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  • 151. Camarri, Simone
    et al.
    Trip, Renzo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fransson, Jens
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Investigation of passive control of the wake past a thick plate by stability and sensitivity analysis of experimental dataManuskript (preprint) (Annet vitenskapelig)
  • 152.
    Campagne, Antoine
    et al.
    LEGI, Université Grenoble Alpes.
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Chassagne, Rémi
    LEGI, Université Grenoble Alpes.
    Micard, Diane
    LMFA, École Centrale de Lyon.
    Mordant, Nicolas
    LEGI, Université Grenoble Alpes.
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Sommeria, Joel
    LEGI, Université Grenoble Alpes.
    Viboud, Samuel
    LEGI, Université Grenoble Alpes.
    Mohanan, Ashwin Vishnu
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Augier, Pierre
    LEGI, Université Grenoble Alpes.
    First report of the MILESTONE experiment: strongly stratified turbulence and mixing efficiency in the Coriolis platform2016Inngår i: VIIIth International Symposium on Stratified Flows (ISSF), 2016, 2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Strongly stratified turbulence is a possible interpretation of oceanic and atmospheric mea-surements. However, this regime has never been produced in a laboratory experiment be-cause of the two conditions of very small horizontal Froude number Fh and large buoyancy Reynolds number R which require a verily large experimental facility. We present a new attempt to study strongly stratified turbulence experimentally in the Coriolis platform.The flow is forced by a slow periodic movement of an array of six vertical cylinders of 25 cm diameter with a mesh of 75 cm. Five cameras are used for 3D-2C scanned horizontalparticles image velocimetry (PIV) and stereo 2D vertical PIV. Five density-temperatureprobes are used to measure vertical and horizontal profiles and signals at fixed positions.The first preliminary results indicate that we manage to produce strongly stratified tur-bulence at very small Fh and large R in a laboratory experiment.

  • 153.
    Canton, Jacopo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Linear stability of the flow in a toroidal pipe2015Inngår i: 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015, TSFP-9 , 2015Konferansepaper (Fagfellevurdert)
    Abstract [en]

    While hydrodynamic stability and transition to turbulence in straight pipes - being one of the most fundamental problems in fluid mechanics - has been studied extensively, the stability of curved pipes has received less attention. In the present work, the first (linear) instability of the canonical flow inside a toroidal pipe is investigated as a first step in the study of the related laminar-turbulent transition process. The impact of the curvature of the pipe, in the range 8 e [0.002,1], on the stability properties of the flow is studied in the framework of linear stability analysis. Results show that the flow is indeed modally unstable for all curvatures investigated and that the wave number corresponding to the critical mode depends on the curvature, as do several other features of this problem. The critical modes are mainly located in the region of the Dean vortices, and are characterised by oscillations which are symmetric or antisymmetric as a function of the curvature. The neutral curve associated with the first bifurcation is the result of a complex interaction between isolated modes and branches composed by several modes characterised by a common structure. This behaviour is in obvious contrast to that of straight pipes, which are linearly stable for all Reynolds numbers.

  • 154.
    Canton, Jacopo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Modal instability of the flow in a toroidal pipe2016Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 792, s. 894-909Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The modal instability encountered by the incompressible flow inside a toroidal pipe is studied, for the first time, by means of linear stability analysis and direct numerical simulation (DNS). In addition to the unquestionable aesthetic appeal, the torus represents the smallest departure from the canonical straight pipe flow, at least for low curvatures. The flow is governed by only two parameters: the Reynolds number (Formula presented.) and the curvature of the torus (Formula presented.), i.e. the ratio between pipe radius and torus radius. The absence of additional features, such as torsion in the case of a helical pipe, allows us to isolate the effect that the curvature has on the onset of the instability. Results show that the flow is linearly unstable for all curvatures investigated between 0.002 and unity, and undergoes a Hopf bifurcation at (Formula presented.) of about 4000. The bifurcation is followed by the onset of a periodic regime, characterised by travelling waves with wavelength (Formula presented.) pipe diameters. The neutral curve associated with the instability is traced in parameter space by means of a novel continuation algorithm. Tracking the bifurcation provides a complete description of the modal onset of instability as a function of the two governing parameters, and allows a precise calculation of the critical values of (Formula presented.) and (Formula presented.). Several different modes are found, with differing properties and eigenfunction shapes. Some eigenmodes are observed to belong to groups with a set of common characteristics, deemed ‘families’, while others appear as ‘isolated’. Comparison with nonlinear DNS shows excellent agreement, confirming every aspect of the linear analysis, its accuracy, and proving its significance for the nonlinear flow. Experimental data from the literature are also shown to be in considerable agreement with the present results.

  • 155.
    Canton, Jacopo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Chin, C.
    Hutchins, N.
    Monty, J.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    On Large-Scale Friction Control in Turbulent Wall Flow in Low Reynolds Number Channels2016Inngår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 97, nr 3, s. 811-827Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present study reconsiders the control scheme proposed by Schoppa & Hussain (Phys. Fluids 10, 1049–1051 1998), using a new set of numerical simulations. The computations are performed in a turbulent channel at friction Reynolds numbers of 104 (the value employed in the original study) and 180. In particular, the aim is to better characterise the physics of the control as well as to investigate the optimal parameters. The former purpose lead to a re-design of the control strategy: moving from a numerical imposition of the mean flow to the application of a volume force. A comparison between the two is presented. Results show that the original method only gave rise to transient drag reduction. The forcing method, on the other hand, leads to sustained drag reduction, and thus shows the superiority of the forcing approach for all wavelengths investigated. A clear maximum efficiency in drag reduction is reached for the case with a viscous-scaled spanwise wavelength of the vortices of 1200, which yields a drag reduction of 18 %, as compared to the smaller wavelength of 400 suggested as the most efficient vortex in Schoppa & Hussain. Various turbulence statistics are considered, in an effort to elucidate the causes of the drag-reducing effect. For instance, a region of negative production was found, which is quite unusual for developed turbulent channel flow.

  • 156.
    Canton, Jacopo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Chin, Cheng
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Reynolds number dependence of large-scale friction control in turbulent channel flow2016Inngår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 1, nr 8, artikkel-id 081501Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present work investigates the effectiveness of the control strategy introduced by Schoppa and Hussain [Phys. Fluids 10, 1049 (1998)] as a function of Reynolds number (Re). The skin-friction drag reduction method proposed by these authors, consisting of streamwise-invariant, counter-rotating vortices, was analyzed by Canton et al. [Flow, Turbul. Combust. 97, 811 (2016)] in turbulent channel flows for friction Reynolds numbers (Re t) corresponding to the value of the original study (i.e., 104) and 180. For these Re, a slightly modified version of the method proved to be successful and was capable of providing a drag reduction of up to 18%. The present study analyzes the Reynolds number dependence of this drag-reducing strategy by performing two sets of direct numerical simulations (DNS) for Re-tau = 360 and 550. A detailed analysis of the method as a function of the control parameters (amplitude and wavelength) and Re confirms, on the one hand, the effectiveness of the large-scale vortices at low Re and, on the other hand, the decreasing and finally vanishing effectiveness of this method for higher Re. In particular, no drag reduction can be achieved for Re t = 550 for any combination of the parameters controlling the vortices. For low Reynolds numbers, the large-scale vortices are able to affect the near-wall cycle and alter the wall-shear-stress distribution to cause an overall drag reduction effect, in accordance with most control strategies. For higher Re, instead, the present method fails to penetrate the near-wall region and cannot induce the spanwise velocity variation observed in other more established control strategies, which focus on the near-wall cycle. Despite the negative outcome, the present results demonstrate the shortcomings of the control strategy and show that future focus should be on methods that directly target the near-wall region or other suitable alternatives.

