology's outcomes was compared to that of scenarios based on landfilling and incineration with energy recovery. The results show that the inclusion of EOL modelling in the LCEO methodology can alter material use patterns and significantly effect the life cycle energy of the optimal designs.

    Furthermore, the previous model is expanded to enable holistic vehicle product system design with the LCEO methodology. The constrained optimisation of a vehicle sub-system, and the design of a subset of the processes which are applied to it during its life cycle, are simultaneously optimised for a minimal product system life cycle energy. In particular, a subset of the EOL processes' parameters are considered as continuous design variables with associated barrier functions that control their feasibility. The results show that the LCEO methodology can be used to find an optimal design along with its associated ideal synthetic EOL scenario. Moreover, the ability of the method to identify the underlying mechanisms enabling the optimal solution's trade-offs is further demonstrated.

    Finally, the functional scope of the methodology is expanded through the inclusion of shape-related variables and aerodynamic drag estimations. Here, vehicle curvature is taken into account in the LCEO methodology through its impact on the aerodynamic drag and therewith its related operational energy demand. In turn, aerodynamic drag is considered through the estimation of the drag coefficient of a vehicle body shape using computational fluid dynamics simulations. The aforementioned coefficient is further used to estimate the energy required by the vehicle to overcome aerodynamic drag. The results demonstrate the ability of the LCEO methodology to capitalise on the underlying functional alignment of the structural and aerodynamic requirements, as well as the need for an allocation strategy for the aerodynamic drag energy within the context of vehicle sub-system redesign.

    Overall, these methodological developments contributed to the exploration of the ability of the LCEO methodology to handle life cycle and functional trade-offs to achieve life cycle energy optimal vehicle designs.

  • 7.
    Canton, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Rinaldi, Enrico
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Royal Inst Technol, Linne FLOW Ctr KTH Mech, SE-10044 Stockholm, Sweden..
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Critical Point for Bifurcation Cascades and Featureless Turbulence2020In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 124, no 1, article id 014501Article in journal (Refereed)
    Abstract [en]

    In this Letter we show that a bifurcation cascade and fully sustained turbulence can share the phase space of a fluid flow system, resulting in the presence of competing stable attractors. We analyze the toroidal pipe flow, which undergoes subcritical transition to turbulence at low pipe curvatures (pipe-to-torus diameter ratio) and supercritical transition at high curvatures, as was previously documented. We unveil an additional step in the bifurcation cascade and provide evidence that, in a narrow range of intermediate curvatures, its dynamics competes with that of sustained turbulence emerging through subcritical transition mechanisms.

  • 8.
    Casanueva, Carlos
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Rail Vehicles.
    Dirks, Babette
    Trafikverket.
    Tohmmy, Bustad
    Trafikverket.
    Rail Vehicle Dynamics Simulation-based decision support for novel block brake material implementation in Sweden2019In: THE 26TH IAVSD INTERNATIONAL SYMPOSIUM ON DYNAMICS OF VEHICLES ON ROADS AND TRACKS, 12-16 August 2019, Gothenburg, Sweden, 2019Conference paper (Other academic)
    Abstract [en]

    . The application of TSI Noise in Sweden needs decision support that can objectively state system-wide benefits and disadvantages, as there are issues with the introduction of novel block brakes: reduced braking performance and increased equivalent conicity. The Roll2Rail Universal Cost Model (UCM) is used to analyse Life Cycle Cost (LCC), as it was also conceived so that it could be used within the decision making processes of infrastructure managers. The simulated characteristics are mainly track Rolling Contact Fatigue (RCF) due to the worsened dynamic train-track interaction.

    The full text will be freely available from 2020-04-30 21:46
  • 9.
    Ceccato, Chiara
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Barbier, C.
    BillerudKorsnäs, Grums, Värmland County, Sweden.
    Investigation of rolling contact between metal and rubber-covered cylinders governing the paper compaction process2019In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 163, article id 105156Article in journal (Refereed)
    Abstract [en]

    With the goal of partial or complete replacement of plastic components with paper-based products, 3D paper structures have seen a growing interest in industrial applications: from packaging to more complex daily life objects. However, the extensibility of paper has become a key issue within this context and is the main factor determining the formability of these products and determining the depth of the achievable shapes. The most effective way to increase paper's stretch potential is by subjecting the moist paper web to a compaction process, which can be achieved through an extensible unit that is located in the drying section of a paper machine, where the network experiences in-plane compression in the machine direction (MD), under out-of-plane lateral constraints. The objective of this work is to clarify the mechanisms governing the compression process and to evaluate the influence of various parameters on the final material properties to optimize the industrial production of extensible paper. The system operation has been simulated realistically and shows that paper experiences a compressive state passing through the nip, with plastic strains in MD direction being of the same order of the applied speed difference, which was expected experimentally in optimal conditions of compaction. Starting from an initial reference case, a sensitivity study has been performed to identify and address the following factors that may affect the compression state: (a) friction coefficients; (b) indentation level; (c) speed difference; and (d) rubber properties. The analysis of the numerical results gives an insight into the mechanisms governing the compaction operations and allows a better comprehension of the features controlling the process outcome.

  • 10.
    Chaparian, Emad
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    De Vita, Francesco
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Royal Inst Technol KTH, Mech Dept, SERC, Stockholm, Sweden..
    Brandt, Luca
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Royal Inst Technol KTH, Mech Dept, SERC, Stockholm, Sweden..
    Tammisola, Outi
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Yield-stress fluids in porous media: a comparison of viscoplastic and elastoviscoplastic flows2020In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 55, no 2, p. 331-342Article in journal (Refereed)
    Abstract [en]

    A numerical and theoretical study of yield-stress fluid flows in two types of model porous media is presented. We focus on viscoplastic and elastoviscoplastic flows to reveal some differences and similarities between these two classes of flows. Small elastic effects increase the pressure drop and also the size of unyielded regions in the flow which is the consequence of different stress solutions compare to viscoplastic flows. Yet, the velocity fields in the viscoplastic and elastoviscoplastic flows are comparable for small elastic effects. By increasing the yield stress, the difference in the pressure drops between the two classes of flows becomes smaller and smaller for both considered geometries. When the elastic effects increase, the elastoviscoplastic flow becomes time-dependent and some oscillations in the flow can be observed. Focusing on the regime of very large yield stress effects in the viscoplastic flow, we address in detail the interesting limit of 'flow/no flow': yield-stress fluids can resist small imposed pressure gradients and remain quiescent. The critical pressure gradient which should be exceeded to guarantee a continuous flow in the porous media will be reported. Finally, we propose a theoretical framework for studying the 'yield limit' in the porous media.

  • 11.
    Chauvat, Guillaume
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI).
    Peplinski, Adam
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI). KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI). KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Global linear analysis of a jet in cross-flow at low velocity ratios2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 889, article id A12Article in journal (Refereed)
    Abstract [en]

    The stability of the jet in cross-flow is investigated using a complete set-up including the flow inside the pipe. First, direct simulations were performed to find the critical velocity ratio as a function of the Reynolds number, keeping the boundary-layer displacement thickness fixed. At all Reynolds numbers investigated, there exists a steady regime at low velocity ratios. As the velocity ratio is increased, a bifurcation to a limit cycle composed of hairpin vortices is observed. The critical bulk velocity ratio is found at approximately for the Reynolds number , above which a global mode of the system becomes unstable. An impulse response analysis was performed and characteristics of the generated wave packets were analysed, which confirmed results of our global mode analysis. In order to study the sensitivity of this flow, we performed transient growth computations and also computed the optimal periodic forcing and its response. Even well below this stability limit, at , large transient growth ( in energy amplification) is possible and the resolvent norm of the linearized Navier-Stokes operator peaks above . This is accompanied with an extreme sensitivity of the spectrum to numerical details, making the computation of a few tens of eigenvalues close to the limit of what can be achieved with double precision arithmetic. We demonstrate that including the meshing of the jet pipe in the simulations does not change qualitatively the dynamics of the flow when compared to the simple Dirichlet boundary condition representing the jet velocity profile. This is in agreement with the recent experimental results of Klotz et al. (J. Fluid Mech., vol. 863, 2019, pp. 386-406) and in contrast to previous studies of Cambonie & Aider (Phys. Fluids, vol. 26, 2014, 084101). Our simulations also show that a small amount of noise at subcritical velocity ratios may trigger the shedding of hairpin vortices.

