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May 2010

Volume 22, Issue 5, Articles (05xxxx)

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Phys. Fluids 22, 051301 (2010); http://dx.doi.org/10.1063/1.3407662 (20 pages)

H. J. S. Fernando, D. Zajic, S. Di Sabatino, R. Dimitrova, B. Hedquist, and A. Dallman
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back to top Turbulent Flows

Lattice Boltzmann methods for thermal flows: Continuum limit and applications to compressible Rayleigh–Taylor systems

A. Scagliarini, L. Biferale, M. Sbragaglia, K. Sugiyama, and F. Toschi

Phys. Fluids 22, 055101 (2010); http://dx.doi.org/10.1063/1.3392774 (21 pages) | Cited 11 times

Online Publication Date: 4 May 2010

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We compute the continuum thermohydrodynamical limit of a new formulation of lattice kinetic equations for thermal compressible flows, recently proposed by Sbragaglia et al. [J. Fluid Mech. 628, 299 (2009)] . We show that the hydrodynamical manifold is given by the correct compressible Fourier–Navier–Stokes equations for a perfect fluid. We validate the numerical algorithm by means of exact results for transition to convection in Rayleigh–Bénard compressible systems and against direct comparison with finite-difference schemes. The method is stable and reliable up to temperature jumps between top and bottom walls of the order of 50% the averaged bulk temperature. We use this method to study Rayleigh–Taylor instability for compressible stratified flows and we determine the growth of the mixing layer at changing Atwood numbers up to At ∼ 0.4. We highlight the role played by the adiabatic gradient in stopping the mixing layer growth in the presence of high stratification and we quantify the asymmetric growth rate for spikes and bubbles for two dimensional Rayleigh–Taylor systems with resolution up to Lx×Lz = 1664×4400 and with Rayleigh numbers up to Ra ∼ 2×1010.
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47.40.-x Compressible flows; shock waves
47.20.-k Flow instabilities
02.70.Bf Finite-difference methods
47.10.A- Mathematical formulations
47.55.Hd Stratified flows
47.10.ad Navier-Stokes equations

Turbulent drag reduction in nonionic surfactant solutions

Shinji Tamano, Motoyuki Itoh, Katsuo Kato, and Kazuhiko Yokota

Phys. Fluids 22, 055102 (2010); http://dx.doi.org/10.1063/1.3407666 (12 pages) | Cited 4 times

Online Publication Date: 4 May 2010

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There are only a few studies on the drag-reducing effect of nonionic surfactant solutions which are nontoxic and biodegradable, while many investigations of cationic surfactant solutions have been performed so far. First, the drag-reducing effects of a nonionic surfactant (AROMOX), which mainly consisted of oleyldimethylamineoxide, was investigated by measuring the pressure drop in the pipe flow at solvent Reynolds numbers Re between 1000 and 60 000. Second, we investigated the drag-reducing effect of a nonionic surfactant on the turbulent boundary layer at momentum-thickness Reynolds numbers Reθ from 443 to 814 using two-component laser-Doppler velocimetry and particle image velocimetry systems. At the temperature of nonionic surfactant solutions, T = 25 °C, the maximum drag reduction ratio for AROMOX 500 ppm was about 50%, in the boundary layer flow, although the drag reduction ratio was larger than 60% in pipe flow. Turbulence statistics and structures for AROMOX 500 ppm showed the behavior of typical drag-reducing flow such as suppression of turbulence and modification of near-wall vortices, but they were different from those of drag-reducing cationic surfactant solutions, in which bilayered structures of the fluctuating velocity vectors were observed in high activity.
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47.27.nf Flows in pipes and nozzles
47.32.-y Vortex dynamics; rotating fluids
47.60.Dx Flows in ducts and channels
47.80.Cb Velocity measurements
47.80.Jk Flow visualization and imaging
47.85.lb Drag reduction
47.50.-d Non-Newtonian fluid flows
47.27.nb Boundary layer turbulence

Influence of global rotation and Reynolds number on the large-scale features of a turbulent Taylor–Couette flow

F. Ravelet, R. Delfos, and J. Westerweel

Phys. Fluids 22, 055103 (2010); http://dx.doi.org/10.1063/1.3392773 (8 pages) | Cited 7 times

Online Publication Date: 7 May 2010

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We experimentally study the turbulent flow between two coaxial and independently rotating cylinders. We determined the scaling of the torque with Reynolds numbers at various angular velocity ratios (Rotation numbers) and the behavior of the wall shear stress when varying the Rotation number at high Reynolds numbers. We compare the curves with particle image velocimetry analysis of the mean flow and show the peculiar role of perfect counter-rotation for the emergence of organized large scale structures in the mean part of this very turbulent flow that appear in a smooth and continuous way: the transition resembles a supercritical bifurcation of the secondary mean flow.
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47.27.N- Wall-bounded shear flow turbulence
47.27.Cn Transition to turbulence
47.80.Jk Flow visualization and imaging

A seamless hybrid RANS-LES model based on transport equations for the subgrid stresses and elliptic blending

Atabak Fadai-Ghotbi, Christophe Friess, Rémi Manceau, and Jacques Borée

Phys. Fluids 22, 055104 (2010); http://dx.doi.org/10.1063/1.3415254 (19 pages) | Cited 5 times

