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• 201.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Interaction between hairy surfaces and turbulence for different surface time scales2018Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 861, s. 556-584Artikel i tidskrift (Refereegranskat)

Surfaces with filamentous structures are ubiquitous in nature on many different scales, ranging from forests to micrometre-sized cilia in organs. Hairy surfaces are elastic and porous, and it is not fully understood how they modify turbulence near a wall. The interaction between hairy surfaces and turbulent flows is here investigated numerically in a turbulent channel flow configuration at friction Reynolds number Re-tau approximate to 180. We show that a filamentous bed of a given geometry can modify a turbulent flow very differently depending on the resonance frequency of the surface, which is determined by the elasticity and mass of the filaments. Filaments having resonance frequencies lower than the main frequency content of the turbulent wall-shear stress conform to slowly travelling elongated streaky structures, since they are too slow to adapt to fluid forces of higher frequencies. On the other hand, a bed consisting of stiff and low-mass filaments has a high resonance frequency and shows local regions of increased permeability, which results in large entrainment and a vast increase in drag.

• 202.
KTH, Tidigare Institutioner                               , Mekanik.
Absolute and convective instability of a relaxational plane liquid jet2003Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 493, s. 89-119Artikel i tidskrift (Refereegranskat)

The stability of a plane relaxational liquid jet has been studied theoretically and experimentally through linear stability analysis and flow visualizations. The relaxational liquid jet is obtained by the outflow of a liquid from a plane channel with an upstream fully developed Poiseuille flow into an ambient stagnant gas. Linear spatial stability calculations show that there are five convectively unstable modes, three sinuous and two dilatational. The spatial stability calculations are compared to experimental results for wavenumber variation and the growth of waves found in the visualizations. These variations have been quantified with a wavelet transform and through a comparison with the spatial stability results the type of mode observed in the visualizations has been determined. For this type of mode the calculated wavenumber variation is in good agreement with the experimental results. Also, in the experiments the breakup of the jet has been observed when the Reynolds number reaches a certain value, and as the Reynolds number increases this breakup moves closer to the channel exit. This upstream movement of the breakup can be explained by the linear stability results. Finally the relaxational liquid jet is shown to be absolutely unstable for a certain parameter region. Close to the nozzle both a sinuous mode and a dilatational mode are shown to be absolutely unstable. As the jet profile relaxes to uniform, the sinuous mode is shown to be the only unstable mode. This occurs for Weber numbers We < 1, which is in agreement with the theory for liquid jets with uniform velocity profile. The frequency selection for the observed waves is believed to be related to the region of absolute instability located closest to the channel exit.

• 203.
Osaka Univ, Grad Sch Engn Sci, 1-3 Machikaneyama, Toyonaka, Osaka 5608531, Japan..
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Kobe Univ, Grad Sch Engn, Nada Ku, 1-1 Rokkodai, Kobe, Hyogo 6578501, Japan.. Osaka Univ, Grad Sch Engn Sci, 1-3 Machikaneyama, Toyonaka, Osaka 5608531, Japan.. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
Haemorheology in dilute, semi-dilute and dense suspensions of red blood cells2019Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 872, s. 818-848Artikel i tidskrift (Refereegranskat)

We present a numerical analysis of the rheology of a suspension of red blood cells (RBCs) in a wall-bounded shear flow. The flow is assumed as almost inertialess. The suspension of RBCs, modelled as biconcave capsules whose membrane follows the Skalak constitutive law, is simulated for a wide range of viscosity ratios between the cytoplasm and plasma, D 0 : 1-10, for volume fractions up to D 0 : 41 and for different capillary numbers (Ca). Our numerical results show that an RBC at low Ca tends to orient to the shear plane and exhibits so-called rolling motion, a stable mode with higher intrinsic viscosity than the so-called tumbling motion. As Ca increases, the mode shifts from the rolling to the swinging motion. Hydrodynamic interactions (higher volume fraction) also allow RBCs to exhibit tumbling or swinging motions resulting in a drop of the intrinsic viscosity for dilute and semi-dilute suspensions. Because of this mode change, conventional ways of modelling the relative viscosity as a polynomial function of cannot be simply applied in suspensions of RBCs at low volume fractions. The relative viscosity for high volume fractions, however, can be well described as a function of an effective volume fraction, defined by the volume of spheres of radius equal to the semi-middle axis of a deformed RBC. We find that the relative viscosity successfully collapses on a single nonlinear curve independently of except for the case with Ca > 0 : 4, where the fit works only in the case of low/ moderate volume fraction, and fails in the case of a fully dense suspension.

• 204.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Oscillatory sensitivity patterns for global modes in wakes2012Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 701, s. 251-277Artikel i tidskrift (Refereegranskat)

Globally unstable wakes with co-flow at intermediate Reynolds numbers are studied, to quantify important spatial regions for the development and control of the global instability. One region of high structural sensitivity is found close to the inlet for all wakes, in agreement with previous findings for cylinder wakes. A second, elongated region of high structural sensitivity is seen downstream of the first one for unconfined wakes at Re = 400. When base-flow modifications are considered, a spatially oscillating sensitivity pattern is found inside the downstream high-structural-sensitivity region. This implies that the same change in the base flow can either destabilize or stabilize the flow, depending on the exact position where it is applied. It is shown that the sensitivity pattern remains unchanged for different choices of streamwise boundary conditions and numerical resolution. The actual base-flow is modified in selected configurations, and the linear global modes recomputed. It is confirmed that the linear global eigenvalues move according to the predicted sensitivity pattern for small-amplitude base-flow modifications, for which the theory applies. We also look at the implications of a small control cylinder for the flow. Only the upstream high-sensitivity region proves to be robust in terms of control, but one should be careful not to disturb the flow in the downstream high-sensitivity region, in order to achieve control. The findings can have direct implications for the numerical resolution requirements for wakes at higher Reynolds numbers. Furthermore, they provide one more possible explanation for why confined wakes have a more narrow frequency spectrum than unconfined wakes.

