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  • 51. Alveroglu, B.
    et al.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Garrett, S. J.
    The effect of surface roughness on the convective instability of the BEK family of boundary-layer flows2016In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 56, p. 178-187Article in journal (Refereed)
    Abstract [en]

    A Chebyshev polynomial discretisation method is used to investigate the effect of both anisotropic (radially and azimuthally) and isotropic surface roughnesses on the convective instability of the BEK family of rotating boundary-layer flows. The mean-flow profiles for the velocity components are obtained by modelling surface roughness with a partial-slip approach. A linear stability analysis is then performed to investigate the effect of roughness on the convective instability characteristics of the inviscid Type I (cross-flow) instability and the viscous Type II instability. It is revealed that all roughness types lead to a stabilisation of the Type I mode in all flows within the BEK family, with the exception of azimuthally-anisotropic roughness (radial grooves) within the Bödewadt layer which causes a mildly destabilising effect. In the case of the Type II mode, the results reveal the destabilising effect of radially-anisotropic roughness (concentric grooves) on all the boundary layers, whereas both azimuthally-anisotropic and isotropic roughnesses have a stabilising effect on the mode for Ekman and von Kármán layers. Complementary results are also presented by considering the effects of roughness on the growth rates of each instability mode within the Ekman layer.

  • 52.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics. Södertörn Univ, Alfred Nobels Alle 7, S-14189 Huddinge, Sweden..
    Detailed modelling of contact line motion in oscillatory wetting2022In: NPJ MICROGRAVITY, ISSN 2373-8065, Vol. 8, no 1, article id 1Article in journal (Refereed)
    Abstract [en]

    The experimental results of Xia and Steen for the contact line dynamics of a drop placed on a vertically oscillating surface are analyzed by numerical phase field simulations. The concept of contact line mobility or friction is discussed, and an angle-dependent model is formulated. The results of numerical simulations based on this model are compared to the detailed experimental results of Xia and Steen with good general agreement. The total energy input in terms of work done by the oscillating support, and the dissipation at the contact line, are calculated from the simulated results. It is found that the contact line dissipation is almost entirely responsible for the dissipation that sets the amplitude of the response. It is argued that angle-dependent line friction may be a fruitful interpretation of the relations between contact line speed and dynamic contact angle that are often used in practical computational fluid dynamics.

  • 53.
    Amini, Kasra
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Janabadi, Ehsan Dehghani
    Univ Art, Fac Architecture & Urban Planning, Dept Architectural Technol, Tehran, Iran..
    Fayaz, Rima
    Univ Art, Fac Architecture & Urban Planning, Dept Architectural Technol, Tehran, Iran..
    Lighting and illumination investigation of long-term residence on Mars for the case of a set of designed Martian Habitat Units (MHUs)2022In: Acta Astronautica, ISSN 0094-5765, E-ISSN 1879-2030, Vol. 192, p. 210-232Article in journal (Refereed)
    Abstract [en]

    Addressing the subject matter of human missions on Mars, the Martian Habitat Units (MHUs) are presented as a comprehensive solution. MHUs are designed in clusters of 10 units, each capable of serving as long-term habitats for 9 scientific crew members. The life-style requirements of the units are targeted at an imitation of cultural thriving life we all know of, and not a mere survival-type shelter expecting the first people to step foot on Mars. One aspect of many challenging issues to be addressed in such complex settings is the lighting and illuminance condition of the said habitats, which in the context of Mars, and generally deep space missions being far from the sun will certainly lead to an arduous task. To check the validity of the argument and assess the extent to which the natural light level available on the surface of Mars will be sufficient for the daily requirements of the crew and mission in terms of illuminance, the current manuscript presents thorough and detailed simulations and analyses on the availability evaluation of natural lighting in the site location of MHUs, namely Valles Marineris, Melas Chasma. In this paper solar irradiation parameters on Mars are calculated based on the previous research which resulted in global, direct and diffuse irradiance at 12 different Martian solar times. The simulations are distributed over the Martian year and its day-time, and for two extreme orientations of MHUs in their circular surrounding cluster, namely East/West and South/North units. The distribution of illuminance for each case, and trend comparison studies are then accompanied by numerical values and analyses on the percentage to which the natural lighting conditions on Mars have been shown to be sufficient as a fraction of the whole lighting load of the habitats, which is to be compensated using artificial sources. The corresponding values are shown to fall well in the range of 35-45% of the total lighting loads. Also, as the results of the simulations show, due to the consistency of the glazed parts of the designed facade through all exterior surface of the MHU, natural lighting sufficiency percentage does not show a significant difference between two simulated orientations. This fact further approves the circular orientation premise of the MHUs in their cluster.

  • 54.
    Amini, Kasra
    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.
    Mishra, Ases Akas
    Department of Industrial and Materials Science, Chalmers University of Technology, 412 96 Gothenburg, Sweden.
    Sivakumar, Amit Kumar
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden.
    Arlov, Dragana
    Tetra Pak Processing Systems, 221 86 Lund, Sweden.
    Innings, Fredrik
    Tetra Pak Processing Systems, 221 86 Lund, Sweden.
    Kádár, Roland
    Department of Industrial and Materials Science, Chalmers University of Technology, 412 96 Gothenburg, Sweden.
    Tammisola, Outi
    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.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Scaling laws for near-wall flows of thixo-elasto-viscoplastic fluids in a millifluidic channel2024In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 36, no 2, article id 023107Article in journal (Refereed)
    Abstract [en]

    Thixo-elasto-viscoplastic (TEVP) fluids are very complex fluids. In addition to elasticity and viscoplasticity, they exhibit thixotropy, i.e., time-dependent rheology due to breakdown and recovery of internal structures at different length- and timescales. General and consistent methods for a priori flow prediction of TEVP fluids based on rheological characteristics are yet to be developed. We report a combined study of the rheology and flow of 18 samples of different TEVP fluids (three yogurts and three concentrations of Laponite and Carbopol, respectively, in water in both the unstirred and a stirred state). The rheology is determined both with standard protocols and with an ex situ protocol aiming at reproducing the shear history of the fluid in the flow. Micrometer resolution flow measurements in a millimeter scale rectangular duct are performed with Doppler Optical Coherence Tomography (D-OCT). As expected, the results show the existence of a plug flow region for samples with sufficiently high yield stress. At low flow rates, the plug extends almost all the way to the wall and the extent of the plug decreases not only with increased flow rate but also with increased thixotropy. The ex situ rheology protocol enables estimation of the shear rate and shear stress close to the wall, making it possible to identify two scaling laws that relates four different non-dimensional groups quantifying the key properties wall-shear stress and slip velocity. The scaling laws are suggested as an ansatz for a priori prediction of the near-wall flow of TEVP fluids based on shear flow-curves obtained with a rheometer.

  • 55.
    Amini, Kasra
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics. Univ Stuttgart, Inst Aerosp Thermodynam ITLR, Stuttgart, Germany..
    Moradi, Mojgan
    Univ Art, Fac Architecture & Urban Planning, Dept Architectural Technol, Tehran, Iran..
    Teymoori, Peyman Ebadi Belfeh
    Univ Art, Fac Architecture & Urban Planning, Dept Architectural Technol, Tehran, Iran..
    Vossoughi, Bahareh
    Rhein Westfal TH Aachen, Fac Mech Engn, Aachen, Germany.;Rhein Westfal TH Aachen, Inst Combust Engines Verbrennungskraftmaschinen, Aachen, Germany..
    Janabadi, Ehsan Dehghani
    Univ Art, Fac Architecture & Urban Planning, Dept Architectural Technol, Tehran, Iran..
    Fayaz, Rima
    Univ Art, Fac Architecture & Urban Planning, Dept Architectural Technol, Tehran, Iran..
    Design of a set of habitat units and the corresponding surrounding cluster for long-term scientific missions in the pre-terraforming era on mars2022In: Icarus, ISSN 0019-1035, E-ISSN 1090-2643, Vol. 385, p. 115119-, article id 115119Article in journal (Refereed)
    Abstract [en]

    We are living in a point in the history of science and technology, where space travel for research and settlement is inevitable. As the utmost crucial technology pieces for leaving Earth and travelling into the cosmos is being established one after another, it is just a matter of decades, until it all gets integrated together, solving the engineering problems ahead of the way and being able to step on the planets and moons of the solar system. In this quest, as has been the case for most of the technological advancements so far, there ought to be mind experiments, in which one skips one step, assumes the availability of responses to the skipped-over step, and searches for the solution to the questions of the next level. This way, by getting passed the first, i.e. current step, the solution to the next one is already available. The current manuscript is addressing this very 'next step', on the long path to eventually colonize Mars and inhabit it for long-term research-based missions; let it be for terraforming, or other agenda to be defined by the research strategists, then. And as mentioned earlier, the current step; being setting foot on Mars, is well-deservedly taken for granted, as is to come forth undoubtedly. Having that realized, we might find ourselves faced by the engineering complexities of surviving and thriving on Mars, which is the subject matter of the current research, from the aspect point of space technological and architectural design. The design procedure beginning from setting the philosophy of design upon the concerns of sustaining in the hostile environment of Mars, to the stepwise emergence of the final design of a cluster of Martian Habitat Units (MHUs) considering the high-criteria of the case, is the subject matter covered in this manuscript.

