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  • 1751.
    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.

  • 1752.
    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.

  • 1753.
    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.

  • 1754.
    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.

  • 1755.
    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.

     

  • 1756.
    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)
  • 1757.
    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.

  • 1758.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Stone, Howard A.
    Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
    Harnessing elasticity to generate self-oscillation via an electrohydrodynamic instability2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 888, article id A31Article in journal (Refereed)
    Abstract [en]

    Under a steady DC electric field of sufficient strength, a weakly conducting dielectric sphere in a dielectric solvent with higher conductivity can undergo spontaneous spinning (Quincke rotation) through a pitchfork bifurcation. We design an object composed of a dielectric sphere and an elastic filament. By solving an elasto-electro-hydrodynamic (EEH) problem numerically, we uncover an EEH instability exhibiting diverse dynamic responses. Varying the bending stiffness of the filament, the composite object displays three behaviours: a stationary state, undulatory swimming and steady spinning, where the swimming results from a self-oscillatory instability through a Hopf bifurcation. By conducting a linear stability analysis incorporating an elastohydrodynamic model, we theoretically predict the growth rates and critical conditions, which agree well with the numerical counterparts. We also propose a reduced model system consisting of a minimal elastic structure which reproduces the EEH instability. The elasto-viscous response of the composite structure is able to transform the pitchfork bifurcation into a Hopf bifurcation, leading to self-oscillation. Our results imply a new way of harnessing elastic media to engineer self-oscillations, and more generally, to manipulate and diversify the bifurcations and the corresponding instabilities. These ideas will be useful in designing soft, environmentally adaptive machines.

  • 1759.
    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.

  • 1760.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Acoustic multi-ports with application to duct acoustics2010In: 17th International Congress on Sound and Vibration 2010, ICSV 2010: Volume 5, 2010, p. 3930-3942Conference paper (Refereed)
    Abstract [en]

    In this paper the development of multi-port models to describe linear acoustic problems in ducts with flow is presented. From an engineering point of view this field covers many important applications ranging from ventilation ducts in vehicles or buildings to intake/exhaust ducts on ICengines and power plants. Historically the use of multi-port models for ducts started in the 1920's, when the four-pole (2-port) filter models used by electrical engineers were applied to analyse transmission of low frequency 1D (plane) waves with application to automotive mufflers. An important step was then taken in 1971, when Cremer presented the idea that such "black box" models can be applied to describe aerodynamically generated sound in ducts. This implies that any fluid machine or unsteady flow process can be modelled as a "black box", with a "passive part" that describes how incident waves are scattered and an "active part" that describes the sound generation. The active part is normally assumed independent of the acoustic state, which makes the "black box" or acoustic multi-port model consistent with Lighthills acoustic analogy.

  • 1761.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Marcus Wallenberg Laboratory MWL.
    Jacob, Stefan
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Marcus Wallenberg Laboratory MWL.
    A comment on the correct boundary conditions for the Cremer impedance2021In: JASA EXPRESS LETTERS, ISSN 2691-1191, Vol. 1, no 2, article id 022801Article in journal (Refereed)
    Abstract [en]

    Mode merging and the creation of exceptional points can be used to create optimum damping in a lined duct, as pointed out by Cremer [Acustica 3, 249-263 (1953)]. The effect of a mean flow has traditionally been analyzed by assuming the Ingard-Myer boundary condition at the wall. For low frequencies, however, the classical boundary condition is a better alternative. This paper shows that this choice removes two problems with the low-frequency solution: the negative real part of the optimum wall impedance and the non-valid solution for the upstream case. Theoretical derivations are complemented by numerical results to support these conclusions.