• 205. Tammisola, Outi
Department of Engineering, Cambridge University, Cambridge.
Second-order perturbation of global modes and implications for spanwise wavy actuation2014Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, s. 314-335Artikel i tidskrift (Refereegranskat)

Sensitivity analysis has successfully located the most efficient regions in which to apply passive control in many globally unstable flows. As is shown here and in previous studies, the standard sensitivity analysis, which is linear (first order) with respect to the actuation amplitude, predicts that steady spanwise wavy alternating actuation/modification has no effect on the stability of planar flows, because the eigenvalue change integrates to zero in the spanwise direction. In experiments, however, spanwise wavy modification has been shown to stabilize the flow behind a cylinder quite efficiently. In this paper, we generalize sensitivity analysis by examining the eigenvalue drift (including stabilization/destabilization) up to second order in the perturbation, and show how the second-order eigenvalue changes can be computed numerically by overlapping the adjoint eigenfunction with the first-order global eigenmode correction, shown here for the first time. We confirm the prediction against a direct computation, showing that the eigenvalue drift due to a spanwise wavy base flow modification is of second order. Further analysis reveals that the second-order change in the eigenvalue arises through a resonance of the original (2-D) eigenmode with other unperturbed eigenmodes that have the same spanwise wavelength as the base flow modification. The eigenvalue drift due to each mode interaction is inversely proportional to the distance between the eigenvalues of the modes (which is similar to resonance), but also depends on mutual overlap of direct and adjoint eigenfunctions (which is similar to pseudoresonance). By this argument, and by calculating the most sensitive regions identified by our analysis, we explain why an in-phase actuation/modification is better than an out-of-phase actuation for control of wake flows by spanwise wavy suction and blowing. We also explain why wavelengths several times longer than the wake thickness are more efficient than short wavelengths.

• 206.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. Univ Nottingham, England.
Coherent structures in a swirl injector at Re=4800 by nonlinear simulations and linear global modes2016Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 792, s. 620-657Artikel i tidskrift (Refereegranskat)

The large-scale coherent motions in a realistic swirl fuel-injector geometry are analysed by direct numerical simulations (DNS), proper orthogonal decomposition (POD), and linear global modes. The aim is to identify the origin of instability in this turbulent flow in a complex internal geometry. The flow field in the nonlinear simulation is highly turbulent, but with a distinguishable coherent structure: the precessing vortex core (a spiralling mode). The most energetic POD mode pair is identified as the precessing vortex core. By analysing the fast Fourier transform (FFT) of the time coefficients of the POD modes, we conclude that the first four POD modes contain the coherent fluctuations. The remaining POD modes (incoherent fluctuations) are used to form a turbulent viscosity field, using the Newtonian eddy model. The turbulence sets in from convective shear layer instabilities even before the nonlinear flow reaches the other end of the domain, indicating that equilibrium solutions of the Navier-Stokes are never observed. Linear global modes are computed around the mean flow from DNS, applying the turbulent viscosity extracted from POD modes. A slightly stable discrete m = 1 eigenmode is found, well separated from the continuous spectrum, in very good agreement with the POD mode shape and frequency. The structural sensitivity of the precessing vortex core is located upstream of the central recirculation zone, identifying it as a spiral vortex breakdown instability in the nozzle. Furthermore, the structural sensitivity indicates that the dominant instability mechanism is the Kelvin-Helmholtz instability at the inflection point forming near vortex breakdown. Adjoint modes are strong in the shear layer along the whole extent of the nozzle, showing that the optimal initial condition for the global mode is localized in the shear layer. We analyse the qualitative influence of turbulent dissipation in the stability problem (eddy viscosity) on the eigenmodes by comparing them to eigenmodes computed without eddy viscosity. The results show that the eddy viscosity improves the complex frequency and shape of global modes around the fuel-injector mean flow, while a qualitative wavemaker position can be obtained with or without turbulent dissipation, in agreement with previous studies. This study shows how sensitivity analysis can identify which parts of the flow in a complex geometry need to be altered in order to change its hydrodynamic stability characteristics.

• 207.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
Global linear and nonlinear stability of viscous confined plane wakes with co-flow2011Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 675, s. 397-434Artikel i tidskrift (Refereegranskat)

The global stability of confined wakes is studied numerically, using two-dimensionallinear global modes and nonlinear direct numerical simulations (DNS).The wake inflow velocity is varied between different amounts of co-flow (basebleed), while the density and viscosity are assumed to be constant everywherein the flow domain. In accordance with previous studies, we find that thefrequencies of both the most unstable linear and the saturated nonlinear globalmode increase with confinement. Here, we also find that for wake Reynoldsnumber Re = 100, the confinement is stabilising. It decreases both the growthrate of the linear and the saturation amplitude of the nonlinear modes. Weconclude that the dampening effect is connected to the streamwise developmentof the base flow, and for higher Reynolds numbers this effect decreases, sincethe flow becomes more parallel. The linear analysis reveals that the criticalwake velocities below which the flow becomes unstable are almost identicalfor unconfined and confined wakes at Re ≈ 400. Also, the present resultsare compared with literature data for an inviscid parallel wake due to thesimilarity of inflow profile. The confined wake is found to be more stable thanits inviscid counterpart, while the unconfined wake is more unstable than theinviscid wake. The main reason to both can be explained by the base flowdevelopment. A detailed comparison of the linear and nonlinear results revealsthat the most unstable linear global mode gives an excellent prediction of theinitial nonlinear behaviour and therefore the stability boundary, in all cases.However, the nonlinear saturated state is quite different in particular for higherReynolds numbers. For Re = 100, the saturated frequency also differs less than5% from the linear frequency, and trends regarding confinement observed in thelinear analysis are confirmed.141

• 208.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
Surface tension-induced global instability of planar jets and wakes2012Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 713, s. 632-658Artikel i tidskrift (Refereegranskat)

The effect of surface tension on global stability of co-flow jets and wakes at a moderate Reynolds number is studied. The linear temporal two-dimensional global modes are computed without approximations. All but one of the flow cases under study are globally stable without surface tension. It is found that surface tension can cause the flow to be globally unstable if the inlet shear (or, equivalently, the inlet velocity ratio) is strong enough. For even stronger surface tension, the flow is restabilized. As long as there is no change of the most unstable mode, increasing surface tension decreases the oscillation frequency. Short waves appear in the high-shear region close to the nozzle, and their wavelength increases with increasing surface tension. The critical shear (the weakest inlet shear at which a global instability is found) gives rise to antisymmetric disturbances for the wakes and symmetric disturbances for the jets. However, at stronger shear, the opposite symmetry can be the most unstable one, in particular for wakes at high surface tension. The results show strong effects of surface tension that should be possible to reproduce experimentally as well as numerically.

• 209.
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
Stabilizing effect of surrounding gas flow on a plane liquid sheet2011Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 672, s. 5-32Artikel i tidskrift (Refereegranskat)

The stability of a plane liquid sheet is studied experimentally and theoretically, with an emphasis on the effect of the surrounding gas. Co-blowing with a gas velocity of the same order of magnitude as the liquid velocity is studied, in order to quantify its effect on the stability of the sheet. Experimental results are obtained for a water sheet in air at Reynolds number Re-t = 3000 and Weber number We = 300, based on the half-thickness of the sheet at the inlet, water mean velocity at the inlet, the surface tension between water and air and water density and viscosity. The sheet is excited with different frequencies at the inlet and the growth of the waves in the streamwise direction is measured. The growth rate curves of the disturbances for all air flow velocities under study are found to be within 20% of the values obtained from a local spatial stability analysis, where water and air viscosities are taken into account, while previous results from literature assuming inviscid air overpredict the most unstable wavelength with a factor 3 and the growth rate with a factor 2. The effect of the air flow on the stability of the sheet is scrutinized numerically and it is concluded that the predicted disturbance growth scales with (i) the absolute velocity difference between water and air (inviscid effect) and (ii) the square root of the shear from air on the water surface (viscous effect).