  • 56.
    Amo-Navarro, Jesus
    et al.
    Univ Politecn Valencia, Inst Univ Matemat Pura & Aplicada, Valencia 46022, Spain..
    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.
    Conejero, J. Alberto
    Univ Politecn Valencia, Inst Univ Matemat Pura & Aplicada, Valencia 46022, Spain..
    Hoyas, Sergio
    Univ Politecn Valencia, Inst Univ Matemat Pura & Aplicada, Valencia 46022, Spain..
    Two-Dimensional Compact-Finite-Difference Schemes for Solving the bi-Laplacian Operator with Homogeneous Wall-Normal Derivatives2021In: Mathematics, E-ISSN 2227-7390, Vol. 9, no 19, article id 2508Article in journal (Refereed)
    Abstract [en]

    In fluid mechanics, the bi-Laplacian operator with Neumann homogeneous boundary conditions emerges when transforming the Navier-Stokes equations to the vorticity-velocity formulation. In the case of problems with a periodic direction, the problem can be transformed into multiple, independent, two-dimensional fourth-order elliptic problems. An efficient method to solve these two-dimensional bi-Laplacian operators with Neumann homogeneus boundary conditions was designed and validated using 2D compact finite difference schemes. The solution is formulated as a linear combination of auxiliary solutions, as many as the number of points on the boundary, a method that was prohibitive some years ago due to the large memory requirements to store all these auxiliary functions. The validation has been made for different field configurations, grid sizes, and stencils of the numerical scheme, showing its potential to tackle high gradient fields as those that can be found in turbulent flows.

  • 57.
    Amor, Christian
    et al.
    Univ Politecn Madrid, Sch Aerosp Engn, Madrid 28040, Spain..
    Perez, Jose M.
    Univ Politecn Madrid, Sch Aerosp Engn, Madrid 28040, Spain..
    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.
    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.
    Le Clainche, Soledad
    Univ Politecn Madrid, Sch Aerosp Engn, Madrid 28040, Spain..
    Modeling the Turbulent Wake Behind a Wall-Mounted Square Cylinder2020In: Logic journal of the IGPL (Print), ISSN 1367-0751, E-ISSN 1368-9894, Vol. 30, no 2, p. 263-276Article in journal (Refereed)
    Abstract [en]

    This article introduces some soft computing methods generally used for data analysis and flow pattern detection in fluid dynamics. These techniques decompose the original flow field as an expansion of modes, which can be either orthogonal in time (variants of dynamic mode decomposition), or in space (variants of proper orthogonal decomposition) or in time and space (spectral proper orthogonal decomposition), or they can simply be selected using some sophisticated statistical techniques (empirical mode decomposition). The performance of these methods is tested in the turbulent wake of a wall-mounted square cylinder. This highly complex flow is suitable to show the ability of the aforementioned methods to reduce the degrees of freedom of the original data by only retaining the large scales in the flow. The main result is a reduced-order model of the original flow case, based on a low number of modes. A deep discussion is carried out about how to choose the most computationally efficient method to obtain suitable reduced-order models of the flow. The techniques introduced in this article are data-driven methods that could be applied to model any type of non-linear dynamical system, including numerical and experimental databases.

  • 58.
    Amor, Christian
    et al.
    Univ Politecn Madrid, Sch Aerosp Engn, E-28040 Madrid, Spain..
    Perez, Jose M.
    Univ Politecn Madrid, Sch Aerosp Engn, E-28040 Madrid, Spain..
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Le Clainche, Soledad
    Univ Politecn Madrid, Sch Aerosp Engn, E-28040 Madrid, Spain..
    Soft Computing Techniques to Analyze the Turbulent Wake of a Wall-Mounted Square Cylinder2020In: 14th International Conference on Soft Computing Models in Industrial and Environmental Applications, SOCO 2019 / [ed] Alvarez, FM Lora, AT Munoz, JAS Quintian, H Corchado, E, Springer, 2020, Vol. 950, p. 577-586Conference paper (Refereed)
    Abstract [en]

    This paper introduces several methods, generally used in fluid dynamics, to provide low-rank approximations. The algorithm describing these methods are mainly based on singular value decomposition (SVD) and dynamic mode decomposition (DMD) techniques, and are suitable to analyze turbulent flows. The application of these methods will be illustrated in the analysis of the turbulent wake of a wall-mounted cylinder, a geometry modeling a skyscraper. A brief discussion about the large and small size structures of the flow will provide the key ideas to represent the general dynamics of the flow using low-rank approximations. If the flow physics is understood, then it is possible to adapt these techniques, or some other strategies, to solve general complex problems with reduced computational cost. The main goal is to introduce these methods as machine learning strategies that could be potentially used in the field of fluid dynamics, and that can be extended to any other research field.

  • 59.
    Amor, Christian
    et al.
    Technol Grad Univ, Okinawa Inst Sci, Complex Fluids & Flows Unit, 1919-1 Tancha, Onna, Okinawa 9040495, Japan..
    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.
    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.
    Le Clainche, Soledad
    Univ Politecn Madrid, ETSI Aeronaut & Espacio, Plaza Cardenal Cisneros 3, Madrid 28040, Spain..
    Higher-order dynamic mode decomposition on-the-fly: A low-order algorithm for complex fluid flows2023In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 475, article id 111849Article in journal (Refereed)
    Abstract [en]

    This article presents a new method to identify the main patterns describing the flow motion in complex flows. The algorithm is an extension of the higher-order dynamic mode decomposition (HODMD), which compresses the snapshots from the analysed database and progressively updates new compressed snapshots on-the-fly, so it is denoted as HODMD on -the-fly (HODMD-of). This algorithm can be applied in parallel to the numerical simulations (or experiments), and it exhibits two main advantages over offline algorithms: (i) it automatically selects on-the-fly the number of necessary snapshots from the database to identify the relevant dynamics; and (ii) it can be used from the beginning of a numerical simulation (or experiment), since it uses a sliding-window to automatically select, also on-the-fly, the suitable interval to perform the data analysis, i.e. it automatically identifies and discards the transient dynamics. The HODMD-of algorithm is suitable to build reduced order models, which have a much lower computational cost than the original simulation. The performance of the method has been tested in three different cases: the axi-symmetric synthetic jet, the three-dimensional wake of a circular cylinder and the turbulent wake behind a wall-mounted square cylinder. The obtained speed-up factors are around 7 with respect to HODMD; this value depends on the simulation and the configuration of the hyperparameters. HODMD-of also provides a significant reduction of the memory requirements, between 40 - 80% amongst the two-and three-dimensional cases studied in this paper.

