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  • 2351.
    Zhang, Mengqi
    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.
    Lashgari, Iman
    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.
    Zaki, Tamer A.
    Brandt, Luca
    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.
    Linear stability analysis of channel flow of viscoelastic Oldroyd-B and FENE-P fluids2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 737, p. 249-279Article in journal (Refereed)
    Abstract [en]

    We study the modal and non-modal linear instability of inertia-dominated channel flow of viscoelastic fluids modelled by the Oldroyd-B and FENE-P closures. The effects of polymer viscosity and relaxation time are considered for both fluids, with the additional parameter of the maximum possible extension for the FENE-P. We find that the parameter explaining the effect of the polymer on the instability is the ratio between the polymer relaxation time and the characteristic instability time scale (the frequency of a modal wave and the time over which the disturbance grows in the non-modal case). Destabilization of both modal and non-modal instability is observed when the polymer relaxation time is shorter than the instability time scale, whereas the flow is more stable in the opposite case. Analysis of the kinetic energy budget reveals that in both regimes the production of perturbation kinetic energy due to the work of the Reynolds stress against the mean shear is responsible for the observed effects where polymers act to alter the correlation between the streamwise and wall-normal velocity fluctuations. In the subcritical regime, the non-modal amplification of streamwise elongated structures is still the most dangerous disturbance-growth mechanism in the flow and this is slightly enhanced by the presence of polymers. However, viscoelastic effects are found to have a stabilizing effect on the amplification of oblique modes.

  • 2352.
    Zhou, Guang-Quan
    et al.
    Southeast Univ, Sch Biol Sci & Med Engn, Nanjing, Jiangsu, Peoples R China.;Southeast Univ, Natl Demonstrat Ctr Expt Biomed Engn Educ, Nanjing, Jiangsu, Peoples R China..
    Zhang, Yi
    Southeast Univ, Sch Biol Sci & Med Engn, Nanjing, Jiangsu, Peoples R China.;Southeast Univ, Natl Demonstrat Ctr Expt Biomed Engn Educ, Nanjing, Jiangsu, Peoples R China..
    Wang, Ruo-Li
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, BioMEx. Karolinska Inst, Dept Womens & Childrens Hlth, Stockholm, Sweden..
    Zhou, Ping
    Southeast Univ, Sch Biol Sci & Med Engn, Nanjing, Jiangsu, Peoples R China.;Southeast Univ, Natl Demonstrat Ctr Expt Biomed Engn Educ, Nanjing, Jiangsu, Peoples R China..
    Zheng, Yong-Ping
    Hong Kong Polytech Univ, Dept Biomed Engn, Hong Kong, Hong Kong, Peoples R China..
    Tarassova, Olga
    Swedish Sch Sport & Hlth Sci, Stockholm, Sweden..
    Arndt, Anton
    Swedish Sch Sport & Hlth Sci, Stockholm, Sweden.;Karolinska Inst, Dept Clin Intervent & Technol, Stockholm, Sweden..
    Chen, Qiang
    Southeast Univ, Sch Biol Sci & Med Engn, Nanjing, Jiangsu, Peoples R China.;Southeast Univ, Natl Demonstrat Ctr Expt Biomed Engn Educ, Nanjing, Jiangsu, Peoples R China..
    Automatic Myotendinous Junction Tracking in Ultrasound Images with Phase-Based Segmentation2018In: BioMed Research International, ISSN 2314-6133, E-ISSN 2314-6141, article id 3697835Article in journal (Refereed)
    Abstract [en]

    Displacement of the myotendinous junction (MTJ) obtained by ultrasound imaging is crucial to quantify the interactive length changes of muscles and tendons for understanding the mechanics and pathological conditions of the muscle-tendon unit during motion. However, the lack of a reliable automatic measurement method restricts its application in human motion analysis. This paper presents an automated measurement of MTJ displacement using prior knowledge on tendinous tissues and MTJ, precluding the influence of nontendinous components on the estimation of MTJ displacement. It is based on the perception of tendinous features from musculoskeletal ultrasound images using Radon transform and thresholding methods, with information about the symmetric measures obtained from phase congruency. The displacement of MTJ is achieved by tracking manually marked points on tendinous tissues with the Lucas-Kanade optical flow algorithm applied over the segmented MTJ region. The performance of this method was evaluated on ultrasound images of the gastrocnemius obtained from 10 healthy subjects (26.0 +/- 2.9 years of age). Waveform similarity between the manual and automatic measurements was assessed by calculating the overall similarity with the coefficient ofmultiple correlation (CMC). In vivo experiments demonstrated that MTJ tracking with the proposedmethod (CMC = 0.97 +/- 0.02) was more consistent with the manual measurements than existing optical flow tracking methods (CMC = 0.79 +/- 0.11). This study demonstrated that the proposed method was robust to the interference of nontendinous components, resulting in a more reliable measurement of MTJ displacement, whichmay facilitate further research and applications related to the architectural change of muscles and tendons.

  • 2353.
    Zhou, Yang
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Numerical instability investigations for thin membranes2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Membrane structures are commonly used in many fields. The studies of these structures are of increasing interest. The projects in this thesis focus on the evaluations of equilibrium states for pressurized membranes under different problem settings, using finite element methods, and the corresponding instability behaviors.

    The first part of the current work discusses the instability behavior of a thin, planar, circular and initially horizontal membrane subjected to downwards or upwards fluid pressure. The membrane structures exhibit large deformations under pressure. The method for evaluating fluid pressure from gravity was developed in finite element context, and used in numerical simulations. Limit and bifurcation points have been detected for different loading parameters and conditions. The effects on instabilities of parameters, the initial states of the membrane, and the chosen mesh are discussed.

    The second part of the current work discusses instability behavior of a thin, spherical and closed membrane containing gas and fluid, when placed on a horizontal rigid and non-friction plane. A multi-parametric loading is described. By adding practically relevant controlling equations, different classes of equilibrium paths were followed using a generalized path following algorithm. Stability conclusions were made, according to the considered load parameters and the constraints. A generalized eigenvalue analysis was used to evaluate the stability behavior including the constraint effects. Fold line evaluations were performed to analyze the parametric dependence. A solution surface approach is used to visualize the mechanical response under this multi-parametric setting.

    The third part of the current work focuses on instability response of a truncated sphere, containing gas and fluid, and in contact with two vertical rigid and non-friction planes. Different penalty formulations were used and compared. The effects of contact implementations on instability behaviors were investigated. Bifurcation points induced by contacts have been observed. Multi-parametric problems were defined, and generalized paths were followed. The multi-parametric stability was evaluated using generalized eigenvalue analysis, based on the mass and total differential matrices. The effects of augmenting equations on bifurcation points and limit points are discussed.

    The fourth part of the current work analyses the instability response of a truncated sphere, completely filled with fluid, placed on a horizontal plane and spinning around the vertical axis. The loads from fluid pressure and the constraints, e.g., fluid volume, were formulated to generate a symmetric differential matrix. Several mesh patterns with different symmetries were used to simulate the model, and the obtained results are compared. Various problem settings were considered, and generalized paths were followed. The effects of symmetry aspects of the chosen meshes on instability behaviors are discussed, as are the effects of parameters.