  • 12. Dall'Ara, Enrico
    Palanca, Marco
    Giorgi, Mario
    Cristofolini, Luca
    Tozzi, Gianluca
    Precision of Digital Volume Correlation approaches for strain analysis in bone imaged with micro-computed tomography at different dimensional levels2017In: Frontiers in MaterialsArticle in journal (Refereed)
  • 13.
    De Mori, Arianna
    et al.
    School of Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth PO1 2DT, UK..
    Peña Fernández, Marta
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Biomechanics. Zeiss Global Centre, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, PO1 3DJ, UK.
    Blunn, Gordon
    School of Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth PO1 2DT, UK..
    Tozzi, Gianluca
    Zeiss Global Centre, School of Engineering, University of Portsmouth, Portsmouth PO1 3DJ, UK..
    Roldo, Marta
    School of Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth PO1 2DT, UK..
    3D Printing and Electrospinning of Composite Hydrogels for Cartilage and Bone Tissue Engineering.2018In: Polymers, ISSN 2073-4360, E-ISSN 2073-4360, Vol. 10, no 3, article id E285Article in journal (Refereed)
    Abstract [en]

    Injuries of bone and cartilage constitute important health issues costing the National Health Service billions of pounds annually, in the UK only. Moreover, these damages can become cause of disability and loss of function for the patients with associated social costs and diminished quality of life. The biomechanical properties of these two tissues are massively different from each other and they are not uniform within the same tissue due to the specific anatomic location and function. In this perspective, tissue engineering (TE) has emerged as a promising approach to address the complexities associated with bone and cartilage regeneration. Tissue engineering aims at developing temporary three-dimensional multicomponent constructs to promote the natural healing process. Biomaterials, such as hydrogels, are currently extensively studied for their ability to reproduce both the ideal 3D extracellular environment for tissue growth and to have adequate mechanical properties for load bearing. This review will focus on the use of two manufacturing techniques, namely electrospinning and 3D printing, that present promise in the fabrication of complex composite gels for cartilage and bone tissue engineering applications.

  • 14.
    De Vita, Francesco
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Lagrée, P. -Y
    Sorbonne Université, CNRS, Institut Jean le Rond d'Alembert, Paris, 75005, France.
    Chibbaro, S.
    Sorbonne Université, CNRS, Institut Jean le Rond d'Alembert, Paris, 75005, France.
    Popinet, S.
    Sorbonne Université, CNRS, Institut Jean le Rond d'Alembert, Paris, 75005, France.
    Beyond Shallow Water: Appraisal of a numerical approach to hydraulic jumps based upon the Boundary Layer theory2020In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 79, p. 233-246Article in journal (Refereed)
    Abstract [en]

    We study the flow of a thin layer of fluid over a flat surface. Commonly, the 1-D Shallow-water or Saint-Venant set of equations are used to compute the solution of such flows. These simplified equations may be obtained through the integration of the Navier–Stokes equations over the depth of the fluid, but their solution requires the introduction of constitutive relations based on strict hypothesis on the flow régime. Here, we present an approach based on a kind of boundary layer system with hydrostatic pressure. This relaxes the need for closure relations which are instead obtained as solutions of the computation. It is then demonstrated that the corresponding closures are very dependent on the type of flow considered, for example laminar viscous slumps or hydraulic jumps. This has important practical consequences as far as the applicability of standard closures is concerned.

  • 15.
    De Vita, Francesco
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rosti, Marco E.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Caserta, Sergio
    Univ Naples Federico II, Dept Chem Mat & Ind Prod Engn, Piazzale V Tecchio 80, I-80125 Naples, Italy..
    Brandt, Luca
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the effect of coalescence on the rheology of emulsions2019In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 880, p. 969-991Article in journal (Refereed)
    Abstract [en]

    We present a numerical study of the rheology of a two-fluid emulsion in dilute and semidilute conditions. The analysis is performed for different capillary numbers, volume fractions and viscosity ratios under the assumption of negligible inertia and zero buoyancy force. The effective viscosity of the system increases for low values of the volume fraction and decreases for higher values, with a maximum for approximately 20% concentration of the disperse phase. When the dispersed fluid has lower viscosity, the normalised effective viscosity becomes smaller than 1 for high enough volume fractions. To single out the effect of droplet coalescence on the rheology of the emulsion we introduce an Eulerian force which prevents merging, effectively modelling the presence of surfactants in the system. When the coalescence is inhibited the effective viscosity is always greater than 1 and the curvature of the function representing the emulsion effective viscosity versus the volume fraction becomes positive, resembling the behaviour of suspensions of deformable particles. The reduction of the effective viscosity in the presence of coalescence is associated with the reduction of the total surface of the disperse phase when the droplets merge, which leads to a reduction of the interface tension contribution to the total shear stress. The probability density function of the flow topology parameter shows that the flow is mostly a shear flow in the matrix phase, with regions of extensional flow when the coalescence is prohibited. The flow in the disperse phase, instead, always shows rotational components. The first normal stress difference is positive, except for the smallest viscosity ratio considered, whereas the second normal difference is negative, with their ratio being constant with the volume fraction. Our results clearly show that the coalescence efficiency strongly affects the system rheology and that neglecting droplet merging can lead to erroneous predictions.

  • 16.
    Eliasson, Sara
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Barsoum, Zuheir ()
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.
    Wennhage, Per ()
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Development of fatigue testing procedure for unidirectional carbon fiber composites2019In: Procedia Structural Integrity, ISSN 2452-3216, Vol. 19, p. 81-89Article in journal (Refereed)
  • 17.
    Everitt, Carl-Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Öberg, Martin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    An imprint method to produce surface asperities for EHL and RCF experiments2020Report (Other academic)
    Abstract [en]

    A method was developed for creating single well defined surface asperities using an imprint technique. The proposed method can be used to create asperities of different heights and widths in the micrometre range. The technique for creating single surface asperities is based on rolling a hard disc with indents against a soft disc. The contact pressure will cause plastic deformation forcing material into the indents to create the asperities. The height of the asperities can be controlled by adjusting the applied force. After initial reshaping during the run-in process, the asperities were strong enough to survive more than 35 million EHL contact cycles. The method should thus be of great interest for the researchers investigating rolling contact fatigue experimentally. The method could also aid the research of the run in process by enabling tracing the development of specific surface defects. Since the method can produce high and strong asperities it might also prove useful for investigations of exactly how asperities deform under sever contacts conditions.  

  • 18.
    Friederici, Anke
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Köpp, Wiebke
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Atzori, Marco
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Weinkauf, Tino
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Distributed Percolation Analysis for Turbulent Flows2019In: 2019 IEEE 9th Symposium on Large Data Analysis and Visualization, LDAV 2019, Institute of Electrical and Electronics Engineers (IEEE), 2019, p. 42-51, article id 8944383Conference paper (Refereed)
    Abstract [en]

    Percolation analysis is a valuable tool to study the statistical properties of turbulent flows. It is based on computing the percolation function for a derived scalar field, thereby quantifying the relative volume of the largest connected component in a superlevel set for a decreasing threshold. We propose a novel memory-distributed parallel algorithm to finely sample the percolation function. It is based on a parallel version of the union-find algorithm interleaved with a global synchronization step for each threshold sample. The efficiency of this algorithm stems from the fact that operations in-between threshold samples can be freely reordered, are mostly local and thus require no inter-process communication. Our algorithm is significantly faster than previous algorithms for this purpose, and is neither constrained by memory size nor number of compute nodes compared to the conceptually related algorithm for extracting augmented merge trees. This makes percolation analysis much more accessible in a large range of scenarios. We explore the scaling of our algorithm for different data sizes, number of samples and number of MPI processes. We demonstrate the utility of percolation analysis using large turbulent flow data sets.