• 210.
KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
Effect of adverse pressure gradients on turbulent wing boundary layers2020Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 883, nr A8, s. 1-28Artikel i tidskrift (Refereegranskat)

The characteristics of turbulent boundary layers (TBLs) subjected to adverse pressure gradients are analysed through well-resolved large-eddy simulations. The geometries under study are the NACA0012 and NACA4412 wing sections, at 0 and 5 degrees angle of attack, respectively, both of them at a Reynolds number based on inflow velocity and chord length Rec = 400,000. The turbulence statistics show that adverse pressure gradients (APGs) have a significant effect on the mean velocity, velocity fluctuations and turbulent kinetic energy budget, and this effect is more prominent on the outer region of the boundary layer. Furthermore, the effect of flow history is assessed by means of an integrated Clauser pressure-gradient parameter, β, through the study of cases with matching local values of β and the friction Reynolds number, Reτ, to isolate this effect. Our results show a noticeable effect of the flow history on the outer region, however the differences in the near-wall peak of the tangential velocity fluctuations appear to be mostly produced by the local APG magnitude. The one-dimensional power-spectral density shows energetic small scales in the outer region of APG TBLs, whereas these energetic scales do not appear in zero-pressure-gradient (ZPG) TBLs, suggesting that small scales near the wall are advected towards the outer layer by the APG. Moreover, the linear coherence spectra show that the spectral outer peak of high-Reynolds-number ZPG TBLs is highly correlated with the near-wall region , unlike APG TBLs which do not show such a correlation. This result, together with the different two-dimensional spectra of APG and high-Reynolds-number ZPG TBLs, suggests different energisation mechanisms due to APG and increase in Reynolds number. To the authors' knowledge, this is the first in-depth analysis of the TBL characteristics over wings, including detailed single-point statistics, spectra and coherence.

• 211.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Spatial optimal growth in three-dimensional compressible boundary layers2012Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 704, s. 251-279Artikel i tidskrift (Refereegranskat)

This paper represents a continuation of the work by Tempelmann et al. (JFM 2010) on spatial optimal growth in incompressible boundary layers over swept flat plates . We present an extension of the methodology to compressible flow. Also we account for curvature effects. Spatial optimal growth is studied for boundary layers over both flat and curved swept plates with adiabatic and cooled walls. We find that optimal growth increases for higher Mach numbers. In general, extensive non-modal growth is observed for all boundary layer cases even in subcritical regions, i.e. where the flow is stable with respect to modal crossflow disturbances. Wall cooling, though stabilising crossflow modes, destabilises disturbances of non-modal nature. Curvature acts similarly on modal as well as non-modal disturbances. Convex walls have a stabilising effect on the boundary layer whereas concave walls act destabilising. The physical mechanisms of optimal growth in all studied boundary layers are found to be similar to those identified for incompressible flat-plate boundary layers.

• 212.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Swept-wing boundary-layer receptivity2012Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 700, s. 490-501Artikel i tidskrift (Refereegranskat)

Adjoint solutions of the linearized incompressible Navier-Stokes equations are presented for a cross-flow-dominated swept-wing boundary layer. For the first time these have been computed in the region upstream of the swept leading edge and may therefore be used to predict receptivity to any disturbances of the incoming free stream as well as to surface roughness. In this paper we present worst-case scenarios, i.e. those external disturbances yielding maximum receptivity amplitudes of a steady cross-flow disturbance. In the free stream, such an 'optimal' disturbance takes the form of a streak which, while being convected downstream, penetrates the boundary layer and smoothly turns into a growing cross-flow mode. The 'worst-case' surface roughness has a wavy shape and is distributed in the chordwise direction. It is shown that, under such optimal conditions, the boundary layer is more receptive to surface roughness than to incoming free stream disturbances.

• 213.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Spatial optimal growth in three-dimensional boundary layers2010Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 646, s. 5-37Artikel i tidskrift (Refereegranskat)

A parabolized set of linear equations is derived, which, in combination with the proposed solution procedure, allows for the study of both non-modal and modal disturbance growth in three-dimensional boundary layers. The method is applicable to disturbance waves whose lines of constant phase are closely aligned with the external streamline. Moreover, strongly growing disturbances may fall outside the scope of application. These equations are used in conjunction with a variational approach to compute optimal disturbances in Falkner Skan Cooke boundary layers subject to adverse and favourable pressure gradients. The disturbances associated with maximum energy growth initially take the form of streamwise vortices which are tilted against the mean crossflow shear. While travelling downstream these vortical structures rise into an upright position and evolve into bent streaks. The physical mechanism responsible for non-modal growth in three-dimensional boundary layers is therefore identified as a combination of the lift-up effect and the Orr mechanism. Optimal disturbances smoothly evolve into crossflow modes when entering the supercritical domain of the flow. Non-modal growth is thus found to initiate modal instabilities in three-dimensional boundary layers. Optimal growth is first studied for stationary disturbances. Influences of parameters such as sweep angle, spanwise wavenumber and position of inception are studied, and the initial optimal amplification of stationary crossflow modes because of non-modal growth is investigated. Finally, general disturbances are considered, and envelopes yielding the maximum growth at each position are computed. In general, substantial growth is already found upstream of the first neutral point. The computations show that at supercritical conditions, maximum growth of optimal disturbances in accelerated boundary layers can exceed the growth predicted for modal instabilities by several orders of magnitude.

• 214.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Swept-wing boundary-layer receptivity to localised surface roughness2011Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Artikel i tidskrift (Övrigt vetenskapligt)

The receptivity to localised surface roughness of a swept-wing boundary layer is studied by direct numerical simulation (DNS) and computations using the parabolised stability equations (PSE). The DNS is laid out to reproduce wind tunnel experiments performed by Saric & coworkers, where micron-sized cylinders were used to trigger steady crossflow modes. The amplitudes of the roughness-induced fundamental crossflow wave and its superharmonics obtained from nonlinear PSE solutions agree excellently with the DNS results. A receptivity model using the direct and adjoint PSE is shown to provide reliable predictions of the receptivity to roughness cylinders of different heights and chordwise locations. Being robust and computationally efficient, the model is well suited as a predictive tool of receptivity in flows of practical interest. The crossflow mode amplitudes obtained based on both DNS and PSE are 40% of those measured in the experiments.Additional comparisons between experimental and PSE data for various disturbance wavelengths reveal that the measured disturbance amplitudes are consistently larger than those predicted by the PSE-based receptivity model by a nearly constant factor. Supplementary DNS and PSE computations suggest that possible natural leading-edge roughness and free-stream turbulence in the experiments are unlikely to account for this discrepancy. It is more likely that experimental uncertainties in the streamwise location of the roughness array and cylinder height are responsible for the additional receptivity observed in the experiments.

