Hybrid RANS LES simulation of non-equilibrium boundary layers

Abstract

Hybrid simulations that couple the solution of the Reynolds-Averaged Navier-Stokes equations (RANS) to Large-Eddy Simulations (LES) have the ability to apply the high accuracy of LES only in regions of the flow that demand it, while using the less expensive RANS approach in regions of the flow where standard turbulence models are expected to be accurate, while LES is used in non-equilibrium flow regions. One issue that arises in these applications is the behavior of the flow in the transition zone between the RANS and LES regions. In the RANS zone the flow solution is either steady, or only contains information on the largest scales of motion; most or all of the Reynolds shear stress is provided by the turbulence model. In the LES region the resolved scales must supply most of the Reynolds shear stress. Typically, a transition zone exists in which the resolved, energy-containing eddies are gradually generated and grow.

In this work, methodologies for the improvement of hybrid LES/RANS are studied, to shorten the transition from the smooth RANS field to the LES, which requires energy- and momentum-supporting eddies. The method tested is based on the generation of synthetic turbulence with a realistic spectrum, and statistics obtained from the RANS. The eddies thus generated are then selectively forced by a control method that amplifies the bursts, and maintains a desired Reynolds shear stress downstream of the RANS/LES interface. This method allows to match the two techniques smoothly, and to minimize the extent of the region required to develop the realistic turbulent eddies. It was found to perform well in several non-equilibrium boundary layers achieved by imposing either a variable freestream velocity or a spanwise pressure gradient on a flat-plate boundary layer. A finely resolved LES of the flow in a boundary layer subjected to strong acceleration was also performed. The flow in this configuration reverts to a laminar state and then retransitions to turbulence. Statistics and flow visualizations of this flow indicate the presence of two of the mechanisms that have been conjectured to cause the re-laminarization.