  • 157.
    Canton, Jacopo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Characterisation of the steady, laminar incompressible flow in toroidal pipes covering the entire curvature range2017Inngår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 66, s. 95-107Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work is concerned with a detailed investigation of the steady (laminar), incompressible flow inside bent pipes. In particular, a toroidal pipe is considered in an effort to isolate the effect of the curvature, δ, on the flow features, and to compare the present results to available correlations in the literature. More than 110 000 numerical solutions are computed, without any approximation, spanning the entire curvature range, 0 ≤ δ ≤ 1, and for bulk Reynolds numbers Re up to 7 000, where the flow is known to be unsteady. Results show that the Dean number De provides a meaningful non-dimensional group only below very strict limits on the curvature and the Dean number itself. For δ>10−6 and De > 10, in fact, not a single flow feature is found to scale well with the Dean number. These considerations are also valid for quantities, such as the Fanning friction factor, that were previously considered Dean-number dependent only. The flow is therefore studied as a function of two equally important, independent parameters: the curvature of the pipe and the Reynolds number. The analysis shows that by increasing the curvature the flow is fundamentally changed. Moderate to high curvatures are not only quantitatively, but also qualitatively different from low δ cases. A complete description of some of the most relevant flow quantities is provided. Most notably the friction factor f for laminar flow in curved pipes by Ito [J. Basic Eng. 81:123–134 (1959)] is reproduced, the influence of the curvature on f is quantified and the scaling is discussed. A complete database including all the computed solutions is available at www.flow.kth.se.

  • 158.
    Carlson, Andreas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Bellani, Gabriele
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Contact line dissipation in short-time dynamic wetting2012Inngår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 97, nr 4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dynamic wetting of a solid surface is a process that is ubiquitous in Nature, and also of increasing technological importance. The underlying dissipative mechanisms are, however, still unclear. We present here short-time dynamic wetting experiments and numerical simulations, based on a phase field approach, of a droplet on a dry solid surface, where direct comparison of the two allows us to evaluate the different contributions from the numerics. We find that an important part of the dissipation may arise from a friction related to the motion of the contact line itself, and that this may be dominating both inertia and viscous friction in the flow adjacent to the contact line. A contact line friction factor appears in the theoretical formulation that can be distinguished and quantified, also in room temperature where other sources of dissipation are present. Water and glycerin-water mixtures on various surfaces have been investigated where we show the dependency of the friction factor on the nature of the surface, and the viscosity of the liquid.

  • 159.
    Carlson, Andreas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Bellani, Gabriele
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Measuring contact line dissipation in dynamic wetting2011Rapport (Annet vitenskapelig)
    Abstract [en]

    Dynamic wetting of a solid surface is a process that is ubiquitous in nature, and also of increasing technological importance. The underlying dissipative mechanisms are however still unclear. We present here dynamic wetting experiments of a droplet on a dry surface, showing that an important part of the dissipation may arise from a friction related to the motion of the contact line itself, and that this may be dominating the viscous friction in the flow adjacent to the contact line. By a combination of simulations and experiments, values of a corresponding friction factor are obtained. By this procedure the contact line friction factor can be distinguished and quantified, also in room temperature where other sources of dissipation are present. Water and glycerin-water mixtures on various surfaces have been investigated. We show the dependency of the friction factor on the nature of the surface, and the viscosity of the liquid.

  • 160.
    Carlson, Andreas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Bellani, Gabriele
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Universality in dynamic wetting dominated by contact-line friction2012Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 85, nr 4, s. 045302-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report experiments on the rapid contact-line motion present in the early stages of capillary-driven spreading of drops on dry solid substrates. The spreading data fail to follow a conventional viscous or inertial scaling. By integrating experiments and simulations, we quantify a contact-line friction mu(f) which is seen to limit the speed of the rapid dynamic wetting. A scaling based on this contact-line friction is shown to yield a universal curve for the evolution of the contact-line radius as a function of time, for a range of fluid viscosities, drop sizes, and surface wettabilities.

  • 161.
    Carlson, Andreas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Dissipation in rapid dynamic wetting2011Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 682, s. 213-240Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this article, we present a modelling approach for rapid dynamic wetting based on the phase field theory. We show that in order to model this accurately, it is important to allow for a non-equilibrium wetting boundary condition. Using a condition of this type, we obtain a direct match with experimental results reported in the literature for rapid spreading of liquid droplets on dry surfaces. By extracting the dissipation of energy and the rate of change of kinetic energy in the flow simulation, we identify a new wetting regime during the rapid phase of spreading. This is characterized by the main dissipation to be due to a re-organization of molecules at the contact line, in a diffusive or active process. This regime serves as an addition to the other wetting regimes that have previously been reported in the literature.

  • 162.
    Carlson, Andreas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Droplet dynamics in a bifurcating channel2010Inngår i: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 36, nr 5, s. 397-405Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In the present paper we present a phenomenological description of droplet dynamics in a bifurcating channel that is based on three-dimensional numerical experiments using the Phase Field theory. Droplet dynamics is investigated in a junction, which has symmetric outflow conditions in its daughter branches. We identify two different flow regimes as the droplets interact with the tip of the bifurcation, splitting and non-splitting. A distinct criterion for the flow regime transition is found based on the initial droplet volume and the Capillary (Ca) number. The Rayleigh Plateau instability is identified as a driving mechanism for the droplet breakup close to the threshold between the splitting and non-splitting regime.

  • 163.
    Carlson, Andreas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Modeling of dynamic wetting far from equilibrium2009Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, nr 12Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper we present simulations of dynamic wetting far from equilibrium based on phase field theory. In direct simulations of recent experiments [J. C. Bird, S. Mandre, and H. A. Stone, Phys. Rev. Lett. 100, 234501 (2008)], we show that in order to correctly capture the dynamics of rapid wetting, it is crucial to account for nonequilibrium at the contact line, where the gas, liquid, and solid meet. A term in the boundary condition at the solid surface that naturally arises in the phase field theory is interpreted as allowing for the establishment of a local structure in the immediate vicinity of the contact line. A direct qualitative and quantitative match with experimental data of spontaneously wetting liquid droplets is shown.