• 215.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Swept wing boundary-layer receptivity to localized surface roughness2012Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 711, s. 516-544Artikel i tidskrift (Refereegranskat)

The receptivity to localized surface roughness of a swept-wing boundary layer is studied by direct numerical simulation (DNS) and computations using the parabolized stability equations (PSEs). The DNS is laid out to reproduce wind tunnel experiments performed by Saric and coworkers, where micron-sized cylinders were used to trigger steady crossflow modes. The amplitudes of the roughness-induced fundamental crossflow wave and its superharmonics obtained from nonlinear PSE solutions agree excellently with the DNS results. A receptivity model using the direct and adjoint PSEs is shown to provide reliable predictions of the receptivity to roughness cylinders of different heights and chordwise locations. Being robust and computationally efficient, the model is well suited as a predictive tool of receptivity in flows of practical interest. The crossflow mode amplitudes obtained based on both DNS and PSE methods are 40% of those measured in the experiments. Additional comparisons between experimental and PSE data for various disturbance wavelengths reveal that the measured disturbance amplitudes are consistently larger than those predicted by the PSE-based receptivity model by a nearly constant factor. Supplementary DNS and PSE computations suggest that possible natural leading-edge roughness and free-stream turbulence in the experiments are unlikely to account for this discrepancy. It is more likely that experimental uncertainties in the streamwise location of the roughness array and cylinder height are responsible for the additional receptivity observed in the experiments.

• 216. Theofanous, T. G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnkraftsäkerhet.
Aerobreakup in disturbed subsonic and supersonic flow fields2007Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 593, s. 131-170Artikel i tidskrift (Refereegranskat)

This work concerns the breakup of millimetre-scale liquid droplets in gaseous flow fields that are disturbed from free-stream conditions by the presence of solid obstacles or other drops. A broad range of flow conditions is considered - from subsonic to supersonic, from highly rarefied to ambient pressures, and from fixed cylindrical obstacles to free liquid droplets (as obstacles). The liquid is water or tributyl phosphate, a water-like low-viscosity fluid of very low vapour pressure. We present data on deformation and breakup regimes, and, aided by numerical simulations, we discuss governing mechanisms and the time scaling of these events. Thereby a methodology is demonstrated for conveniently forecasting first-order behaviours in disturbed flow fields more generally. The highly resolved images lend themselves to testing/benchmarking numerical simulations of interfacial flows. These results, along with the experimental capability developed, constitute one of the key building blocks for our overall long-term aim towards predicting ultimate particle-size distributions from such intense aerodynamic interactions involving very large quantities of Newtonian and viscoelastic liquids.

• 217.
KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Lättkonstruktioner.
A solution technique for longitudinal Stokes flow around multiple aligned cylinders2001Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 439, s. 199-216Artikel i tidskrift (Refereegranskat)
• 218.
Univ Oxford, Oxford OX2 6GG, England.;Yale Univ, New Haven, CT 06520 USA..
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
The combined effects of shear and buoyancy on phase boundary stability2019Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 868, s. 648-665Artikel i tidskrift (Refereegranskat)

We study the effects of externally imposed shear and buoyancy driven flows on the stability of a solid-liquid interface. A linear stability analysis of shear and buoyancy-driven flow of a melt over its solid phase shows that buoyancy is the only destabilizing factor and that the regime of shear flow here, by inhibiting vertical motions and hence the upward heat flux, stabilizes the system. It is also shown that all perturbations to the solid-liquid interface decay at a very modest shear flow strength. However, at much larger shear-flow strength, where flow instabilities coupled with buoyancy might enhance vertical motions, a re-entrant instability may arise.

• 219. Tsuji, Y.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
Pressure statistics and their scaling in high-Reynolds-number turbulent boundary layers2007Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 585, s. 1-40Artikel i tidskrift (Refereegranskat)

Pressure fluctuations are an important ingredient in turbulence, e.g. in the pressure strain terms which redistribute turbulence among the different fluctuating velocity components. The variation of the pressure fluctuations inside a turbulent boundary layer has hitherto been out of reach of experimental determination. The mechanisms of non-local pressure-related coupling between the different regions of the boundary layer have therefore remained poorly understood. One reason for this is the difficulty inherent in measuring the fluctuating pressure. We have developed a new technique to measure pressure fluctuations. In the present study, both mean and fluctuating pressure, wall pressure, and streamwise velocity have been measured simultaneously in turbulent boundary layers up to Reynolds numbers based on the momentum thickness R-theta similar or equal to 20 000. Results on mean and fluctuation distributions, spectra, Reynolds number dependence, and correlation functions are reported. Also, an attempt is made to test, for the first time, the existence of Kolmogorov's -7/3 power-law scaling of the pressure spectrum in the limit of high Reynolds numbers in a turbulent boundary layer.

• 220.
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
Flow regimes in a plane Couette flow with system rotation2010Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 648, s. 5-33Artikel i tidskrift (Refereegranskat)

Flow states in plane Couette flow in a spanwise rotating frame of reference have been mapped experimentally in the parameter space spanned by the Reynolds number and rotation rate. Depending on the direction of rotation, the flow is either stabilized or destabilized. The experiments were made through flow visualization in a Couette flow apparatus mounted on a rotating table, where reflected flakes are mixed with the water to visualize the flow. Both short- and long-time exposures have been used: the short-time exposure gives an instantaneous picture of the turbulent flow field, whereas the long-time exposure averages the small, rapidly varying scales and gives a clearer representation of the large scales. A correlation technique involving the light intensity of the photographs made it possible to obtain, in an objective manner, both the spanwise and streamwise wavelengths of the flow structures. During these experiments 17 different flow regimes have been identified, both laminar and turbulent with and without roll cells, as well as states that can be described as transitional, i.e. states that contain both laminar and turbulent regions at the same time. Many of these flow states seem to be similar to those observed in Taylor Couette flow.

