Reynolds number effect on the velocity derivative flatness factor

International audienceThis paper investigates the effect of a finite Reynolds number (FRN) on the flatness factor (F) of the velocity derivative in decaying homogeneous isotropic turbulence by applying the eddy damped quasi-normal Markovian (EDQNM) method to calculate all terms in an analytic expression for F (Djenidi et al., Phys. Fluids, vol. 29 (5), 2017b, 051702). These terms and hence F become constant when the Taylor microscale Reynolds number, Re λ exceeds approximately 10 4. For smaller values of Re λ , F, like the skewness −S, increases with Re λ ; this behaviour is in quantitative agreement with experimental and direct numerical simulation data. These results indicate that one must first ensure that Re λ is large enough for the FRN effect to be negligibly small before the hypotheses of Kolmogorov (Dokl

The Effects of Magnetic Field on the Fluid Flow through a Rotating Straight Duct with Large Aspect Ratio

AbstractThis paper presents a numerical study of an investigation of a fluid flow through a rotating rectangular straight duct in the presence of magnetic field. The straight duct of rectangular cross-section rotates at a constant angular velocity about the centre of the duct cross- section is same as the axis of the magnetic field along the positive direction in the stream wise direction of the flows. Numerical calculation is based on the Magneto hydrodynamics incompressible viscous steady fluid model whereas Spectral method is applied as a main tool. Flow depends on the Magnetic parameter, Dean number and Taylor number. One of the interesting phenomena of the fluid flow is the solution curve and the flow structures in case of rotation of the duct axis. The calculation are carried out for 5≤Mg≤50000, 50≤Tr≤100000, Dn=500, 1000, 1500 and 2000 where the aspect ratio 3.0. The maximum axial flow will be shifted to the centre from the wall and turn into the ring shape under the effects of high magnetic parameter and large Taylor number whereas the fluid particles strength is weak

Diderot, l’Encyclopédie & autres études, sillages de Jacques Proust.

Pour tous les spécialistes de Diderot, pour tous les dix-huitiémistes mais aussi pour tous les universitaires, la figure de Jacques Proust (1926- 2005) conserve une présence singulière. En témoigne ce beau livre préparé par Marie Leca-Tsiomis qui réunit un ensemble d’évocations et d’études variées suscitées par le souvenir du grand professeur et du grand critique. Les cinq années écoulées depuis sa disparition permettent de donner à cet hommage quelque chose qui va plus loin que la piété et l..

Contribution a l'etude de couches limites sur parois rainurees

SIGLEINIST T 71616 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Lattice-Boltzmann simulation of grid-generated turbulence

The lattice-Boltzmann method (LBM) is used to carry out a direct numerical simulation (DNS) of grid-generated turbulence with the view to improve comparison between experimental and numerical results on approximate isotropic turbulence. The grid is made up of four by four floating flat square elements in an aligned arrangement. The Reynolds number based on the Taylor microscale is about 40 at a distance of 70 times the separation between the elements downstream of the grid; this value is comparable to that of many experiments. While the results compare relatively well with existing experimental data on grid turbulence (grid made up of bars), they highlight the importance of the mesh resolution of the simulation and computational domain size in the decay of turbulence. For example, while a power-law decay could be identified, at least over a short distance, its decay exponent proves to be difficult to determine with good accuracy. This points out the need for simulations (and perhaps experiments too) where all scales are properly solved before conclusions can be drawn

Structure of a turbulent crossbar near-wake studied by means of lattice Boltzmann simulation

The turbulent near-wake of a crossbar is investigated numerically with the lattice Boltzmann method (LBM). The crossbar is made up of two perpendicular square bars arranged in a biplane configuration and is included in the computational domain. The Reynolds number based on a bar diameter is about 1600. The numerical results are first tested against results of both particle image velocimetry (PIV) and laser Doppler velocimetry (LDV). The LBM data compare well with the PIV and LDV data. In particular, the LBM reproduces the generation of vortical structures at the crossbar as observed in the PIV data. The numerical results reveal the presence of intermittent lateral motions along the span of the two bars, yielding fingerlike structures. It is argued that these motions contribute to the formation of streamwise vortical structures just behind the crossbar.These streamwise structures interlace with lateral structures also generated at the crossbar. The region over which this activity takes place is about four diameters. Within this region, the turbulent kinetic energy at the crossbar centerline increases and reaches a maximum at a distance of about three diameters. As the downstream distance increases, the individual wakes merge to form a single wake with features, for x/D≥20, similar to those observed in grid-generated turbulence

Velocity and passive scalar characteristics in a round jet with grids at the nozzle exit

Velocity and passive scalar (temperature) measurements have been made in the near field of a round jet with and without obstructing grids placed at the jet exit. The Reynolds number ReD (based on the exit centreline velocity and nozzle diameter) is 4.9 × 104 and the flow is incompressible, while the temperature rise does not affect the velocity behaviour. The streamwise development and radial spreading of the passive scalar are attenuated, relative to the unobstructed jet. Close to the jet outlet, the spatial similarity of the moments (up to the third-order) of velocity fluctuations is improved, when the jet is perturbed. An explanation, based on the reduced effect of the large coherent structures in the developing region, is provided.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Investigation of flow around a pair of side-by-side square cylinders using the lattice Boltzmann method

The low-Reynolds number flow around two square cylinders placed side-by-side is investigated using the lattice Boltzmann method (LBM). The effects of the gap ratio s/d (s is the separation between the cylinders and d is the characteristic dimension) on the flow are studied. These simulations reveal the existence of regimes with either synchronized or non-synchronized vortex-shedding, with transition occurring at s/d ≈ 2, which is larger than for circular cylinders. Detailed results are presented at Re = 73 for s/d = 2.5 and 0.7 corresponding to the synchronized and flip-flop regimes, respectively. Vortex-shedding from the cylinder occurs either in-phase or in-antiphase in the synchronized regime. However, linear stochastic estimate (LSE) calculations show that in-phase locking is the predominant mode. LSE is also employed to educe the underlying modes in the flip-flop regime, where evidence for both in-phase and anti-phase locked vortices is found, indicating that this regime is in a quasi-stable state between these two modes. The merging of the wakes, which is gradual for the synchronized regime, occurs rapidly in the flip-flop regime. The mean pressure on the upstream surface is symmetric and asymmetric for the synchronized and flip-flop regimes, respectively. Differences in results between the two regimes are interpreted in terms of the interaction of the jet formed between the cylinders with the adjoining wakes, the strength of this interaction depending on the spacing.© Elsevie

On the outer layer controversy for a turbulent boundary layer over a rough wall

An impressive number of experimental and numerical studies of turbulent flows over rough walls has appeared over the last 20 years. Much useful information has been obtained in terms of the turbulence structure both near the roughness canopy and in the outer flow region. However, the issue of whether or not the outer region of the boundary layer is affected by the nature of the wall has yet to be resolved satisfactorily. While the available data, mostly at sufficiently large values of the Reynolds numbers and δ/k (δ and k are the boundary layer thickness and characteristic roughness height, respectively), seem to suggest that 3D and transverse 2D rough surfaces may affect the outer layer differently, this suggestion can only be tested rigorously once the measurement of the wall-normal velocity fluctuation over the transverse 2D roughness is improved. With the benefit of the channel flow DNS data, it is argued that, for this latter surface type, the wall shear stress, as inferred from the form drag or drag balance, has actually been measured reasonably accurately in the past