Hybrid LES-RANS: Inlet Boundary Conditions

Paper in proceeding, 2005

The paper evaluates a method to prescribe synthesized turbulent inlet boundary conditions. When doing LES, DES or hybrid LES-RANS a precursor channel DNS is often used. The disadvantage of this method is that it is difficult to re-scale the DNS fluctuations to higher Reynolds numbers. In the present work synthesized isotropic turbulent fluctuations are generated at the inlet plane with a prescribed turbulent length scale and energy spectrum. A large number of independent realizations are generated. A time correlation at time step m is introduced through a linear interpolation of the running average (time step 1 to m-1) of the inlet profiles and realization number m. In this way an autocorrelation is prescribed. The method is validated for hybrid LES-RANS of channel flow at Re_tau=2000 on a coarse mesh using different length and time scales of the inlet fluctuations. It is found that the time scale is more important than the length scale. Furthermore it is found that inlet time and length scales should not be equal to the correct, physical values, but should be related to the grid. The inlet boundary conditions have much in common with forcing fluctuations at the interface in hybrid LES-RANS. In both methods the object is to trig the equations into describing turbulence. The method to generate inlet boundary conditions is also relevant in embedded LES, where LES is used on a mesh embedded in a global steady or unsteady RANS computation.