Title: “Recent DNS and LES Studies with Separation”
Speaker: Philippe Spalart, Boeing
Date: Friday, April 14, 2017
Location: NASA/LaRC, IESB Bldg. 2102, Reid 2
Sponsor/Hosts: Mujeeb Malik/Christopher Rumsey
Abstract: We use two presentations from 2016. In the first, the Bachalo-Johnson transonic bump, with shock-induced separation, is studied by Wall-Modeled Large-Eddy Simulation and by Direct Numerical Simulation, with grid counts in the billions. Disturbingly, two WMLES runs with widely different grid counts produced essentially the same answer, giving an indication of grid convergence, but with an inaccurate shock position and pressure distribution. Changes to the SGS and wall modeling failed to resolve the problem. A DNS in a reduced domain, on the other hand, produced close agreement with the experiment. The prospects for WMLES are discussed in that light. In the second study, DNS is used for a Couette-Poiseuille flow with parameters adjusted so that one wall has zero skin friction. Stratford’s theory then predicts a square-root layer, analogous to the logarithmic layer. Indeed, DNS over a range of 4 in Reynolds number produces the square-root layer, as well as “Townsend scaling” in the core of the channel and a long log layer along the other wall. In this academic problem, the success of Turbulence Theory is convincing, although again it does not extend to Reynolds stresses other than the shear stress. The value of this knowledge to spatially-separating flows and to turbulence modeling is considered.
Bio: Philippe Spalart studied Math and Engineering in Paris, and obtained an Aerospace PhD at Stanford/NASA-Ames in 1982. Still at Ames, he then conducted DNS of transitional and turbulent boundary layers. After moving to Boeing in 1990, he created the Spalart-Allmaras one-equation RANS model. He wrote a review article and co-holds a patent on airplane trailing vortices. In 1997 he proposed the Detached-Eddy Simulation approach, blending RANS and LES to address separated flows at high Reynolds numbers at a manageable cost. Recent work includes RANS modeling, jet and airframe noise, and theoretical contributions in classical aerodynamics.