  • 1762.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Karlsson, Mikael
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Can acoustic multi-port models be used to predict whistling2010In: 16th AIAA/CEAS Aeroacoustics Conference (31st AIAA Aeroacoustics Conference), 2010, p. 2010-4009-Conference paper (Refereed)
    Abstract [en]

    In duct aeroacoustic problems can be described using so called acoustic multi-port models. Such models represent a linear and time-invariant aeroacoustic model, which split the problem in a passive part, a scattering matrix in the frequency domain, describing the reflection and transmission and an active part describing the source strength. In accordance with Lighthill one normally assumes in this kind of model that the source part is uncoupled from the acoustic field. However, this assumption can be relaxed and it is fully possible to assume that the source strength can be affected by incident sound waves via a linear and time-invariant mechanism. The most general frequency domain model for this is a matrix which formally can be added to the scattering matrix describing the passive part. This leads to a model that has the same structure as the traditional multi-port model, but where the scattering matrix also contains information about fluid-acoustic interaction effects which is the origin for creating fluid driven feedback loops or whistles. The implication of these ideas is that multi-port models can be used to analyze amplification of sound and whistling.

  • 1763.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Karlsson, Mikael
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kierkegaard, Axel
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the use of linear aero-acoustic methods to predict whistling2009In: 16th International Congress on Sound and Vibration 2009, ICSV 2009, 2009, p. 2406-2413Conference paper (Refereed)
    Abstract [en]

    In duct aero-acoustic problems can be described using so called acoustic multi-port models. Such models represent a linear and time-invariant aero-acoustic model, which split the problem in a passive part, a scattering matrix in the frequency domain, describing the reflection and transmission and an active part describing the source strength. In accordance with Lighthill one normally assumes in this type of model that the source part is uncoupled from the acoustic field. However, this assumption can be relaxed and it is fully possible to assume that the source strength can be affected by incident sound waves via a linear and time-invariant mechanism. The most general frequency domain model for this is a matrix which formally can be added to the scattering matrix describing the passive part. This leads to a model that has the same structure as the traditional multi-port model, but where the scattering matrix also contains information about fluid-acoustic interaction effects which is the origin for creating fluid driven feedback loops or whistles.

  • 1764.
    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.

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  • 1765.
    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.

  • 1766.
    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.

  • 1767.
    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.

  • 1768.
    Åsén, Per-Olov
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kreiss, Gunilla
    Uppsala Univ, Dept Informat Technol..
    Rempfer, Dietmar
    IIT, Chicago.
    Direct numerical simulations of localized disturbances in pipe Poiseuille flow2010In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 39, no 6, p. 926-935Article in journal (Refereed)
    Abstract [en]

    We consider pipe Poiseuille flow subjected to a disturbance which is highly localized in space. Experiments by Peixinho and Mullin have shown this disturbance to be efficient in triggering turbulence, yielding a threshold dependence on the required amplitude as R-1.5 on the Reynolds number, R. The experiments also indicate an initial formation of hairpin vortices, with each hairpin having a length of approximately one pipe radius, independent of the Reynolds number in the range of R = 2000-3000. We perform direct numerical simulations for R = 5000. The results show a packet of hairpin vortices traveling downstream, each having a length of approximately one pipe radius. The perturbation remains highly localized in space while being advected downstream for approximately 10 pipe diameters. Beyond that distance from the disturbance origin, the flow becomes severely disordered.

  • 1769.
    Çinar, O.Y.
    et al.
    Linnæus University, International Centre of Mathematical Modelling, School of Computer Science, Physics and Mathematics, Sweden. Gebze Institute of Technology, Department of Mathematics, Turkey.
    Boij, Susann
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Çinar, G.
    Linnæus University, International Centre of Mathematical Modelling, School of Computer Science, Physics and Mathematics, Sweden. Gebze Institute of Technology, Department of Mathematics, Turkey.
    Nilsson, B.
    Linnæus University, International Centre of Mathematical Modelling, School of Computer Science, Physics and Mathematics, Sweden.
    Jet pipe reflections - Influence of geometrical and flow exit conditions2010In: 16th AIAA/CEAS Aeroacoustics Conference (31st AIAA Aeroacoustics Conference), 2010, p. 2010-4013-Conference paper (Refereed)
    Abstract [en]

    In this paper, we study the influence of the exit conditions on the acoustic reflections at a jet pipe opening. For the flow exit conditions, the model of Munt assuming a vortex sheet at the exit of a semi-infinite jet pipe is improved by considering more general coupling conditions. We focus on modelling the acoustic properties at the shear layer in general and the stability properties in particular. Experimental methods are used for studying primarily the geometric exit conditions. Both theoretical and experimental results are presented and discussed. Numerical results related to the theory show that the thickness of the shear layer causes an expected increase in the magnitude of the reflection coefficient compared to the original infinitely thin shear layer in the Munt model. The experimental results, on the other hand, show that there are additional effects due to the presence of thick walls at the exit of the straight pipes used in the experiments. These effects are observed to be more dominant than the effects due to the shape of the edges. Based on the discussions, further theoretical and experimental investigations are proposed.