  • 19.
    Ge, Zhouyang
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    On droplet interactions and suspension flow: If you want to follow the dissertation but are not able to do so via zoom, please contact luca@mech.kth.se for further information2020Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Micron to millimetre sized droplets, precisely generated or sustained in controlled environment, have great potential in myriads of engineering applications functioning as the basic element to assemble metamaterials, deliver drugs, host surfactant, reduce friction and damp turbulence. The interaction of droplets from pairwise to collective levels is the most important factor in controlling these processes, yet little is known about the detailed mechanisms in various nonideal conditions. The present thesis combines a number of studies aiming to elucidate the physical principles of droplet interactions and suspension flow using both high- and low-fidelity numerical simulations.

    We first study flow-assisted droplet assembly in microfluidic channels, seeking to harness the droplet interactions to produce photonic bandgap materials. A novel interface-correction level set/ghost fluid method (ICLS/GFM) is developed to directly simulate liquid droplets under depletion forces. Comparing to previous methods, ICLS/GFM conserves the global mass of each fluid using a simple mass-correction scheme, accurately computes the surface tension and depletion forces under the same framework, and has subsequently been applied to investigate the droplet clustering observed in a microfluidic experiment. Our simulations, supported by theoretical derivations, suggest that the observed fast self-assembly arises from a combination of strong depletion forces, confinement-mediated shear alignments of the droplets, and fine-tuned inflow conditions of the microchannel. However, the interplay of these 3D effects negates a simple droplet interaction model of parametric dependence, rendering the design of microfluidic chips for photonic crystal fabrications difficult in practice.

    The next objective of the thesis is the implementation of a minimal hybrid lubrication/granular dynamics (HLGD) model for simulation of dense particle suspensions. The main ingredients of HLGD include (i) a frame-invariant, short-range lubrication model for spherical particles, and (ii) a soft-core, stick/slide frictional contact model activated when particles overlap. Since contact interactions dominate at high particle concentrations, we expect the methodology to be applicable for probing the jamming of non-spherical particles and the rheology of foams as well.

    Finally, we include two miscellaneous studies concerning the slippage property of liquid-infused surfaces and droplets statistics in a homogeneous turbulent shear flow. Overall, results of these simulations provide detailed flow visualisations and qualitative dependence of the target functional on various governing parameters, facilitating experimental and theoretical investigations to design more robust drag-reducing surfaces and predict droplet distributions in emulsions.

  • 20.
    Gualtieri, P.
    et al.
    Dipartimento di Ingegneria Meccanica e Aerospaziale, 'Sapienza' University, Via Eudossiana 18, 00184 Roma, Italy.
    Picano, F.
    KTH, Superseded Departments (pre-2005), Mechanics. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics. Department of Mechanics and Aeronautics, "Sapienza" University of Rome, Via Eudossiana 18, 00184 Rome, Italy;.
    Sardina, G.
    Department of Mechanics and Aeronautics, "Sapienza" University of Rome, Via Eudossiana 18, 00184 Rome; UKE - Università Kore di ENNA Facoltà di Ingegneria, Architettura e Scienze Motorie, Via delle Olimpiadi, 94100 Enna, Ital.
    Casciola, C. M.
    Department of Mechanics and Aeronautics, 'Sapienza' University of Rome, Via Eudossiana 18, 00184 Rome, Italy.
    Exact regularized point particle method for particle-laden flows in the two-way coupling regime2012In: Proceedings of the Seventh International Symposium On Turbulence Heat and Mass Transfer, 2012, p. 1342-1351Conference paper (Refereed)
    Abstract [en]

    In this paper we present a new methodology which is proved to capture the momentum exchange between a carrier turbulent flow and thousands of sub-Kolmogorov inertial particles. The velocity disturbance produced by the disperse phase is described in terms of exact regularized unsteady Stokes solutions. The approach is validated by addressing the motion of a single particle in still fluid and by comparing data of actual turbulent spatially homogeneous flows against results provided by the classical particle-in-cell method.

  • 21.
    Hörberg, Erik
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.
    An Experimental Investigation of Shape Distortions in Aerospace Composites: Du som saknar dator/datorvana kan kontakta akermo@kth.se för information2020Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Composite materials are increasingly used in primary structure of modern commercial aircraft. Its excellent material characteristics enables reduction of structural weight compared to traditional metal solutions and thereby offers reduction of fuel consumption and carbon dioxide (CO2) emissions. In the aerospace industry, carbon fibre reinforced plastics or CFRP is the most commonly used composite material, where the reinforcement is held together by a thermoset resin, often epoxy, referred to as the matrix.

    When manufacturing aircraft composite parts, the curing temperature is usually in-between 120°C to 180°C. As the constituents, i.e. fibre and matrix, have significantly different thermal expansion, the temperature difference from manufacturing of parts to assembly and in-service use results in shape distortions and/or development of residual stresses. With an increased size and complexity of structural parts used in modern aircraft, the development of efficient methods for shape distortion analysis are therefore becoming increasingly important. Shape distortions come from numerous sources and some of them like thermal expansion and chemical shrinkage during curing are fairly well studied and understood. The focus of this thesis is on less researched parameters such as the laminate bending stiffness and effects of moisture content.

    The bending stiffness of a laminate can be controlled by varying the thickness of the laminate, or by changing the layup sequence of individual plies. Paper A presents an experimental study on shape distortion were the effect of laminate bending stiffness is separated from that of the laminate thickness. The results show that it is possible to tailor the laminate layup in a way that is beneficial for in-plane loads, while still reducing the built-in stresses that occur in a composite component due to shape distortions.

    The second parameter investigated in this thesis is the laminate moisture content. Composite materials used in aircraft structures will be exposed to environmental effects such as varying temperatures and moisture. The exposure is seldom constant but varies over time, depending on seasonal change and geographical area of aircraft operation. In Paper B, the influence of laminate moisture content on shape distortions is experimentally investigated. It becomes clear that laminate moisture content has such a strong effect on shape distortions that it is important to control and predict for all composite structures.

    The results presented in this thesis show that both laminate bending stiffness and laminate moisture content have a great influence on shape distortions, and that further research and development is needed to improve the simulation methodology used within the aerospace industry. This is key to future cost-efficient production and assembly of large composite parts.

  • 22.
    Hörberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.
    Nyman, Tonny
    Åkermo, Malin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.
    Thickness effect on spring-in of prepreg composite L-profiles – An experimental study2019In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 209, p. 499-507Article in journal (Refereed)
    Abstract [en]

    This paper presents the results and analysis of an experimental study of laminate thickness effects on the springin and shape distortion of thermoset composite L profiles. The primary objective is to achieve a broader understanding of how shape distortion is affected by laminate bending stiffness and part thickness of L-shaped laminates whose thickness varies between 1 and 12 mm. The larger thicknesses in particular have not received much attention in previous research. This work further aims at distinguishing the pure (geometrical) thickness effect from that of the coupled laminate bending stiffness by comparing laminates with different lay-ups. The work is performed on test specimens subjected to both a standard cure cycle and post-cure heat treatment at elevated temperatures. In parallel, finite element (FE) analysis is performed to evaluate if variation in the bending stiffness or the laminate thickness affects the predicted spring-in angle. The results clearly show springin dependence on laminate thickness and bending stiffness, whereas this dependence is not well predicted by the FE approaches. It is concluded that both effects exist and that shape distortions are more strongly related to bending stiffness than to laminate thickness.

  • 23. Hörberg, Erik
    et al.
    Åkermo, Malin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.
    Moisture effect on shape distortions of curved prepreg laminatesManuscript (preprint) (Other academic)
    Abstract [en]

    The influence from moisture content on shape distortion of curved thermoset composite laminates is investigated experimentally. The particular objects of study are L-shaped carbon/epoxy specimens with a quasi-isotropic layup and the thickness varying between 1 and 12 mm. The effect on the shape distortion is quantified by means of spring-in angle measurements vs. accelerated moisture uptake utilising a climate chamber at 90°C and 95% relative humidity. The results clearly show a strong dependence on spring-in angles from laminate moisture content – the effect is in fact in the same order of magnitude as the spring-in from thermal and chemical shrinkage during curing. Moisture does thus not only affect shape distortions but also has to be taken into consideration and be carefully controlled when assessing spring-in due to other parameters. Finally, a model based on the experimental results predicting the spring-in angle is presented.

