Unstructured Large-Eddy Simulation Code for Simulation of Reacting Flows in Complex Geometries
K. Mahesh, G. Constantinescu, S. Apte, G. Iaccarino, P. Moin
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A large-eddy simulation (LES) unstructured mesh code for high-fidelity simulation of turbulent reacting flows has been developed. The code is designed to run on massively parallel supercomputers. It can handle complex geometries and is being used to compute the flow and associated combustion phenom-ena in an industrial gas turbine combustor in collaboration with Pratt and Whitney (P&W). LES is chosen because of its demonstrated superiority in predicting turbulent mixing over Reynolds-averaged Navier-Stokes (RANS) formulation. Accurate simulations of chemically reacting flows are critically dependent on the ability to accurately simulate turbulent mixing.
The numerical algorithm allows for the use of hybrid grids. It is a conservative non-dissipative formulation--second-order accurate on uniform grids. The energy-conserving properties of the algorithm allow us to obtain a robust method without the need to introduce numerical dissipation, as is generally done in RANS codes. Keeping the numerical dissipation at very low levels is essential to maintaining the accuracy of LES simulations. The code solves the incompressible flow equations or the low-Mach-number variable-density equations, the latter being used in simulations of reacting flows.
The dynamic LES model developed at the Center for Turbulence Research (CTR) is used to represent the subgrid stresses. The dynamic formulation offers many advantages when used in unstructured grid formulations, including dynamically computing the model coefficient (no empirical constants), eliminating the need for damping functions near solid surfaces, and eliminating the need for computing the dis-tance to the wall.
Considerable effort was devoted to making the design of the code efficient. The code is fully parallel and uses Message Passing Interface. A novel algorithm was developed for grid-ordering, with the aim of minimizing processsor-to-processor communication. The code was ported to several platforms (e.g., Origin2000, IBM SP2, ASCI RED-Intel) and was shown to scale well for computations using hundreds of processors, provided that the grid partition is such that each processor partition contains at least 5,000 nodes.
Validation simulations in the combustor geometry (a 1/18-combustor sector corresponding to one injector) provided by P&W are under way (see fig. 1). In particular, a special P&W cold-flow combustor rig, for which detailed experimental data are available, will serve as the key validation test for complex geometries.
Point of Contact: Parviz Moin
(650) 604-5127
moin@rft33.nas.nasa.gov
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Fig. 1. Section through the PW6000 combustor (plane cutting through the injector symmetry plane). The simulation shows a 20-degree sector of the combustor that corresponds to one injector. Periodic boundary conditions are used on the lateral walls. Reynolds number based on the diffuser inlet section is Re = 36,000. The figure shows contours of velocity magnitude. The color scale is 0 (white) to 400 feet per second (dark blue), 40 contours. The results are from a 1.2-million node simulation. The grid is hybrid combination of tets, hexes, and pyramids.
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