  • 1770.
    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.

  • 1771.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Low Reynolds number Zero Pressure-Gradient Equilibrium Turbulent Boundary-Layer ExperimentsManuscript (preprint) (Other academic)
  • 1772.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the determination of the wall position in wall-bounded turbulent flowsManuscript (preprint) (Other academic)
  • 1773.
    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.

  • 1774.
    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.

  • 1775.
    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.

  • 1776.
    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.

  • 1777.
    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.

  • 1778.
    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.

  • 1779.
    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'.

  • 1780.
    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)
  • 1781.
    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)
  • 1782.
    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.

  • 1783.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Peinke, J.
    Talamelli, A.
    Oberlack, M.
    9th iTi Conference on Turbulence, iTi 2021,  Virtual, Online2021Conference paper (Refereed)
  • 1784.
    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.

  • 1785.
    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.
    Inflow length and tripping effects in turbulent boundary layers2011In: 13TH European Turbulence Conference (ETC13): Wall-Bounded Flows And Control Of Turbulence, Institute of Physics Publishing (IOPP), 2011, p. 022018-Conference paper (Refereed)
    Abstract [en]

    A recent assessment of available direct numerical simulation (DNS) data from turbulent boundary layer flows [Schlatter & Orlu, J. Fluid Mech. 659, 116 (2010)] showed surprisingly large differences not only in the skin friction coefficient or shape factor, but also in their predictions of mean and fluctuation profiles far into the sublayer. For the present paper the DNS of a zero pressure-gradient turbulent boundary layer flow by Schlatter et al. [Phys. Fluids 21, 051702 (2009)] serving as the baseline simulation, was re-simulated, however with physically different inflow conditions and tripping effects. The downstream evolution of integral and global quantities as well as mean and fluctuation profiles are presented and results indicate that different inflow conditions and tripping effects explain most of the differences observed when comparing available DNS. It is also found, that if transition is initiated at a low enough Reynolds number (based on the momentum-loss thickness) Re-theta < 300, all data agree well for both inner and outer layer for Re-theta > 2000; a result that gives a lower limit for meaningful comparisons between numerical and/or wind tunnel experiments.

  • 1786.
    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.
    On the fluctuating wall-shear stress in zero pressure-gradient turbulent boundary layer flows2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 2, p. 021704-Article in journal (Refereed)
    Abstract [en]

    Recent direct numerical simulation (DNS) results relating to the behavior of the fluctuating wall-shear stress tau(+)(w,rms) in turbulent boundary layer flows are discussed. This new compilation is motivated by a recent article [Wu and Moin, "Transitional and turbulent boundary layer with heat transfer," Phys. Fluids 22, 085105 (2010)], which indicates a need for clarification of the value of tau(+)(w,rms). It is, however, shown here, based on other recent DNS data, that most results, both in boundary layer and channel geometry, yield tau(+)(w,rms)approximate to 0.4 plus a small increase with Reynolds number coming from the growing influence of the outer spectral peak. The observed discrepancy in experimental data is mainly attributed to spatial resolution effects, as originally described by Alfredsson et al. [" The fluctuating wall-shear stress and the velocity field in the viscous sublayer, "Phys. Fluids 31, 1026 (1988)].

