Interface Conditions for Hybrid RANS/LES Calculations

Abstract

This chapter explains interface conditions for hybrid Reynolds averaged Navier–Stokes (RANS) /LES calculations. Hybrid RANS/LES methods, in which the attached boundary layers are simulated by RANS while the non-equilibrium regions of the flow are computed by LES. One issue that may affect the accuracy of the results in hybrid methods is the generation of turbulent eddies capable of supporting the Reynolds stresses in the LES region from a smooth RANS field (in which the Reynolds stress is due entirely to the model). The chapter provides results obtained in spatially developing turbulent channel flow in which synthetic turbulence is coupled with a controlled forcing technique that generates physically realistic turbulent eddies. This technique substantially reduces the distance required for the development of realistic turbulence. The effect of the free parameters used in this technique, and their optimization are discussed. Simulations are also performed that evaluate the effect of the approximations required when the full Reynolds-stress tensor is not known, and only statistics typically available from RANS simulations (the mean velocity and the Reynolds shear stress) are available. The effect of the quality of the synthetic turbulence at the inflow plane and the overall computational cost is also discussed. Finally, results from a simulation of an accelerating boundary layer are given.