  • 24. Karnama, A.
    et al.
    Haghighi, E. B.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Organic data centers: A sustainable solution for computing facilities2019In: Results in Engineering, ISSN 2590-1230, Vol. 4, article id 100063Article in journal (Refereed)
    Abstract [en]

    In the present perspective article we provide an overview of on-going work in the literature and possible future development of organic data centers (ODC). These are defined as the combined operation of a data center and a greenhouse, and given their compatible thermal and operation requirements, ODCs have the potential to provide an excellent solution in terms of sustainability. In particular, we identify possible positive impacts of ODCs on at least 5 of the 17 United Nations (UN) Sustainable Development Goals (SDGs), including SDGs 2 and 13 on zero hunger and climate change, respectively.

  • 25. Krishna, Visakh V
    Henning, Arne
    DLR.
    Taracjak, Peter
    Tatravagonka Poporad.
    Deliverable 4.1- Development of Functional Requirements for Sustainable and Attractive European Rail Freight: A state of the art on running gears and wagon design2017Report (Other (popular science, discussion, etc.))
    Abstract [en]

    A state of the art on 

    • Existing wagon design
    • Existing braking designs and technologies
    • Running gear and wheelsets
    • Aerodynamics
    • Noise and Sound & Vibration of Freight Trains
  • 26.
    Krishna, Visakh V
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Six, Klaus
    Virtaul Vehicle Research Centre, Graz.
    Deliverable 4.3- Development of Functional Requirements for Sustainable and Attractive European Rail Freight : A state of the art on running gears and wagon design.: Preliminary running gear and wagon design concepts2018Report (Other (popular science, discussion, etc.))
    Abstract [en]

    The next generation freight wagons shall improve the competitiveness of rail freight logistics by providing more flexible and reliable high-capacity assets at competitive costs. This report describes the developed and chosen concepts on running gear, wheelsets and wagon design concepts. The proposed concepts for running gear, wagon designs, wheelsets design and materials assessment but also braking technologies, were assessed and evaluated comparing to the state of the art solutions for freight wagon technologies according the 5L-approach. A concept for verification and validation of aero-acoustics and aerodynamics optimization measures based on experimental and numerical analysis was developed. Furthermore, the designed concepts were assessed and benchmarked on their track and wheel friendliness (RCF and wear) by means of Vehicle/Track Model simulation applying the whole-system approach by means of stochastic simulation.

  • 27.
    Krishna, Visakh V
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hossein Nia, Saeed
    Stichel, Sebastian
    KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design. KTH, Superseded Departments (pre-2005), Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics.
    New Methodology to Estimate Costs Caused by Rail Wear and RCF Depending on the Type of Running Gear2020In: IAVSD 2019: Advances in Dynamics of Vehicles on Roads and Tracks / [ed] Matthijs Klomp, Fredrik Bruzelius, Jens Nielsen and Angela Hillemyr, 2020Conference paper (Refereed)
    Abstract [en]

    A new methodology to estimate costs for wear and Rolling Contact Fatigue (RCF) on rails that cause a major portion of track maintenance costs is presented. It is demonstrated for a standard UIC-Y25 bogie and a FR8RAIL bogie, a softer and cross-braced iteration of the former. The rail profile evolution and the surface-initiated fatigue on the rail surface for different track radii with progressive tonnage are calculated using multi-body simulations. ‘Fastrip’ is used to calculate tangential stresses on the contact patch. Various non-linearities in the vehicle and track models have been considered, based on running conditions on the Swedish Iron-ore line. A cyclic rail grinding procedure at fixed tonnage intervals based on recommendations from EN13231-5 is implemented, to also account for the effect of track maintenance on the rate of rail surface damage. The wear depths (W), worn cross-sectional areas (A) and the number of axle passes that carry a risk for RCF initiation (Nr) are presented and discussed for 100 Mega Gross Tonnes passage. In doing so, the methodology addresses ‘track-friendliness’ of the running gear considering both its design and the track maintenance strategies.

  • 28.
    Krishna, Visakh V
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Jobstfinke, Daniel
    Melzi, Stefano
    Berg, Mats
    KTH, Superseded Departments (pre-2005), Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design. KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    An integrated numerical framework to investigate the running safety of overlong freight trains2020In: Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit, ISSN 0954-4097, E-ISSN 2041-3017Article in journal (Refereed)
    Abstract [en]

    Long freight trains up to 1500 m in length are currently not in regular operation in Europe. One of the important reasons for the same is high inter-wagon forces generated during the operation, especially when pneumatic (P-type) brake systems are used. For long trains with multiple locomotives at different positions along the train, radio communication with necessary fail-safe mechanisms can be used to apply the brakes. Long freight train operation on a given line is subjected to various attributes such as braking/traction scenarios, loading patterns, wagon geometries, brake-block materials, buffer types, track design geometries, etc., which are referred to as heterogeneities. The complex longitudinal train dynamics arising in the train due to various heterogeneities play a major role in determining its running safety. In this context, the maximum in-train force refers to the maximum force developed between any two wagons along the train during operation. The tolerable longitudinal compressive force is the maximum compressive force that can be exerted on a wagon without resulting in its derailment. Here, the authors adopt a bottom-up approach to model pneumatic braking systems and inter-wagon interactions in multibody simulation environments to study the complex longitudinal train dynamics behavior and estimate maximum in-train forces and tolerable longitudinal compressive forces, subjected to various heterogeneities. These two force quantities intend to facilitate a given freight train operation by providing guidelines regarding the critical heterogeneities, that currently limit its safe operation. In doing so, the authors propose the notion to have an operation-based approval for long freight trains using the simulations-based tool.

  • 29. Krishna, Visakh V
    et al.
    Stichel, Sebastian
    KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design. KTH, Superseded Departments (pre-2005), Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics.
    Hossein Nia, Saeed
    et, al
    Deliverable 4.1- Development of Functional Requirements for Sustainable and Attractive European Rail Freight: Detailed running gear and wagon design concept2018Report (Other (popular science, discussion, etc.))
    Abstract [en]

    This Deliverable D4.4 “Detailed running gear and wagon design concept” describes the detailed design of a new concept for the core market wagon design. Firstly, the needs and the challenges of the actual market for freight wagons are listed in order to address the objectives and define the target parameters for the new freight wagon. Different subsystems of the wagon are developed and virtually designed as 3D model to be assessed, comprehensively analysed and compared. The assembly is taken into account from the beginning to provide an easier implementation of the new concept. This document focus on:

    • Wagon structure and chassis. The design of the wagon structure describes the coupling system, the adjustable end carriages, as well as the buffer compensation and it includes a structural analysis with different load schemes and boundary conditions.

    • Bogies and wheelset concepts. For this part, the use of break disks as a breaking system is defined in order to significantly reduce the noise level, and this document presents investigations focused on novel wheelset design and materials. Therefore, it is important the definition of wheel material, axle material as well as definition of noise damping systems for wheels.