  • 60.
    Andreolli, Andrea
    et al.
    Karlsruhe Inst Technol, Inst Fluid Mech, Kaiserstr 10, D-76131 Karlsruhe, Germany..
    Gatti, Davide
    Karlsruhe Inst Technol, Inst Fluid Mech, Kaiserstr 10, D-76131 Karlsruhe, Germany..
    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.
    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. Karlsruhe Inst Technol, Karlsruhe, Germany..
    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.
    Separating large-scale superposition and modulation in turbulent channels2023In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 958, article id A37Article in journal (Refereed)
    Abstract [en]

    The presence of very-large-scale motions in wall-bounded turbulent flows is commonly associated with their footprint in the form of the superposition of the large scales at the wall and the additional amplitude modulation of small-scale near-wall turbulence. These two phenomena are currently understood to be interlinked, with the superposed large-scale velocity gradient causing the modulation of small-scale activity in the proximity of the wall. To challenge this idea, we devise a numerical strategy that selectively suppresses either superposition or amplitude modulation, in an effort to isolate and study the remaining phenomenon. Results from our direct numerical simulations indicate that a positive correlation between the amplitude of the small scales in the near-wall region and the large-scale signal in the outer flow persists even when near-wall large-scale motions are suppressed - i.e. in absence of superposition. Clearly, this kind of correlation cannot be caused by the near-wall large-scale velocity or its gradients, as both are absent. Conversely, when modulation is blocked, the near-wall footprints of the large scales seem to disappear. This study has been carried out on channel flows at friction Reynolds number Re-tau = 1000 in both standard simulation domains and minimal streamwise units (MSUs), where the streamwise fluctuation energy is enhanced. The consistency of the results obtained by the two approaches suggests that MSUs can capture correctly this kind of scale interaction at a much reduced cost.

  • 61.
    Appelquist, Elinor
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. Swedish e-Science Research Centre (SeRC).
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. Swedish e-Science Research Centre (SeRC).
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Lingwood, Rebecca J.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. University of Cambridge, Cambridge .
    Investigation of the Global Instability of the Rotating-disk Boundary Layer2015In: Procedia IUTAM, Elsevier, 2015, p. 321-328Conference paper (Refereed)
    Abstract [en]

    The development of the flow over a rotating disk is investigated by direct numerical simulations using both the linearized and fully nonlinear incompressible Navier-Stokes equations. These simulations allow investigation of the transition to turbulence of the realistic spatially-developing boundary layer. The current research aims to elucidate further the global linear stability properties of the flow, and relate these to local analysis and discussions in literature. An investigation of the nonlinear upstream (inward) influence is conducted by simulating a small azimuthal section of the disk (1/68). The simulations are initially perturbed by an impulse disturbance where, after the initial transient behaviour, both the linear and nonlinear simulations show a temporally growing upstream mode. This upstream global mode originates in the linear case close to the end of the domain, excited by an absolute instability at this downstream position. In the nonlinear case, it instead originates where the linear region ends and nonlinear harmonics enter the flow field, also where an absolute instability can be found. This upstream global mode can be shown to match a theoretical mode from local linear theory involved in the absolute instability at either the end of the domain (linear case) or where nonlinear harmonics enter the field (nonlinear case). The linear simulation grows continuously in time whereas the nonlinear simulation saturates and the transition to turbulence moves slowly upstream towards smaller radial positions asymptotically approaching a global upstream mode with zero temporal growth rate, which is estimated at a nondimensional radius of 582.

  • 62.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Imayama, Shintaro
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Revisiting the stability analysis of the flow over a rotating diskManuscript (preprint) (Other academic)
    Abstract [en]

    Local linear stability analysis applied to the rotating-disk flow is discussed.This flow case is an exact similarity solution to the cylindrical incompressible Navier–Stokes equations also called the von K ́arm ́an flow. The laminar mean velocity profiles are obtained by solving the resulting ordinary differential equations assuming the flow is axisymmetric and time independent. Two stability-analyses methods are used to investigate the local linear stability of this flow: i)the ‘shooting method’; and ii) the ‘Chebyshev polynomial method’. This theoretical investigation focuses on convectively unstable disturbances. Results obtained from the two methods are compared and the methods are shown togive similar results. These theoretical results are also compared with direct numerical simulations and experimental results showing good agreement.

    Download full text (pdf)
    fulltext
  • 63.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Imayama, Shintaro
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Revisiting the stability analysis of the flow over a rotating disk2014Report (Other academic)
    Abstract [en]

    Local linear stability analysis applied to the rotating-disk flow is discussed.This flow case is an exact similarity solution to the cylindrical incompressibleNavier–Stokes equations also called the von Karman flow. The laminar mean velocity profiles are obtained by solving the resulting ordinary differential equa-tions assuming the flow is axisymmetric and time independent. Two stability-analyses methods are used to investigate the local linear stability of this flow: i)the ‘shooting method’; and ii) the ‘Chebyshev polynomial method’. This the-oretical investigation focuses on convectively unstable disturbances. Resultsobtained from the two methods are compared and the methods are shown togive similar results. These theoretical results are also compared with directnumerical simulations and experimental results showing good agreement.

    Download full text (pdf)
    Technical report
  • 64.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Imayama, Shintaro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Alfredsson, Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lingwood, Rebecca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Simulating the linear behaviour of the flow over a rotating disk due to roughness elements2014Report (Other academic)
  • 65.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philip
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lingwood, Rebecca J.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of London, United Kingdom.
    On the global nonlinear instability of the rotating-disk flow over a finite domain2016In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 803, p. 332-355Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations based on the incompressible nonlinear Navier-Stokes equations of the flow over the surface of a rotating disk have been conducted. An impulsive disturbance was introduced and its development as it travelled radially outwards and ultimately transitioned to turbulence has been analysed. Of particular interest was whether the nonlinear stability is related to the linear stability properties. Specifically three disk-edge conditions were considered; (i) a sponge region forcing the flow back to laminar flow, (ii) a disk edge, where the disk was assumed to be infinitely thin and (iii) a physically realistic disk edge of finite thickness. This work expands on the linear simulations presented by Appelquist el al. (J. Fluid. Mech., vol. 765, 2015, pp. 612-631), where, for case (i), this configuration was shown to be globally linearly unstable when the sponge region effectively models the influence of the turbulence on the flow field. In contrast, case (ii) was mentioned there to he linearly globally stable, and here, where nonlinearity is included, it is shown that both cases (ii) and (iii) are nonlinearly globally unstable. The simulations show that the flow can he globally linearly stable if the linear wavepacket has a positive front velocity. However, in the same flow field, a nonlinear global instability can emerge, which is shown to depend on the outer turbulent region generating a linear inward-travelling mode that sustains a transition front within the domain. The results show that the front position does not approach the critical Reynolds number for the local absolute instability, R = 507. Instead, the front approaches R = 583 and both the temporal frequency and spatial growth rate correspond to a global mode originating at this position.

  • 66.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Simulating the laminar von Karman flow in Nek50002014Report (Other academic)
    Abstract [en]

    The laminar incompressible boundary layer over a rotating disk, also called the von Karman flow, is investigated. The goal is to set up a direct numericalsimulation (DNS) environment for further use to investigate the transition from laminar to turbulent flow for this boundary layer. For this the spectral-element code Nek5000 is used. A set of ODE-equations are first derived from the incompressible cylindrical Navier–Stokes equations, which are solved for the exact von Karman solution. Further, Nek5000 is prepared to solve for the same laminar solution. Comparing the two solutions give a quantification of the accuracy of the DNS solver Nek5000. Different scalings of the equations are investigated, together with quantifications of how good the different available boundary conditions are, also investigating different reference frames and grid dependency of the solution. The general conclusion is that the von K ́rm ́na aflow is possible to simulate in Nek5000. The method was robust when it cameto using different scalings, reference frames and resolutions.

    Download full text (pdf)
    Technical report
  • 67.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Alfredsson, Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lingwood, Rebecca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Global linear instability and the radial boundary of the rotating-disk flowManuscript (preprint) (Other academic)
  • 68.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lingwood, Rebecca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. nstitute of Continuing Education, University of Cambridge, Madingley Hall, Madingley Cambridge, United Kingdom .
    Global linear instability of the rotating-disk flow investigated through simulations2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 765, p. 612-631Article in journal (Refereed)
    Abstract [en]

    Numerical simulations of the flow developing on the surface of a rotating disk are presented based on the linearized incompressible Navier-Stokes equations. The boundary-layer flow is perturbed by an impulsive disturbance within a linear global framework, and the effect of downstream turbulence is modelled by a damping region further downstream. In addition to the outward-travelling modes, inward-travelling disturbances excited at the radial end of the simulated linear region, r(end), by the modelled turbulence are included within the simulations, potentially allowing absolute instability to develop. During early times the flow shows traditional convective behaviour, with the total energy slowly decaying in time. However, after the disturbances have reached r(end), the energy evolution reaches a turning point and, if the location of r(end) is at a Reynolds number larger than approximately R = 594 (radius non-dimensionalized by root v/Omega*, where v is the kinematic viscosity and Omega* is the rotation rate of the disk), there will be global temporal growth. The global frequency and mode shape are clearly imposed by the conditions at r(end). Our results suggest that the linearized Ginzburg-Landau model by Healey (J. Fluid Mech., vol. 663, 2010, pp. 148-159) captures the (linear) physics of the developing rotating-disk flow, showing that there is linear global instability provided the Reynolds number of r(end) is sufficiently larger than the critical Reynolds number for the onset of absolute instability.