• 221.
KTH, Skolan för teknikvetenskap (SCI), Mekanik.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
The separating flow in a plane asymmetric diffuser with 8.5 degrees opening angle: mean flow and turbulence statistics, temporal behaviour and flow structures2009Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 636, s. 337-370Artikel i tidskrift (Refereegranskat)

The flow in a plane asymmetric diffuser with an opening angle of 8.5 degrees has been studied experimentally using time-resolving stereoscopic particle image velocimetry. The inlet condition is fully developed turbulent channel flow at a Reynolds number based on the inlet channel height and bulk velocity of Re = 38 000. All mean velocity and Reynolds stress components have been measured. A separated region is found on the inclined wall with a mean separation point at 7.4 and a mean reattachment point at 30.5 inlet channel heights downstream the diffuser inlet (the inclined wall ends 24.8 channel heights downstream the inlet). Instantaneous flow reversal never occurs upstream of five inlet channel heights but may occur far downstream the point of reattachment. A strong shear layer in which high rates of turbulence production are found is located in a region outside the separation. The static wall pressure through the diffuser is presented and used in an analysis of the balance between pressure forces and momentum change. It is demonstrated that production of turbulence causes a major part of the losses of mean flow kinetic energy. The character of the large turbulence structures is investigated by means of time-resolved sequences of velocity fields and spatial auto-correlation functions. Pronounced inclined structures are observed in the spanwise velocity and it is suggested that these are due to the legs of hairpin-like vortices.

• 222.
Technion Israel Inst Technol, Dept Comp Sci, IL-32000 Haifa, Israel..
KTH, Skolan för teknikvetenskap (SCI), Mekanik. Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
Inertial gravity current produced by the drainage of a cylindrical reservoir from an outer or inner edge2019Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 874, s. 185-209Artikel i tidskrift (Refereegranskat)

We consider the time-dependent flow of a fluid of density rho(1) in a vertical cylindrical container embedded in a fluid of density rho(2) (<rho(1)) whose side boundary is suddenly removed and the fluid drains freely from the edge. We show that in the inertial-buoyancy regime (large initial Reynolds number) the flow is modelled by the shallow-water equations and bears similarities to a gravity current released from a lock (the dam-break problem) driven by the reduced gravity g' = (1 - rho(2)/rho(1))g. This formulation is amenable to an efficient finite-difference solution. Moreover, we demonstrate that similarity solutions exist, and show that the flow created by the dam break approaches the predicted self-similar behaviour when the volume ratio nu(t)/nu(0) approximate to 1/2 where t is time elapsed from the dam break. We considered two cases of drainage: (i) outward from the outer boundary in a full-radius reservoir; and (ii) inward from the inner radius in an annular-shaped reservoir. For the first case the similarity solution is expressed analytically, while the second case is more complicated and requires a numerical solution. In both cases nu(t)/nu(0) decays like t(-2), but the details are different. The similarity solutions admit an adjustable virtual-origin constant, which we determine by matching with the finite-difference solution. The analysis is valid for both Boussinesq and non-Boussinesq systems, and a wide range of geometric parameters (inner and outer radii, and height). The importance of the neglected viscous terms increases with time, and eventually the inertial-buoyancy model becomes invalid. An estimate for this occurrence is also provided. The predictions of the model are compared to results of direct numerical simulations; there is good agreement for the position of the interface and for the averaged radial velocity, and excellent agreement for nu(t)/nu(0). A box model is used for estimating the effect of a partial (over a sector) dam break. This study is an extension of the work for a rectangular reservoir of Momen et al. (J. Fluid Mech., vol. 827, 2017, pp. 640-663). We demonstrate that there are some similarities, but also significant differences, between the rectangular and the cylindrical reservoirs concerning the velocity, shape of the interface and rate of drainage, which are of interest in applications. The overall conclusion is that this simple model captures very well the flow field under consideration.

• 223.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Infrared Reynolds number dependency of the two-dimensional inverse energy cascade2011Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 667, s. 463-473Artikel i tidskrift (Refereegranskat)

High resolution simulations of forced two-dimensional turbulence reveal that the inverse cascade range is sensitive to an infrared Reynolds number $Re_{\alpha}=k_f/k_{\alpha}$, where $k_f$ is the forcing wave number and $k_{\alpha}$ is a frictional wave number based on linear friction. In the limit of high $Re_{\alpha}$, the classic $k^{-5/3}$-scaling is lost and we obtain steeper energy spectra. The sensitivity is traced to the formation of vortices in the inverse energy cascade range. Thus, it is hypothesized that the dual limit $Re_{\alpha} \rightarrow \infty$ and $Re_{\nu}=k_d/k_f \rightarrow \infty$, where $k_d$ is the small-scale dissipation wave number, will lead to a steeper energy spectrum than $k^{-5/3}$ in the inverse energy cascade range. It is also found that the inverse energy cascade is maintained by nonlocal triad interactions.

• 224.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Charney isotropy and equipartition in quasi-geostrophic turbulence2010Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 656, s. 448-457Artikel i tidskrift (Refereegranskat)

High-resolution simulations of forced quasi-geostrophic (QG) turbulence reveal that Charney isotropy develops under a wide range of conditions, and constitutes a preferred state also in beta-plane and freely decaying turbulence. There is a clear analogy between two-dimensional and QG turbulence, with a direct enstrophy cascade that is governed by the prediction of Kraichnan (J. Fluid Mech., vol. 47, 1971, p. 525) and an inverse energy cascade following the classic k(-5/3) scaling. Furthermore, we find that Charney's prediction of equipartition between the potential and kinetic energy in each of the two horizontal velocity components is approximately fulfilled in the inertial ranges.

• 225.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
The enstrophy cascade in forced two-dimensional turbulence2011Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 671, s. 168-183Artikel i tidskrift (Refereegranskat)

We carry out direct numerical simulations of two-dimensional turbulence with forcing at different wave numbers and resolutions up to 32768^2 gridpoints. In the absence of large scale drag, a state is reached where enstrophy is quasistationary while energy is growing. In the enstrophy cascade range the energy spectrum has the form $E(k) = K \epsilon_{\omega} ^{2/3} k^{-3}$, without any logarithmic correction, where$\epsilon_{\omega}$ is the enstrophy dissipation and K is of the order of unity. However, K is varying between different simulations and is thus not a perfect constant. This variation can be understood as a consequence of large-scale dissipation intermittency, following the argument by Landau (Landau \& Lifshitz 1959).  In the presence of a large scale drag, we obtain a slightly steeper spectrum. When forcing is applied at a scale which is somewhat smaller than the computational domain no vortices are formed and the statistics remain close to Gaussian in the enstrophy cascade range. When forcing is applied at a smaller scale, long lived coherent vortices form at larger scales  than the forcing scale and intermittency measures become very large at all scales, including the scales of the enstrophy cascade. We conclude that the enstrophy cascade with a $k^{-3}$-spectrum, is a robust feature of the two-dimensional Navier-Stokes equations. However, there is a complete lack of universality of higher order statistics of vorticity increments in the enstrophy cascade range.