  • 30.
    KULKARNI, ROHAN
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Rail Vehicles.
    Qazizadeh, Alireza
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Rail Vehicles.
    Berg, Mats
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Rail Vehicles.
    Stichel, Sebastian
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Rail Vehicles.
    FAULT DETECTION AND ISOLATION METHOD FOR VEHICLE RUNNING INSTABILITY FROM VEHICLE DYNAMICS RESPONSE USING MACHINE LEARNING2019In: Proceedings of 11th International Conference on Railway Bogies and Running Gears (BOGIE'19) / [ed] Prof. István ZOBORY, Budapest, 2019Conference paper (Refereed)
    Abstract [en]

    In this paper, a Fault Detection and Isolation (FDI) method is proposed for monitoring the vehicle running stability in a high-speed railway bogie. The objective is to detect and isolate the different faults of bogie components which are critical to vehicle stability, especially degraded yaw dampers and high equivalent conicity caused by wheel wear. The proposed method has two steps; firstly, signal features sensitive to the characteristics of running instability are extracted based on frequency domain and time domain analysis of lateral accelerations of bogie frame and axlebox; then these features along with vehicle speed are fed into machine learning based fault classifiers. The supervised machine learning based fault classifier are trained to identify the cause of observed running instability among yaw damper degradation and wheel-rail profile pair with high equivalent conicity. The Support Vector Machine (SVM) classifier with Linear and Gaussian kernels are trained by k-fold crossvalidation method and the hyperparameters are optimized with a bayesian optimization algorithm to minimize the classification error. These fault classifiers are trained and tested with an extensive database generated from numerical experiments performed by multibody simulation (MBS) software. The performance of Linear and Gaussian SVM fault classifiers is compared with each other to identify the best performing classifier. The results underline the ability of machine learning based fault classifiers to be used for FDI of vehicle running instability and outline the possibility of detecting and isolating bogie faults critical to the vehicle stability based on onboard measurement of vehicle dynamic response.

  • 31.
    Lacis, Ugis
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sudhakar, Y.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. School of Mechanical Sciences, Indian Institute of Technology Goa, Ponda, 403401, India.
    Pasche, Simon
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Transfer of mass and momentum at rough and porous surfaces2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 884, article id A21Article in journal (Refereed)
    Abstract [en]

    The surface texture of materials plays a critical role in wettability, turbulence and transport phenomena. In order to design surfaces for these applications, it is desirable to characterise non-smooth and porous materials by their ability to exchange mass and momentum with flowing fluids. While the underlying physics of the tangential (slip) velocity at a fluid-solid interface is well understood, the importance and treatment of normal (transpiration) velocity and normal stress is unclear. We show that, when the slip velocity varies at an interface above the texture, a non-zero transpiration velocity arises from mass conservation. The ability of a given surface texture to accommodate a normal velocity of this kind is quantified by a transpiration length. We further demonstrate that normal momentum transfer gives rise to a pressure jump. For a porous material, the pressure jump can be characterised by so-called resistance coefficients. By solving five Stokes problems, the introduced measures of slip, transpiration and resistance can be determined for any anisotropic non-smooth surface consisting of regularly repeating geometric patterns. The proposed conditions are a subset of the effective boundary conditions derived from formal multi-scale expansion. We validate and demonstrate the physical significance of the effective conditions on two canonical problems - a lid-driven cavity and a turbulent channel flow, both with non-smooth bottom surfaces.

  • 32.
    Le Clainche, S.
    et al.
    Univ Politecn Madrid, Sch Aerosp Engn, E-28040 Madrid, Spain..
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rosti, Marco E.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Coherent structures in the turbulent channel flow of an elastoviscoplastic fluid2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 888, article id A5Article in journal (Refereed)
    Abstract [en]

    In this numerical and theoretical work, we study the turbulent channel flow of Newtonian and elastoviscoplastic fluids. The coherent structures in these flows are identified by means of higher order dynamic mode decomposition (HODMD), applied to a set of data non-equidistant in time, to reveal the role of the near-wall streaks and their breakdown, and the interplay between turbulent dynamics and non-Newtonian effects. HODMD identifies six different high-amplitude modes, which either describe the yielded flow or the yielded-unyielded flow interaction. The structure of the low- and high-frequency modes suggests that the interaction between high- and low-speed streamwise velocity structures is one of the mechanisms triggering the streak breakdown, dominant in Newtonian turbulence where we observe shorter near-wall streaks and a more chaotic dynamics. As the influence of elasticity and plasticity increases, the flow becomes more correlated in the streamwise direction, with long streaks disrupted for short times by localised perturbations, reflected in reduced drag. Finally, we present streamwise-periodic dynamic mode decomposition modes as a viable tool to describe the highly complex turbulent flows, and identify simple well-organised groups of travelling waves.

  • 33.
    Lind, Petter N.
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Augmented Single Loop Single Vector Algorithm Using Nonlinear Approximations of Constraints in Reliability-Based Design Optimization2019In: Journal of mechanical design (1990), ISSN 1050-0472, E-ISSN 1528-9001, Vol. 141, no 10, article id 101403Article in journal (Refereed)
    Abstract [en]

    Reliability-based design optimization (RBDO) aims at minimizing a function of probabilistic design variables, given a maximum allowed probability of failure. The most efficient methods available for solving moderately nonlinear problems are single loop single vector (SLSV) algorithms that use a first-order approximation of the probability of failure in order to rewrite the inherently nested structure of the loop into a more efficient single loop algorithm. The research presented in this paper takes off from the fundamental idea of this algorithm. An augmented SLSV algorithm is proposed that increases the rate of convergence by making nonlinear approximations of the constraints. The nonlinear approximations are constructed in the following way: first, the SLSV experiments are performed. The gradient of the performance function is known, as well as an estimate of the most probable failure point (MPP). Then, one extra experiment, a probe point, per performance function is conducted at the first estimate of the MPP. The gradient of each performance function is not updated but the probe point facilitates the use of a natural cubic spline as an approximation of an augmented MPP estimate. The SLSV algorithm using probing (SLSVP) also incorporates a simple and effective move limit (ML) strategy that also minimizes the heuristics needed for initiating the optimization algorithm. The size of the forward finite difference design of experiment (DOE) is scaled proportionally with the change of the ML and so is the relative position of the MPP estimate at the current iteration. Benchmark comparisons against results taken from the literature show that the SLSVP algorithm is more efficient than other established RBDO algorithms and converge in situations where the SLSV algorithm fails.

  • 34.
    Liu, Hailong
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Polymer Technology. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Ahlinder, Astrid
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Polymer Technology.
    Yassin, M. A.
    Finne Wistrand, Anna
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Polymer Technology.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Computational and experimental characterization of 3D-printed PCL structures toward the design of soft biological tissue scaffolds2020In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 188, article id 108488Article in journal (Refereed)
    Abstract [en]

    Degradable porous polymeric structures are attractive candidates for biological tissue scaffolds, and adequate mechanical, transport, chemical and biological properties determine their functionality. Aside from the properties of polymer-based materials, the scaffold's meso-structure controls its elasticity at the organ length-scale. This study investigated the effect of the meso-structure on scaffolds' mechanical and transport properties using finite element analysis (FEA) and computational fluid dynamics (CFD). A number of poly (ε-caprolactone) (PCL) - based scaffolds were 3D printed, analyzed by microcomputed tomography (micro-CT) and mechanically tested. We found that the gradient (G) and gradient and staggered (GS) meso-structure designs led to a higher scaffold permeability, a more homogeneous flow inside the scaffold, and a lower wall shear stress (WSS) in comparison with the basic (B) meso-structure design. The GS design resulted in scaffold stiffness as low as 1.07/0.97 MPa under compression/tension, figures that are comparative with several soft tissues. Image processing of micro-CT data demonstrated that the imposed meso-structures could have been adequately realized through 3D printing, and experimental testing validated FEA analysis. Our results suggest that the properties of 3D-printed PCL-based scaffolds can be tuned via meso-structures toward soft tissue engineering applications. The biological function of designed scaffolds should be further explored in-situ studies.