  • 69.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lingwood, R. J.
    Transition to turbulence in the rotating-disk boundary-layer flow with stationary vortices2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 836, p. 43-71Article in journal (Refereed)
    Abstract [en]

    This paper proposes a resolution to the conundrum of the roles of convective and absolute instability in transition of the rotating-disk boundary layer. It also draws some comparison with swept-wing flows. Direct numerical simulations based on the incompressible Navier-Stokes equations of the flow over the surface of a rotating disk with modelled roughness elements are presented. The rotating-disk flow has been of particular interest for stability and transition research since the work by Lingwood (J.FluidMech., vol.299, 1995, pp.17-33) where an absolute instability was found. Here stationary disturbances develop from roughness elements on the disk and are followed from the linear stage, growing to saturation and finally transitioning to turbulence. Several simulations are presented with varying disturbance amplitudes. The lowest amplitude corresponds approximately to the experiment by Imayama etal. (J.FluidMech., vol.745, 2014a, pp.132-163). For all cases, the primary instability was found to be convectively unstable, and secondary modes were found to be triggered spontaneously while the flow was developing. The secondary modes further stayed within the domain, and an explanation for this is a proposed globally unstable secondary instability. For the low-amplitude roughness cases, the disturbances propagate beyond the threshold for secondary global instability before becoming turbulent, and for the high-amplitude roughness cases the transition scenario gives a turbulent flow directly at the critical Reynolds number for the secondary global instability. These results correspond to the theory of Pier (J.EngngMaths, vol.57, 2007, pp.237-251) predicting a secondary absolute instability. In our simulations, high temporal frequencies were found to grow with a large amplification rate where the secondary global instability occurred. For smaller radial positions, low-frequency secondary instabilities were observed, tripped by the global instability.

  • 70.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lingwood, Rebecca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Transition to turbulence in the rotating-disk boundary layer2020In: ETC 2013 - 14th European Turbulence Conference, Zakon Group LLC , 2020Conference paper (Refereed)
    Abstract [en]

    The development of the flow over a rotating disk is investigated by direct numerical simulations using both the linearised and fully nonlinear Navier-Stokes equations. The nonlinear simulations allow investigation of the transition to turbulence of the realistic spatially-developing boundary layer, and these simulations can be directly validated by physical experiments of the same case. The current research aims to elucidate further the global stability properties of the flow. So far, there are no conclusive simulations available in the literature for the fully nonlinear case for this flow, and since the nonlinearity is particularly relevant for transition to turbulence an increased understanding of this process is expected. 

  • 71.
    Appelquist, Ellinor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lingwood, Rebecca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Queen Mary University of London, Mile End Road, London, United Kingdom.
    Turbulence in the rotating-disk boundary layer investigated through direct numerical simulations2018In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 70, p. 6-18Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations (DNS) are reported for the turbulent rotating-disk boundary layer for the first time. Two turbulent simulations are presented with overlapping small and large Reynolds numbers, where the largest corresponds to a momentum-loss Reynolds number of almost 2000. Simulation data are compared with experimental data from the same flow case reported by Imayama et al. (2014), and also a comparison is made with a numerical simulation of a two-dimensional turbulent boundary layer (2DTBL) over a flat plate reported by Schlatter and Örlü (2010). The agreement of the turbulent statistics between experiments and simulations is in general very good, as well as the findings of a missing wake region and a lower shape factor compared to the 2DTBL. The simulations also show rms-levels in the inner region similar to the 2DTBL. The simulations validate Imayama et al.’s results showing that the rotating-disk turbulent boundary layer in the near-wall region contains shorter streamwise (azimuthal) wavelengths than the 2DTBL, probably due to the outward inclination of the low-speed streaks. Moreover, all velocity components are available from the simulations, and hence the local flow angle, Reynolds stresses and all terms in the turbulent kinetic energy equation are also discussed. However there are in general no large differences compared to the 2DTBL, hence the three-dimensional effects seem to have only a small influence on the turbulence.

  • 72.
    Ardekani, Mehdi Niazi
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Costa, Pedro
    Breugem, Wim Paul
    Brandt, Luca
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical study of the sedimentation of spheroidal particles2016In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 87, p. 16-34Article in journal (Refereed)
    Abstract [en]

    The gravity-driven motion of-rigid particles in a viscous fluid is relevant in many natural and industrial processes, yet this has mainly been investigated for spherical particles. We therefore consider the sedimentation of non-spherical (spheroidal) isolated and particle pairs in a viscous fluid via numerical simulations using the Immersed Boundary Method. The simulations performed here show that the critical Galileo number for the onset of secondary motions decreases as the spheroid aspect ratio departs from 1. Above this critical threshold, oblate particles perform a zigzagging motion whereas prolate particles rotate around, the vertical axis while having their broad side facing the falling direction. Instabilities of the vortices in the wake follow when farther increasing the Galileo number. We also study the drafting kissing-tumbling associated with the settling of particle pairs. We find that the interaction time increases significantly for non-spherical particles and, more interestingly, spheroidal particles are attracted from larger lateral displacements. This has important implications for the estimation of collision kernels and can result its increasing clustering in suspensions of sedimenting spheroids.

  • 73.
    Arnal, Daniel
    et al.
    ONERA.
    Tran, Dac
    Dassault Aviation.
    Hein, Stefan
    DLR.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Engelbrecht, T.
    SUPERsonic TRAnsition Control Contract N° AST4-CT-2005-516100: Final Technical Report2008Report (Other academic)
  • 74. Arnqvist, J.
    et al.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Dellwik, E.
    Bergström, H.
    Wind Statistics from a Forested Landscape2015In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 156, no 1, p. 53-71Article in journal (Refereed)
    Abstract [en]

    An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensure high data quality. A 40 wide wind-direction sector is selected as the most representative for large-scale forest conditions, and from that sector first-, second- and third-order statistics, as well as analyses regarding the characteristic length scale, the flux-profile relationship and surface roughness are presented for a wide range of stability conditions. The results are discussed with focus on the validity of different scaling regimes. Significant wind veer, decay of momentum fluxes and reduction in shear length scales with height are observed for all stability classes, indicating the influence of the limited depth of the boundary layer on the measured profiles. Roughness sublayer characteristics are however not detected in the presented analysis. Dimensionless gradients are shown to follow theoretical curves up to 100 m in stable conditions despite surface-layer approximations being invalid. This is attributed to a balance of momentum decay and reduced shear length scale growth with height. The wind profile shows a strong stability dependence of the aerodynamic roughness length, with a 50 % decrease from neutral to stable conditions.

  • 75.
    Atif, Abdul-Raouf
    et al.
    Uppsala Univ, Dept Mat Sci & Engn, S-75122 Uppsala, Sweden..
    Lacis, Ugis
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Engqvist, Håkan
    Uppsala Univ, Dept Mat Sci & Engn, S-75122 Uppsala, Sweden..
    Tenje, Maria
    Uppsala Univ, Dept Mat Sci & Engn, S-75122 Uppsala, Sweden.;Uppsala Univ, Sci Life Lab, S-75122 Uppsala, Sweden..
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mestres, Gemma
    Uppsala Univ, Dept Mat Sci & Engn, S-75122 Uppsala, Sweden.;Uppsala Univ, Sci Life Lab, S-75122 Uppsala, Sweden..
    Experimental Characterization and Mathematical Modeling of the Adsorption of Proteins and Cells on Biomimetic Hydroxyapatite2022In: ACS Omega, E-ISSN 2470-1343, Vol. 7, no 1, p. 908-920Article in journal (Refereed)
    Abstract [en]

    Biomaterial development is a long process consisting of multiple stages of design and evaluation within the context of both in vitro and in vivo testing. To streamline this process, mathematical and computational modeling displays potential as a tool for rapid biomaterial characterization, enabling the prediction of optimal physicochemical parameters. In this work, a Langmuir isotherm-based model was used to describe protein and cell adhesion on a biomimetic hydroxyapatite surface, both independently and in a one-way coupled system. The results indicated that increased protein surface coverage leads to improved cell adhesion and spread, with maximal protein coverage occurring within 48 h. In addition, the Langmuir model displayed a good fit with the experimental data. Overall, computational modeling is an exciting avenue that may lead to savings in terms of time and cost during the biomaterial development process.