  • 2354.
    Zhou, Yang
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Parametric stability analyses for fluid-loaded thin membranes2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Membrane structures are commonly used in many elds. The studies of thesestructures are of increasing interest. The two projects focus on the evaluations ofequilibrium states for uid-pressurized membranes under dierent loading conditions,and the corresponding instability behavior.The rst part of the current work discusses the instability behavior of a thin,planar, circular and initially horizontal membrane subjected to downwards or upwards uid pressure. The membrane structures exhibit large deformations under uid pressure. Various instability behaviors have been observed for dierent loadingparameters. Limit and bifurcation points have been detected for dierent loadingconditions. Dierent loading parameters have been used to interpret the instabilitybehavior. The eects on instability of parameters, the initial states of the membrane,and the chosen mesh have been discussed.The second part of the current work discusses instability behavior of a thin,spherical and closed membrane containing gas and uid placed on a horizontal rigidand non-friction plane. A multi-parametric loading has been described. By addingthe practically relevant controlling equations, the complex equilibrium paths werefollowed using the generalized path following algorithm, and the stability conclusionswere made dierently, according to the considered load parameters and theconstraints. A generalized eigenvalue analysis was used to evaluate the stabilitybehavior including the constraint eects. Fold line evaluations were performed toanalyze the parametric dependence of the instability behavior. A solution surfaceapproach was used to visualize the mechanical response under this multi-parametricsetting.

  • 2355.
    Zhou, Yang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Hussain, M.
    Guanglin, Kuang
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Zhang, J.
    Tu, Yaoquan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Mechanistic insights into peptide and ligand binding of the ATAD2-bromodomain via atomistic simulations disclosing a role of induced fit and conformational selection2018In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 36, p. 23222-23232Article in journal (Refereed)
    Abstract [en]

    ATAD2 has emerged as a promising bromodomain (BRD)-containing therapeutic drug target in multiple human cancers. However, recent druggability assessment studies predicted ATAD2's BRD as a target 'difficult to drug' because its binding pocket possesses structural features that are unfeasible for ligand binding. Here, by using all-atom molecular dynamics simulations and an advanced metadynamics method, we demonstrate a dynamic view of the binding pocket features which can hardly be obtained from the "static" crystal data. The most important features disclosed from our simulation data, include: (1) a distinct 'open-to-closed' conformational switch of the ZA loop region in the context of peptide or ligand binding, akin to the induced fit mechanism of molecular recognition, (2) a dynamic equilibrium of the BC loop "in" and "out" conformations, highlighting a role in the conformational selection mechanism for ligand binding, and (3) a new binding region identified distal to the histone-binding pocket that might have implications in bromodomain biology and in inhibitor development. Moreover, based on our simulation results, we propose a model for an "auto-regulatory" mechanism of ATAD2's BRD for histone binding. Overall, the results of this study will not only have implications in bromodomain biology but also provide a theoretical basis for the discovery of new ATAD2's BRD inhibitors.

  • 2356.
    Zhou, Yang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Nordmark, Arne
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Instability investigation for rotating thin spherical membrane2017In: International Journal of Non-Linear Mechanics, ISSN 0020-7462, E-ISSN 1878-5638, Vol. 97, p. 96-106Article in journal (Refereed)
    Abstract [en]

    A fluid-filled truncated spherical membrane fixed along its truncated edge to a horizontal, rigid and frictionless plane and spinning around a center axis was investigated. A two-parameter Mooney-Rivlin model was used to describe the material of the membrane. The truncated sphere was modeled in 3D using finite element meshes with different symmetry properties. A quadratic function was used for interpolating hydro-static pressure, giving a symmetric tangent stiffness matrix, thereby reproducing the conservative problem. Various problem settings were considered, related to the spinning, and different instability behaviors were observed. Multi-parametric problems were defined, generalized paths including primary and secondary paths were followed. Stability of the multi parametric problem was evaluated using generalized eigenvalue analysis based on the total differential matrix for the constrained problem. Numerical results showed that mesh symmetry affected the simulated stability behavior. Fold line evaluations showed the parametric effects on critical solutions.

  • 2357.
    Zhou, Yang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Nordmark, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Instability investigation for rotating thin spherical membranesManuscript (preprint) (Other academic)
  • 2358.
    Zhou, Yang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Nordmark, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Instability of thin circular membranes subjected to hydro-static loads2015In: International Journal of Non-Linear Mechanics, ISSN 0020-7462, E-ISSN 1878-5638, Vol. 76, p. 144-153Article in journal (Refereed)
    Abstract [en]

    Membrane structures subjected to hydrostatic load are prone to undergo large deformations and lose stability. This paper investigates different instability phenomena for a thin, circular and initially flat and horizontal membrane. The Mooney-Rivlin hyper-elastic model is used to provide the material description. An axisymmetric and a 3D model have been set up to show the large deformations and instability behavior with different parameter settings. Numerical examples show that the methods developed are capable to describe the deformation dependent loading conditions and the instability phenomena. The numerical simulations show fundamental differences in the response and instability behavior when the horizontal membrane is loaded from above or below. The parameters of fluids and membranes and the means for introducing the pressure are of essence for interpreting the instability behavior.

  • 2359.
    Zhou, Yang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Nordmark, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Multi-parametric stability investigation for gas and uid pressurized thin membranesManuscript (preprint) (Other academic)
  • 2360.
    Zhou, Yang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Nordmark, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Multi-parametric stability investigation for thin membranes with contactsManuscript (preprint) (Other academic)
  • 2361.
    Zhou, Yang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Nordmark, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Multi-parametric stability investigation for thin spherical membranes filled with gas and fluid2016In: International Journal of Non-Linear Mechanics, ISSN 0020-7462, E-ISSN 1878-5638, Vol. 82, p. 37-48Article in journal (Refereed)
    Abstract [en]

    The instability behavior of spherical membranes completely or partially filled with fluid, also with internal gas over-pressure, placed on a friction-less rigid plane was investigated. The two-parameter Mooney-Rivlin model was used for material description. A third order penalty function was used to describe the rigid support. Different problem settings were considered, and different instability responses were observed. For the partially fluid-filled membrane, a multi-parametric problem was defined and analyzed. Augmenting equations were introduced to impose control constraints on variables chosen. These equations also affect the instability analysis. A generalized eigenvalue analysis was used for the stability conclusions. Numerical simulations showed that appropriate control constraints are of essence to interpret the instability conclusions. Fold line evaluations were performed to analyze the dependence of the instability behavior on the parameters. A solution surface algorithm was utilized to analyze and visualize the mechanical responses to multi-variable loading.

  • 2362.
    Zhou, Yang
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Nordmark, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Multi-parametric stability investigation for thin spherical membranes with contacts2017In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 131-132, p. 334-344Article in journal (Refereed)
    Abstract [en]

    The instability behavior for a thin truncated spherical membrane completely filled with fluid or containing both gas and fluid, fixed on a circular platform and in contact with two vertical planes was investigated. Different penalty functions for contacts, and symmetry aspects of the discretized model were studied, and gave effects on instability behavior. Stability conclusions for the multi-parametric problems were made using generalized eigenvalue analyses, showing limit points, bifurcation points and turning point. Contact conditions were shown to introduce bifurcations and secondary paths, dependent on the contact implementations and discretizations. Their effects on stability behaviors in connection with various controlling equations are discussed.

  • 2363.
    Zhu, Hongli
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
    Helander, Mikaela
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Moser, Carl
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Ståhlkranz, Adam
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henriksson, Gunnar
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Lindström, Mikael
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    A novel nano cellulose preparation method and size fraction by cross flow ultra- filtration2012In: Current organic chemistry, ISSN 1385-2728, E-ISSN 1875-5348, Vol. 16, no 16, p. 1871-1875Article in journal (Refereed)
    Abstract [en]

    A novel energy-efficient method called nanopulping (patent pending) to produce nanocellulose from chemical pulp, and a novel cross-flow ultra-filtration method to separate nanofibrils fractions of different size were applied in this study. Pretreatment with endoglucanase or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation seems to enhance the nanopulping process. Results were evaluated with atomic force microscope and ultrafiltration. The nanopulping produced a relatively inhomogeneous material with larger particles/ fibers in addition to nanofibers. However, by ultrafiltration of the material it was possible to obtain more homogeneous material in different dimensions with methods industrially acceptable.