  • 35. Lu, Xuekun
    et al.
    Peña Fernández, Marta
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Biomechanics. Zeiss Global Centre, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, PO1 3DJ, UK.
    Bradley, Robert S
    Rawson, Shelley D
    O'Brien, Marie
    Hornberger, Benjamin
    Leibowitz, Marty
    Tozzi, Gianluca
    Withers, Philip J
    Anisotropic crack propagation and deformation in dentin observed by four-dimensional X-ray nano-computed tomography.2019In: Acta Biomaterialia, ISSN 1742-7061, E-ISSN 1878-7568, Vol. 96, p. 400-411, article id S1742-7061(19)30464-7Article in journal (Refereed)
    Abstract [en]

    Understanding the cracking behaviour of biological composite materials is of practical importance. This paper presents the first study to track the interplay between crack initiation, microfracture and plastic deformation in three dimensions (3D) as a function of tubule and collagen fibril arrangement in elephant dentin using in situ X-ray nano-computed tomography (nano-CT). A nano-indenter with a conical tip has been used to incrementally indent three test-pieces oriented at 0°, 45° and 70° to the long axis of the tubules (i.e. radial to the tusk). For the 0° sample two significant cracks formed, one of which linked up with microcracks in the axial-radial plane of the tusk originating from the tubules and the other one occurred as a consequence of shear deformation at the tubules. The 70° test-piece was able to bear the greatest loads despite many small cracks forming around the indenter. These were diverted by the microstructure and did not propagate significantly. The 45° test-piece showed intermediate behaviour. In all cases strains obtained by digital volume correlation were well in excess of the yield strain (0.9%), indeed some plastic deformation could even be seen through bending of the tubules. The hoop strains around the conical indenter were anisotropic with the smallest strains correlating with the primary collagen orientation (axial to the tusk) and the largest strains aligned with the hoop direction of the tusk. STATEMENT OF SIGNIFICANCE: This paper presents the first comprehensive study of the anisotropic nature of microfracture, crack propagation and deformation in elephant dentin using time-lapse X-ray nano-computed tomography. To unravel the interplay of collagen fibrils and local deformation, digital volume correlation (DVC) has been applied to map the local strain field while the crack initiation and propagation is tracked in real time. Our results highlight the intrinsic and extrinsic shielding mechanisms and correlate the crack growth behavior in nature to the service requirement of dentin to resist catastrophic fracture. This is of wide interest not just in terms of understanding dentin fracture but also can extend beyond dentin to other anisotropic structural composite biomaterials such as bone, antler and chitin.

  • 36.
    Lupi, Valerio
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Canton, J.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    On the onset of transition in 90°-bend pipe flow2019In: 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019, International Symposium on Turbulence and Shear Flow Phenomena, TSFP , 2019Conference paper (Refereed)
    Abstract [en]

    The present work deals with the global stability analysis of the flow in a 90◦-bend pipe with curvature δ = R/Rc = 0.3, being R the radius of the cross-section of the pipe and Rc the radius of curvature at the pipe centreline. Direct numerical simulations (DNS) for values of the bulk Reynolds number Reb = U-------D/v between 2000 and 3000 are performed. The bulk Reynolds number is based on the bulk velocity Ub, the pipe diameter D, and the kinematic viscosity ν. It is found that the flow is steady for Reb ≤ 2500, and two pairs of symmetric, counter-rotating vortices are observed in the section of the pipe downstream of the bend. Moreover, two recirculation regions are present inside the bend, one on the outer wall and the other on the inner one. For Reb ≥ 2550, the flow becomes periodic, oscillating with a fundamental non-dimensional frequency St = fD/Ub = 0.23. A global stability analysis reveals a pair of complex conjugate eigenvalues with positive real part. The velocity components of the unstable direct and adjoint eigenmodes are investigated, and it is observed that a large spatial separation occurs because of the non-normality of the linearised Navier–Stokes operator. Thus, an analysis of the structural sensitivity of the unstable eigenmode to spatially localised feedbacks is performed, in order to identify the core of the instability, the so-called wavemaker. It is found that the region located 15° downstream of the bend inlet, on the outer wall, is where the instability originates. Since flow separation is observed in this region, it is concluded that the instability is linked with the strong shear by the backflow phenomena.

  • 37. Mansour, Rami
    et al.
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Second-order reliability method based on Edgeworth expansion with application to reliability-based design optimization2019In: Proceedings of the ASME Design Engineering Technical Conference, 2019Conference paper (Refereed)
    Abstract [en]

    In the Second-Order Reliability Method, the limit-state function is approximated by a hyper-parabola in standard normal and uncorrelated space. However, there is no exact closed form expression for the probability of failure based on a hyper-parabolic limit-state function and the existing approximate formulas in the literature have been shown to have major drawbacks. Furthermore, in applications such as Reliability-based Design Optimization, analytical expressions, not only for the probability of failure but also for probabilistic sensitivities, are highly desirable for efficiency reasons. In this paper, a novel Second-Order Reliability Method is presented. The proposed expression is a function of three statistical measures: the Cornell Reliability Index, the skewness and the Kurtosis of the hyper-parabola. These statistical measures are functions of the First-Order Reliability Index and the curvatures at the Most Probable Point. Furthermore, analytical sensitivities with respect to mean values of random variables and deterministic variables are presented. The sensitivities can be seen as the product of the sensitivities computed using the First-Order Reliability Method and a correction factor. The proposed expressions are studied and their applicability to Reliability-based Design Optimization is demonstrated.

  • 38.
    Mansour, Rami
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Zhu, Jinchao
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Edgren, Martin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Barsoum, Zuheir
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    A probabilistic model of weld penetration depth based on process parameters2019In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 105, no 1-4, p. 499-514Article in journal (Refereed)
    Abstract [en]

    In welded structures using robotized metal active gas (MAG) welding, unwanted variation in penetration depth is typically observed. This is due to uncertainties in the process parameters which cannot be fully controlled. In this work, an analytical probabilistic model is developed to predict the probability of satisfying a target penetration, in the presence of these uncertainties. The proposed probabilistic model incorporates both aleatory process parameter uncertainties and epistemic measurement uncertainties. The latter is evaluated using a novel digital tool for weld penetration measurement. The applicability of the model is demonstrated on fillet welds based on an experimental investigation. The studied input process parameters are voltage, current, travel speed, and torch travel angle. The uncertainties in these parameters are modelled using adequate probability distributions and a statistical correlation based on the volt-ampere characteristic of the power source. Using the proposed probabilistic model, it is shown that a traditional deterministic approach in setting the input process parameters typically results in only a 50% probability of satisfying a target penetration level. It is also shown that, using the proposed expressions, process parameter set-ups satisfying a desired probability level can be simply identified. Furthermore, the contribution of the input uncertainties to the variation of weld penetration is quantified. This work paves the way to make effective use of the robotic welding, by targeting a specified probability of satisfying a desired weld penetration depth as well as predicting its variation.

  • 39.
    Mao, Huina
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Rumpler, Romain
    KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design. KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Marcus Wallenberg Laboratory MWL.
    Göransson, Peter
    KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    An inverse method for characterisation of the static elastic Hooke's tensors of solid frame of anisotropic open-cell materials2020In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 147, article id 103198Article in journal (Refereed)
    Abstract [en]

    This paper proposes an inverse estimation method for the extraction of the equivalent, static elastic, Hooke's tensor. The inversion is based on a fitting of the displacements, obtained from a combination of static compression and shear traction loads, on the faces of a sample specimen. An equivalent, homogenised material model is found by varying the elastic moduli until a defined cost function, based on the error measured as the difference between the displacement fields, has reached a minimum, at which an anisotropic constitutive solid model has been identified. The method is built on a multi-level step-wise approach, both from a computational as well as an assumed constitutive model symmetry point of view. The principle of the method is validated for a target anisotropic solid material model. The proposed multi-level approach is developed and refined for a known open-cell structure based on the Kelvin cell geometry. The accuracy of the method is verified and various strategies for increasing the rate of convergence in the inversion are discussed.

  • 40.
    Mao, Huina
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Rumpler, Romain
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Göransson, Peter
    KTH, Superseded Departments (pre-2005), Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    An inverse method for design and characterisation of acoustic materials2019Conference paper (Refereed)
    Abstract [en]

    This paper presents applications of an inverse method for the design and characterisation of anisotropic elastic material properties of acoustic porous materials. Full field 3D displacements under static surface loads are used as targets in the inverse estimation to fit a material model of an equivalent solid to the measurement data. Test cases of artificial open-cell foams are used, and the accuracy of the results are verified. The method is shown to be able to successfully characterise both isotropic and anisotropic elastic material properties. The paper demonstrates a way to reduce costs by characterising material properties based on the design model without a need for manufacturing and additional experimental tests.