  • 1787.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Royal Inst Technol, Linn FLOWCtr, KTHMech, SE-10044 Stockholm, Sweden..
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Royal Inst Technol, Linn FLOWCtr, KTHMech, SE-10044 Stockholm, Sweden..
    Turbulent Boundary-Layer Flow: Comparing Experiments with DNS2012In: Progress in Turbulence and Wind Energy IV / [ed] Oberlack, M Peinke, J Talamelli, A Castillo, L Holling, M, Springer Nature , 2012, p. 213-216Conference paper (Refereed)
    Abstract [en]

    Since the first direct numerical simulation (DNS) of turbulent channel flows became available more than two decades ago a large number of DNS have been performed for various flow cases, and steadily increased the Reynolds number (Re) extent, finally reaching those ranges put forward by well-resolved experiments. Hence, it was not surprising that a detailed comparison between DNS and experiments in turbulent channel flow [1] showed good agreement for the first and second order moment as well as spectral distributions of the streamwise velocity component.

  • 1788.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    Alfredsson, Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the passive control of the near-field of coaxial jets by means of vortex shedding2008In: Int. Conf. on Jets, Wakes and Separated Flows, ICJWSF-2, September 16-19, 2008, Technical University of Berlin, Germany, 2008, p. 1-16Conference paper (Refereed)
  • 1789.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    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.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Passive control of mixing in a coaxial jet2008In: Proc. 7th Int. ERCOFTAC Symp. on Engineering Turbulence Modelling and Measurements (ETMM7), 2008, p. 450-455Conference paper (Refereed)
    Abstract [en]

    An experimental investigation regarding interacting shear layers in a coaxial jet geometry has been performed. The present paper confirms experimentally the theoretical result by Talamelli and Gavarini (2006), who proposed that the wake behind the separation wall between the two stream of a coaxial jet creates the condition for an absolute instability. This instability, by means of the induced vortex shedding, may provide 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 has been demonstrated.

  • 1790.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. 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.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulence Enhancement in Coaxial Jet Flows by Means of Vortex Shedding2009In: PROGRESS IN TURBULENCE III / [ed] Peinke, J.; Oberlack, M.; Talamelli, A., 2009, Vol. 131, p. 235-238Conference paper (Refereed)
    Abstract [en]

    Over the past decades a variety of passive and active flow control mechanisms have been tested and applied in a variety of canonical as well as applied flow cases. An example for the latter is the coaxial jet flow, which has mainly been investigated regarding the receptivity to active flow control strategies (see e.g. [1]), probably due to the multitude of parameters characterising the complex flow field [2]. Physical and numerical experiments (see e.g. [3] and [5]) have established that the vortical motion in coaxial jet flows is dominated by the vortices emerging from the outer shear layer. The frequency of these vortices is related to the Kelvin- Helmholtz instability as predicted by linear stability analysis for single jets. The vortices in the inner shear layer, on the other hand, are trapped in the spaces left free between two consecutive outer shear layer vortices, and are therefore sharing the frequencies of the most amplified modes of the outer shear layer and do not relate to the values one would expect from linear stability analysis. This fact has become known as the “locking phenomenon”, which describes the mutual interaction of both shear layers. Nevertheless it is believed that only the outer shear layer is able to significantly control the evolution of the inner shear layer [7], which may explain the focus of control strategies on the outer shear layer.

  • 1791.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Klewicki, J.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Generalization of the diagnostic plot to higher-order moments in turbulent boundary layers2016In: Springer Proceedings in Physics, Springer, 2016, p. 333-338Conference paper (Refereed)
    Abstract [en]

    The present work extends the diagnostic plot concept for the streamwise turbulence intensity in wall-bounded turbulent flows [Alfredsson and Örlü, Eur. J. Mech. B/Fluids 42, 403 (2010)], and generalizes it for higher-order (even and odd) moments, thereby providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layer data up to a friction Reynolds number of 20000 are employed and demonstrate the feasibility of the diagnostic plot to scale data throughout the logarithmic and wake regions.

  • 1792.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Klewicki, J.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Generalized diagnostic scaling for high-order moments in turbulent boundary layers2015In: Proceedings - 15th European Turbulence Conference, ETC 2015, TU Delft , 2015Conference paper (Refereed)
    Abstract [en]

    The present work builds upon the diagnostic plot for the streamwise turbulence intensity [Alfredsson & Örlü, 2010] and generalises it for higher-order (even and odd) moments providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layers (up to a friction Reynolds number of 20’000) are employed and demonstrate its feasibility to scale data throughout the overlap and outer region. 