  • 164.
    Carlson, Andreas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Kim, P.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Stone, H. A.
    Short and long time drop dynamics on lubricated substrates2013Inngår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 104, nr 3, s. 34008-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Liquid infiltrated solids have been proposed as functional solvent-phobic surfaces for handling single and multiphase flows. Implementation of such surfaces alters the interfacial transport phenomenon as compared to a dry substrate. To better understand the interface characteristics in such systems we study experimentally the dynamics of a pendant water drop in air that contacts a substrate coated by thin oil films. At short times the water drop is deformed by the oil that spreads onto the water-air interface, and the dynamics are characterized by inertial and viscous regimes. At late times, the the oil film under the drop relaxes either to a stable thin film or ruptures. In the thin film rupture regime, we measure the waiting time for the rupture as a function of the drop equilibrium contact angle on a dry substrate and the initial film height. The waiting time is rationalized by lubrication theory, which indicates that long-range intermolecular forces destabilize the oil-water interface and is the primary mechanism for the film drainage.

  • 165.
    Carlsson, Allan
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Near wall fibre orientation in flowing suspensions2009Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis deals with fibre orientation in wall-bounded shear flows. The primary application in mind is papermaking. The study is mainly experimental,but is complemented with theoretical considerations.The main part of the thesis concerns the orientation of slowly settlingfibres in a wall-bounded viscous shear flow. This is a flow case not dealt withpreviously even at small Reynolds numbers. Experiments were conducted usingdilute suspensions with fibres having aspect ratios of rp ≈ 7 and 30. It is foundthat the wall effect on the orientation is small for distances from the wall wherethe fibre centre is located farther than half a fibre length from the wall. Farfrom the wall most fibres were oriented close to the flow direction. Closer tothe wall than half a fibre length the orientation distribution first shifted to bemore isotropic and in the very proximity of the wall the fibres were orientedclose to perpendicular to the flow direction, nearly aligned with the vorticityaxis. This was most evident for the shorter fibres with rp ≈ 7.Due to the density difference between the fibres and the fluid there is anincreased concentration near the wall. Still, a physical mechanism is requiredin order for a fibre initially oriented close to the flow direction at about half afibre length from the wall to change its orientation to aligned with the vorticityaxis once it has settled down to the wall. A slender body approach is usedin order to estimate the effect of wall reflection and repeated wall contacts onthe fibre rotation. It is found that the both a wall reflection, due to settlingtowards the wall, and contact between the fibre end and the wall are expectedto rotate the fibre closer to the vorticity axis. A qualitative agreement withthe experimental results is found in a numerical study based on the theoreticalestimation.In addition an experimental study on fibre orientation in the boundarylayers of a headbox is reported. The orientation distribution in planes parallelto the wall is studied. The distribution is found to be more anisotropic closerto the wall, i.e. the fibres tend to be oriented closer to the flow direction nearthe wall. This trend is observed sufficiently far upstream in the headbox.Farther downstream no significant change in the orientation distribution couldbe detected for different distances from the wall.

  • 166.
    Carlsson, Allan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Koch, Donald L.
    Orbit drift of a slowly settling fibre in a wall-bounded shear flowManuskript (Annet vitenskapelig)
  • 167.
    Carlsson, Allan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fibre orientation control related to papermaking2007Inngår i: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 129, nr 4, s. 457-465Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The orientation of fibers suspended in a shear flow flowing over a solid wall has been studied experimentally. The possibility to control this orientation with physical surface modifications, ridges, has also been studied. The fiber suspension was driven by gravity down a slightly inclined glass plate and a CCD-camera was used to capture images of the fibers in the flow. Image analysis based on the concept of steerable filters extracted the position and orientation of the fibers in the plane of the image. From these data, the velocity of the fibers was determined. When viewing the flow from the side, the velocity of the fibers at different heights was measured and found to agree with the theoretical solution for Newtonian flow down an inclined plate. Moving the camera so that the flow was filmed from below, the orientation and velocity of fibers in the plane parallel to the solid surface was determined. The known relationship between the velocity and the wall normal position of the fibers made it possible to determine the height above the plate for each identified fiber. Far away from the wall, the fibers were aligned with the flow direction in both cases. In a region close to the smooth plate surface the fibers oriented themselves perpendicular to the flow direction. This change in orientation did not occur when the surface structure was modified with ridges.

  • 168.
    Carlsson, Allan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Söderberg, L. Daniel
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Evaluation of a steerable filter for detection of fibres in flowing suspensionsManuskript (Annet vitenskapelig)
    Abstract [en]

    Steerable filters are concluded to be useful in order to determine the orientation of fibres captured in digital images. The fibre orientation is a key variable in the study of flowing fibre suspensions. Here digital image analysis based on a filter within the class of steerable filters is evaluated for suitability of finding the position and orientation of fibres suspended in flowing suspensions. In sharp images with small noise levels the steerable filter succeeds in determining the orientation of artificially generated fibres with well-defined angles. The influence of reduced image quality on the orientation has been quantified. The effect of unsharpness and noise is studied and the results show that the error in orientation is less than 1◦ for moderate levels. A set of images with fibres suspended in a shear flow is also analyzed. The fibre orientation distribution is determined in the flow-vorticity plane. In this analysis a comparison is also made to a robust, but computationally more expensive, method involving convolutions with an oriented elliptic filter. A good agreement is found when comparing the resulting fibre orientation distributions obtained with the two methods.

  • 169.
    Carlsson, Allan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Söderberg, L. Daniel
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Orientation of slowly sedimenting fibers in a flowing suspensionnear a plane wall2007Inngår i: Svenska Mekanikdagarna, 2007Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The effect of a wall on the orientation of slowly sedimenting fibers suspended in a shear flow has been studied experimentally. Experiments were performed at two concentrations with two aspect ratios, rp ≈ 7 and rp ≈ 30, where rp is defined as the fiber length divided by the diameter. For all cases the majority of the fibers were oriented close to parallel to the flow direction for distances farther away from the wall than half a fiber length. As the distance from the wall decreased a change in orientation was observed. At distances from the wall closer than about an eighth of a fiber length a significant amount of the fibers were oriented close to perpendicular to the flow. This was particularly clear for the shorter fibers. Due to the density difference between the fibers and the surrounding fluid the fiber concentration was increased in the near wall region. An increased concentration was found in a limited region close to half a fiber length from the wall. For the shorter fibers a large number of fibers was also detected in the very proximity of the wall.

  • 170.
    Carlsson, Allan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Söderberg, Daniel
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fibre Orientation Control Related To Papermaking2006Inngår i: PROCEEDINGS OF THE ASME FLUIDS ENGINEERING DIVISION SUMMER CONFERENCE, VOL 1, PTS A AND B, 2006, s. 1501-1509Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The wall effect on the orientation of fibres suspended in a shear flow has been studied experimentally. A fibre suspension, driven by gravity down an inclined glass plate, constitutes the shear flow field. A CCD-camera was mounted underneath the flow in order to visualize the flow. The orientation of fibres in the plane perpendicular to the plate was determined, by using the concept of steerable filters. In a region close to the smooth plate surface the fibres oriented themselves perpendicular to the flow direction. This did not occur when the surface structure was modified with ridges.

  • 171.
    Carlsson, Allan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Söderberg, L. D.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Fibre orientation measurements near a headbox wall2010Inngår i: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 25, nr 2, s. 204-212Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Experimental results on the fibre orientation in a laboratory scale headbox are reported. Images containing fibres in approximately 1 mm thick slices parallel to the wall were captured at different wall distances. A steerable filter was used to determine the orientation of bleached and unbeaten birch fibres, suspended in water, at different distances from one of the inclined walls of the headbox contraction. Due to optical limitations only dilute suspensions were studied. It is shown that the fibre orientation distribution varies with the distance from the wall. Sufficiently far upstream in the headbox a more anisotropic distribution is found closer to the wall.