  • 2364.
    Zhu, Lailai
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical investigation of swimming micro-organisms in complex environments2012Licentiate thesis, comprehensive summary (Other academic)
  • 2365.
    Zhu, Lailai
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Simulation of individual cells in flow2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, simulations are performed to study the motion ofindividual cells in flow, focusing on the hydrodynamics of actively swimming cells likethe self-propelling microorganisms, and of passively advected objects like the red bloodcells. In particular, we develop numerical tools to address the locomotion ofmicroswimmers in viscoelastic fluids and complex geometries, as well as the motion ofdeformable capsules in micro-fluidic flows.

    For the active movement, the squirmer is used as our model microswimmer. The finiteelement method is employed to study the influence of the viscoelasticity of fluid on theperformance of locomotion. A boundary element method is implemented to study swimmingcells inside a tube. For the passive counterpart, the deformable capsule is chosen as the modelcell. An accelerated boundary integral method code is developed to solve thefluid-structure interaction, and a global spectral method is incorporated to handle theevolving cell surface and its corresponding membrane dynamics.

    We study the locomotion of a neutral squirmer with anemphasis on the change of swimming kinematics, energetics, and flowdisturbance from Newtonian to viscoelastic fluid. We also examine the dynamics of differentswimming gaits resulting in different patterns of polymer deformation, as well as theirinfluence on the swimming performance. We correlate the change of swimming speed withthe extensional viscosity and that of power consumption with the phase delay of viscoelasticfluids. Moreover, we utilise the boundary element method to simulate the swimming cells in astraight and torus-like bent tube, where the tube radius is a few times the cell radius. Weinvestigate the effect of tube confinement to the swimming speed and power consumption. Weanalyse the motions of squirmers with different gaits, which significantly affect thestability of the motion. Helical trajectories are produced for a neutralsquirmer swimming, in qualitative agreement with experimental observations, which can beexplained by hydrodynamic interactions alone.

    We perform simulations of a deformable capsule in micro-fluidic flows. We look atthe trajectory and deformation of a capsule through a channel/duct with a corner. Thevelocity of capsule displays an overshoot as passing around the corner, indicating apparentviscoelasticity induced by the interaction between the deformable membrane and viscousflow. A curved corner is found to deform the capsule less than the straight one. In addition, we propose a new cell sorting device based on the deformability of cells. Weintroduce carefully-designed geometric features into the flow to excite thehydrodynamic interactions between the cell and device. This interaction varies andclosely depends on the cell deformability, the resultant difference scatters the cellsonto different trajectories. Our high-fidelity computations show that the new strategy achievesa clear and robust separation of cells. We finally investigate the motion of capsule in awall-bounded oscillating shear flow, to understand the effect of physiological pulsation to thedeformation and lateral migration of cells. We observe the lateral migration velocity of a cellvaries non-monotonically with its deformability.

  • 2366.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    The motion of a deforming capsule through a corner2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 770, p. 374-397Article in journal (Refereed)
    Abstract [en]

    A three-dimensional deformable capsule convected through a square duct with a corner is studied via numerical simulations. We develop an accelerated boundary integral implementation adapted to general geometries and boundary conditions. A global spectral method is adopted to resolve the dynamics of the capsule membrane developing elastic tension according to the neo-Hookean constitutive law and bending moments in an inertialess flow. The simulations show that the trajectory of the capsule closely follows the underlying streamlines independently of the capillary number. The membrane deformability, on the other hand, significantly influences the relative area variations, the advection velocity and the principal tensions observed during the capsule motion. The evolution of the capsule velocity displays a loss of the time-reversal symmetry of Stokes flow due to the elasticity of the membrane. The velocity decreases while the capsule is approaching the corner, as the background flow does, reaches a minimum at the corner and displays an overshoot past the corner due to the streamwise elongation induced by the flow acceleration in the downstream branch. This velocity overshoot increases with confinement while the maxima of the major principal tension increase linearly with the inverse of the duct width. Finally, the deformation and tension of the capsule are shown to decrease in a curved corner.

  • 2367.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lauga, Eric
    Dept. of Mechanical and Aerospace Engineering, University of California, San Diego, USA.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Locomotion by tangential deformation in a polymeric fluid2011In: Physical Reivew E, ISSN 1539-3755, Vol. 83, no 1, p. 011901-Article in journal (Refereed)
    Abstract [en]

    In several biologically relevant situations, cell locomotion occurs in polymeric fluids with Weissenberg number larger than 1. Here we present results of three-dimensional numerical simulations for the steady locomotion of a self-propelled body in a model polymeric (Giesekus) fluid at low Reynolds number. Locomotion is driven by steady tangential deformation at the surface of the body (the so-called squirming motion). In the case of a spherical squirmer, we show that the swimming velocity is systematically less than that in a Newtonian fluid, with a minimum occurring for Weissenberg numbers of order 1. The rate of work done by the swimmer always goes up compared to that occurring in the Newtonian solvent alone but is always lower than the power necessary to swim in a Newtonian fluid with the same viscosity. The swimming efficiency, defined as the ratio between the rate of work necessary to pull the body at the swimming speed in the same fluid and the rate of work done by swimming, is found to always be increased in a polymeric fluid. Further analysis reveals that polymeric stresses break the Newtonian front-back symmetry in the flow profile around the body. In particular, a strong negative elastic wake is present behind the swimmer, which correlates with strong polymer stretching, and its intensity increases with Weissenberg number and viscosity contrasts. The velocity induced by the squirmer is found to decay in space faster than in a Newtonian flow, with a strong dependence on the polymer relaxation time and viscosity. Our computational results are also extended to prolate spheroidal swimmers and smaller polymer stretching are obtained for slender shapes compared to bluff swimmers. The swimmer with an aspect ratio of two is found to be the most hydrodynamically efficient.

  • 2368.
    Zhu, Lailai
    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. Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland.;Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08540 USA.
    Gallaire, Francois
    Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland..
    Bifurcation Dynamics of a Particle-Encapsulating Droplet in Shear Flow2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 6, article id 064502Article in journal (Refereed)
    Abstract [en]

    To understand the behavior of composite fluid particles such as nucleated cells and double emulsions in flow, we study a finite-size particle encapsulated in a deforming droplet under shear flow as a model system. In addition to its concentric particle-droplet configuration, we numerically explore other eccentric and time-periodic equilibrium solutions, which emerge spontaneously via supercritical pitchfork and Hopf bifurcations. We present the loci of these solutions around the codimension-two point. We adopt a dynamic system approach to model and characterize the coupled behavior of the two bifurcations. By exploring the flow fields and hydrodynamic forces in detail, we identify the role of hydrodynamic particle-droplet interaction which gives rise to these bifurcations.

