15:00
Control 2
Chair: Rudie Kunnen
15:00
15 mins
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'Synthetic' Large Scale Motions Organize Small Scale Motions in the Turbulent Boundary Layer
Ian Jacobi, Beverley McKeon
Abstract: The relationship between large- and small-scale motions in a non-equilibrium turbulent boundary layer was studied experimentally. A zero-pressure-gradient flat plate turbulent boundary layer was perturbed by a short array of two-dimensional roughness elements, both statically and under dynamic actuation. The dynamic forcing generated a `synthetic' very-large-scale motion (VLSM) within the flow which was observed by phase-locked flow measurements. The phase-relationship between both synthetic and natural VLSMs and the small scale motions within the boundary layer was studied by cross-correlation and cospectral techniques, to reveal an organizing effect of the artificial VLSM on smaller scales.
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15:15
15 mins
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SUBHARMONIC LOCK-IN AND PHASE SYNCHRONIZATON IN KARMAN STREET DUE TO SURFACE WAVES
Hans GUNNOO, Nizar ABCHA, Isabel GARCIA-HERMOSA, Alexander Ezersky
Abstract: It is experimentally shown that the action of harmonic surface waves on the turbulent Karman vortex street can provide the shedding frequency lock-in. In our experimental conditions, this effect occurs if the frequency of the surface wave is approximately two times higher than the frequency of vortex shedding. The dynamics of phase fluctuations in the vortex wake of the cylinder is studied using the Hilbert transform technique. On the plane wave frequency – wave amplitude we find an area where phase synchronization of oscillations in the vortex wake occurs.
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15:30
15 mins
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Modulation of the wall-heat transfer in turbulent thermomagnetic convection by magnetic field gradients
Sasa Kenjeres, Ruben Zinsmeester, Lykasz Pyrda, Elzbieta Fornalik-Wajs, Janusz Szmyd
Abstract: We present combined experimental and numerical studies of the heat transfer of paramagnetic or diamagnetic fluid inside a differentially heated cubical enclosure subjected to the magnetic field gradients of different strength and orientation. In contrast to the previously reported studies in literature, which observed solely laminar flow regimes, here we focused on the fully developed turbulent flow regimes. That was possible by using a combination of the state-of-art superconducting magnets
(with a strength up to 10 T and magnetic field gradients up to 900 T$^2$/m) and by selecting various paramagnetic or diamagnetic working fluids (in a range of $10$$\le$Pr$\le$$1000$). Detailed comparison between experiments (integral wall-heat transfer, temperature time-series at different locations within the enclosure) and direct numerical simulations (DNS) are performed and generally very good agreements are obtained in predicting the integral heat transfer. In addition, analysis of the long-term averaged first- and second-moments of velocity and thermal fields is performed. Finally, budgets of the turbulent kinetic energy and of the temperature variance are analyzed and the mean mechanism of the thermal plume reorganization in terms of the proper-orthogonal decomposition (POD) modes is presented.
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15:45
15 mins
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Drag reduction in homogeneous shear flow turbulence diluted with contravariant and covariant polymers
Taketoshi Fujiwara, Kiyosi Horiuti, Shu Suzuki
Abstract: This study conducts a multi-scale analysis on the drag reduction (DR) in the fluid flow diluted with the polymers. A mesoscopic description of ensemble of elastic dumbbells using Brownian dynamics (BDS) is connected to the macroscopic description for the solvent Newtonian fluid using DNS [1]. In Horiuti \etal [2], non-affinity in which the motion of dumbbells does not precisely correspond to macroscopically-imposed deformation was introduced and its effect on DR was elucidated in homogeneous isotropic turbulence (HIT). This work aims to carry out assessment on the influence of shear on DR in non-affine viscoelastic turbulence placed under constant mean shear. It is shown that the occurrence of DR and its mechanism are in agreement with those in HIT. More drastic DR is achieved when non-affinity is maximum than in the complete affine case. This difference is attributable to the convective motions of dumbbells. In the complete affine case, the connector vector of dumbbell is convected as a contravariant vector representing material line element, whereas, when non-affinity is the largest, it is convected as a covariant vector representing material surface element. In the latter case, the dumbbells directs outward perpendicularly on the planar structures and exert an extra tension on vortex sheet, which leads to attenuation of energy cascade, causing larger DR. The effect of presence of the streaks on the alignment of the dumbbells is discussed.
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16:00
15 mins
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TORQUE MEASUREMENTS AND FLOWVISUALISATIONS IN A WIDE GAP TAYLOR-COUETTE FLOW
Andreas Froitzheim, Sebastian Merbold, Christoph Egbers
Abstract: The Torque acting on the inner cylinder in a differentially rotating Taylor-Couette flow is investigated. In the past a torque maximum has been found in the counter rotating regime. Using flow visualisations and LDA measurements a deeper look inside the
flow in the vicinity of this maximum will be done.
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16:15
15 mins
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Drag reduction in a turbulent boundary layer using periodic blowing through one array of streamwise slits
Yinzhe Li, Yu Zhou
Abstract: Active drag reduction of a turbulent boundary layer has been experimentally investigated using periodic blowing through one array of streamwise slits. Local skin-friction drag exhibits a strong dependence on control parameters, including the blowing amplitude A+ and frequency f +. A maximum drag reduction of almost 70% has been achieved at x+=33 downstream of the actuators. The near-wall structure under control are measured using smoke-wire flow visualization, hot-wire and PIV techniques. All the results point to a pronounced change in the structure of the boundary layer and a significant increase of the mean energy dissipation rate.
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16:30
15 mins
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The influence of steady blowing and roughness on transitional separated boundary layers
Mark Phil Simens, Ayse Gungor
Abstract: This paper presents the results of a study between two types of forcing, namely steady blowing and a tripwire, on the control of laminar separated boundary layers.
The analysis focuses on the differences in the transition process between these two types of forcing. This effect will be studied using direct numerical simulation.
The main differences consists in the coherent structures formed during transition and the overall kinetic energy growth.
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16:45
15 mins
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Interaction of flexible filaments with the wake of cylinder at low Reynolds numbers
Mohammad Omidyeganeh, Alfredo Pinelli
Abstract: This work is the very first attempt to understand and optimize the configuration of flexible filaments placed on the lee side of a bluff body to manipulate flow transitions and bifurcations. It is found that the presence of a sparse set of flexible filaments on the lee side of a cylinder can interfere with the 2D-3D transition process resulting in elongation of recirculation bubble, inhibition of higher order unstable modes, and narrowing the global energy content about a particular shedding frequency. Filaments become effective when spacing between them is smaller than the dominant unstable mode at each particular Reynolds number, i.e. A and B modes.
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