Online Publication Date: 7 May 2010

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The aim of the present work is to develop a seamless hybrid Reynolds-averaged Navier–Stokes (RANS) large-eddy simulation (LES) model based on transport equations for the subgrid stresses, using the elliptic-blending method to account for the nonlocal kinematic blocking effect of the wall. It is shown that the elliptic relaxation strategy of Durbin is valid in a RANS (steady) as well as a LES context (unsteady). In order to reproduce the complex production and redistribution mechanisms when the cutoff wavenumber is located in the productive zone of the turbulent energy spectrum, the model is based on transport equations for the subgrid-stress tensor. The partially integrated transport model (PITM) methodology offers a consistent theoretical framework for such a model, enabling to control the cutoff wavenumber κc, and thus the transition from RANS to LES, by making the Cε2 coefficient in the dissipation equation of a RANS model a function of κc. The equivalence between the PITM and the Smagorinsky model is shown when κc is in the inertial range of the energy spectrum. The extension of the underlying RANS model used in the present work, the elliptic-blending Reynolds-stress model, to the hybrid RANS-LES context, brings out some modeling issues. The different modeling possibilities are compared in a channel flow at Reτ = 395. Finally, a dynamic procedure is proposed in order to adjust during the computation the dissipation rate necessary to drive the model toward the expected amount of resolved energy. The final model gives very encouraging results in comparison to the direct numerical simulation data. In particular, the turbulence anisotropy in the near-wall region is satisfactorily reproduced. The contribution of the resolved and modeled fields to the Reynolds stresses behaves as expected: the modeled part is dominant in the near-wall zones (RANS mode) and decreases toward the center of the channel, where the relative contribution of the resolved part increases. Moreover, when the mesh is modified, the amount of resolved energy changes but the total Reynolds stresses remain nearly constant.
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47.10.ad Navier-Stokes equations
47.27.-i Turbulent flows
47.27.nd Channel flow
47.60.Dx Flows in ducts and channels

Attenuation of the wake of a sphere in an intense incident turbulence with large length scales

Zouhir Amoura, Véronique Roig, Frédéric Risso, and Anne-Marie Billet

Phys. Fluids 22, 055105 (2010); http://dx.doi.org/10.1063/1.3425628 (9 pages) | Cited 5 times

Online Publication Date: 13 May 2010

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We report an investigation of the wake of a sphere immersed in a uniform turbulent flow for sphere Reynolds numbers ranging from 100 to 1000. An original experimental setup has been designed to generate a uniform flow convecting an isotropic turbulence. At variance with previous works, the integral length scale of the turbulence is of the same order as the sphere diameter and the turbulence intensity is large. In consequence, the most intense turbulent eddies are capable of influencing the flow in the close vicinity of the sphere. Except in the attached region downstream of the sphere where the perturbation of the mean velocity is larger than the standard deviation of the incident turbulence, the flow is controlled by the incident turbulence. The distortion of the turbulence while the flow goes round the sphere leads to an increase in the longitudinal fluctuation and a decrease in the transversal one. The attenuation of the transversal fluctuations is still significant at 30 radii downstream of the sphere whereas the longitudinal fluctuations relax more rapidly toward the incident value. The more striking result however concerns the evolution of the mean velocity defect with the distance x from the sphere. It decays as x−2 and scales with the standard deviation of the incident turbulence instead of scaling with the mean incident velocity.
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47.27.-i Turbulent flows
47.27.wb Turbulent wakes
47.27.te Turbulent convective heat transfer

Large-eddy simulation of turbulent collision of heavy particles in isotropic turbulence

Guodong Jin, Guo-Wei He, and Lian-Ping Wang

Phys. Fluids 22, 055106 (2010); http://dx.doi.org/10.1063/1.3425627 (13 pages) | Cited 11 times

Online Publication Date: 20 May 2010

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The small-scale motions relevant to the collision of heavy particles represent a general challenge to the conventional large-eddy simulation (LES) of turbulent particle-laden flows. As a first step toward addressing this challenge, we examine the capability of the LES method with an eddy viscosity subgrid scale (SGS) model to predict the collision-related statistics such as the particle radial distribution function at contact, the radial relative velocity at contact, and the collision rate for a wide range of particle Stokes numbers. Data from direct numerical simulation (DNS) are used as a benchmark to evaluate the LES using both a priori and a posteriori tests. It is shown that, without the SGS motions, LES cannot accurately predict the particle-pair statistics for heavy particles with small and intermediate Stokes numbers, and a large relative error in collision rate (up to 60%) may arise when the particle Stokes number is near StK = 0.5. The errors from the filtering operation and the SGS model are evaluated separately using the filtered-DNS (FDNS) and LES flow fields. The errors increase with the filter width and have nonmonotonic variations with the particle Stokes numbers. It is concluded that the error due to filtering dominates the overall error in LES for most particle Stokes numbers. It is found that the overall collision rate can be reasonably predicted by both FDNS and LES for StK>3. Our analysis suggests that, for StK<3, a particle SGS model must include the effects of SGS motions on the turbulent collision of heavy particles. The spectral analysis of the concentration fields of the particles with different Stokes numbers further demonstrates the important effects of the small-scale motions on the preferential concentration of the particles with small Stokes numbers.
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47.55.Kf Particle-laden flows
47.27.ek Direct numerical simulations
47.27.em Eddy-viscosity closures; Reynolds stress modeling
47.27.ep Large-eddy simulations
47.27.er Spectral methods
66.20.Cy Theory and modeling of viscosity and rheological properties, including computer simulation
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