  • 76. Atzori, M.
    et al.
    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.
    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.
    Control effects on coherent structures in a non-uniform adverse-pressure-gradient boundary layer2022In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 97, article id 109036Article in journal (Refereed)
    Abstract [en]

    We examine the effects of three basic but effective control strategies, namely uniform blowing, uniform suction, and body-force damping, on the intense Reynolds-stress events in the turbulent boundary layer (TBL) developing on the suction side of a NACA4412 airfoil. This flow is subjected to a non-uniform adverse pressure gradient (APG), which substantially modifies its turbulence statistics with respect to a zero-pressure-gradient (ZPG) boundary layer, and it also changes how control strategies affect the flow. The strong APG results in intense events that are shorter and more often detached from the wall than in ZPG TBLs. In a quadrant analysis, ejections remain the most relevant structures, but sweeps become more important than in ZPG TBLs, a fact that results in a lower contribution to the wall-normal velocity from intense Reynolds-stress events. Control effects are relatively less important on intense events than on the turbulent statistics. Uniform blowing has an impact similar to that of an even more intense APG, while uniform suction has more complex effects, most likely due to the particular behavior of the wall-normal velocity component near the wall. Body-force damping also reduces the probability of occurrence of very-large attached structures and that of intense events in the proximity of the actuation region. Our results show that intense Reynolds-stress events are robust features of the flow. If control strategies do not target directly these structures, their effects on the strong events is less pronounced than the effects on the mean flow. 

  • 77.
    Atzori, Marco
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Köpp, Wiebke
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Chien, Wei Der
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Theoretical Computer Science, TCS. KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Massaro, Daniele
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Mallor, Fermin
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Rezaei, Mohammadtaghi
    KTH, School of Electrical Engineering and Computer Science (EECS), Centres, Centre for High Performance Computing, PDC.
    Jansson, Niclas
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Markidis, Stefano
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Laure, E.
    Schlatter, Philipp
    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).
    In situ visualization of large-scale turbulence simulations in Nek5000 with ParaView Catalyst2022In: Journal of Supercomputing, ISSN 0920-8542, E-ISSN 1573-0484, Vol. 78, no 3, p. 3605-3620Article in journal (Refereed)
    Abstract [en]

    In situ visualization on high-performance computing systems allows us to analyze simulation results that would otherwise be impossible, given the size of the simulation data sets and offline post-processing execution time. We develop an in situ adaptor for Paraview Catalyst and Nek5000, a massively parallel Fortran and C code for computational fluid dynamics. We perform a strong scalability test up to 2048 cores on KTH’s Beskow Cray XC40 supercomputer and assess in situ visualization’s impact on the Nek5000 performance. In our study case, a high-fidelity simulation of turbulent flow, we observe that in situ operations significantly limit the strong scalability of the code, reducing the relative parallel efficiency to only ≈ 21 % on 2048 cores (the relative efficiency of Nek5000 without in situ operations is ≈ 99 %). Through profiling with Arm MAP, we identified a bottleneck in the image composition step (that uses the Radix-kr algorithm) where a majority of the time is spent on MPI communication. We also identified an imbalance of in situ processing time between rank 0 and all other ranks. In our case, better scaling and load-balancing in the parallel image composition would considerably improve the performance of Nek5000 with in situ capabilities. In general, the result of this study highlights the technical challenges posed by the integration of high-performance simulation codes and data-analysis libraries and their practical use in complex cases, even when efficient algorithms already exist for a certain application scenario.

  • 78.
    Atzori, Marco
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Mallor, Fermin
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Pozuelo, Ramon
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Fukagata, Koji
    Department of Mechanical Engineering, Keio University, 223-8522 Yokohama, Japan.
    Vinuesa, Ricardo
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Schlatter, Philipp
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    A new perspective on skin-friction contributions in adverse-pressure-gradient turbulent boundary layersManuscript (preprint) (Other academic)
    Abstract [en]

    For adverse-pressure-gradient turbulent boundary layers, the study of integral skin-friction contributions still poses significant challenges. Beyond questions related to the integration boundaries and the derivation procedure, which have been thoroughly investigated in the literature, an important issue is how different terms should be aggregated. The nature of these flows, which exhibit significant in-homogeneity in the streamwise direction, usually results in cancellation between several contributions with high absolute values. We propose a formulation of the identity derived by Fukagata, Iwamoto \& Kasagi (Phys. Fluids, vol. 14, 2002, pp. 73--76), which we obtained from the convective form of the governing equations. A new skin-friction contribution is defined, considering wall-tangential convection and pressure gradient together. This contribution is related to the evolution of the dynamic pressure in the mean flow. The results of the decomposition are examined for a broad range of pressure-gradient conditions and different flow-control strategies. We found that the new formulation of the identity allows to readily identify the different regimes of near-equilibrium conditions and approaching separation. It also provides a more effective description of control effects. A similar aggregation between convection and pressure-gradient terms is also possible for any other decomposition where in-homogeneity contributions are considered explicitly. 

  • 79.
    Atzori, Marco
    et al.
    Johannes Kepler Univ Linz, Dept Particulate Flow Modelling, A-4040 Linz, Austria..
    Mallor, Fermin
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Pozuelo, Ramon
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fukagata, Koji
    Keio Univ, Dept Mech Engn, Yokohama 2238522, Japan..
    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.
    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.
    A new perspective on skin-friction contributions in adverse-pressure-gradient turbulent boundary layers2023In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 101, article id 109117Article in journal (Refereed)
    Abstract [en]

    For adverse-pressure-gradient turbulent boundary layers, the study of integral skin-friction contributions still poses significant challenges. Beyond questions related to the integration boundaries and the derivation procedure, which have been thoroughly investigated in the literature, an important issue is how different terms should be aggregated. The nature of these flows, which exhibit significant in-homogeneity in the streamwise direction, usually results in cancellation between several contributions with high absolute values. We propose a formulation of the identity derived by Fukagata et al. (2002), which we obtained from the convective form of the governing equations. A new skin-friction contribution is defined, considering wall-tangential convection and pressure gradient together. This contribution is related to the evolution of the dynamic pressure in the mean flow. The results of the decomposition are examined for a broad range of pressure-gradient conditions and different flow-control strategies. We found that the new formulation of the identity allows to readily identify the different regimes of near-equilibrium conditions and approaching separation. It also provides a more effective description of control effects. A similar aggregation between convection and pressure-gradient terms is also possible for any other decomposition where in-homogeneity contributions are considered explicitly.

  • 80.
    Atzori, Marco
    et al.
    Department of Particulate Flow Modelling Johannes Kepler University 4040 Linz, Austria.
    Stroh, Alexander
    Institute of Fluid Mechanics (ISTM) Karlsruhe Institute of Technology 76131 Karlsruhe, Germany.
    Gatti, Davide
    Institute of Fluid Mechanics (ISTM) Karlsruhe Institute of Technology 76131 Karlsruhe, Germany.
    Fukagata, Koji
    Department of Mechanical Engineering Keio University 223-8522 Yokohama, Japan.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    A New Point of View On Skin-Friction Contributions ni Adverse-Pressure-Gradient Turbulent Boundary Layers2022In: 12th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2022, International Symposium on Turbulence and Shear Flow Phenomena, TSFP , 2022Conference paper (Refereed)
    Abstract [en]

    Skin-friction decompositions such as the so-called FIK identity (Fukagata et al., 2002) are useful tools in identifying relevant contributions to the friction, but may also lead to results difficult to interpret when the total friction is recovered from cancellation of multiple terms with large values. We propose a new formulation of the FIK contributions related to streamwise inhomogeneity, which is derived from the convective form of the momentum equation and using the concept of dynamic pressure. We examine turbulent boundary layers subjected to various pressure-gradient conditions, including cases with drag-reducing control. The new formulation distinguishes more precisely the roles of the free-stream pressure distribution, wall-normal convection, and turbulent fluctuations. Our results allow to identify different regimes in adverse-pressure-gradient turbulent boundary layers, corresponding to different proportions of the various contributions, and suggest a possible direction towards studying the onset of mean separation.

