Abstract: The paper proposes a method to prescribe synthesized turbulent inlet boundary conditions. When making 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 correlation in time between these realization is introduced via an asymmetric, non-truncated time filter. In this way the turbulent time scale of the synthesized isotropic turbulent fluctuations is prescribed.
The method is first validated for DNS atIt is then employed in
hybrid LES-RANS of channel flow at on a coarse mesh. The sensitivity
to different prescribed inlet length scales, time scales and amplitudes of the
fluctuations is investigated.
The inlet boundary conditions have much in common with forcing fluctuations in the interface region in hybrid LES-RANS. In both cases the object is to trigger the equations to resolve turbulence. The method of generating 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.