  • 2369.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lauga, Eric
    Dept. of Mechanical and Aerospace Engineering, University of California, San Diego, USA.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Low-Reynolds number swimming in a capillary tube2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 726, p. 285-311Article in journal (Refereed)
    Abstract [en]

    We use the boundary element method to study the low-Reynolds-number locomotion of a spherical model microorganism in a circular tube. The swimmer propels itself by tangential or normal surface motion in a tube whose radius is of the order of the swimmer size. Hydrodynamic interactions with the tube walls significantly affect the average swimming speed and power consumption of the model microorganism. In the case of swimming parallel to the tube axis, the locomotion speed is always reduced (respectively, increased) for swimmers with tangential (respectively, normal) deformation. In all cases, the rate of work necessary for swimming is increased by confinement. Swimmers with no force dipoles in the far field generally follow helical trajectories, solely induced by hydrodynamic interactions with the tube walls, and in qualitative agreement with recent experimental observations for Paramecium. Swimmers of the puller type always display stable locomotion at a location which depends on the strength of their force dipoles: swimmers with weak dipoles (small alpha) swim in the centre of the tube while those with strong dipoles (large alpha) swim near the walls. In contrast, pusher swimmers and those employing normal deformation are unstable and end up crashing into the walls of the tube. Similar dynamics is observed for swimming into a curved tube. These results could be relevant for the future design of artificial microswimmers in confined geometries.

  • 2370.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lauga, Eric
    Dept. of Mechanical and Aerospace Engineering, University of California, San Diego, USA.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Self-propulsion in viscoelastic fluids: pushers vs. pullers2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 5, p. 051902-Article in journal (Refereed)
    Abstract [en]

    We use numerical simulations to address locomotion at zero Reynolds number in viscoelastic (Giesekus) fluids. The swimmers are assumed to be spherical, to self-propel using tangential surface deformation, and the computations are implemented using a finite element method. The emphasis of the study is on the change of the swimming kinematics, energetics, and flow disturbance from Newtonian to viscoelastic, and on the distinction between pusher and puller swimmers. In all cases, the viscoelastic swimming speed is below the Newtonian one, with a minimum obtained for intermediate values of the Weissenberg number, We. An analysis of the flow field places the origin of this swimming degradation in non-Newtonian elongational stresses. The power required for swimming is also systematically below the Newtonian power, and always a decreasing function of We. A detail energetic balance of the swimming problem points at the polymeric part of the stress as the primary We-decreasing energetic contribution, while the contributions of the work done by the swimmer from the solvent remain essentially We-independent. In addition, we observe negative values of the polymeric power density in some flow regions, indicating positive elastic work by the polymers on the fluid. The hydrodynamic efficiency, defined as the ratio of the useful to total rate of work, is always above the Newtonian case, with a maximum relative value obtained at intermediate Weissenberg numbers. Finally, the presence of polymeric stresses leads to an increase of the rate of decay of the flow velocity in the fluid, and a decrease of the magnitude of the stresslet governing the magnitude of the effective bulk stress in the fluid.

  • 2371.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rabault, Jean
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The dynamics of a capsule in a wall-bounded oscillating shear flow2014Report (Other academic)
    Abstract [en]

    The motion of an initially spherical capsule in a wall-bounded oscillating shear flow is studied via an accelerated boundary integral implementation. Neo-Hookean model is used as the constitutive law of the membrane of capsule. The lateral migration velocity of the capsule varies non-monotonically with its capillary number. It is negatively related with the initial height of the capsule above the wall. A positive correlation between the lateral migration velocity and normal stress difference is identified. The correlation becomes strongest for the capsule with the highest lateral migration velocity. For a fixed capillary number, the lateral migration velocity decreases linearly with the frequency of oscillating shear, and approaches an asymptotic value of zero for high frequency. The deformation of capsule displays a wave-like variation in time and its frequency is twice that of the underlying shear. A phase delay is observed between the variation of capsule deformation with that of the oscillatory flow, more pronounced for a more deformable capsule.

     

  • 2372.
    Zhu, LaiLai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rabault, Jean
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Ecole Polytech, F-91128 Palaiseau, France.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The dynamics of a capsule in a wall-bounded oscillating shear flow2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 071902Article in journal (Refereed)
    Abstract [en]

    The motion of an initially spherical capsule in a wall-bounded oscillating shear flow is investigated via an accelerated boundary integral implementation. The neo-Hookean model is used as the constitutive law of the capsule membrane. The maximum wall-normal migration is observed when the oscillation period of the imposed shear is of the order of the relaxation time of the elastic membrane; hence, the optimal capillary number scales with the inverse of the oscillation frequency and the ratio agrees well with the theoretical prediction in the limit of high-frequency oscillation. The migration velocity decreases monotonically with the frequency of the applied shear and the capsule-wall distance. We report a significant correlation between the capsule lateral migration and the normal stress difference induced in the flow. The periodic variation of the capsule deformation is roughly in phase with that of the migration velocity and normal stress difference, with twice the frequency of the imposed shear. The maximum deformation increases linearly with the membrane elasticity before reaching a plateau at higher capillary numbers when the deformation is limited by the time over which shear is applied in the same direction and not by the membrane deformability. The maximum membrane deformation scales as the distance to the wall to the power 1/3 as observed for capsules and droplets in near-wall steady shear flows.

  • 2373.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rorai, Cecilia
    Mitra, Dhrubaditya
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A microfluidic device to sort capsules by deformabilityManuscript (preprint) (Other academic)
  • 2374.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. Laboratory of Fluid Mechanics and Instabilities, Switzerland .
    Rorai, Cecilia
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Mitra, Dhrubaditya
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Brandt, Luca
    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.
    A microfluidic device to sort capsules by deformability: a numerical study2014In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 10, no 39, p. 7705-7711Article in journal (Refereed)
    Abstract [en]

    Guided by extensive numerical simulations, we propose a microfluidic device that can sort elastic capsules by their deformability. The device consists of a duct embedded with a semi-cylindrical obstacle, and a diffuser which further enhances the sorting capability. We demonstrate that the device can operate reasonably well under changes in the initial position of the capsule. The efficiency of the device remains essentially unaltered under small changes of the obstacle shape (from semi-circular to semi-elliptic cross-section). Confinement along the direction perpendicular to the plane of the device increases its efficiency. This work is the first numerical study of cell sorting by a realistic microfluidic device.

  • 2375.
    Zhu, Lailai
    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. Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA.
    Stone, Howard A.
    Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
    Propulsion driven by self-oscillation via an electrohydrodynamic instability2019In: Physical Review Fluids, ISSN 2469-990X, Vol. 4, no 6, article id 061701Article in journal (Refereed)
    Abstract [en]

    Oscillations of flagella and cilia play an important role in biology, which motivates the idea of functional mimicry as part of bioinspired applications. Nevertheless, it still remains challenging to drive their artificial counterparts to oscillate via a steady, homogeneous stimulus. Combining theory and simulations, we demonstrate a strategy to achieve this goal by using an elastoelectrohydrodynamic instability (based on the Quincke rotation instability). In particular, we show that applying a uniform dc electric field can produce self-oscillatory motion of a microrobot composed of a dielectric particle and an elastic filament. Upon tuning the electric field and filament elasticity, the microrobot exhibits three distinct behaviors: a stationary state, undulatory swimming, and steady spinning, where the swimming behavior stems from an instability emerging through a Hopf bifurcation. Our results imply the feasibility of engineering self-oscillations by leveraging the elastoviscous response to control the type of bifurcation and the form of instability. We anticipate that our strategy will be useful in a broad range of applications imitating self-oscillatory natural phenomena and biological processes.