  • 172.
    Carlsson, Allan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Söderberg, L. Daniel
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. STFI-Packforsk AB, Sweden.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fibre orientation in the boundary layers of a planar converging channel2008Inngår i: TAPPI Press - Paper Conference and Trade Show, PaperCon '08, 2008, s. 384-408Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Experimental results on the fibre orientation in a laboratory scale headbox are reported. A steerable filter was used to determine the orientation of bleached and unbeaten birch fibres at different distances from one of the inclined walls of the headbox contraction. Due to optical limitations only low concentrations were studied. It is shown that the orientation varies with the distance from the wall. For most studied cases a more anisotropic profile was found closer to the wall.

  • 173.
    Carlsson, Allan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Söderberg, L. Daniel
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Fibre orientation near a wall of a headbox.2010Inngår i: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 25, s. 204-212Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Experimental results on the fibre orientation in a laboratory scale headbox are reported. A steerable filter was used to determine the orientation of bleached unbeaten birch fibres at different distances from one of the inclined walls of the headbox contraction. Due to optical limitations only dilute suspensions were studied. It is shown that the fibre orientation distribution varies with the distance from the wall. Sufficiently far upstream in the headbox a more anisotropic distribution is found closer to the wall as compared to farther away from the wall.

  • 174. Castellani, F.
    et al.
    Astolfi, D.
    Mana, M.
    Becchetti, M.
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wake losses from averaged and time-resolved power measurements at full scale wind turbines2017Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 854, nr 1, artikkel-id 012006Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work deals with the experimental analysis of wake losses fluctuations at full-scale wind turbines. The test case is a wind farm sited on a moderately complex terrain: 4 turbines are installed, having 2 MW of rated power each. The sources of information are the time-resolved data, as collected from the OPC server, and the 10-minutes averaged SCADA data. The objective is to compare the statistical distributions of wake losses for far and middle wakes, as can be observed through the "fast" lens of time-resolved data, for certain selected test-case time series, and through the "slow" lens of SCADA data, on a much longer time basis that allow to set the standards of the mean wake losses along the wind farm. Further, time-resolved data are used for an insight into the spectral properties of wake fluctuations, highlighting the role of the wind turbine as low-pass filter. Summarizing, the wind rose, the layout of the site and the structure of the data sets at disposal allow to study middle and far wake behavior, with a "slow" and "fast" perspective.

  • 175. Castro, Ian P.
    et al.
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Outer-layer turbulence intensities in smooth- and rough-wall boundary layers2013Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 727, s. 119-131Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Clear differences in turbulence intensity profiles in smooth, transitional and fully rough zero-pressure-gradient boundary layers are demonstrated, using the diagnostic plot introduced by Alfredsson, Segalini & Orlu (Phys. Fluids, vol. 23, 2011, p. 041702) u'/U versus U/U-e, where u' and U are the local (root mean square) fluctuating and mean velocities and U-e is the free stream velocity. A wide range of published data are considered and all zero-pressure-gradient boundary layers yield outer flow u'/U values that are roughly linearly related to U/U-e, just as for smooth walls, but with a significantly higher slope which is completely independent of the roughness morphology. The difference in slope is due largely to the influence of the roughness parameter (Delta U+ in the usual notation) and all the data can be fitted empirically by using a modified form of the scaling, dependent only on Delta U/U-e. The turbulence intensity, at a location in the outer layer where U/U-e is fixed, rises monotonically with increasing Delta U/U-e which, however, remains of O(1) for all possible zero-pressure-gradient rough-wall boundary layers even at the highest Reynolds numbers. A measurement of intensity at a point in the outer region of the boundary layer can provide an indication of whether the surface is aerodynamically fully rough, without having to determine the surface stress or effective roughness height. Discussion of the implication for smooth/rough flow universality of differences in outer-layer mean velocity wake strength is included.

  • 176.
    Chandramouli, Sathyanarayanan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Gojon, Romain
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fridh, Jens
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Kraft- och värmeteknologi. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Numerical characterization of entropy noise with a density based solver2017Inngår i: 12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017, KTH Royal Institute of Technology, 2017Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this work, dbnsTurbFoam, a new coupled density based solver, written in the framework of FOAM-EXTEND, is considered. The solver is first assessed on two canonical compressible flow scenarios, namely the Sod's shock tube and the ONERA S8 transonic channel. Results are compared with analytical formulations and experiments, respectively. 2-D Unsteady Reynolds Averaged Navier-Stokes simulations and 3-D Large Eddy Simulations of the flow within the passages of a geometrically simplified High Pressure Turbine Nozzle Guide Vane are then performed. Results are compared against experimental data in order to justify the geometrical simplifications made. Finally, the case of a sinusoidal temperature forcing at the inlet is considered in order to study the phenomenon of indirect combustion noise. Notably, the impact of the forcing on the vortex shedding dynamics and on the losses is discussed.

  • 177.
    Chaparian, Emad
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Tammisola, Outi
    An adaptive finite element method for elastoviscoplastic fluid flows2019Inngår i: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 271, artikkel-id UNSP 104148Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Elastoviscoplastic fluids are a class of yield-stress fluids that behave like neoHookean (or viscoelastic) solids when the imposed stress is less than the yield stress whereas after yielding, their behaviour is described by a viscoplastic fluid with an additional elastic history. This exceptional behaviour has been recently observed by many yield stress fluids in rheometric tests such as waxy crude oil, Carbopol gel, etc. Moreover, interesting phenomena have been evidenced experimentally such as the presence of a negative wake and a loss of fore-aft symmetry about a settling particle which are predominantly related to the elastic behaviour of yield-stress fluids (i.e., coupling of elasticity and plasticity). Here, we present a numerical scheme based on the so-called augmented Lagrangian method for numerical simulation of elastoviscoplastic fluid flows. The method is benchmarked by two rheometric flows: Poiseuille and circular Couette flows for which analytical solutions are derived. Moreover, anisotropic adaptive mesh procedure (which was previously introduced for viscoplastic fluid flows by Saramito and Roquet, Comput. Meth. Appl. Mech. Eng., vol. 190, 2001, pp. 5391-5412) is coupled to obtain a fine resolution of the yield surfaces. Finally, the presented method is applied to study more complex flows: elastoviscoplastic fluid flow in a wavy channel.