  • 1793.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Klewicki, J.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    High-order generalisation of the diagnostic scaling for turbulent boundary layers2016In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 17, no 7, p. 664-677Article in journal (Refereed)
    Abstract [en]

    The diagnostic scaling concept, introduced for the streamwise turbulence intensity in wall-bounded turbulent flows (Alfredsson, Segalini and Örlü, Phys. Fluids 2011;23:041702), is here extended and generalised not only for the higher even-order central statistical moments, but also for the odd moments and thereby the probability density distribution of the streamwise velocity fluctuations. Turbulent boundary layer data up to a friction Reynolds number of 60,000 are employed and demonstrate the feasibility of the diagnostic scaling for the data throughout the logarithmic and wake regions. A comparison with the generalised logarithmic law for even-order moments by Meneveau and Marusic (J. Fluid Mech. 2013;719:R1) based on the attached-eddy hypothesis, is reported. The diagnostic plot provides an apparent Reynolds-number-independent scaling of the data, and is exploited to reveal the functional dependencies of the constants needed in the attached-eddy-based model. In particular, the invariance of the lowest order diagnostic scaling poses an intriguing incompatibility with the asymptotic constancy of the Townsend–Perry constant.

  • 1794.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Segalini, Antonio
    Talamelli, Alessandro
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Effect of oblique waves on jet turbulence2010In: SEVENTH IUTAM SYMPOSIUM ON LAMINAR-TURBULENT TRANSITION / [ed] Schlatter P; Henningson DS, 2010, Vol. 18, p. 541-544Conference paper (Refereed)
    Abstract [en]

    The paper describes experiments on acoustically excited axisymmetric turbulent jet flows The investigation is based on 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 for 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. Even though it was not possible to detect the presence of streamwise streaks, for a certain range of the excitation frequencies. the turbulence intensity, at a fixed stream wise position, is found to be significantly reduced.

  • 1795.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Talamelli, A.
    Department of Industrial Engineering, Università di Bologna, Forlì, Italy.
    Oberlack, M.
    Department of Mechanical Engineering, TU Darmstadt, Darmstadt, Germany.
    Peinke, J.
    ForWind and Institute of Physics, AG TWiSt—Turbulence, Wind Energy and Stochastics, Carl-von-Ossietzky University Oldenburg, Oldenburg, Germany.
    Preface2019In: Computer Algebra and Differential Equations / [ed] Evelyne Tournier, Cambridge University Press (CUP) , 2019, Vol. 226, p. v-viChapter in book (Refereed)
    Abstract [en]

    When the Organizing Commitee of CADE began to choose the program of CADE-92, it was decided that D-modules would be a central topic at this conference.

    The theory of D-modules is quite recent. It began in the late sixties and at first was considered to be quite abstract and difficult. Over the years the situation improved with the development of the theory and its applications. The organizers felt that it was time to try to introduce it to a larger audience interested in differential equations and computer algebra, since the theory of D-modules offers an excellent way to effectively handle linear systems of analytic PDEs.

    Once this decision was made it was natural to ask Bernard Malgrange to be the “invité d'honneur” at CADE-92, with the task of lecturing about D-modules in a way adapted to an audience interested in effectivity. This was natural because Bernard Malgrange is not only one of the most famous mathematicians in this field, but also because he is perhaps the true originator of this direction. It is generally admitted that D-module theory began in the early seventies with the fundamental work of I. N. Berstein and of the Japanese school around M. Sato, but in fact Bernard Malgrange introduced the basic concepts ten years ago for the constant coefficients case (see his 1962 Bourbaki report “systèmes différentiels à coefficients constants”), and later for the general case (see his lectures at Orsay Cohomologie de Spencer (d'après Quillen)).