  • 41.
    Marchioli, C.
    et al.
    Univ Udine, Dept Engn & Architecture, I-33100 Udine, Italy.;CISM, Dept Fluid Mech, I-33100 Udine, Italy..
    Bhatia, H.
    Univ Udine, Dept Engn & Architecture, I-33100 Udine, Italy..
    Sardina, G.
    Chalmers Univ Technol, Dept Mech & Maritime Sci, Div Fluid Dynam, S-41258 Gothenburg, Sweden..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Soldati, A.
    Univ Udine, Dept Engn & Architecture, I-33100 Udine, Italy.;TU Wien, Inst Fluid Mech & Heat Transfer, A-1040 Vienna, Austria..
    Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers2019In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 12, article id 124304Article in journal (Refereed)
    Abstract [en]

    In this work, we use direct-numerical-simulation-based Eulerian-Lagrangian simulations to investigate the dynamics of small gyrotactic swimmers in free-surface turbulence. We consider open-channel flow turbulence in which bottom-heavy swimmers are dispersed. Swimmers are characterized by different vertical stability, so that some realign to swim upward with a characteristic time smaller than the Kolmogorov timescale, while others possess a reorientation time longer than the Kolmogorov timescale. We cover one order of magnitude in the flow Reynolds number and two orders of magnitude in the stability number, which is a measure of bottom heaviness. We observe that large-scale advection dominates vertical motion when the stability number, scaled on the local Kolmogorov timescale of the flow, is larger than unity: This condition is associated to enhanced migration toward the surface, particularly at low Reynolds number, when swimmers can rise through surface renewal motions that originate directly from the bottom-boundary turbulent bursts. Conversely, small-scale effects become more important when the Kolmogorov-based stability number is below unity: Under this condition, migration toward the surface is hindered, particularly at high Reynolds, when bottom-boundary bursts are less effective in bringing bulk fluid to the surface. In an effort to provide scaling arguments to improve predictions of models for motile microorganisms in turbulent water bodies, we demonstrate that a Kolmogorov-based stability number around unity represents a threshold beyond which swimmer capability to reach the free surface and form clusters saturates.

  • 42.
    Marin, Gustav
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. RISE Innventia.
    Nygårds, Mikael
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Stiffness and strength properties of five paperboards and their moisture dependency2019In: Proceedings of the 2019 International Paper Physics Conference / [ed] Doug Coffin, Tappi , 2019Conference paper (Refereed)
    Abstract [en]

    Five folding box boards made on the same paperboard machine have been analyzed. The paperboards were from the same product series but had different grammage (235, 255, 270, 315, 340 g/m2) and different bending stiffness. The paperboards are normally used to make packages, and since the bending stiffness and grammage varies the packages performance will be different. Finite element simulations can be used to predict these differences. However, the stiffness and strength properties then need to be known. For efficient determination of the three-dimensional properties in MD, CD and ZD, it is proposed that the whole paperboard should be characterized with the following tests: in-plane tension, ZD tension, shear strength profiles and two-point bending. The stiffness and strength properties have with the proposed setups been determined at different relative humidity (20, 50, 70 and 90 % RH), and the mechanical properties have been evaluated as function of moisture ratio.

    The results showed a linear relation between mechanical properties and moisture ratio for each paperboard. The data was then normalized with data for the standard climate (50 % RH) and investigated as a function of moisture ratio. The results indicated that the normalized mechanical properties for all paperboards coincided along one single line and could therefore be expressed as a linear function of moisture ratio and two constants.

    Consequently, the study indicates that it is possible to obtain the mechanical properties of a paperboard, by knowing the structural properties for the preferred level of RH and the mechanical property for the standard climate (50 % RH and 23 °C).

  • 43.
    Matsubara, Masaharu
    et al.
    Shinshu Univ, Dept Mech Syst Engn, Wakasato 4-17-1, Nagano 3808553, Japan..
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Linear modes in a planar turbulent jet2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 888, article id A26Article in journal (Refereed)
    Abstract [en]

    A planar jet issuing from a fully developed two-dimensional turbulent channel flow is studied, with a focus on the transverse flapping of the jet core. The streamwise and transverse velocities were measured with hot-wire anemometry using an X-type probe. The mean velocity field and the velocity covariances were first characterised to assess the undisturbed flow field. Periodic excitations were introduced from a slot mounted at the channel exit and the coherent fluctuating part of the signal was obtained by using a phase-locked averaging technique, where the periodic initial forcing was used as trigger. This enabled the eduction of the coherent structure associated with the introduced perturbation. Its amplitude was found to be directly proportional to the intensity of the initial forcing and, within a certain range of the initial forcing amplitude, the growth curves were identical as well as the spatial distribution of the extracted fluctuations. Parallel and non-parallel linear stability theory captures qualitatively and quantitatively the features of the educed coherent structure. The existence of the linear mode in the turbulent jet implies that the large-scale perturbations observed in natural (unforced) jets can be regarded as an incoherent set of linear modes.

  • 44.
    Mittal, Nitesh
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Fluid Physics. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Massachusetts Institute of Technology, Cambridge, MA 02142, United States.
    Benselfelt, Tobias
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fibre Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Ansari, F.
    Gordeyeva, Korneliya
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Roth, Stephan Volkher
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Coating Technology. DESY, D-22607 Hamburg, Germany.
    Wågberg, Lars
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Ion-Specific Assembly of Strong, Tough, and Stiff Biofibers2019In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 58, no 51, p. 18562-18569Article in journal (Refereed)
    Abstract [en]

    Designing engineering materials with high stiffness and high toughness is challenging as stiff materials tend to be brittle. Many biological materials realize this objective through multiscale (i.e., atomic- to macroscale) mechanisms that are extremely difficult to replicate in synthetic materials. Inspired from the architecture of such biological structures, we here present flow-assisted organization and assembly of renewable native cellulose nanofibrils (CNFs), which yields highly anisotropic biofibers characterized by a unique combination of high strength (1010 MPa), high toughness (62 MJ m−3) and high stiffness (57 GPa). We observed that properties of the fibers are primarily governed by specific ion characteristics such as hydration enthalpy and polarizability. A fundamental facet of this study is thus to elucidate the role of specific anion binding following the Hofmeister series on the mechanical properties of wet fibrillar networks, and link this to the differences in properties of dry nanostructured fibers. This knowledge is useful for rational design of nanomaterials and is critical for validation of specific ion effect theories. The bioinspired assembly demonstrated here is relevant example for designing high-performance materials with absolute structural control.

  • 45.
    Nagib, H. M.
    et al.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Vidal, A.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vorticity fluxes: A tool for three-dimensional and secondary flows in turbulent shear flows2019In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 89, p. 39-48Article in journal (Refereed)
    Abstract [en]

    In this work we extend the vorticity-flux approach, proposed by Brown and Roshko (2012) for the analysis of turbulent shear layers and wakes, to the study of secondary flows of Prandtl's second kind. To this end, we assess direct numerical simulations (DNSs) of turbulent flow through sinusoidal channels (Vidal et al., 2018a) at bulk Reynolds numbers Re h = 2500 and 5000, and with various wall wave parameters, leading to a range of secondary flow intensities. We find that the fluctuating vorticity-flux difference (w'omega(y)') over bar (+) - (v'omega(z)') over bar (+) is closely connected to the in-plane cross-flow, in particular the large negative values present around the wall peak, which enhance the transport of near-wall momentum towards the channel core. The tilting of sweep events at the wall valley is also connected to the secondary flow magnitude, and is associated with positive values of the fluctuating vorticity-flux difference. Furthermore, conditionally averaged fields show that, unlike what is observed in channels with flat walls, the behavior in the vorticity-flux field at the peak is mostly due to Q1 and Q4 events, which essentially tilt momentum towards the peak.