  • 81.
    Atzori, Marco
    et al.
    Johannes Kepler Univ Linz, Dept Particulate Flow Modelling, A-4040 Linz, Austria..
    Torres, Pablo
    Univ Politecn Valencia, Inst Univ Matemat Pura & Aplicada, Valencia 46022, Spain..
    Vidal, Alvaro
    Parallel Works, Chicago, IL 60654 USA..
    Le Clainche, Soledad
    Univ Politecn Madrid, Sch Aerosp Engn, Madrid 28040, Spain..
    Hoyas, Sergio
    Univ Politecn Valencia, Inst Univ Matemat Pura & Aplicada, Valencia 46022, Spain..
    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.
    High-resolution simulations of a turbulent boundary layer impacting two obstacles in tandem2023In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, no 6, article id 063801Article in journal (Refereed)
    Abstract [en]

    High-fidelity large-eddy simulations of the flow around two rectangular obstacles are carried out at a Reynolds number of 10 000 based on the freestream velocity and the obstacle height. The incoming flow is a developed turbulent boundary layer. Mean-velocity components, turbulence fluctuations, and the terms of the turbulent-kinetic-energy budget are analyzed for three flow regimes: skimming flow, wake interference, and isolated roughness. Three regions are identified where the flow undergoes the most significant changes: the first obstacle's wake, the region in front of the second obstacle, and the region around the second obstacle. In the skimming-flow case, turbulence activity in the cavity between the obstacles is limited and mainly occurs in a small region in front of the second obstacle. In the wake-interference case, there is a strong interaction between the freestream flow that penetrates the cavity and the wake of the first obstacle. This interaction results in more intense turbulent fluctuations between the obstacles. In the isolated-roughness case, the wake of the first obstacle is in good agreement with that of an isolated obstacle. Separation bubbles with strong turbulent fluctuations appear around the second obstacle.

  • 82.
    Atzori, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Fahland, G.
    Stroh, A.
    Gatti, D.
    Frohnapfel, B.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Aerodynamic Effects of Uniform Blowing and Suction on a NACA4412 Airfoil2020In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987Article in journal (Refereed)
    Abstract [en]

    We carried out high-fidelity large-eddy simulations to investigate the effects of uniform blowing and uniform suction on the aerodynamic efficiency of a NACA4412 airfoil at the moderate Reynolds number based on chord length and incoming velocity of Rec= 200 , 000. We found that uniform blowing applied at the suction side reduces the aerodynamics efficiency, while uniform suction increases it. This result is due to the combined impact of blowing and suction on skin friction, pressure drag and lift. When applied to the pressure side, uniform blowing improves aerodynamic efficiency. The Reynolds-number dependence of the relative contributions of pressure and friction to the total drag for the reference case is analysed via Reynolds-averaged Navier–Stokes simulations up to Rec= 10 , 000 , 000. The results suggest that our conclusions on the control effect can tentatively be extended to a broader range of Reynolds numbers. 

  • 83.
    Atzori, Marco
    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.
    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. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Gatti, D.
    Stroh, A.
    Frohnapfel, B.
    Schlatter, Philipp
    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.
    Effects of Different Friction Control Techniques on Turbulence Developing Around Wings2020In: ERCOFTAC Workshop Direct and Large Eddy Simulation: Direct and Large Eddy Simulation XII, Springer, 2020, p. 305-311Chapter in book (Refereed)
    Abstract [en]

    Developing efficient flow control techniques remain a challenging task due to the complexity of turbulent flows in industrial applications, a relevant example of which are turbulent boundary layers (TBL) subjected to pressure gradients. In the present study, we employ high-fidelity numerical simulations to assess the impact of different control strategies on the flow around a NACA4412 airfoil at a Reynolds number Rec=200,000 based on the chord length c and the inflow velocity U∞. The choice of this specific study case is motivated by the relatively weak dependence of the pressure distribution around the airfoil on the Reynolds number [6], which allows distinguishing the effects of increasing Reynolds number and those of the non-uniform adverse pressure gradient (APG).

  • 84.
    Atzori, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lozano-Durán, A.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Characterization of turbulent coherent structures in square duct flow2018In: Journal of Physics: Conference Series, Institute of Physics Publishing (IOPP), 2018, Vol. 1001, no 1Conference paper (Refereed)
    Abstract [en]

    This work is aimed at a first characterization of coherent structures in turbulent square duct flows. Coherent structures are defined as connected components in the domain identified as places where a quantity of interest (such as Reynolds stress or vorticity) is larger than a prescribed non-uniform threshold. Firstly, we qualitatively discuss how a percolation analysis can be used to assess the effectiveness of the threshold function, and how it can be affected by statistical uncertainty. Secondly, various physical quantities that are expected to play an important role in the dynamics of the secondary flow of Prandtl's second kind are studied. Furthermore, a characterization of intense Reynolds-stress events in square duct flow, together with a comparison of their shape for analogous events in channel flow at the same Reynolds number, is presented.

  • 85.
    Atzori, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Lozano-Durán, A.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Contribution of Reynolds-stress structures to the secondary flow in turbulent ducts2019In: 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 is aimed at evaluating the contribution to the secondary flow in duct flow with square and rectangular cross section from three-dimensional coherent structures, defined as intense Reynolds-stress events. The contribution to a certain mean quantity is defined as the ensemble average over the detected coherent structures, weighted with their own occupied volume fraction. Our analysis unveils that the contribution to the cross-stream components of the mean velocity is either very similar to the same contribution in channel flow, or almost negligible in respect to the contribution from the portion of the domain not occupied by coherent structures. These results suggest that the most intense events are not directly responsible for the secondary flow.

  • 86.
    Atzori, Marco
    et al.
    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.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Lozano-Durán, Adrián
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Coherent structures in turbulent boundary layers over an airfoil2020In: Journal of Physics: Conference Series, ISSN 1742-6588, Vol. 1522, article id 012020Article in journal (Refereed)
    Abstract [en]

    This preliminary study is concerned with the identification of three-dimensional coherent structures, defined as intense Reynolds-stress events, in the turbulent boundary layer developing over the suction side of a NACA4412 airfoil at a Reynolds number based on the chord length and the incoming velocity of $Re_c=200,000$. The scientific interest for such flows originates from the non-uniform adverse pressure gradient that affects the boundary-layer development. Firstly, we assess different methods to identify the turbulent-non-turbulent interface, in order to exclude the irrotational region from the analysis. Secondly, we evaluate the contribution of the considered coherent structures to the enhanced wall-normal velocity, characteristic of adverse pressure gradients. Our results show that it is necessary to limit the detection of coherent structures to the turbulent region of the domain, and that the structures reveal qualitative differences between the contributions of intense events to the wall-normal velocity in adverse-pressure-gradient and zero-pressure-gradient turbulent boundary layers.

  • 87.
    Atzori, Marco
    et al.
    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.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Lozano-Durán, Adrián
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Intense Reynolds-stress events in turbulent ducts2021In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 89, article id 108802Article in journal (Refereed)
    Abstract [en]

    The aim of the present work is to investigate the role of intense Reynolds shear-stress events in the generation of the secondary flow in turbulent ducts. We consider the connected regions of flow where the product of the instantaneous fluctuations of two velocity components is higher than a threshold based on the long-time turbulence statistics, in the spirit of the three-dimensional quadrant analysis proposed by Lozano-Dur\'an \textit{et al.} (\textit{J.~Fluid Mech.}, vol. 694, 2012, pp. 100--130). We examine both the geometrical properties of these structures and their contribution to the mean in-plane velocity components, and we perform a comparison with turbulent channel flow at similar Reynolds number. The contribution to a certain mean quantity is defined as the ensemble average over the detected coherent structures, weighted with their own occupied volume fraction. In the core region of the duct, the contribution of intense events to the wall-normal component of the mean velocity is in very good agreement with that in the channel, despite the presence of the secondary flow in the former. Additionally, the shapes of the three-dimensional objects do not differ significantly in both flows. In the corner region of the duct, the proximity of the walls affects both the geometrical properties of the coherent structures and the contribution to the mean component of the vertical velocity. However, such contribution is less relevant than that of the complementary portion of the flow not included in such objects. Our results show that strong Reynolds shear-stress events are affected by the presence of a corner but, despite the important role of these structures in the dynamics of wall-bounded turbulent flows, their contribution to the secondary flow is relatively low, both in the core and in the corner.