• 226.
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Montana State Univ, Dept Phys, Bozeman, MT 59717 USA.
Oscillatory superfluid Ekman pumping in helium II and neutron stars2015Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 783, s. 251-282Artikel i tidskrift (Refereegranskat)

The linear response of a superfluid, rotating uniformly in a cylindrical container and threaded with a large number of vortex lines, to an impulsive increase in the angular velocity of the container is investigated. At zero temperature and with perfect pinning of vortices to the top and bottom of the container, we demonstrate that the system oscillates persistently with a frequency proportional to the vortex line tension parameter to the quarter power. This low-frequency mode is generated by a secondary flow analogous to classical Ekman pumping that is periodically reversed by the vortex tension in the boundary layers. We compare analytic solutions to the two-fluid equations by Chandler & Bay in (J. Low Temp. Phys., vol. 62, 1986, pp. 119-142) with the spin-up experiments by Tsakadze & Tsakadze (J. Low Temp. Phys., vol. 39, 19)0, pp. 649-688) in helium II and find that the frequency agrees within a factor of four, although the experiment is not perfectly suited to the application of linear theory. We argue that this oscillatory Ekman pumping mode, and not Tkachenko modes, provides a natural explanation for the observed oscillation. In neutron stars, the oscillation period depends on the pinning interaction between neutron vortices and flux tubes in the outer core. Using a simplified pinning model, we demonstrate that strong pinning can accommodate modes with periods of days to years, which are only weakly damped by mutual friction over longer time scales.

• 227.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Scania CV AB, Sweden.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Flow separation control behind a cylindrical bump using dielectric-barrier-discharge vortex generator plasma actuators2017Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 835, s. 852-879Artikel i tidskrift (Refereegranskat)

Dielectric-barrier-discharge plasma actuators are arranged to produce counter-rotating streamwise vortices to control flow separation on a cylindrical bump on a flat plate that is approached by a turbulent boundary layer. The control was tested for different free-stream velocities and actuation driving voltages. The recirculation area downstream of the bump was reduced by the actuation for velocities up to 15 m s(-1) at the highest voltage achievable of the present set-up. However, the flow shows a bi-modality, the nominal two-dimensional wake flow is shown to consist of large-scale streamwise vortices, which are energised by the actuation until a phenomenon of lock-on of these vortices occurs at sufficiently high driving voltages. The wavelength of the actuation is half that of the large-scale vortices. The lock-on shifts sometimes, i.e. the large streamwise vortices centre switch spanwise location, explaining the bi-modality in the flow. The details of the bi-modality are further investigated by conditional averaging and proper orthogonal decomposition.

• 228.
IIT, Dept MMAE, Chicago, IL 60616 USA..
IIT, Dept MMAE, Chicago, IL 60616 USA.. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
Secondary flow in spanwise-periodic in-phase sinusoidal channels2018Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 851, s. 288-316Artikel i tidskrift (Refereegranskat)

Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl's second kind in fully developed spanwise-periodic channels with in-plane sinusoidal walls. The secondary flow is characterized for different combinations of wave parameters defining the wall geometry at Re-h = 2500 and 5000, where h is the half-height of the channel. The total cross-flow rate in the channel Q(yz) is defined along with a theoretical model to predict its behaviour. Interaction between the secondary flows from opposite walls is observed if lambda similar or equal to h similar or equal to A, where A and lambda are the amplitude and wavelength of the sinusoidal function defining the wall geometry. As the outer-scaled wavelength (lambda/h) is reduced, the secondary vortices become smaller and faster, increasing the total cross-flow rate per wall. However, if the inner-scaled wavelength (lambda(+)) is below 130 viscous units, the cross-flow decays for smaller wavelengths. By analysing cases in which the wavelength of the wall is much smaller than the half-height of the channel lambda << h, we show that the cross-flow distribution depends almost entirely on the separation between the scales of the instantaneous vortices, where the upper and lower bounds are determined by lambda/h and lambda(+), respectively. Therefore, the distribution of the secondary flow relative to the size of the wave at a given Re-h can be replicated at higher Re-h by decreasing lambda/h and keeping lambda(+) constant. The mechanisms that contribute to the mean cross-flow are analysed in terms of the Reynolds stresses and using quadrant analysis to evaluate the probability density function of the bursting events. These events are further classified with respect to the sign of their instantaneous spanwise velocities. Sweeping events and ejections are preferentially located in the valleys and peaks of the wall, respectively. The sweeps direct the instantaneous cross-flow from the core of the channel towards the wall, turning in the wall-tangent direction towards the peaks. The ejections drive the instantaneous cross-flow from the near-wall region towards the core. This preferential behaviour is identified as one of the main contributors to the secondary flow.

• 229.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Laminarization mechanisms and extreme-amplitude states in rapidly rotating plane channel flow2013Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 730, s. 193-219Artikel i tidskrift (Refereegranskat)

Fully developed plane channel flow rotating in the spanwise direction has been studied analytically and numerically. Linear stability analysis reveals that all cross-flow modes are stable for supercritical rotation numbers, Ro &gt; Roc , where Roc will approach 3 from below for increasing Reynolds number. Plane Tollmien-Schlichting (TS) waves are independent of rotation and always linearly unstable for supercritical Reynolds numbers. Direct numerical simulation (DNS) of the laminarization process reveals that the turbulence is damped when Ro approaches Roc. Hence, the laminarization is dominated by linear mechanisms. The flow becomes periodic for supercritical Reynolds numbers and rotation rates, i.e. when Ro &gt; Ro c and Re &gt; Rec. At such rotation rates, all oblique (cross-flow) modes are damped and when the disturbance amplitude becomes small enough, the TS modes start to grow exponentially. Secondary instabilities are initially blocked by the rotation since all cross-flow modes are linearly stable and the breakdown to turbulence will be strongly delayed. Hence, the TS waves will reach extremely high amplitudes, much higher than for typical turbulent fluctuations. Eventually, the extreme-amplitude state with TS-like waves will break down to turbulence and the flow will laminarize due to the influence of the rapid rotation, thus completing the cycle that will then be repeated. This flow is strongly dominated by linear mechanisms, which is remarkable considering the extremely high amplitudes involved in the processes of laminarization of the turbulence at Ro ≥ Roc and the growth of the unstable TS waves.

