GPU-Accelerated Large-Eddy Simulation of Turbulent Channel Flows

High performance computing clusters that are augmented with cost and power efficient graphics processing unit (GPU) provide new opportunities to broaden the use of large-eddy simulation technique to study high Reynolds number turbulent flows in fluids engineering applications. In this paper, we extend our earlier work on multi-GPU acceleration of an incompressible Navier-Stokes solver to include a large-eddy simulation (LES) capability. In particular, we implement the Lagrangian dynamic subgrid scale model and compare our results against existing direct numerical simulation (DNS) data of a turbulent channel flow at Reτ = 180. Overall, our LES results match fairly well with the DNS data. Our results show that the Reτ = 180 case can be entirely simulated on a single GPU, whereas higher Reynolds cases can benefit from a GPU cluster

Small-Scale Dissipation in Supercritical, Transitional Mixing Layers

The dissipation and small-scale dissipation is calculated for transitional states obtained elsewhere from Direct Numerical Simulations (DNS) of temporal, supercritical mixing layers for two species systems, O₂/H₂ and C₇H₁₆/N₂, so as to understand their species-independent and species-dependent aspects. The effect of filter size on the results was also investigated, with filtering exclusively performed in the dissipation regime of the energy spectrum. Both domain-average dissipation and the small-scale dissipation were analyzed in terms of the three mode contributions to them due to the viscous, heat and species-mass fluxes. The species-mass flux originated contribution dominates both the dissipation and the small-scale dissipation for all simulations and its percentage of the total dissipation or of the small-scale dissipation varies in a very small range across the species system, the initial Reynolds number and the perturbation wavelength used to excite the layer. For a filter size that is four times the DNS grid size, the proportion of each small-scale dissipation mode in the total small-scale dissipation is similar to that obtained at the DNS scale, indicating a scale similarity. It was also found that the percentage of total small-scale dissipation in the total DNS dissipation is only species-system and filter size dependent but nearly independent of the initial conditions. With filter size increase, the increase in the small-scale dissipation portion of the DNS dissipation has similar functional variation for both species systems, although the fraction reached by C₇H₁₆/N₂ layers is much larger than for O₂/H₂ ones. Normalization by the results obtained at the smallest filter size led to highlighting several aspects that are only species-system dependent with increasing the filter size. Backscatter was shown to occur over a substantial percentage of the computational domain, and its magnitude was found to be a substantial fraction of the positive small-scale dissipation. A four fold increase in filter size decreased the spatial extent of backscatter by only at most 32%, 13% and 7.5% for the viscous, heat and species-mass flux originated modes. The implications or these results for Larger Eddy Simulation modeling are discussed

An a Posteriori Study of a DNS Database Describing Supercritical Binary-Species Mixing

Large Eddy Simulation (LES) a posteriori study is conducted for a mixing layer which initially contains different species in the lower and upper streams, and where the initial pressure is larger than the critical pressure of either species. An initially imposed-vorticity perturbation promotes roll-up and a double pairing of four initial spanwise vortices to reach a transitional state. The LES equations consist of the differential conservation equations coupled with a real-gas equation of state, and the equation set utilizes transport properties depending on the thermodynamic variables. Unlike all LES models to date, the differential equations contain, additional to the Subgrid Scale (SGS) fluxes, a new SGS term which is a pressure correction in the momentum equation. This additional term results from altering of the Direct Numerical Simulation (DNS) equations and represents the gradient of the difference between the filtered pressure and the pressure computed from the filtered flow field. A previous a priori analysis, using a DNS database for the same configuration, found this term to be of leading order in the momentum equation, a fact traced to the existence of high density-gradient magnitude regions that populated the entire flow; in that study, models were proposed for the SGS fluxes as well as this new term. In the present study, the previously-proposed constant-coefficient SGS-flux models of the a priori investigation are tested a posteriori in LES devoid or including the SGS pressure correction term. The present pressure-correction model is different from, and more accurate and less computationally intensive than that of the a priori study. The constant-coefficient SGS-flux models encompass the Smagorinsky (SMC), in conjunction with the Yoshizawa (YO) model for the trace, the Gradient (GRC) and the Scale Similarity (SSC) models, all exercised with the a priori study constant coefficients calibrated at the transitional state. The LES comparison is performed with the fitered-and-coarsened (FC) DNS which represents an ideal LES solution. Expectably, when the LES model is devoid of SGS terms, it is shown to be considerably inferior to models containing SGS effects. Among models containing SGS effects, those including the pressure-correction term are substantially superior to those devoid of it. The sensitivity of the predictions to the initial conditions and grid size are also investigated