  • 178.
    Chen, Song
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik.
    Nozzle Pressure Ratio Effects on Aerodynamics and Acoustics of a Highly-Heated Rectangular Supersonic Jet2019Inngår i: 25th AIAA/CEAS Aeroacoustics Conference, 2019, s. 16-, artikkel-id AIAA 2019-2753Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Implicit large-eddy simulations (LES) are performed in this work to study the flow-field and acoustic characteristics of a highly-heated rectangular supersonic jet. The focus is on the nozzle pressure ratio(NPR) effects. Three NPRs are investigated including 3.0, 3.67, and 4.0, which correspond to the nozzle over-expansion, perfect-expansion, and under-expansion conditions respectively. The current hot jet has a nozzle temperature ratio (NTR) of 7.0, corresponding to a total temperature of around 2100K. The rectangular nozzle has an aspect ratio of 2.0 and has been extensively tested at the Universityof Cincinnati. An in-house CFD code with an artificial dissipation mechanism is used to perform the large-scale implicit LES computations. By studying the pressure contours, density gradients and dilatation, it is found that the three-dimensional jet shock/expansion wave structure changes signifi-cantly when the jet NPR increases from an over-expanded to under-expanded condition. The length of the laminar shear layer right outside the nozzle is extended to the downstream before transitioning to be turbulent. The distance between the nozzle lip and the first shock cell is doubled while the total number of shock cells keeps the same, which results in a longer jet potential core. The increase of nozzle NPR also provides about an 11% increase in jet velocity and a 25% increase in shear layer convection Mach number, which leads to a stronger Mach wave radiation noise component in the acoustic fields. Pressure spectra in the near field reveal that screech only exists in the over-expansion case and the broadband shock-associated noise is enhanced in the perfect and under expansion cases.The far-field acoustics at 40Deq is characterized by about 4 dB increase of the overall sound pressure level in the Mach wave radiation direction and about 2 – 3 dB increase in all other directions. The far-field pressure spectra also confirm that the screech noise component vanishes when the nozzle NPR is increased to perfect- and under-expanded conditions.

  • 179.
    Chevalier, Mattias
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI).
    Hein, Stefan
    DLR.
    Sousa, Joao M.M.
    Technical University of Lisbon.
    Additional computationsfor the RWG results Final analysis2008Rapport (Annet vitenskapelig)
  • 180. Chin, C
    et al.
    Monty, J
    Hutchins, N
    Ooi, A
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Simulation of a large-eddy-break-up device (LEBU) in a moderate Reynolds number turbulent boundary layer2015Inngår i: Proc 9th Turbulence and Shear Flow Phenomena Conference, 2015Konferansepaper (Fagfellevurdert)
  • 181. Chin, C.
    et al.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Cardesa, J. I.
    Noorani, Azad
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Chong, M. S.
    Flow topology of rare back flow events and critical points in turbulent channels and toroidal pipes2018Inngår i: Journal of Physics: Conference Series, Institute of Physics Publishing (IOPP), 2018, Vol. 1001, nr 1, artikkel-id 012002Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A study of the back flow events and critical points in the flow through a toroidal pipe at friction Reynolds number Reτ ≈ 650 is performed and compared with the results in a turbulent channel flow at Reτ ≈ 934. The statistics and topological properties of the back flow events are analysed and discussed. Conditionally-averaged flow fields in the vicinity of the back flow event are obtained, and the results for the torus show a similar streamwise wall-shear stress topology which varies considerably for the spanwise wall-shear stress when compared to the channel flow. The comparison between the toroidal pipe and channel flows also shows fewer back flow events and critical points in the torus. This cannot be solely attributed to differences in Reynolds number, but is a clear effect of the secondary flow present in the toroidal pipe. A possible mechanism is the effect of the secondary flow present in the torus, which convects momentum from the inner to the outer bend through the core of the pipe, and back from the outer to the inner bend through the pipe walls. In the region around the critical points, the skin-friction streamlines and vorticity lines exhibit similar flow characteristics with a node and saddle pair for both flows. These results indicate that back flow events and critical points are genuine features of wall-bounded turbulence, and are not artifacts of specific boundary or inflow conditions in simulations and/or measurement uncertainties in experiments.

  • 182. Chin, Cheng
    et al.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Monty, Jason
    Hutchins, Nicholas
    Influence of a Large-Eddy-Breakup-Device on the Turbulent Interface of Boundary Layers2017Inngår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, nr 3-4, s. 823-835Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effects of implementing a large-eddy break-up device (LEBU) in a turbulent boundary layer on the interaction with the boundary layer is investigated with particular emphasis on the turbulent/non-turbulent interface (TNTI). The simulation data is taken from a recent well-resolved large eddy simulation (Chin et al. Flow Turb. Combust. 98, 445-460 2017), where the LEBU was implemented at a wall-normal distance of 0.8 delta (local boundary layer thickness) from the wall. A comparison of the TNTI statistics is performed between a zero-pressure-gradient boundary layer with and without the LEBU. The LEBU is found to delay the growth of the turbulent boundary layer and also attenuates the fluctuations of the TNTI. The LEBU appears to alter the structure size at the interface, resulting in a narrower and shorter dominant structure (in an average sense). Further analysis beneath the TNTI using two-point correlations shows that the LEBU affects the turbulent structures in excess of 100 delta downstream of the LEBU.

  • 183.
    Cifuentes, Luis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Univ Duisburg Essen, Chair Fluid Dynam, Inst Combust & Gasdynam IVG, D-47057 Duisburg, Germany..
    Fooladgar, Ehsan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Duwig, Christophe
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Chemical Explosive Mode Analysis for a Jet-in-Hot-Coflow burner operating in MILD combustion2018Inngår i: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 232, s. 712-723Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Large Eddy Simulations (LES) of Moderate and Intense Low oxygen Dilution (MILD) combustion of a Jet-in-HotCoflow (JHC) burner were performed using detailed chemistry. On the contrary to traditional flames, where heat release is occurring in very thin fronts, MILD combustion occurs in the distributed reaction regime where the reaction zone is broad, thus, this paper applies a direct Arrhenius closure with detailed chemistry to resolve important details of the fuel oxidation reactions. Comparisons of LES results are in good agreement with experiments, demonstrating that the simulations capture the intermediate species and finite reaction rate effects. A Chemical Explosive Mode Analysis (CEMA) was used to determine the flame structure and to detect the pre-and post-ignition regions, including the contributions to the CEMs analyzing the Explosion Index (EI) and Participation Index (PI). To the best of our knowledge, a detailed study of CEMA on MILD or flameless regime has never been reported. The flame structure was clearly visualized with CEMA, as well as the lean and the rich flame fronts. Different flame zones close to the anchoring points of these turbulent lifted flames were selected and the analysis demonstrates the contributions of dominant chemical species, such as HO2 and O. The reactions related to the dominant local CEM were obtained to highlight the nature of the stabilization in these highly diluted operating conditions.

  • 184. Cimarelli, A.
    et al.
    De Angelis, E.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Talamelli, A.
    Casciola, C. M.
    Sources and fluxes of scale energy in the overlap layer of wall turbulence2015Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 771, s. 407-423Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulations of turbulent channel flows at friction Reynolds numbers (Re) of 550, 1000 and 1500 are used to analyse the turbulent production, transfer and dissipation mechanisms in the compound space of scales and wall distances by means of the Kolmogorov equation generalized to inhomogeneous anisotropic flows. Two distinct peaks of scale-energy source are identified. The first, stronger one, belongs to the near-wall cycle. Its location in the space of scales and physical space is found to scale in viscous units, while its intensity grows slowly with Re, indicating a near-wall modulation. The second source peak is found further away from the wall in the putative overlap layer, and it is separated from the near-wall source by a layer of significant scale-energy sink. The dynamics of the second outer source appears to be strongly dependent on the Reynolds number. The detailed scale-by-scale analysis of this source highlights well-defined features that are used to make the properties of the outer turbulent source independent of Reynolds number and wall distance by rescaling the problem. Overall, the present results suggest a strong connection of the observed outer scale-energy source with the presence of an outer region of turbulence production whose mechanisms are well separated from the near-wall region and whose statistical features agree with the hypothesis of an overlap layer dominated by attached eddies. Inner-outer interactions between the near-wall and outer source region in terms of scale-energy fluxes are also analysed. It is conjectured that the near-wall modulation of the statistics at increasing Reynolds number can be related to a confinement of the near-wall turbulence production due to the presence of increasingly large production scales in the outer scale-energy source region.

