Turbulent Separated Flow over a Ramp: A Numerical Investigation
Final technical rept. Nov 1997-Mar 2001
ARIZONA STATE UNIV TEMPE DEPT OF MECHANICAL AND AEROSPACE ENGINEERING
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Large Eddy Simulation LES and Reynolds-averaged Navier-Stokes RANS calculations have been performed of the turbulent flow over a smoothly-contoured ramp. The upstream conditions are prescribed as canonical turbulent boundary layer, obtained from a separate pre-computation. The flow in the primary calculation develops downstream, first experiencing an increase in the surface skin friction due to ramp curvature, then exhibiting a shallow separation with subsequent reattachment on a flat section downstream. The computational domain includes the downstream recovery region, enabling an assessment of model predictions of the recovery process. The turbulence models implemented are the Spalart-Allmaras one-equation model, referred to as S-A throughout Spalart and Allmaras 1994, V2F Durbin 1991, and a modified version, referred to as M2VF Lien and Durbin 1996. Visualizations of the instantaneous flow shows that the separation location fluctuates at a high frequency than the reattachment location, with the instantaneous reattachment trajectory giving indications of short shedding events. The mean velocities collapse to the universal log law at about four ramp lengths downstream of the beginning of the curved section whereas the turbulence quantities recover more slowly. The recovery process is not monotonic in that it is governed by decay of the outer layer perturbations and initiation and growth of inner layer following attachment.
- Fluid Mechanics