  • 2376.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Stone, Howard A.
    Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
    Rotation of a low-Reynolds-number watermill: theory and simulations2018In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 849, p. 57-75Article in journal (Refereed)
    Abstract [en]

    Recent experiments have demonstrated that small-scale rotary devices installed in a microfluidic channel can be driven passively by the underlying flow alone without resorting to conventionally applied magnetic or electric fields. In this work, we conduct a theoretical and numerical study on such a flow-driven 'watermill' at low Reynolds number, focusing on its hydrodynamic features. We model the watermill by a collection of equally spaced rigid rods. Based on the classical resistive force (RF) theory and direct numerical simulations, we compute the watermill's instantaneous rotational velocity as a function of its rod number N, position and orientation. When N >= 4, the RF theory predicts that the watermill's rotational velocity is independent of N and its orientation, implying the full rotational symmetry (of infinite order), even though the geometrical configuration exhibits a lower-fold rotational symmetry; the numerical solutions including hydrodynamic interactions show a weak dependence on N and the orientation. In addition, we adopt a dynamical system approach to identify the equilibrium positions of the watermill and analyse their stability. We further compare the theoretically and numerically derived rotational velocities, which agree with each other in general, while considerable discrepancy arises in certain configurations owing to the hydrodynamic interactions neglected by the RP theory. We confirm this conclusion by employing the RP-based asymptotic framework incorporating hydrodynamic interactions for a simpler watermill consisting of two or three rods and we show that accounting for hydrodynamic interactions can significantly enhance the accuracy of the theoretical predictions.

  • 2377. Zhu, M. -M
    et al.
    Zhao, P. -H
    Chen, Y. -L
    Bai, X. -S
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    The vorticity field of acoustic modulated free jets2012In: Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics, ISSN 0253-231X, Vol. 33, no 10, p. 1811-1814Article in journal (Refereed)
    Abstract [en]

    The motivation of this paper is to investigate the effects of acoustic modulation on the structures of free jet. Large eddy simulation is performed on three dimensional free jet of Re number 2200 with multigrid method. Comparisons of the vorticity fields are made between the unmodulated jet and perturbed jets with different modulation frequencies. When the modulation frequency is near the preferred mode of the natural jet, the flow field changes significantly. The azimuthal vorticity rolls up into large vortex rings, and the jet spreads more widely. When the modulation frequency is about the twice of the preferred mode, vortex pairing happens and the subharmonic wave becomes dominant. The flow fields are almost the same as the natural jet when the modula-tion frequency is much higher than the preferred mode. The distribution of streamwise vorticity is examined for the modulated jet near the preferred mode, and agrees qualitatively with the experiments.

  • 2378. Zolesi, V. S.
    et al.
    Ganga, P. L.
    Scolamiero, L.
    Micheletti, A.
    Podio-Guidugli, P.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Donati, A.
    Ghiozzi, M.
    On an innovative deployment concept for large space structures2012In: 42nd International Conference on Environmental Systems 2012, ICES 2012, American Institute of Aeronautics and Astronautics, 2012Conference paper (Refereed)
    Abstract [en]

    Large deployable space structures are mission-critical technologies for which deployment failure cannot be an option. The difficulty to fully reproduce and test on ground the deployment of large systems dictates the need for extremely reliable architectural concepts. In 2010, ESA promoted a study focused at the pre-development of breakthrough architectural concepts offering superior reliability. The study, which was performed as an initiative of ESA Small Medium Enterprises Office (http://www.esa.int/SME/), by Kayser Italia at its premises in Livorno (Italy), with Universita' di Roma TorVergata (Rome, Italy) as sub-contractor and consultancy from KTH (Stockholm, Sweden), led to the identification of an innovative large deployable structure of "tensegrity" type, which achieves the required reliability because it permits a drastic reduction in the number of articulated joints in comparison with non-tensegrity architectures. The identified target application was in the field of large antenna reflectors. The project focused on the overall architecture of a deployable system and the related design implications. With a view toward verifying experimentally the performance of the deployable structure, a reduced-scale breadboard model was designed and manufactured. A gravity off-loading system was designed and implemented, so as to check deployment functionality in a 1-g environment. Finally, a test campaign was conducted, to validate the main design assumptions as well as to ensure the concept's suitability for the selected target application. The test activities demonstrated satisfactory stiffness, deployment repeatability, and geometric precision in the fully deployed configuration. The test data were also used to validate a finite element model, which predicts a good static and dynamic behavior of the full-scale deployable structure.

  • 2379.
    Zou, Rongfeng
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH).
    Zhou, Yang
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Wang, Yong
    Structural Biology and NMR Laboratory, Linderstrøm-Lang Centre for Protein Science, Department of Biology, University of Copenhagen.
    Guanglin, Kuang
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH).
    Wu, Junchen
    Key Laboratory for Advanced Materials & Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology.
    Tu, Yaoquan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Free Energy Profile and Kinetics for Coupled Folding and Binding of the Intrinsically Disordered Protein p53 with MDM2Manuscript (preprint) (Other academic)
    Abstract [en]

    Intrinsically disordered proteins (IDPs) exert their functions by binding to partner proteins via a complex process that includes coupled folding and binding. Motivated by that inhibiting the binding of the IDP p53 to its partner MDM2 has become a promising strategy for drug design and that understanding of this process poses a most significant challenging task, we present an atomistic level simulation of the coupled folding and binding process linking the IDP p53 to MDM2. Using bias-exchange metadynamics (BE-MetaD) and infrequent metadynamics (InMetaD) we estimate the binding free energy, the unbinding rate and the binding rate. By analyzing the stable intermediates, we uncover the role of nonnative interactions played in the p53-MDM2 binding/unbinding process. We use a three-state model to describe the whole binding/unbinding process and to obtain the corresponding rate constants. Our work shows that the binding of p53 favors an induced fit mechanism which proceeds in a stepwise fashion. In general, InMetaD gave consistent results with BE-MetaD in terms of binding mechanism and intermediates, proving the robustness of our studies of the p53-MDM2 system using metadynamics. The results contribute to the in-depth understanding for the coupled folding and binding process that is needed for the design of MDM2 inhibitors.

  • 2380.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Feedback Control of Spatially Evolving Flows2007Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In this thesis we apply linear feedback control to spatially evolving flows in order to minimize disturbance growth. The dynamics is assumed to be described by the linearized Navier--Stokes equations. Actuators and sensor are designed and a Kalman filtering technique is used to reconstruct the unknown flow state from noisy measurements. This reconstructed flow state is used to determine the control feedback which is applied to the Navier--Stokes equations through properly designed actuators. Since the control and estimation gains are obtained through an optimization process, and the Navier--Stokes equations typically forms a very high-dimensional system when discretized there is an interest in reducing the complexity of the equations. One possible approach is to perform Fourier decomposition along (almost) homogeneous spatial directions and another is by constructing a reduced order model by Galerkin projection on a suitable set of vectors. The first strategy is used to control the evolution of a range of instabilities in the classical family of Falkner--Skan--Cooke flows whereas the second is applied to a more complex cavity type of geometry.

  • 2381.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Global stability and feedback control of boundary layer flows2008Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In this thesis the stability of generic boundary layer flows is studied from a global viewpoint using optimization methods. Global eigenmodes of the incompressible linearized Navier-Stokes equations are computed using the Krylov subspace Arnoldi method. These modes serve as a tool both to study asymptotic stability and as a reduced basis to study transient growth. Transient growth is also studied using adjoint iterations. The knowledge obtained from the stability analysis is used to device systematic feedback control in the Linear Quadratic Gaussian framework. The dynamics is assumed to be described by the linearized Navier-Stokes equations. Actuators and sensors are designed and a Kalman filtering technique is used to reconstruct the unknown flow state from noisy measurements. This reconstructed flow state is used to determine the control feedback which is applied to the Navier-Stokes equations through properly designed actuators. Since the control and estimation gains are obtained through an optimization process, and the Navier-Stokes equations typically forms a very high-dimensional system when discretized there is an interest in reducing the complexity of the equations. A standard method to construct a reduced order model is to perform a Galerkin projection of the full equations onto the subspace spanned by a suitable set of vectors, such as global eigenmodes and balanced truncation modes.

