In this paper, the eddy-damped quasi-normal Markovian closure is used to study the behavior of the scalar flux spectrum in isotropic turbulence as the Reynolds number Re varies in a range between 30 and 10 7 . The different contributions to the evolution equation of the scalar flux spectrum are studied. One-dimensional spectra are in good agreement with direct numerical simulation ͑DNS͒ and experiments at moderate Re . The closure shows that at high Reynolds numbers, a K −7/3 scaling is found for the scalar flux spectrum, in agreement with Lumley's prediction ͓Phys. Fluids 10, 855 ͑1967͔͒, but enormous Re are needed before it can be clearly observed. In the range of wind tunnel experiments, the spectral exponent for the scalar flux is closer to −2 in agreement with existing measurements ͓Mydlarski and Warhaft, J. Fluid Mech. 358, 135 ͑1998͔͒. The results for the molecular dissipation of scalar flux are in agreement with the DNS results of Overholt and Pope ͓Phys. Fluids A 8, 3128 ͑1996͔͒. The large Re behavior of this quantity is also addressed.

The velocity-scalar cross spectrum (or scalar flux spectrum) is generally presumed to have a K −7/3 wavenumber dependence in the inertial range, in agreement with the dimensional analysis proposed by Lumley [Phys. Fluids 10, 855 (1967)]. Such a behavior is, however, clearly not observed in experiments in which spectra closer to K −2 (or even less steep) are generally found. It is shown in the paper that dimensional analysis is compatible with a K −2 scaling if a spectral flux of the velocity-scalar cross correlation is introduced. An analysis of the different terms in the equation of the scalar flux spectrum shows that two nonlinear contributions can be identified: a transfer term and the pressure contribution. Direct numerical simulations and large eddy simulation calculations are performed to obtain spectral information about the scalar flux spectrum and to analyze some key properties of the associated nonlinear transfer and pressure terms.

No abstract

In order to properly address the simulation of complex (weakly compressible) turbulent flows, the lattice Boltzmann method, originally designed for uniform structured grids, needs to be extended to composite multi-domain grids displaying various levels of spatial resolution. Therefore, physical conditions must be specified to determine the mapping of statistical information (about the populations of moving particles) at the interface between two domains of different resolutions. It is here argued that these conditions can express quite simply in terms of the probability distributions of the underlying discrete-velocity Boltzmann equation. Namely, the continuity of the mass density and fluid momentum is fulfilled by imposing the continuity of the equilibrium part of these distributions, whereas the discontinuity of the rate-of-strain tensor is ensured by applying a "spatial transformation" to the collision term of the discrete-velocity Boltzmann equation. This latter condition allows us to explicitly account for the subgrid-scale modeling in the treatment of resolution changes. Test computations of a turbulent plane-channel flow have been considered. The lattice Boltzmann scheme relies on the standard D3Q19 lattice in a cell-vertex representation, and uses the BGK approximation for the collision term. A shear-improved Smagorinsky viscosity is used for the subgrid-scale modeling. In a quasi-Direct Numerical Simulation at Re τ = 180 (with two levels of resolution) the results are found in excellent agreement with reference data obtained by a highresolution pseudo-spectral simulation. In a Large-Eddy Simulation at Re τ = 395 (with three levels of resolution) the results compare very well with high-resolution reference data. The accuracy is improved in comparison with a large-eddy simulation based on finite-volume discretization with the same subgrid-scale viscosity model and comparable grid resolution. This study demonstrates the good capabilities of the lattice Boltzmann method to handle both Direct and Large-Eddy Simulations of turbulent flows with grid resolutions comparable to those commonly used in simulations based on standard discretization methods, e.g. pseudo-spectral or finite-volume methods.

Purpose-The lattice Boltzmann (LB) method offers an alternative to conventional computational fluid dynamics (CFD) methods. However, its practical use for complex turbulent flows of engineering interest is still at an early stage. In this article, a LB wallmodeled large-eddy simulation (WMLES) solver is outlined. The flow past a rod-airfoil tandem in the sub-critical turbulent regime is examined as a challenging benchmark. Design/methodology/approach-Fluid dynamics are discretized upon the LB principles. The large-eddy simulation is accounted straightforwardly by including a modeled subgrid-scale viscosity in the LB scheme, whereas a wall-law model enforces the boundary condition at the first off-wall node. This physical modeling is briefly introduced and relevant references are given for details. The flow past a rod-airfoil tandem at Reynolds number Re = 4.8 × 10 4 and Mach number Ma 0.2 is simulated on a composite multiresolution grid; the numerical setup is detailed. Unsteady aerodynamic and aeroacoustic features including spectral analysis and far-field pressure fluctuations are discussed. Findings-Extensive quantitative comparisons with both experimental and numerical reference data indicate that aerodynamic and aeroacoustic features are well captured by the LB simulation. Originality/value-Our study shows that WMLES within the LB framework provides a workable and efficient alternative to Navier-Stokes CFD solvers in the context of complex turbulent flows. The LB method permits to access an attractive turnaround time while 1 preserving engineering accuracy.

No abstract

No abstract

Lattice-Boltzmann simulations of corner separation flow in a compressor cascade are presented. The lattice Boltzmann approach is rather new in the context of turbomachinery and the configuration is known to be particularly challenging for turbulence modelling. The present methodology is characterized by a quasi-autonomous meshing strategy and a limited computational cost (a net ratio of 5 compared to a previous finite-volume compressible Navier-Stokes simulation). The simulation of the reference case (4° incidence) shows a good agreement with the experimental data concerning the wall pressure distribution or the distribution of losses. A good description is also obtained when incidence angle is increased to 7°, with a span-wise development of the separation. Subsequently, the methodology is used to investigate the sensitivity of the flow to the end-wall boundary-layer thickness. A thinner boundary-layer results in a smaller corner separation, but not a complete elimination. Finally, the ingredients of the wall modelling are analysed in details. On the one hand, the curvature correction term promotes transition to turbulence on the blade suction side and avoids a spurious separation. On the other hand, the addition of the pressure-gradient correction term allows a wider and more realistic corner separation.