Hybrid LES/RANS Using Synthesized Turbulence for Forcing at the Interface

Paper in proceeding, 2004

The main bottle neck for using Large Eddy Simulations at high Reynolds number is the requirement of very fine meshes near walls. Hybrid LES-RANS was invented to get rid of this limitation. In this method unsteady RANS (URANS) is used near walls and away from walls LES is used. The matching between URANS and LES takes place in the inner log-region. In the present paper a method to improve standard LES-RANS is evaluated. The improvement consists of adding instantaneous turbulent fluctuations (forcing conditions) at the matching plane between the LES and URANS regions in order to provide the equations in the LES region with relevant turbulent structures. The turbulent fluctuations are taken from synthesized homogeneous turbulence assuming a modified von Karman spectrum. Both isotropic and non-isotropic fluctuations are generated. The turbulent length scale k^1.5/eps and the turbulent kinetic energy are used as input and they are taken from time-averaged DNS data at the location of the interface plane (y^+=60). The on-isotropic fluctuations are generated in the principal coordinate system of the DNS Reynolds stress tensor. They are then scaled to fit the DNS Reynolds tensor in the principal coordinate system. After that they are transformed back to the original coordinate system. The new approach is applied to fully developed channel flow. It is found that the prescribed turbulent length scale of the synthesized turbulence has a large effect on the predicted results. As an alternative to changing the prescribed turbulent length scale, the turbulent length scale of the synthesized fluctuations can artificially be increased in the streamwise direction by changing the mapping of the fluctuations to the interface plane in the CFD domain. Good results are obtained when the turbulent length scale is increased by a factor of four, either through the prescribed turbulent length scale or through the mapping. Very similar results are obtained when replacing the synthesized fluctuations with instantaneous DNS fluctuations (a time series). Also in this case better results are obtained when the streamwise turbulent length scale is enhanced by a factor of four by changing the mapping (i.e. the Taylor hypothesis) of the DNS fluctuations from the time domain to the interface plane.