• 230.
FFA.
KTH, Skolan för teknikvetenskap (SCI), Mekanik.
An explicit algebraic Reynolds stress model for incompressible and compressible turbulent flows2000Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 403, s. 89-132Artikel i tidskrift (Refereegranskat)

Some new developments of explicit algebraic Reynolds stress turbulence models (EARSM) are presented. The new developments include a new near-wall treatment ensuring realizability for the individual stress components, a formulation for compressible flows, and a suggestion for a possible approximation of diffusion terms in the anisotropy transport equation. Recent developments in this area are assessed and collected into a model for both incompressible and compressible three-dimensional wall-bounded turbulent flows. This model represents a solution of the implicit ARSM equations, where the production to dissipation ratio is obtained as a solution to a nonlinear algebraic relation. Three-dimensionality is fully accounted for in the mean flow description of the stress anisotropy. The resulting EARSM has been found to be well suited to integration to the wall and all individual Reynolds stresses can be well predicted by introducing wall damping functions derived from the van Driest damping function. The platform for the model consists of the transport equations for the kinetic energy and an auxiliary quantity. The proposed model can be used with any such platform, and examples are shown for two different choices of the auxiliary quantity.

• 231.
Staz Zool Anton Dohrn, Lab Ecol & Evolut Plankton, I-80121 Naples, Italy..
Chalmers Univ Technol, Dept Mech & Maritime Sci, Fluid Dynam, SE-41296 Gothenburg, Sweden.. KTH, Skolan för teknikvetenskap (SCI), Mekanik. Staz Zool Anton Dohrn, Lab Ecol & Evolut Plankton, I-80121 Naples, Italy..
On the time scales and structure of Lagrangian intermittency in homogeneous isotropic turbulence2019Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 867, s. 438-481, artikel-id 025301(R)Artikel i tidskrift (Refereegranskat)

We present a study of Lagrangian intermittency and its characteristic time scales. Using the concepts of flying and diving residence times above and below a given threshold in the magnitude of turbulence quantities, we infer the time spectra of the Lagrangian temporal fluctuations of dissipation, acceleration and enstrophy by means of a direct numerical simulation in homogeneous and isotropic turbulence. We then relate these time scales, first, to the presence of extreme events in turbulence and, second, to the local flow characteristics. Analyses confirm the existence in turbulent quantities of holes mirroring bursts, both of which are at the core of what constitutes Lagrangian intermittency. It is shown that holes are associated with quiescent laminar regions of the flow. Moreover, Lagrangian holes occur over few Kolmogorov time scales while Lagrangian bursts happen over longer periods scaling with the global decorrelation time scale, hence showing that loss of the history of the turbulence quantities along particle trajectories in turbulence is not continuous. Such a characteristic partially explains why current Lagrangian stochastic models fail at reproducing our results. More generally, the Lagrangian dataset of residence times shown here represents another manner for qualifying the accuracy of models. We also deliver a theoretical approximation of mean residence times, which highlights the importance of the correlation between turbulence quantities and their time derivatives in setting temporal statistics. Finally, whether in a hole or a burst, the straining structure along particle trajectories always evolves self-similarly (in a statistical sense) from shearless two-dimensional to shear bi-axial configurations. We speculate that this latter configuration represents the optimum manner to dissipate locally the available energy.

• 232.
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. FOI, Swedish Def Res Agcy,Sweden.
Numerical and theoretical investigation of pulsatile turbulent channel flows2016Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 792, s. 98-133Artikel i tidskrift (Refereegranskat)

A turbulent channel flow subjected to imposed harmonic oscillations is studied by direct numerical simulation (DNS) and theoretical models. Simulations have been performed for different pulsation frequencies. The time- and phase-averaged data have been used to analyse the flow. The onset of nonlinear effects during the production of the perturbation Reynolds stresses is discussed based on the DNS data, and new physical features observed in the DNS are reported. A linear model proposed earlier by the present authors for the coherent perturbation Reynolds shear stress is reviewed and discussed in depth. The model includes the non-equilibrium effects during the response of the Reynolds stress to the imposed periodic shear straining, where a phase lag exists between the stress and the strain. To validate the model, the perturbation velocity and Reynolds shear stress from the model are compared with the DNS data. The performance of the model is found to be good in the frequency range where quasi-static assumptions are invalid. The viscoelastic characteristics of the turbulent eddies implied by the model are supported by the DNS data. Attempts to improve the model are also made by incorporating the DNS data in the model.

• 233.
Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
KTH, Skolan för teknikvetenskap (SCI), Mekanik. Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA.;KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden.;KTH Mech, Swedish E Sci Res Ctr SeRC, SE-10044 Stockholm, Sweden.. Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA.. Univ Bristol, Sch Math, Bristol BS8 1TW, Avon, England.. Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
Time-dependent motion of a confined bubble in a tube: transition between two steady states2018Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 857, artikel-id R4Artikel i tidskrift (Refereegranskat)

When a confined bubble translates steadily in a cylindrical capillary tube, without the consideration of gravity effects, a uniform thin film of liquid separates the bubble surface and the tube wall. In this work, we investigate how this steady state is established by considering the transitional motion of the bubble as it adjusts its film thickness profile between two steady states, characterized by two different bubble speeds. During the transition, two uniform film regions coexist, separated by a step-like transitional region. The transitional motion also requires modification of the film solution near the rear of the bubble, which depends on the ratio of the two capillary numbers. These theoretical results are verified by experiments and numerical simulations.

• 234.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden.;KTH Mech, SeRC Swedish E Sci Res Ctr, SE-10044 Stockholm, Sweden..
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden.;KTH Mech, SeRC Swedish E Sci Res Ctr, SE-10044 Stockholm, Sweden.. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden.;KTH Mech, SeRC Swedish E Sci Res Ctr, SE-10044 Stockholm, Sweden.. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden.;KTH Mech, SeRC Swedish E Sci Res Ctr, SE-10044 Stockholm, Sweden.. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden.;KTH Mech, SeRC Swedish E Sci Res Ctr, SE-10044 Stockholm, Sweden..
Experimental investigation of turbulent suspensions of spherical particles in a squareduct2018Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 857, s. 748-783Artikel i tidskrift (Refereegranskat)

We report experimental observations of turbulent flow with spherical particles in a square duct. Three particle sizes, namely 2H/d(p) = 40, 16 and 9 (2H being the duct full height and d(p) being the particle diameter), are investigated. The particles are nearly neutrally buoyant with a density ratio of 1.0035 and 1.01 with respect to the suspending fluid. Refractive index matched-particle image velocimetry (RIM-PIV) is used for fluid velocity measurement even at the highest particle volume fraction (20 %) and particle tracking velocimetry (PTV) for the particle velocity statistics for the flows seeded with particles of the two largest sizes, whereas only pressure measurements are reported for the smallest particles. Settling effects are seen at the lowest bulk Reynolds number R-e2H approximate to 10 000, whereas, at the highest R-e2H approximate to 27 000, particles are in almost full suspension. The friction factor of the suspensions is found to be significantly larger than that of single-phase duct flow at the lower R-e2H investigated; however, the difference decreases when increasing the flow rate and the total drag approaches the values of the single-phase flow at the higher Reynolds number considered, R-e2H = 27 000. The pressure drop is found to decrease with the particle diameter for volume fractions lower than (sic) = 10% for nearly all R-e2H investigated. However, at the highest volume fraction (sic) = 20 %, we report a peculiar non-monotonic behaviour: the pressure drop first decreases and then increases with increasing particle size. The decrease of the turbulent drag with particle size at the lowest volume fractions is related to an attenuation of the turbulence. The drag increase for the two largest particle sizes at (sic) = 20 %, however, occurs despite this large reduction of the turbulent stresses, and it is therefore due to significant particle-induced stresses. At the lowest Reynolds number, the particles reside mostly in the bottom half of the duct, where the mean velocity significantly decreases; the flow is similar to that in a moving porous bed near the bottom wall and to turbulent duct flow with low particle concentration near the top wall.