  • 185. Citro, Vincenzo
    et al.
    Giannetti, Flavio
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Luchini, Paolo
    Linear three-dimensional global and asymptotic stability analysis of incompressible open cavity flow2015Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 768Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The viscous and inviscid linear stability of the incompressible flow past a square open cavity is studied numerically. The analysis shows that the flow first undergoes a steady three-dimensional bifurcation at a critical Reynolds number of 1370. The critical mode is localized inside the cavity and has a flat roll structure with a spanwise wavelength of about 0.47 cavity depths. The adjoint global mode reveals that the instability is most efficiently triggered in the thin region close to the upstream tip of the cavity. The structural sensitivity analysis identifies the wavemaker as the region located inside the cavity and spatially concentrated around a closed orbit. As the flow outside the cavity plays no role in the generation mechanisms leading to the bifurcation, we confirm that an appropriate parameter to describe the critical conditions in open cavity flows is the Reynolds number based on the average velocity between the two upper edges. Stabilization is achieved by a decrease of the total momentum inside the shear layer that drives the core vortex within the cavity. The mechanism of instability is then studied by means of a short-wavelength approximation considering pressureless inviscid modes. The closed streamline related to the maximum inviscid growth rate is found to be the same as that around which the global wavemaker is concentrated. The structural sensitivity field based on direct and adjoint eigenmodes, computed at a Reynolds number far higher than that of the base flow, can predict the critical orbit on which the main instabilities inside the cavity arise. Further, we show that the sub-leading unstable time-dependent modes emerging at supercritical conditions are characterized by a period that is a multiple of the revolution time of Lagrangian particles along the orbit of maximum growth rate. The eigenfrequencies of these modes, computed by global stability analysis, are in very good agreement with the asymptotic results.

  • 186. Coppola, Gennaro
    et al.
    Semeraro, Onofrio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Interfacial instability of two rotating viscous immiscible fluids in a cylinder2011Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, nr 6, s. 064105-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A complete original study of the linear temporal instability analysis of two viscous and immiscible fluids enclosed in a rigid cylinder rotating about its axis and separated by a cylindrical interface is performed for the case of higher density fluid located in the annulus. The results of the present contribution fill the lack of an overall assessment of the system behavior due to the increase of both the analytical difficulties and the number of the governing parameters when the several physical effects are all included. The analysis is carried out numerically by discretizing the equations of the evolution of disturbances separately in the two phases formulated in a rotating reference frame. Normal mode analysis leads to a generalized eigenvalue problem which is solved by means of a Chebyshev collocation spectral method. The investigation of the preferred modes of instability is carried out over wide ranges of the parameters space. The behavior of the system is physically discussed and is compared to inviscid asymptotic limits and to viscous approximate solutions of the previous literature.

  • 187. Cossu, C.
    et al.
    Chevalier, M.
    Henningson, Dan Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Secondary optimal growth and subcritical transition in the plane Poiseuille flow2010Inngår i: 7th IUTAM Symposium on Laminar-Turbulent Transition, Springer Netherlands, 2010, s. 129-134Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Nonlinear optimal perturbations leading to subcritical transition with minimum threshold energy are searched in the plane Poiseuille flow at Re = 1500. To this end we proceed in two steps. First a family of optimally growing primary streaks U issued by the optimal vortices of the Poiseuille laminar solution is computed by direct numerical simulation for a set of finite amplitudes AI of the primary vortices. An adjoint technique is then used to compute the maximum growth and the finite time Lyapunov exponents of secondary perturbations growing on top of these primary base flows. The secondary optimals take into full account the non-normality and the local instabilities of the tangent operator all along the temporal evolution of the primary flows. The most amplified optimal perturbations are sinuous and realized in correspondence of streaks that are locally unstable. The combinations of primary and secondary perturbations optimal for transition are then explored using direct numerical simulations. It is shown that the minimum initial energy is realized by a large set of these combinations, revealing new paths to transition. Surprisingly we find that transition can be efficiently obtained even using secondary perturbations alone, in the absence of primary optimal vortices.

  • 188. Cossu, Carlo
    et al.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Bagheri, Shervin
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Secondary threshold amplitudes for sinuous streak breakdown2011Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, nr 7, s. 074103-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The nonlinear stability of laminar sinuously bent streaks is studied for the plane Couette flow at Re = 500 in a nearly minimal box and for the Blasius boundary layer at Re(delta)(*)=700. The initial perturbations are nonlinearly saturated streamwise streaks of amplitude A(U) perturbed with sinuous perturbations of amplitude A(W). The local boundary of the basin of attraction of the linearly stable laminar flow is computed by bisection and projected in the A(U) - A(W) plane providing a well defined critical curve. Different streak transition scenarios are seen to correspond to different regions of the critical curve. The modal instability of the streaks is responsible for transition for A(U) = 25%-27% for the considered flows, where sinuous perturbations of amplitude below A(W) approximate to 1%-2% are sufficient to counteract the streak viscous dissipation and induce breakdown. The critical amplitude of the sinuous perturbations increases when the streamwise streak amplitude is decreased. With secondary perturbations amplitude A(W) approximate to 4%, breakdown is induced on stable streamwise streaks with A(U) approximate to 13%, following the secondary transient growth scenario first examined by Schoppa and Hussain [J. Fluid Mech. 453, 57 (2002)]. A cross-over, where the critical amplitude of the sinuous perturbation becomes larger than the amplitude of streamwise streaks, is observed for streaks of small amplitude A(U) < 5%-6%. In this case, the transition is induced by an initial transient amplification of streamwise vortices, forced by the decaying sinuous mode. This is followed by the growth of the streaks and final breakdown. The shape of the critical A(U) - A(W) curve is very similar for Couette and boundary layer flows and seems to be relatively insensitive to the nature of the edge states on the basin boundary. The shape of this critical curve indicates that the stability of streamwise streaks should always be assessed in terms of both the streak amplitude and the amplitude of spanwise velocity perturbations.