  • 2382.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hoepffner, Jérôme
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Marxen, Olaf
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Steady solutions of the Navier-Stokes equations by selective frequency damping2006In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 18, no 6, p. 068102-Article in journal (Refereed)
    Abstract [en]

    A new method, enabling the computation of steady solutions of the Navier-Stokes equations in globally unstable configurations, is presented. We show that it is possible to reach a steady state by damping the unstable (temporal) frequencies. This is achieved by adding a dissipative relaxation term proportional to the high-frequency content of the velocity fluctuations. Results are presented for cavity-driven boundary-layer separation and a separation bubble induced by an external pressure gradient.

  • 2383.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ehrenstein, Uwe
    IRPHÉ, Université de Provence.
    Gallaire, Francois
    Laboratoire J.A. Dieudonné.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Global two-dimensional stability measures of the flat plate boundary-layer flow2008In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 27, no 5, p. 501-513Article in journal (Refereed)
    Abstract [en]

    The stability of the two-dimensional flat plate boundary-layer is studied by means of global eigenmodes. These eigenmodes depend both on the streamwise and wall-normal coordinate, hence there are no assumptions on the streamwise length scales of the disturbances. Expanding the perturbation velocity field in the basis of eigenmodes yields a reduced order model from which the stability characteristics of the flow, i.e. the initial condition and forcing function leading to the largest energy growth, are extracted by means of non-modal analysis. In this paper we show that, even when performing stability analysis using global eigenmodes, it is not sufficient to consider only a few of the least damped seemingly relevant eigenmodes. Instead it is the task of the optimization procedure, inherent in the non-modal analysis, to decide which eigenmodes are relevant. We show that both the optimal initial condition and the optimal forcing structure have the form of upstream tilted structures. Time integration reveals that these structures gain energy through the so called Orr mechanism, where the instabilities extract energy from the mean shear. This provides the optimal way of initiating Tollmien-Schlichting waves in the boundary layer. The optimal initial condition results in a localized Tollmien-Schlichting wavepacket that propagates downstream, whereas the optimal forcing results in a persistent Tollmien-Schlichting wave train.

  • 2384.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Haepffner, Jerome
    Ehrenstein, Uwe
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Model reduction and control of a cavity-driven separated boundary layer2008In: IUTAM Symposium On Flow Control And Mems / [ed] Morrison, JF; Birch, DM; Lavoie, P, 2008, Vol. 7, p. 147-155Conference paper (Refereed)
    Abstract [en]

    The control of a globally unstable boundary-layer flow along a two-dimensional cavity is considered. When perturbed by the worst-case initial condition, the flow exhibits a large transient growth associated with the development of a wave packet along the cavity shear layer followed by a global cycle related to the least stable global eigenmodes. The flow simulation procedure is coupled to a measurement feedback controller, which senses the wall shear stress at the downstream lip of the cavity and actuates at the upstream lip. A reduced model for the control optimization is obtained by a projection on the least stable global eigenmodes. The LQG controller is run in parallel to the Navier-Stokes time integration. It is shown that the controller is able to damp out the global oscillations.

  • 2385.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hoepffner, Jérôme
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Uwe, Eherenstein
    IRPH́E, Université de Provence.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Optimal growth, model reduction and control in a separated boundary-layer flow using global eigenmodes2007In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 579, p. 305-314Article in journal (Refereed)
    Abstract [en]

    Two-dimensional global eigenmodes are used as a projection basis both for analysing the dynamics and building a reduced model for control in a prototype separated boundary-layer flow. In the present configuration, a high aspect ratio smooth cavity-like geometry confines the separation bubble. Optimal growth analysis using the reduced basis shows that the sum of the highly non-normal global eigenmodes are able to describe a localized disturbance. Subject to this worst-case initial condition, a large transient growth associated with the development of a wavepacket along the shear layer followed by a global cycle related to the two unstable global eigenmodes is found. The flow simulation procedure is coupled to a measurement feedback controller, which senses the wall shear stress at the downstream lip of the cavity and actuates at the upstream lip. A reduced model for the control optimization is obtained by a projection on the least stable global eigenmodes, and the resulting linear-quadratic-gaussian controller is applied to the Navier--Stokes time integration. It is shown that the controller is able to damp out the global oscillations.

  • 2386. Åström, J.A.
    et al.
    Carter, A.
    Hetherington, J.
    Ioakimidis, K.
    Lindahl, Erik
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical & Computational Biophysics. KTH, Centres, Science for Life Laboratory, SciLifeLab.
    Mozdzynski, G.
    Nash, R. W.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Signell, A.
    Westerholm, J.
    Preparing scientific application software for exascale computing2013In: Applied Parallel and Scientific Computing: 11th International Conference, PARA 2012, Helsinki, Finland, June 10-13, 2012, Revised Selected Papers / [ed] Pekka Manninen, Per Öster, Springer, 2013, p. 27-42Conference paper (Refereed)
    Abstract [en]

    Many of the most widely used scientifc application software of today were developed largely during a time when the typical amount of compute cores was calculated in tens or hundreds. Within a not too distant future the number of cores will be calculated in at least hundreds of thousands or even millions. A European collaboration group CRESTA has recently been working on a set of renowned scientific software to investigate and develop these codes towards the realm of exascale computing. The codes are ELMFIRE, GROMACS, IFS, HemeLB, NEK5000, and OpenFOAM. This paper contains a summary of the strategies for their development towards exascale and results achieved during the first year of the collaboration project.

  • 2387.
    Örlu, Ramis
    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.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On near wall measurements of wall bounded flows-The necessity of an accurate determination of the wall position2010In: Progress in Aerospace Sciences, ISSN 0376-0421, E-ISSN 1873-1724, Vol. 46, no 8, p. 353-387Article, review/survey (Refereed)
    Abstract [en]

    The present review paper is an account on the experimental determination of the wall position relative to the probe in wall-bounded turbulent flow studies. A thorough review on common measurement techniques as well as correction methods reveals, that there are a number of pitfalls, that-when not accounted for-can lead to wrong conclusions about the wall position and thereby also on the near-wall behaviour of mean and turbulence quantities. Employing the state-of-the-art databases from direct numerical simulations of wall-bounded turbulent flows various indirect methods have been tested and assessed in terms of their robustness and accuracy. It is also demonstrated that accurate measurements reaching the viscous sublayer are necessary in order to ensure a correctly deduced wall position, and dependent quantities as for instance the near-wall scaling of mean (e.g. Reynolds number dependence of the buffer region or the log law constants) and turbulence (e.g. the near-wall peak location of Reynolds stresses) quantities. In experiments using hot-wires near the wall it is well known that heat conduction between the hot-wire and the wall gives errors and mean velocity data from the viscous sublayer can usually not be used to determine the wall position. In this paper we introduce a new method which takes advantage of the similarity of the probability density distributions (pdf) in the near wall region. By using the high velocity data of the pdf, which is shown not to be affected by heat conduction, the heat conduction problem can be circumvented. Extensive appendices are included, describing the history and present knowledge about the scaling of the mean velocity in the near wall and overlap regions in wall bounded turbulent flows.