• 235.
KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Jord- och bergmekanik. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
Finite-size spherical particles in a square duct flow of an elastoviscoplastic fluid: an experimental study2020Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 883, artikel-id A6Artikel i tidskrift (Refereegranskat)

The present experimental study addresses the flow of a yield stress fluid with some elasticity (Carbopol gel) in a square duct. The behaviour of two fluids with lower and higher yield stress is investigated in terms of the friction factor and flow velocities at multiple Reynolds numbers Re* is an element of (1, 200) and, hence, Bingham numbers Bi is an element of (0.01, 0.35). Taking advantage of the symmetry planes in a square duct, we reconstruct the entire 3-component velocity field from two-dimensional particle image velocimetry (PIV). A secondary flow consisting of eight vortices is observed to recirculate the fluid from the core towards the wall centre and from the corners back to the core. The extent and intensity of these vortices grows with increasing Re* or, alternately, as the plug size decreases. The second objective of this study is to explore the change in flow in the presence of particles. To this end, almost neutrally buoyant finite-size spherical particles with a duct height, 2H, to particle diameter, d(p), ratio of 12 are used at two volume fractions phi = 5 and 10 %. Particle tracking velocimetry is used to measure the velocity of these refractive-index-matched spheres in the clear Carbopol gel, and PIV to extract the fluid velocity. Additionally, simple shadowgraphy is also used to qualitatively visualise the development of the particle distribution along the streamwise direction. The particle distribution pattern changes from being concentrated at the four corners, at low flow rates, to being focussed along a diffused ring between the centre and the corners, at high flow rates. The presence of particles induces streamwise and wall-normal velocity fluctuations in the fluid phase; however, the primary Reynolds shear stress is still very small compared to turbulent flows. The size of the plug in the particle-laden cases appears to be smaller than the corresponding single-phase cases. Similar to Newtonian fluids, the friction factor increases due to the presence of particles, almost independently of the suspending fluid matrix. Interestingly, predictions based on an increased effective suspension viscosity agrees quite well with the experimental friction factor for the concentrations used in this study.

• 236.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
Accumulation of motile elongated micro-organisms in turbulence2014Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 739, s. 22-36Artikel i tidskrift (Refereegranskat)

We study the effect of turbulence on marine life by performing numerical simulations of motile micro-organisms, modelled as prolate spheroids, in isotropic homogeneous turbulence. We show that the clustering and patchiness observed in laminar flows, linear shear and vortex flows, are significantly reduced in a three-dimensional turbulent flow mainly because of the complex topology; elongated micro-organisms show some level of clustering in the case of swimmers without any preferential alignment whereas spherical swimmers remain uniformly distributed. Micro-organisms with one preferential swimming direction (e.g. gyrotaxis) still show significant clustering if spherical in shape, whereas prolate swimmers remain more uniformly distributed. Due to their large sensitivity to the local shear, these elongated swimmers react more slowly to the action of vorticity and gravity and therefore do not have time to accumulate in a turbulent flow. These results show how purely hydrodynamic effects can alter the ecology of micro-organisms that can vary their shape and their preferential orientation.

• 237.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
Non-linear disturbance growth during sedimentation in dilute fibre suspensions2013Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 719, s. 268<-294Artikel i tidskrift (Refereegranskat)

Disturbances in a dilute fibre suspension is studied with an Eulerian approach. Based on a linear stability analysis, it is shown that inertia and hydrodynamic diffusion damp perturbations at long wavelengths and short wavelengths, respectively, leading to a wavenumber selection. For small, but finite Reynolds number of the fluid bulk motion, the most unstable wavenumber is a finite value which increases with Reynolds number, and where the diffusion narrows the range of unstable wavenumbers. Numerical simulations of the full non-linear evolution in time of a normal mode perturbation show that the induced flow may either die or saturate on a finite amplitude. The character of this long time behaviour is dictated by the wavenumber and the presence or absence of the translational and rotational diffusivities.

• 238.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
Linear stability analysis of channel flow of viscoelastic Oldroyd-B and FENE-P fluids2013Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 737, s. 249-279Artikel i tidskrift (Refereegranskat)

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.

• 239.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
The motion of a deforming capsule through a corner2015Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 770, s. 374-397Artikel i tidskrift (Refereegranskat)

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.

• 240.
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Dept. of Mechanical and Aerospace Engineering, University of California, San Diego, USA. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Low-Reynolds number swimming in a capillary tube2013Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 726, s. 285-311Artikel i tidskrift (Refereegranskat)

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.

• 241.
KTH, Skolan för teknikvetenskap (SCI), Mekanik.
Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
Rotation of a low-Reynolds-number watermill: theory and simulations2018Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 849, s. 57-75Artikel i tidskrift (Refereegranskat)

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.

• 242.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. IRPH́E, Université de Provence. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
Optimal growth, model reduction and control in a separated boundary-layer flow using global eigenmodes2007Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 579, s. 305-314Artikel i tidskrift (Refereegranskat)

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.

• 243.
KTH, Skolan för datavetenskap och kommunikation (CSC), Numerisk Analys och Datalogi, NADA.
KTH, Skolan för datavetenskap och kommunikation (CSC), Numerisk Analys och Datalogi, NADA.
Resolvent bounds for pipe Poiseuille flow2006Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 568, s. 451-471Artikel i tidskrift (Refereegranskat)

We derive an analytical bound on the resolvent of pipe Poiseuille flow in large parts of the unstable half-plane. We also consider the linearized equations, Fourier transformed in axial and azimuthal directions. For certain combinations of the wavenumbers and the Reynolds number, we derive an analytical bound on the resolvent of the Fourier transformed problem. In particular, this bound is valid for the perturbation which numerical computations indicate to be the perturbation that gives the largest transient growth. Our bound has the same dependence on the Reynolds number as given by the computations.

2345 201 - 243 av 243
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