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We investigate the effect of temperature dependent thermal conductivity λ and isobaric specific heat c_P on the transient amplification of perturbations in a thermally stratified laminar plane Poiseuille flow. It is shown that for decreasing thermal conductivity the maximum transient energy growth is amplified with respect to the λ=1 case, while...conference paper 2015
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Nguyen, M.Q. (author), Simöens, S. (author), Delache, A. (author), El Hajem, M. (author), Bos, W. (author), Poole, R. (author)The comparison of the results of direct numerical simulations of isotropic turbulence of Newtonian and viscoelastic fluid, based on the FENE-P model, provide evidence that viscoelasticity modifies qualitatively the behavior of the smallest scales: we observe a k<sup>−6</sup> law in the far dissipation range of the energy spectrum and we show...conference paper 2015
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In linearly stable shear flows like pipe and plane Couette flows, the transition from the laminar to the turbulent regime occurs abruptly. To better understand this transition, the time evolution of turbulent patches, created by controlled finite amplitude perturbations, have been studied in the literature. These studies mostly focused on pipe flows...conference paper 2015
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The hydrodynamic breakup of small inertial aggregates in homogenous and isotropic turbulence is studied through numerical simulations. Small inertial aggregates are subject to shear stress caused by the local velocity gradient and drag stress caused by the relative velocity of the aggregate and the fluid flow. In our simulations, we follow...conference paper 2015
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Motivated by advances in constrained optimization methods, a fundamentally new autonomic closure for LES is presented that invokes a self-optimization method for the subgrid-scale stresses instead of a predefined turbulence model. This autonomic closure uses the most general dimensionally-consistent expression for the local subgrid-scale...conference paper 2015
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This abstract examines the evolution of perturbations generated by various trips in a turbulent boundary layer. Measurements taken using hot wire anemometry show that the evolution towards the natural state is strongly dependent on the formation mechanisms of the boundary layer, this being different for different wall normal distribution of the...conference paper 2015
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Jacobitz, F. (author), Schneider, K. (author), Farge, M. (author)Direct numerical simulation data of homogeneous turbulence with shear and stratification are analyzed to study the Lagrangian and Eulerian acceleration statistics. Richardson numbers from <em>Ri</em>=0, corresponding to unstratified shear flow, to <em>Ri</em>=1, corresponding to strongly stratified shear flow, are considered. The scale...conference paper 2015
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Longitudinal libration of the cylinder side walls of a rotating annulus in the supercritical regime induces a centrifugally unstable Stokes boundary layer which generates Görtler vortices only in a portion of a libration cycle. We show for the first time that these vortices propagate into the fluid bulk and generate an azimuthal mean flow which...conference paper 2015
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Turbulent patches are localized events of turbulence, typically characterized by sharp differences between the flow characteristics across their interfaces. These localized events might add to the global mixing, heat exchange and mass transfer, playing a non-negligible role in the total energy balance in lakes or the ocean. This study takes a...conference paper 2015
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Stratified turbulent mixing remains an unsolved problem. Turbulent mixing is complicated by its intermittent nature, its highly vortical motion and the large range of scales of its coherent structures. In order to help reduce the problem to a more tractable form, we consider vortex rings as a reproducible, idealized form of a turbulent coherent...conference paper 2015
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We report on results from a set of numerical simulations of isothermal compressible turbulence forced at an intermediate scale with grid resolutions up to 8192^2. At a moderate pumping rate, the total energy of a typical frictionless system, subject to periodic boundary conditions, exhibits linear growth characteristic of an inverse cascade...conference paper 2015
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This study aims to explore how the flow transition from one state to the other in rotating convection will affect the Lagrangian statistics of (fluid) particles. 3D Particle Tracking Velocimetry (3D-PTV) is employed in a water-filled cylindrical tank of equal height and diameter 200 mm. The measurements are performed in the central volume of 50 ...conference paper 2015
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We present a priori and a posteriori analysis of the assumptions and predictions of the hybrid two-level large-eddy simulation (TLS-LES) method for high Reynolds number complex flows. The TLS-LES methodology is a multi-scale framework for simulation of turbulent flows in complex configurations at practically relevant Reynolds number. It...conference paper 2015
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We report new experimental results that use tools from braid theory to characterize two-dimensional turbulent flows driven by Faraday waves. The average topological length of the material fluid lines is found to grow exponentially with time. It allows us to compute the braid’s topological entropy SBraid. We show that SBraid increases as the...conference paper 2015
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The emergence of fluid instabilities in the relevant limit of vanishing fluid inertia (i.e., arbitrarily close to zero Reynolds number) has been investigated for the well-known Kolmogorov flow. The time-lagged viscosity change from lower to higher values due to shear changes is the crucial ingredient for the instabilities to emerge. This...conference paper 2015
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In the context of Large Eddy Simulation (LES) of turbulent flow laden by solid particles, we propose a modelling of the the interaction of a particle with the unresolved scales of the flow. We consider both particles much smaller and larger than the Kolmogorov length scale. The small scales of high Reynolds number flow are characterized by...conference paper 2015
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The recent discovery of rare backflow events in turbulent boundary layer flows based on the analysis of DNS data has raised again the need of experimental visualizations of wall-shear stress (WSS) fields in such unsteady flows. Of importance is the localization of critical points which are thought to strongly correlate with large-scale events in...conference paper 2015
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We perform renormalization group analysis (RG) of the Navier-Stokes equation in the presence of constant mean velocity field $\mathbf U_0$, and show that the renormalized viscosity is unaffected by $\mathbf U_0$, thus negating the ``sweeping effect", proposed by Kraichnan [Phys. Fluids {\bf 7}, 1723 (1964)] using random Galilean invariance....conference paper 2015
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In this contribution we study the clustering of inertial particles using a periodic kinematic simulation. The systematic Lagrangian tracking of particles makes it possible to identify the particles’ clustering patterns for different values of particle’s inertia and drift velocity. The different cases are characterised by different pairs of...conference paper 2015
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The cause of recently observed spanwise-alternating low- and high-momentum pathways [1,2,3] to appear across the entire turbulent boundary layer formed over rough surfaces is experimentally investigated by measuring the flows over roughness elements with systematically varied spanwise spacing. We found that the secondary flows associated with...conference paper 2015