  • 2388.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experimental studies in jet flows and zero pressure-gradient turbulent boundary layers2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis deals with the description and development of two classical turbulent shear flows, namely free jet and flat plate turbulent boundary layer flows. In both cases new experimental data has been obtained and in the latter case comparisons are also made with data obtained from data bases, both of experimental and numerical origin. The jet flow studies comprise three parts, made in three different experimental facilities, each dealing with a specific aspect of jet flows. The first part is devoted to the effect of swirl on the mixing characteristics of a passive scalar in the near-field region of a moderately swirling jet. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of combined X-wire and cold-wire anemometry. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air. The second jet flow part was set out to test the hypothesis put forward by Talamelli & Gavarini (Flow, Turbul. & Combust. 76), who proposed that the wake behind a separation wall between two streams of a coaxial jet creates the condition for an absolute instability. The experiments confirm the hypothesis and show that the instability, by means of the induced vortex shedding, provides a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers as well as its effect towards increased turbulence activity has been shown. The third part of the jet flow studies deals with the hypothesis that so called oblique transition may play a role in the breakdown to turbulence for an axisymmetric jet.For wall bounded flows oblique transition gives rise to steady streamwise streaks that break down to turbulence, as for instance documented by Elofsson & Alfredsson (J. Fluid Mech. 358). The scenario of oblique transition has so far not been considered for jet flows and the aim was to study the effect of two oblique modes on the transition scenario as well as on the flow dynamics. For certain frequencies the turbulence intensity was surprisingly found to be reduced, however it was not possible to detect the presence of streamwise streaks. This aspect must be furher investigated in the future in order to understand the connection between the turbulence reduction and the azimuthal forcing. The boundary layer part of the thesis is also threefold, and uses both new data as well as data from various data bases to investigate the effect of certain limitations of hot-wire measurements near the wall on the mean velocity but also on the fluctuating streamwise velocity component. In the first part a new set of experimental data from a zero pressure-gradient turbulent boundary layer, supplemented by direct and independent skin friction measurements, are presented. The Reynolds number range of the data is between 2300 and 18700 when based on the free stream velocity and the momentum loss thickness. Data both for the mean and fluctuating streamwise velocity component are presented. The data are validated against the composite profile by Chauhan et al. (Fluid Dyn. Res. 41) and are found to fulfil recently established equilibrium criteria. The problem of accurately locating the wall position of a hot-wire probe and the errors this can result in is thoroughly discussed in part 2 of the boundary layer study. It is shown that the expanded law of the wall to forth and fifth order with calibration constants determined from recent high Reynolds number DNS can be used to fix the wall position to an accuracy of 0.1 and 0.25 l_ * (l_* is the viscous length scale) when accurately determined measurements reaching y+=5 and 10, respectively, are available. In the absence of data below the above given limits, commonly employed analytical functions and their log law constants, have been found to affect the the determination of wall position to a high degree. It has been shown, that near-wall measurements below y+=10 or preferable 5 are essential in order to ensure a correctly measured or deduced absolute wall position. A  number of peculiarities in concurrent wall-bounded turbulent flow studies, was found to be associated with a erroneously deduced wall position. The effect of poor spatial resolution using hot-wire anemometry on the measurements of the streamwise velocity is dealt with in the last part. The viscous scaled hot-wire length, L+, has been found to exert a strong impact on the probability density distribution (pdf) of the streamwise velocity, and hence its higher order moments, over the entire buffer region and also the lower region of the log region. For varying Reynolds numbers spatial resolution effects act against the trend imposed by the Reynolds number. A systematic reduction of the mean velocity with increasing L+ over the entire classical buffer region and beyond has been found. A reduction of around 0.3 uƬ, where uƬ is the friction velocity, has been deduced for L+=60 compared to L+=15. Neglecting this effect can lead to a seemingly Reynolds number dependent  buffer or log region. This should be taken into consideration, for instance, in the debate, regarding the prevailing influence of viscosity above the buffer region at high Reynolds numbers. We also conclude that the debate concerning the universality of the pdf within the overlap region has been artificially complicated due to the ignorance of spatial resolution effects beyond the classical buffer region on the velocity fluctuations.

  • 2389.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experimental study of passive scalar mixing in swirling jet flows2006Licentiate thesis, monograph (Other scientific)
    Abstract [en]

    Despite its importance in various industrial applications there is still a lack of experimental studies on the dynamic and thermal field of swirling jets in the near-field region. The present study is an attempt to close this lack and provide new insights on the effect of rotation on the turbulent mixing of a passive scalar, on turbulence (joint) statistics as well as the turbulence structure.

    Swirl is known to increase the spreading of free turbulent jets and hence to entrain more ambient fluid. Contrary to previous experiments, which leave traces of the swirl generating method especially in the near-field, the swirl was imparted by discharging a slightly heated air flow from an axially rotating and thermally insulated pipe (6 m long, diameter 60 mm). This gives well-defined axisymmetric streamwise and azimuthal velocity distributions as well as a well-defined temperature profile at the jet outlet. The experiments were performed at a Reynolds number of 24000 and a swirl number (ratio between the angular velocity of the pipe wall and the bulk velocity in the pipe) of 0.5.

    By means of a specially designed combined X-wire and cold-wire probe it was possible to simultaneously acquire the instantaneous axial and azimuthal velocity components as well as the temperature and compensate the former against temperature variations. The comparison of the swirling and non-swirling cases clearly indicates a modification of the turbulence structure to that effect that the swirling jet spreads and mixes faster than its non-swirling counterpart. It is also shown that the streamwise velocity and temperature fluctuations are highly correlated and that the addition of swirl drastically increases the streamwise passive scalar flux in the near field.

  • 2390.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    The life of a vortex in an axisymmetric jet2011In: Journal of Visualization, ISSN 1343-8875, E-ISSN 1875-8975, Vol. 14, no 1, p. 5-6Article in journal (Refereed)
    Abstract [en]

    An axisymmetric jet is produced whenever a fluid is ejected from a round nozzle. The internal and external disturbances present in and surrounding the jet will trigger the formation of Kelvin–Helmoltz waves that are amplified and grow until they saturate and roll up in discrete vortices undergoing different processes such as (vortex) pairing and tearing until they collapse generating a complete turbulent flow. Concerning the evolution of the vortices within the axisymmetric shear layer, a typical scenario, describing the stages in a life of a vortex, is well represented by the flow visualization sequences shown in Fig. 1 as well as the movie in Online Resource 1.

  • 2391.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    An experimental study of the near-field mixing characteristics of a swirling jet2008In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 80, no 3, p. 323-350Article in journal (Refereed)
    Abstract [en]

    The present experimental investigation is devoted to the mixing charac- teristics of a passive scalar in the near-field region of a moderately swirling jet issuing from a fully developed axially rotating pipe flow. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of a combined X-wire and cold-wire probe. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air.

  • 2392.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. 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.
    BINORMAL COOLING ERRORS IN SINGLE HOT-WIRE MEASUREMENTS2016In: JOURNAL OF THEORETICAL AND APPLIED MECHANICS, ISSN 1429-2955, Vol. 54, no 1, p. 305-310Article in journal (Refereed)
    Abstract [en]

    In single-wire hot-wire measurements, velocity fluctuations acting normal to the hot-wire and its prongs will cause additional heat transfer known as binormal cooling. With respect to wall turbulence, the influence of this additional cooling is well-studied for crossed wires, while it is commonly ignored in single hot-wire measurements. The latter view is challenged in the recent work by Drozdz and Elsner (2014) that claims significant errors in variance measurements when using single-wire probes in turbulent boundary layers. This short communication revisits these claims and quantifies binormal cooling errors through an expansion of the effective-velocity concept and utilisation of direct numerical simulation data. Results support the common habit that binormal cooling errors can safely be ignored in single hot-wire measurements.