  • 1796.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, A.
    Oberlack, M.
    Peinke, J.
    Preface2017In: Springer Proceedings in Physics, Springer Science+Business Media B.V., 2017, Vol. 196, p. v-viConference paper (Refereed)
  • 1797.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Tillmark, Nils
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Measured critical size of roughness elements2021Report (Other academic)
    Abstract [en]

    This report describes wind-tunnel measurements within the RECEPT project to

    determine transition behind a single roughness element placed near the leading

    edge of a 35 degree swept airfoil at negative (-5 degrees) angle of attack with the

    rotation axis parallel to the leading edge. A negative angle of attack decreases the

    sensitivity to Tollmien-Schlichting wave-type instabilities and instead makes the

    boundary layer flow mainly sensitive to cross-flow instability disturbances.

    The airfoil was heated by blowing air through its hollow structure and transition

    was detected through infrared (IR) temperature measurements of the airfoil

    surface. The physical background behind the detection method is that if transition

    occurs, the flow at the wall inside the turbulent wedge behind the roughness element

    will have a larger friction than the laminar flow outside and hence a larger

    heat transfer, making the wall temperature lower in this region. To confirm the

    interpretation of the IR temperature images as well as provide measured boundary

    layer parameters additional hot-wire measurements within the boundary layer

    were performed at few of the parameters covered by the IR measurements. These

    parameters are both related to the roughness elements themselves such as their

    diameters and heights as well as free-stream velocities and turbulence levels of

    the approaching flow.

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  • 1798.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vila, C. S.
    DIscetti, S.
    Ianiro, A.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Towards canonical adverse-pressure-gradient turbulent boundary layers2017In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, International Symposium on Turbulence and Shear Flow Phenomena, TSFP10 , 2017, Vol. 3Conference paper (Refereed)
    Abstract [en]

    The present investigation focuses on the concerted investigation of pressure gradient and streamwise curvature effects on turbulent boundary layers. In particular, a number of direct and large-eddy simulations covering a wide range of pressure gradient parameters and streamwise histories on flat and curved surfaces is performed and will be compared with wind-tunnel experiments utilising hot-wire anemometry and particle image velocimetry that overlap and extend the Reynolds number range. Results are aimed at isolating the effects of pressure gradients, streamwise curvature and streamwise (pressure gradient) histories as well as Reynolds number, which have traditionally inhibited to draw firm conclusions from the available data.

  • 1799.
    Brodin Berggren, Lenita (Contributor)
    KTH, Library.
    Ohlström, Tove (Contributor)
    KTH, Library.
    Bromark, Mikko (Contributor)
    KTH, Library.
    Duwig, Christophe (Creator)
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Process Technology.
    Waesterberg Tomasson, Lotta (Illustrator)
    Öhlén, Emil (Contributor)
    KTH, Library.
    Norsröm Hallberg, Viveka (Contributor)
    KTH, Library.
    Forsberg, Kerstin (Contributor)
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Resource recovery.
    Cornell, Ann M. (Contributor)
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Applied Electrochemistry. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Högselius, Per (Contributor)
    KTH, School of Architecture and the Built Environment (ABE), Philosophy and History, History of Science, Technology and Environment.
    Katzeff, Cecilia (Contributor)
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Strategic Sustainability Studies.
    Dünkelberg Valenca, Mattias (Contributor)
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Exhibition: Towards the energy of the future – the invisible revolution behind the electrical socket2023Artistic output (Unrefereed)
    Abstract [en]

    Energy Crisis! Electricity Price drama! The threat of global energy poverty! Media are generous with spectacular titles. Yes, energy is important, and yes, nearly all societal challenges are connected to how we convert, distribute and use energy. Therefore, the KTH Energy Platform and KTH Library presented an exhibition with the theme Towards the energy of the future – the invisible revolution behind the electrical socket.

    The exhibition displayed showcase illustrations from the book made by Lotta Waesterberg Tomasson, as well as books related to energy and electricity from the KTH Library's collections. In parallell with the exhibition, a series of live popular science lunch seminars with presentations of selected chapters of the book took place. As part of the exhibition, students from KTH's Electrical Engineering program also showcased exciting projects that connect to the anthology’s contents, made with materials and equipment from the student-driven ELAB and “Studentverkstan”. Visitors were also invited to share their reflections and ideas on energy. 

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