  • 189.
    Costa, Pedro
    et al.
    Delft Univ Technol, Proc & Energy Dept Multiphase Syst, Leeghwaterstr 21, NL-2621 CA Delft, Netherlands..
    Picano, Francesco
    Univ Padua, Dept Ind Engn, Via Venezia 1, I-35131 Padua, Italy..
    Brandt, L.uca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Breugem, Wim-Paul
    Delft Univ Technol, Proc & Energy Dept Multiphase Syst, Leeghwaterstr 21, NL-2621 CA Delft, Netherlands..
    Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions2018Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 843, s. 450-478Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use interface-resolved numerical simulations to study finite-size effects in turbulent channel flow of neutrally buoyant spheres. Two cases with particle sizes differing by a factor of two, at the same solid volume fraction of 20% and bulk Reynolds number are considered. These are complemented with two reference single-phase flows: the unladen case, and the flow of a Newtonian fluid with the effective suspension viscosity of the same mixture in the laminar regime. As recently highlighted in Costa etal. (Phys. Rev. Lett., vol.117, 2016, 134501), a particle-wall layer is responsible for deviations of the mesoscale-averaged statistics from what is observed in the continuum limit where the suspension is modelled as a Newtonian fluid with (higher) effective viscosity. Here we investigate in detail the fluid and particle dynamics inside this layer and in the bulk. In the particle-wall layer, the near-wall inhomogeneity has an influence on the suspension microstructure over a distance proportional to the particle size. In this layer, particles have a significant (apparent) slip velocity that is reflected in the distribution of wall shear stresses. This is characterized by extreme events (both much higher and much lower than the mean). Based on these observations we provide a scaling for the particle-to-fluid apparent slip velocity as a function of the flow parameters. We also extend the scaling laws in Costa etal. (Phys. Rev. Lett., vol.117, 2016, 134501) to second-order Eulerian statistics in the homogeneous suspension region away from the wall. The results show that finite-size effects in the bulk of the channel become important for larger particles, while negligible for lower-order statistics and smaller particles. Finally, we study the particle dynamics along the wall-normal direction. Our results suggest that single-point dispersion is dominated by particle-turbulence (and not particle-particle) interactions, while differences in two-point dispersion and collisional dynamics are consistent with a picture of shear-driven interactions.

  • 190. Costa, Pedro
    et al.
    Picano, Francesco
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Breugem, Wim-Paul
    Universal Scaling Laws for Dense Particle Suspensions in Turbulent Wall-Bounded Flows2016Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, nr 13, artikkel-id 134501Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The macroscopic behavior of dense suspensions of neutrally buoyant spheres in turbulent plane channel flow is examined. We show that particles larger than the smallest turbulence scales cause the suspension to deviate from the continuum limit in which its dynamics is well described by an effective suspension viscosity. This deviation is caused by the formation of a particle layer close to the wall with significant slip velocity. By assuming two distinct transport mechanisms in the near-wall layer and the turbulence in the bulk, we define an effective wall location such that the flow in the bulk can still be accurately described by an effective suspension viscosity. We thus propose scaling laws for the mean velocity profile of the suspension flow, together with a master equation able to predict the increase in drag as a function of the particle size and volume fraction.

  • 191. Croze, Ottavio A.
    et al.
    Sardina, Gaetano
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Ahmed, Mansoor
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Bees, Martin A.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Dispersion of swimming algae in laminar and turbulent channel flows: consequences for photobioreactors2013Inngår i: Journal of the Royal Society Interface, ISSN 1742-5689, E-ISSN 1742-5662, Vol. 10, nr 81, s. 20121041-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Shear flow significantly affects the transport of swimming algae in suspension. For example, viscous and gravitational torques bias bottom-heavy cells to swim towards regions of downwelling fluid (gyrotaxis). It is necessary to understand how such biases affect algal dispersion in natural and industrial flows, especially in view of growing interest in algal photobioreactors. Motivated by this, we here study the dispersion of gyrotactic algae in laminar and turbulent channel flows using direct numerical simulation (DNS) and a previously published analytical swimming dispersion theory. Time-resolved dispersion measures are evaluated as functions of the Peclet and Reynolds numbers in upwelling and downwelling flows. For laminar flows, DNS results are compared with theory using competing descriptions of biased swimming cells in shear flow. Excellent agreement is found for predictions that employ generalized Taylor dispersion. The results highlight peculiarities of gyrotactic swimmer dispersion relative to passive tracers. In laminar downwelling flow the cell distribution drifts in excess of the mean flow, increasing in magnitude with Peclet number. The cell effective axial diffusivity increases and decreases with Peclet number (for tracers it merely increases). In turbulent flows, gyrotactic effects are weaker, but discernable and manifested as non-zero drift. These results should have a significant impact on photobioreactor design.

  • 192.
    Dadfar, Reza
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fabbiane, Nicolo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Bagheri, Shervin
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dans S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Centralised Versus Decentralised Active Control of Boundary Layer Instabilities2014Inngår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 93, nr 4, s. 537-553Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use linear control theory to construct an output feedback controller for the attenuation of small-amplitude three-dimensional Tollmien-Schlichting (TS) wavepackets in a flat-plate boundary layer. A three-dimensional viscous, incompressible flow developing on a zero-pressure gradient boundary layer in a low Reynolds number environment is analyzed using direct numerical simulations. In this configuration, we distribute evenly in the spanwise direction up to 72 localised objects near the wall (18 disturbances sources, 18 actuators, 18 estimation sensors and 18 objective sensors). In a fully three-dimensional configuration, the interconnection between inputs and outputs becomes quickly unfeasible when the number of actuators and sensors increases in the spanwise direction. The objective of this work is to understand how an efficient controller may be designed by connecting only a subset of the actuators to sensors, thereby reducing the complexity of the controller, without comprising the efficiency. If n and m are the number of sensor-actuator pairs for the whole system and for a single control unit, respectively, then in a decentralised strategy, the number of interconnections deceases mn compared to a centralized strategy, which has n (2) interconnections. We find that using a semi-decentralized approach - where small control units consisting of 3 estimation sensors connected to 3 actuators are replicated 6 times along the spanwise direction - results only in a 11 % reduction of control performance. We explain how "wide" in the spanwise direction a control unit should be for a satisfactory control performance. Moreover, the control unit should be designed to account for the perturbations that are coming from the lateral sides (crosstalk) of the estimation sensors. We have also found that the influence of crosstalk is not as essential as the spreading effect.

  • 193.
    Dadfar, Reza
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Control of instabilities in an unswept wing boundary layer2018Inngår i: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, nr 5, s. 1750-1759Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Linear control theory is used to construct an output feedback controller to attenuate the amplitude of the Tollmien–Schlichting waves inside the boundary layer developing over an unswept wing. The analysis is based on direct numerical simulations. The studied scenario includes the impulse response of the system to a generic disturbance in the freestream, which triggers a Tollmien–Schlichting wave packet inside the boundary layer. The performance of a linear quadratic Gaussian controller is analyzed to suppress the amplitude of the Tollmien–Schlichting wave packet using a row of sensors and plasma actuators localized at the wall. The target of the controller is chosen as a subset of proper orthogonal decomposition modes describing the dynamics of the unstable disturbances. The plasma actuators are implemented as volume forcing. To account for the limitations of the plasma actuators concerning a unidirectional forcing, several strategies are implemented in the linear quadratic Gaussian framework. Their performances are compared with that for classical linear quadratic Gaussian controller. These controllers successfully reduced the amplitude of the wave packet.

