Hybrid LES Approach for Practical Turbomachinery Flows: Part 1—Hierarchy and Example Simulations

Unlike Reynolds Averaged Navier Stokes (RANS) models which need calibration for different flow classes, LES (where larger turbulent structures are resolved by the grid and smaller modeled in a fashion reminiscent of RANS) offers the opportunity to resolve geometry dependent turbulence as found in complex internal flows — albeit at substantially higher computational cost. Based on the results for a broad range of studies involving different numerical schemes, LES models and grid topologies an LES hierarchy and hybrid LES related approach is proposed. With the latter, away from walls, no LES model is used, giving what can be termed Numerical LES (NLES). This is relatively computationally efficient and makes use of the dissipation present in practical industrial CFD programs. Near walls, RANS modeling is used to cover over numerous small structures, the LES resolution of which is generally intractable with current computational power. The linking of the RANS and NLES zones through a Hamilton-Jacobi equation is advocated. The RANS-NLES hybridization makes further sense for compressible flow solvers, where, as the Mach number tends to zero at walls, excessive dissipation can occur. The hybrid strategy is used to predict flow over a rib roughened surface and a jet impinging on a convex surface. These cases are important for blade cooling and show encouraging results. Further results are presented in a companion paper.