  • 46.
    Peña Fernández, Marta
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Biomechanics. Zeiss Global Centre, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, PO1 3DJ, UK.
    Bodey, Andrew (Contributor)
    Parwani, Rachna
    Blunn, Gordon
    Barber, Asa
    Tozzi, Gianluca
    Full-field strain analysis of bone-biomaterial systems produced by osteoregenerative biomaterials after in vivo service in an ovine model2019In: ACS Biomaterials Science and EngineeringArticle in journal (Refereed)
  • 47.
    Peña Fernández, Marta
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Biomechanics. Zeiss Global Centre, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, PO1 3DJ, UK.
    Barber, A H
    Blunn, G W
    Tozzi, G
    Optimization of digital volume correlation computation in SR-microCT images of trabecular bone and bone-biomaterial systems.2018In: Journal of Microscopy, ISSN 0022-2720, E-ISSN 1365-2818, Vol. 272, no 3, p. 213-228Article in journal (Refereed)
    Abstract [en]

    A micromechanical characterization of biomaterials for bone tissue engineering is essential to understand the quality of the newly regenerated bone, enabling the improvement of tissue regeneration strategies. A combination of microcomputed tomography in conjunction with in situ mechanical testing and digital volume correlation (DVC) has become a powerful technique to investigate the internal deformation of bone structure at a range of dimensional scales. However, in order to obtain accurate three-dimensional strain measurement at tissue level, high-resolution images must be acquired, and displacement/strain measurement uncertainties evaluated. The aim of this study was to optimize imaging parameters, image postprocessing and DVC settings to enhance computation based on 'zero-strain' repeated high-resolution synchrotron microCT scans of trabecular bone and bone-biomaterial systems. Low exposures to SR X-ray radiation were required to minimize irradiation-induced tissue damage, resulting in the need of advanced three-dimensional filters on the reconstructed images to reduce DVC-measured strain errors. Furthermore, the computation of strain values only in the hard phase (i.e. bone, biomaterial) allowed the exclusion of large artefacts localized in the bone marrow. This study demonstrated the suitability of a local DVC approach based on synchrotron microCT images to investigate the micromechanics of trabecular bone and bone-biomaterial composites at tissue level with a standard deviation of the errors in the region of 100 microstrain after a thorough optimization of DVC computation. LAY DESCRIPTION: Understanding the quality of newly regenerated bone after implantation of novel biomaterials is essential to improve bone tissue engineering strategies and formulation of biomaterials. The relationship between microstructure and mechanics of bone has been previously addressed combining microcomputed tomography with in situ mechanical testing. The addition of an image-based experimental technique such as digital volume correlation (DVC) allows to characterize the deformation of materials in a three-dimensional manner. However, in order to obtain accurate information at the micro-scale, high-resolution images, obtained for example by using synchrotron radiation microcomputed tomography, as well as optimization of the DVC computation are needed. This study presents the effect of different imaging parameters, image postprocessing and DVC settings for as accurate investigation of trabecular bone structure and bone-biomaterial interfaces. The results showed that when appropriate image postprocessing and DVC settings are used DVC computation results in very low strain errors. This is of vital importance for a correct understanding of the deformation in bone-biomaterial systems and the ability of such biomaterials in producing new bone comparable with the native tissue they are meant to replace.

  • 48.
    Peña Fernández, Marta
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Biomechanics. Zeiss Global Centre, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, PO1 3DJ, UK.
    Black, Cameron
    Dawson, Jon
    Gibbs, David
    Kanczler, Janos
    Oreffo, Richard O C
    Tozzi, Gianluca
    Exploratory Full-Field Strain Analysis of Regenerated Bone Tissue from Osteoinductive Biomaterials.2020In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 13, no 1, article id E168Article in journal (Refereed)
    Abstract [en]

    Biomaterials for bone regeneration are constantly under development, and their application in critical-sized defects represents a promising alternative to bone grafting techniques. However, the ability of all these materials to produce bone mechanically comparable with the native tissue remains unclear. This study aims to explore the full-field strain evolution in newly formed bone tissue produced in vivo by different osteoinductive strategies, including delivery systems for BMP-2 release. In situ high-resolution X-ray micro-computed tomography (microCT) and digital volume correlation (DVC) were used to qualitatively assess the micromechanics of regenerated bone tissue. Local strain in the tissue was evaluated in relation to the different bone morphometry and mineralization for specimens (n = 2 p/treatment) retrieved at a single time point (10 weeks in vivo). Results indicated a variety of load-transfer ability for the different treatments, highlighting the mechanical adaptation of bone structure in the early stages of bone healing. Although exploratory due to the limited sample size, the findings and analysis reported herein suggest how the combination of microCT and DVC can provide enhanced understanding of the micromechanics of newly formed bone produced in vivo, with the potential to inform further development of novel bone regeneration approaches.

  • 49.
    Peña Fernández, Marta
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Biomechanics. Zeiss Global Centre, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, PO1 3DJ, UK.
    Cipiccia, Silvia
    Dall'Ara, Enrico
    Bodey, Andrew J
    Parwani, Rachna
    Pani, Martino
    Blunn, Gordon W
    Barber, Asa H
    Tozzi, Gianluca
    Effect of SR-microCT radiation on the mechanical integrity of trabecular bone using in situ mechanical testing and digital volume correlation.2018In: Journal of The Mechanical Behavior of Biomedical Materials, ISSN 1751-6161, E-ISSN 1878-0180, Vol. 88, p. 109-119, article id S1751-6161(18)30763-XArticle in journal (Refereed)
    Abstract [en]

    The use of synchrotron radiation micro-computed tomography (SR-microCT) is becoming increasingly popular for studying the relationship between microstructure and bone mechanics subjected to in situ mechanical testing. However, it is well known that the effect of SR X-ray radiation can considerably alter the mechanical properties of bone tissue. Digital volume correlation (DVC) has been extensively used to compute full-field strain distributions in bone specimens subjected to step-wise mechanical loading, but tissue damage from sequential SR-microCT scans has not been previously addressed. Therefore, the aim of this study is to examine the influence of SR irradiation-induced microdamage on the apparent elastic properties of trabecular bone using DVC applied to in situ SR-microCT tomograms obtained with different exposure times. Results showed how DVC was able to identify high local strain levels (> 10,000 µε) corresponding to visible microcracks at high irradiation doses (~ 230 kGy), despite the apparent elastic properties remained unaltered. Microcracks were not detected and bone plasticity was preserved for low irradiation doses (~ 33 kGy), although image quality and consequently, DVC performance were reduced. DVC results suggested some local deterioration of tissue that might have resulted from mechanical strain concentration further enhanced by some level of local irradiation even for low accumulated dose.

  • 50.
    Peña Fernández, Marta
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Biomechanics. Zeiss Global Centre, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, PO1 3DJ, UK.
    Dall'Ara, Enrico
    Kao, Alexander P
    Bodey, Andrew J
    Karali, Aikaterina
    Blunn, Gordon W
    Barber, Asa H
    Tozzi, Gianluca
    Preservation of Bone Tissue Integrity with Temperature Control for In Situ SR-MicroCT Experiments.2018In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 11, no 11, article id E2155Article in journal (Refereed)
    Abstract [en]

    Digital volume correlation (DVC), combined with in situ synchrotron microcomputed tomography (SR-microCT) mechanics, allows for 3D full-field strain measurement in bone at the tissue level. However, long exposures to SR radiation are known to induce bone damage, and reliable experimental protocols able to preserve tissue properties are still lacking. This study aims to propose a proof-of-concept methodology to retain bone tissue integrity, based on residual strain determination using DVC, by decreasing the environmental temperature during in situ SR-microCT testing. Compact and trabecular bone specimens underwent five consecutive full tomographic data collections either at room temperature or 0 °C. Lowering the temperature seemed to reduce microdamage in trabecular bone but had minimal effect on compact bone. A consistent temperature gradient was measured at each exposure period, and its prolonged effect over time may induce localised collagen denaturation and subsequent damage. DVC provided useful information on irradiation-induced microcrack initiation and propagation. Future work is necessary to apply these findings to in situ SR-microCT mechanical tests, and to establish protocols aiming to minimise the SR irradiation-induced damage of bone.

12 1 - 50 of 81
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