  • 2393.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. 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.
    Comment on the scaling of the near-wall streamwise variance peak in turbulent pipe flows2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 1, p. 1431-Article in journal (Refereed)
    Abstract [en]

    Apparent contradictory results regarding the Reynolds number (Re) scaling of the near-wall peak of the streamwise velocity variance distribution in turbulent pipe flows (Ng et al. Exp Fluids 51, 1261 and Vallikivi et al. Exp. Fluids 51, 1521) are discussed. Conclusions from the Princeton University/ONR Superpipe published between 2010 and 2012 are examined, doubts are raised, and recent numerical and experimental results are supplemented. The compiled results support the view that the inner-scaled peak amplitude increases with Re (at least to a Kármán number (R+) of 3,000) in contrast to the view put forward by several publications based on results from the Superpipe. Consequently, there is a need for further experimental results for R+ > 3,000 to ascertain the trend at higher Re.

  • 2394.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. 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.
    On spatial resolution issues related to time-averaged quantities using hot-wire anemometry2010In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 49, no 1, p. 101-110Article in journal (Refereed)
    Abstract [en]

    The effect of spatial resolution on streamwise velocity measurements with single hot-wires is targeted in the present study, where efforts have been made to distinguish between spatial resolution and Reynolds number effects. The basis for measurements to accurately determine the mean velocity and higher order moments is that the probability density distribution is measured correctly. It is well known that the turbulence intensity is increasingly attenuated with increasing wire length. Here, it is also shown (probably for the first time) that besides the probability density distribution and hence the higher order moments, even the mean velocity is affected, albeit to subtle extent, but with important consequences in studies of concurrent wall-bounded turbulence.

  • 2395.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. 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.
    Passive scalar flux measurements in the near-field region of a swirling jet2008In: Heat Transfer Research, ISSN 1064-2285, E-ISSN 2162-6561, Vol. 39, no 7, p. 597-607Article in journal (Refereed)
    Abstract [en]

    The present experimental investigation is devoted to the heat flux characteristics of a passive scalar in the near-field region of a swirling jet issuing from a fully developed axially rotating pipe flow. A combined X-wire and cold-wire probe made it possible to access the instantaneous streamwise and azimuthal velocity components as well as the temperature simultaneously. Results indicate that the addition of swirl increases the integral scales and entrainment and thereby the streamwise passive scalar flux and shortens the distance and hence time needed to mix the jet with the ambient air.

  • 2396.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Fiorini, T.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Bellani, G.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. Università di Bologna, Italy.
    Alfredsson, P Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. Università di Bologna, Italy.
    Reynolds stress scaling in pipe flow turbulence-first results from CICLoPE2017In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 375, no 2089, article id 20160187Article in journal (Refereed)
    Abstract [en]

    This paper reports the first turbulence measurements performed in the Long Pipe Facility at the Center for International Cooperation in Long Pipe Experiments (CICLoPE). In particular, the Reynolds stress components obtained from a number of straight and boundary-layer-type single-wire and X-wire probes up to a friction Reynolds number of 3.8 x 10(4) are reported. In agreement with turbulent boundary-layer experiments as well as with results from the Superpipe, the present measurements show a clear logarithmic region in the streamwise variance profile, with a Townsend-Perry constant of A(2) approximate to 1.26. The wall-normal variance profile exhibits a Reynolds-number-independent plateau, while the spanwise component was found to obey a logarithmic scaling over a much wider wall-normal distance than the other two components, with a slope that is nearly half of that of the Townsend-Perry constant, i.e. A(2,w) approximate to A(2)/2. The present results therefore provide strong support for the scaling of the Reynolds stress tensor based on the attached-eddy hypothesis. Intriguingly, the wall-normal and spanwise components exhibit higher amplitudes than in previous studies, and therefore call for follow-up studies in CICLoPE, as well as other large-scale facilities. This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.

  • 2397.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. 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.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On imperfect hot-wire resolution issues and their effect on mean quantities2009In: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, p. 605-608Conference paper (Refereed)
  • 2398.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. 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).
    Kalpakli Vester, Athanasia
    KTH, School of Engineering Sciences (SCI), Mechanics. 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).
    Flow visualization of an oblique impinging jet: vortices like it downhill, not uphill2015In: Journal of Visualization, ISSN 1343-8875, E-ISSN 1875-8975Article in journal (Refereed)
  • 2399.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Malizia, Fabio
    Cimarelli, Andrea
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Univ Bologna, Italy.
    The influence of temperature fluctuations on hot-wire measurements in wall-bounded turbulence2014In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, no 7, p. 1781-Article in journal (Refereed)
    Abstract [en]

    There are no measurement techniques for turbulent flows capable of reaching the versatility of hot-wire probes and their frequency response. Nevertheless, the issue of their spatial resolution is still a matter of debate when it comes to high Reynolds number near-wall turbulence. Another, so far unattended, issue is the effect of temperature fluctuations-as they are, e. g. encountered in non-isothermal flows-on the low and higher-order moments in wall-bounded turbulent flows obtained through hot-wire anemometry. The present investigation is dedicated to document, understand, and ultimately correct these effects. For this purpose, the response of a hot-wire is simulated through the use of velocity and temperature data from a turbulent channel flow generated by means of direct numerical simulations. Results show that ignoring the effect of temperature fluctuations, caused by temperature gradients along the wall-normal direction, introduces-despite a local mean temperature compensation of the velocity reading-significant errors. The results serve as a note of caution for hot-wire measurements in wall-bounded turbulence, and also where temperature gradients are more prevalent, such as heat transfer measurements or high Mach number flows. A simple correction scheme involving only mean temperature quantities (besides the streamwise velocity information) is finally proposed that leads to a substantial bias error reduction.

  • 2400.
    Örlü, Ramis
    et al.
    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.
    Comparison of experiments and simulations for zero pressure gradient turbulent boundary layers at moderate Reynolds numbers2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 6, p. 1547-Article in journal (Refereed)
    Abstract [en]

    A detailed comparison between recent direct numerical simulation (DNS) and experiments of a turbulent boundary layer under zero pressure gradient at Re-theta = 2,500 and 4,000 (based on the free-stream velocity and momentum-loss thickness) is presented. The well-resolved DNS is computed in a long spatial domain (Schlatter and Orlu in J Fluid Mech 659: 116, 2010a), including the disturbance strip, while the experiments consist of single hot-wire probe and oil-film interferometry measurements. Remarkably, good agreement is obtained for integral quantities such as skin friction and shape factor, as well as mean and fluctuating streamwise velocity profiles, higher-order moments and probability density distributions. The agreement also extends to spectral/structural quantities such as the amplitude modulation of the small scales by the large-scale motion and temporal spectral maps throughout the boundary layer. Differences within the inner layer observed for statistical and spectral quantities could entirely be removed by spatially averaging the DNS to match the viscous-scaled length of the hot-wire sensor, thereby explaining observed differences solely by insufficient spatial resolution of the hot-wire sensor. For the highest Reynolds number, Re-theta = 4,000, the experimental data exhibit a more pronounced secondary spectral peak in the outer region (y/delta(99) = 0.1) related to structures with length on the order of 5-7 boundary layer thicknesses, which is weaker and slightly moved towards lower temporal periods in the DNS. The cause is thought to be related to the limited spanwise box size which constrains the growth of the very large structures. In the light of the difficulty to obtain "canonical" flow conditions, both in DNS and the wind tunnel where effects such as boundary treatment, pressure gradient and turbulence tripping need to be considered, the present cross-validation of the data sets, at least for the present Re-theta-range, provides important reference data for future studies and highlights the importance of taking spatial resolution effects into account when comparing experiment and DNS. For the considered flow, the present data also provide quantitative guidelines on what level of accuracy can be expected for the agreement between DNS and experiments.

4546474849 2351 - 2400 of 2416
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