  • 194.
    Dadfar, Reza
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dans S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Control of instabilities in boundary layer of unswept wingManuskript (preprint) (Annet vitenskapelig)
  • 195.
    Dadfar, Reza
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dans S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Feedback Control for Laminarization of flow over Wings2015Inngår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 94, nr 1, s. 43-62Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An active control strategy is implemented to attenuate the amplitude of the Tollmien-Schlichting (TS) waves inside the boundary layer of an airfoil. The dynamics of the system are modelled by the linearised Navier-Stokes equations. The impulse response to an initial disturbance, initially located outside of the boundary layer and in front of the airfoil is considered. The perturbation evolves and penetrates inside the boundary layer and triggers the TS waves. Different control strategies including the linear quadratic Gaussian (LQG) and model predictive control (MPC) are designed based on a reduced order model where the sensors and actuators are localised near the wall. An output projection is used to identify the unstable disturbances; the objective function of the controller is selected as a set of proper orthogonal decomposition (POD) modes; to isolate the dynamics of the TS waves, the modes with high energy contents in the TS wave frequency band are considered as the objective of the controller. A plasma actuator is modelled and implemented as an external forcing on the flow. To account for the limitations of the plasma actuator several strategies are examined and the results are compared with a classical LQG controller. The outcomes reveal successful performance in mitigating the amplitude of the wavepacket developing inside the boundary layer.

  • 196.
    Dadfar, Reza
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Semeraro, Onofrio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Output Feedback Control of Blasius Flow with Leading Edge Using Plasma Actuator2013Inngår i: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 51, nr 9, s. 2192-2207Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The evolution and control of a two-dimensional wave packet developing on a flat plate with a leading edge is investigated by means of direct numerical simulation. The aim is to identify and suppress the wave packets generated by freestream perturbations. A sensor is placed close to the wall to detect the upcoming wave packet, while an actuator is placed further downstream to control it. A plasma actuator is modeled as an external forcing on the flow using a model based and validated on experimental investigations. A linear quadratic Gaussian controller is designed, and an output projection is used to build the objective function. Moreover, by appropriate selection of the proper orthogonal decomposition modes, we identify the disturbances to be damped. A reduced-order model of the input-output system is constructed by using system identification via the eigensystem realization algorithm. A limitation of the plasma actuators is the unidirectional forcing of the generated wall jet, which is predetermined by the electrodes' location. In this paper, we address this limitation by proposing and comparing two different solutions: 1) introducing an offset in the control signal such that the resulting total forcing is oriented along one direction, and 2) using two plasma actuators acting in opposite directions. The results are compared with the ideal case where constraints are not accounted for the control design. We show that the resulting controllers based on plasma actuators can successfully attenuate the amplitude of the wave packet developing inside the boundary layer.

  • 197.
    Dahan, Jeremy
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aerodynamik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Futrzynski, Romain
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aerodynamik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    O'Reilly, Ciarán
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Efraimsson, Gunilla
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aerodynamik.
    Aero-acoustic source analysis of landing gear noise via dynamic mode decomposition2014Inngår i: 21st International Congress on Sound and Vibration, ICSV21: / [ed] Malcolm J. Crocker, Marek Pawelczyk, Jing Tian, 2014, Vol. 2, s. 1245-1252Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this paper, we apply dynamic mode decomposition (DMD) on time accurate simulationsof the pressure distribution on a realistic full-scale noselanding gear configuration in order toidentify noise generating structures on landing gear surfaces. The simulated pressure data isobtained from DES simulations using the commercial software STAR-CCM+ by CD-adapco.The dynamics of the surface pressure on a tyre are discussed and the DMD modes are com-puted from instantaneous pressure snapshots. The far-fieldnoise is determined via the FfowcsWilliams-Hawkings analogy, where a given frequency band source term can be reconstructedby choosing an appropriate number of DMD modes.

  • 198.
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Finite wavelength selection for the linear instability of a suspension of settling spheroids2011Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 689, s. 183-202Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The instability of an initially homogeneous suspension of spheroids, settling due to gravity, is reconsidered. For non-spherical particles, previous studies in the literature report that normal-mode density perturbations of maximum growth rate are those of arbitrarily large, horizontal wavelength. Using the 'mixture theory' for two-phase flow we show that the maximum growth rate for horizontal density perturbations is obtained for a finite wavelength if the inertia of the bulk motion associated with the normal-mode density perturbation is accounted for. We find that for long wavelengths, lambda -> infinity, the growth rate approaches zero as lambda(-2/3). The maximum growth rate is obtained for lambda similar to d/root alpha(0)Re(L)(1/2), where d is the axis perpendicular to the axis of rotational symmetry of the spheroid, alpha(0) is the undisturbed volume fraction of particles and Re(L), typically << 1, is a Reynolds number of the bulk motion on a typical length scale L similar to d/p root alpha(0) and a velocity scale on the order of the undisturbed settling speed. The theoretical results for the wavelength selection agree qualitatively well with previous experimental results in the literature of measured correlation lengths of vertical streamers in settling fibre suspensions.

  • 199. Daly, C. A.
    et al.
    Schneider, Tobias M.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Peake, N.
    Secondary instability and tertiary states in rotating plane Couette flow2014Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 761, s. 27-61Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Recent experimental studies have shown rich transition behaviour in rotating plane Couette flow (RPCF). In this paper we study the transition in supercritical RPCF theoretically by determination of equilibrium and periodic orbit tertiary states via Floquet analysis on secondary Taylor vortex solutions. Two new tertiary states are discovered which we name oscillatory wavy vortex flow (oWVF) and skewed vortex flow (SVF). We present the bifurcation routes and stability properties of these new tertiary states and, in addition, we describe a bifurcation procedure whereby a set of defected wavy twist vortices is approached. Further to this, transition scenarios at flow parameters relevant to experimental works are investigated by computation of the set of stable attractors which exist on a large domain. The physically observed flow states are shown to share features with states in our set of attractors.

  • 200. Dangendorf, Soenke
    et al.
    Müller-Navarra, Sylvin
    Jensen, Juergen
    Schenk, Frederik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wahl, Thomas
    Weisse, Ralf
    North Sea Storminess from a Novel Storm Surge Record since AD 18432014Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, nr 10, s. 3582-3595Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The detection of potential long-term changes in historical storm statistics and storm surges plays a vitally important role for protecting coastal communities. In the absence of long homogeneous wind records, the authors present a novel, independent, and homogeneous storm surge record based on water level observations in the North Sea since 1843. Storm surges are characterized by considerable interannual-to-decadal variability linked to large-scale atmospheric circulation patterns. Time periods of increased storm surge levels prevailed in the late nineteenth and twentieth centuries without any evidence for significant long-term trends. This contradicts with recent findings based on reanalysis data, which suggest increasing storminess in the region since the late nineteenth century. The authors compare the wind and pressure fields from the Twentieth-Century Reanalysis (20CRv2) with the storm surge record by applying state-of-the-art empirical wind surge formulas. The comparison reveals that the reanalysis is a valuable tool that leads to good results over the past 100 yr; previously the statistical relationship fails, leaving significantly lower values in the upper percentiles of the predicted surge time series. These low values lead to significant upward trends over the entire investigation period, which are in turn supported by neither the storm surge record nor an independent circulation index based on homogeneous pressure readings. The authors therefore suggest that these differences are related to higher uncertainties in the earlier years of the 20CRv2 over the North Sea region.

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