  • 88.
    Augier, Pierre
    et al.
    LEGI, Université Grenoble Alpes.
    Bonamy, Cyrille
    LEGI, Université Grenoble Alpes.
    Campagne, Antoine
    LEGI, Université Grenoble Alpes.
    Mohanan, Ashwin Vishnu
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    FluidImage, a libre framework for scientific treatments of large sets of images: A software for the fluid dynamic community, by the fluid dynamic community2016Conference paper (Other academic)
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  • 89.
    Augier, Pierre
    et al.
    LEGI, Université Grenoble Alpes.
    Mohanan, Ashwin Vishnu
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bonamy, Cyrille
    LEGI, Université Grenoble Alpes.
    FluidDyn: A Python Open-Source Framework for Research and Teaching in Fluid Dynamics by Simulations, Experiments and Data Processing2019In: Journal of Open Research Software, ISSN 2049-9647, Vol. 7, no 1Article in journal (Refereed)
    Abstract [en]

    FluidDyn is a project to foster open-science and open-source in the fluid dynamics community. It is thought of as a research project to channel open-source dynamics, methods and tools to do science. We propose a set of Python packages forming a framework to study fluid dynamics with different methods, in particular laboratory experiments (package fluidlab), simulations (packages fluidfft, fluidsim and fluidfoam) and data processing (package fluidimage). In the present article, we give an overview of the specialized packages of the project and then focus on the base package called fluiddyn, which contains common code used in the specialized packages. Packages fluidfft and fluidsim are described with greater detail in two companion papers [4, 5]. With the project FluidDyn, we demonstrate that specialized scientific code can be written with methods and good practices of the open-source community. The Mercurial repositories are available in Bitbucket (https://bitbucket.org/fluiddyn/). All codes are documented using Sphinx and Read the Docs, and tested with continuous integration run on Bitbucket Pipelines and Travis. To improve the reuse potential, the codes are as modular as possible, leveraging the simple object-oriented programming model of Python. All codes are also written to be highly efficient, using C++, Cython and Pythran to speedup the performance of critical functions.

    Download full text (pdf)
    fulltext
  • 90.
    Augier, Pierre
    et al.
    LEGI, Université Grenoble Alpes.
    Mohanan, Ashwin Vishnu
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Shallow water wave turbulence2019In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 874, p. 1169-1196Article in journal (Refereed)
    Abstract [en]

    The dynamics of irrotational shallow water wave turbulence forced in large scales and dissipated at small scales is investigated. First, we derive the shallow water analogue of the `four-fifths law' of Kolmogorov turbulence for a third order structure function involving velocity and displacement increments. Using this relation and assuming that the flow is dominated by shocks we develop a simple model predicting that the shock amplitude scales as (ϵd)1/3, where ϵ is the mean dissipation rate and d the mean distance between the shocks, and that the pth order displacement and velocity structure functions scale as (ϵd)p/3r/d, where r is the separation. Then we carry out a series of forced simulations with resolutions up to 76802, varying the Froude number, Ff=ϵ1/3/ckf1/3, where kf is the forcing wave number and c is the wave speed. In all simulations a stationary state is reached in which there is a constant spectral energy flux and equipartition between kinetic and potential energy in the constant flux range. The third order structure function relation is satisfied with a high degree of accuracy. Mean energy is found to scale as E∼√(ϵc/kf), and is also dependent on resolution, indicating that shallow water wave turbulence does not fit into the paradigm of a Richardson-Kolmogorov cascade. In all simulations shocks develop, displayed as long thin bands of negative divergence in flow visualisations. The mean distance between the shocks is found to scale as dFf1/2/kf. Structure functions of second and higher order are found to scale in good agreement with the model. We conclude that in the weak limit, Ff→0, shocks will become denser and weaker and finally disappear for a finite Reynolds number. On the other hand, for a given Ff, no matter how small, shocks will prevail if the Reynolds number is sufficiently large.

  • 91.
    Bagge, Joar
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Rosén, Tomas
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Tornberg, Anna-Karin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Parabolic velocity profile causes shape-selective drift of inertial ellipsoids2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 926, article id A24Article in journal (Refereed)
    Abstract [en]

    Understanding particle drift in suspension flows is of the highest importance in numerous engineering applications where particles need to be separated and filtered out from the suspending fluid. Commonly known drift mechanisms such as the Magnus force, Saffman force and Segre-Silberberg effect all arise only due to inertia of the fluid, with similar effects on all non-spherical particle shapes. In this work, we present a new shape-selective lateral drift mechanism, arising from particle inertia rather than fluid inertia, for ellipsoidal particles in a parabolic velocity profile. We show that the new drift is caused by an intermittent tumbling rotational motion in the local shear flow together with translational inertia of the particle, while rotational inertia is negligible. We find that the drift is maximal when particle inertial forces are of approximately the same order of magnitude as viscous forces, and that both extremely light and extremely heavy particles have negligible drift. Furthermore, since tumbling motion is not a stable rotational state for inertial oblate spheroids (nor for spheres), this new drift only applies to prolate spheroids or tri-axial ellipsoids. Finally, the drift is compared with the effect of gravity acting in the directions parallel and normal to the flow. The new drift mechanism is stronger than gravitational effects as long as gravity is less than a critical value. The critical gravity is highest (i.e. the new drift mechanism dominates over gravitationally induced drift mechanisms) when gravity acts parallel to the flow and the particles are small.

  • 92.
    Bagge, Joar
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tornberg, Anna-Karin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Accurate quadrature methods with application to Stokes flow with particles in confined geometries2017In: Proceedings of the Eleventh UK Conference on Boundary Integral Methods (UKBIM 11) / [ed] David J. Chappell, Nottingham: Nottingham Trent University, 2017, p. 15-24Conference paper (Refereed)
    Abstract [en]

    Boundary integral methods are attractive for simulating Stokes flow with particles or droplets due to the reduction in dimensionality and natural handling of the geometry. In many problems walls are present, and it becomes necessary to evaluate singular or nearly singular layer potentials over the wall. In this paper we show how this can be done using quadrature by expansion (QBX), a relatively new method based on local expansions of the layer potential. We present results for the Laplace single layer potential and the Stokes double layer potential. QBX can be used to evaluate the potentials to high accuracy arbitrarily close to the wall and on the wall. We also discuss how some quantities can be precomputed and how geometric symmetries can be used to reduce precomputation and storage.

  • 93.
    Bagge, Joar
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tornberg, Anna-Karin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Fast Ewald summation for Stokes flow with arbitrary periodicityManuscript (preprint) (Other academic)
    Abstract [en]

    A fast and spectrally accurate Ewald summation method for the evaluation of stokeslet, stresslet and rotlet potentials of three-dimensional Stokes flow is presented. This work extends the previously developed Spectral Ewald method for Stokes flow to periodic boundary conditions in any number (three, two, one, or none) of the spatial directions, in a unified framework. The periodic potential is split into a short-range and a long-range part, where the latter is treated in Fourier space using the fast Fourier transform. A crucial component of the method is the modified kernels used to treat singular integration. We derive new modified kernels, and new improved truncation error estimates for the stokeslet and stresslet. An automated procedure for selecting parameters based on a given error tolerance is designed and tested. Analytical formulas for validation in the doubly and singly periodic cases are presented. We show that the computational time of the method scales like O(N log N) for N sources and targets, and investigate how the time depends on the error tolerance and window function, i.e. the function used to smoothly spread irregular point data to a uniform grid. The method is fastest in the fully periodic case, while the run time in the free-space case is around three times as large. Furthermore, the highest efficiency is reached when applying the method to a uniform source distribution in a primary cell with low aspect ratio. The work presented in this paper enables efficient and accurate simulations of three-dimensional Stokes flow with arbitrary periodicity using e.g. boundary integral and potential methods.

  • 94.
    Bagge, Joar
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tornberg, Anna-Karin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Fast Ewald summation for Stokes flow with arbitrary periodicity2023In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 493, p. 112473-, article id 112473Article in journal (Refereed)
    Abstract [en]

    A fast and spectrally accurate Ewald summation method for the evaluation of stokeslet, stresslet and rotlet potentials of three-dimensional Stokes flow is presented. This work extends the previously developed Spectral Ewald method for Stokes flow to periodic boundary conditions in any number (three, two, one, or none) of the spatial directions, in a unified framework. The periodic potential is split into a short-range and a long-range part, where the latter is treated in Fourier space using the fast Fourier transform. A crucial component of the method is the modified kernels used to treat singular integration. We derive new modified kernels, and new improved truncation error estimates for the stokeslet and stresslet. An automated procedure for selecting parameters based on a given error tolerance is designed and tested. Analytical formulas for validation in the doubly and singly periodic cases are presented. We show that the computational time of the method scales like O(Nlog⁡N) for N sources and targets, and investigate how the time depends on the error tolerance and window function, i.e. the function used to smoothly spread irregular point data to a uniform grid. The method is fastest in the fully periodic case, while the run time in the free-space case is around three times as large. Furthermore, the highest efficiency is reached when applying the method to a uniform source distribution in a primary cell with low aspect ratio. The work presented in this paper enables efficient and accurate simulations of three-dimensional Stokes flow with arbitrary periodicity using e.g. boundary integral and potential methods.

  • 95.
    Bagheri, Faranggis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mitra, Dhrubaditya
    NORDITA.
    Perlekar, Prasad
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Statistics of polymer extensions in turbulent channel flow2012In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 86, no 5, p. 056314-Article in journal (Refereed)
    Abstract [en]

    We present direct numerical simulations of turbulent channel flow with passive Lagrangian polymers. To understand the polymer behavior we investigate the behavior of infinitesimal line elements and calculate the probability distribution function (PDF) of finite-time Lyapunov exponents and from them the corresponding Cramer's function for the channel flow. We study the statistics of polymer elongation for both the Oldroyd-B model (for Weissenberg number Wi<1) and the FENE model. We use the location of the minima of the Cramer's function to define the Weissenberg number precisely such that we observe coil-stretch transition at Wi1. We find agreement with earlier analytical predictions for PDF of polymer extensions made by Balkovsky, Fouxon, and Lebedev for linear polymers (Oldroyd-B model) with Wi <1 and by Chertkov for nonlinear FENE-P model of polymers. For Wi >1 (FENE model) the polymer are significantly more stretched near the wall than at the center of the channel where the flow is closer to homogenous isotropic turbulence. Furthermore near the wall the polymers show a strong tendency to orient along the streamwise direction of the flow, but near the center line the statistics of orientation of the polymers is consistent with analogous results obtained recently in homogeneous and isotropic flows.

  • 96.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Computational Hydrodynamic Stability and Flow Control Based on Spectral Analysis of Linear Operators2012In: Archives of Computational Methods in Engineering, ISSN 1134-3060, E-ISSN 1886-1784, Vol. 19, no 3, p. 341-379Article, review/survey (Refereed)
    Abstract [en]

    This paper considers the analysis and control of fluid flows using tools from dynamical systems and control theory. The employed tools are derived from the spectral analysis of various linear operators associated with the Navier-Stokes equations. Spectral decomposition of the linearized Navier-Stokes operator, the Koopman operator, the spatial correlation operator and the Hankel operator provide a means to gain physical insight into the dynamics of complex flows and enables the construction of low-dimensional models suitable for control design. Since the discretization of the Navier-Stokes equations often leads to very large-scale dynamical systems, matrix-free and in some cases iterative techniques have to be employed to solve the eigenvalue problem. The common theme of the numerical algorithms is the use of direct numerical simulations. The theory and the algorithms are exemplified on flow over a flat plate and a jet in crossflow, as prototypes for the laminar-turbulent transition and three-dimensional vortex shedding.

  • 97.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effects of weak noise on oscillating flows: Linking quality factor, Floquet modes, and Koopman spectrum2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 9, p. 094104-Article in journal (Refereed)
    Abstract [en]

    Many fluid flows, such as bluff body wakes, exhibit stable self-sustained oscillations for a wide range of parameters. Here we study the effect of weak noise on such flows. In the presence of noise, a flow with self-sustained oscillations is characterized not only by its period, but also by the quality factor. This measure gives an estimation of the number of oscillations over which periodicity is maintained. Using a recent theory [P. Gaspard, J. Stat. Phys. 106, 57 (2002)], we report on two observations. First, for weak noise the quality factor can be approximated using a linear Floquet analysis of the deterministic system; its size is inversely proportional to the inner-product between first direct and adjoint Floquet vectors. Second, the quality factor can readily be observed from the spectrum of evolution operators. This has consequences for Koopman/Dynamic mode decomposition analyses, which extract coherent structures associated with different frequencies from numerical or experimental flows. In particular, the presence of noise induces a damping on the eigenvalues, which increases quadratically with the frequency and linearly with the noise amplitude. (C) 2014 AIP Publishing LLC.

  • 98.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Koopman-mode decomposition of the cylinder wake2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 726, p. 596-623Article in journal (Refereed)
    Abstract [en]

    The Koopman operator provides a powerful way of analysing nonlinear flow dynamics using linear techniques. The operator defines how observables evolve in time along a nonlinear flow trajectory. In this paper, we perform a Koopman analysis of the first Hopf bifurcation of the flow past a circular cylinder. First, we decompose the flow into a sequence of Koopman modes, where each mode evolves in time with one single frequency/growth rate and amplitude/phase, corresponding to the complex eigenvalues and eigenfunctions of the Koopman operator, respectively. The analytical construction of these modes shows how the amplitudes and phases of nonlinear global modes oscillating with the vortex shedding frequency or its harmonics evolve as the flow develops and later sustains self-excited oscillations. Second, we compute the dynamic modes using the dynamic mode decomposition (DMD) algorithm, which fits a linear combination of exponential terms to a sequence of snapshots spaced equally in time. It is shown that under certain conditions the DMD algorithm approximates Koopman modes, and hence provides a viable method to decompose the flow into saturated and transient oscillatory modes. Finally, the relevance of the analysis to frequency selection, global modes and shift modes is discussed.

  • 99.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Input-output analysis, model reduction and control of the flat-plate boundary layer2009In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 620, p. 263-298Article in journal (Refereed)
    Abstract [en]

    The dynamics and control of two-dimensional disturbances in the spatially evolving boundary layer oil a flat plate are investigated from an input output viewpoint. A set-up of spatially localized inputs (external disturbances and actuators) and Outputs (objective functions and sensors) is introduced for the control design of convectively unstable flow configurations. From the linearized Navier Stokes equations with the inputs and outputs, controllable, observable and balanced modes are extracted using the snapshot method. A balanced reduced-order model (ROM) is constructed and shown to capture the input output behaviour of the linearized Navier Stokes equations. This model is finally used to design H-2-feedback controller to suppress the growth or two-dimensional perturbations inside the boundary layer.

  • 100.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Research on the interaction between streamwise streaks and Tollmien-Schlichting waves at KTH2007In: ERCOFTAC Bulletin, ISSN 2518-0991, Vol. 74, p. 37-43Article in journal (Refereed)
    Abstract [en]

    This paper summarises the experimental and numericalinvestigations on how two different types of disturbancesmay, in a positive way, interact in a flat plateboundary-layer flow. The project, which mainly hasbeen centered at KTH1, has been performed in collaborationwith colleagues from University of Bologna2and LadHyX CNRS Ecole Polytechnique3, duringthe last years. The main phenomena — the stabilisingeffect of streamwise boundary-layer streakson Tollmien-Schlichting waves (and other exponentialdisturbances) — have been captured both in experiments[1, 2] and with different numerical approachessuch as direct numerical simulations [3], parabolicstability equation calculations [5] and large-eddy simulations[6]. We will here briefly review the methodsand the main results of these studies, and discuss howthey correlate with each other. For related referencesoutside KTH the interested reader is referred to thejournal publications in the reference list.

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