Direct numerical simulation of flow separation behind a swept, rearward-facing step at Re-H=3000

The effect of sweep on a transitional separation bubble behind a backward-f acing step is investigated using direct numerical simulation (DNS). The Rey nolds number based on step height and free stream component normal to the s tep, Re-alpha=(HCinfinity cos alpha)/nu, is kept constant at 3000 for sweep angles alpha between 0 degrees and 60 degrees. Results agree well with two experimental investigations. Up to alpha=40 degrees, the mean flow follows the sweep-independence principle. For higher sweep angles, the size of the separated flow region is shortened considerably which is mainly due to an upstream shift of transition in the laminar shear layer emanating from the step and-to a lesser degree-to an increase in turbulent momentum flux <(uv) over bar>. Fluctuations of wall-stress and wall pressure are largest near t he reattachment location and their magnitude scales with (C-infinity cos al pha)(2) for all sweep angles. Transition is dominated by a Kelvin-Helmholtz -type instability of the free shear layer. The influence of skewing on the growth rate of instability waves is weak in accordance with linear stabilit y theory predictions of Lu and Lele [J. Fluid Mech. 249, 441 (1993)]. The u pstream shift of transition for alpha greater than or equal to 50 degrees i s related to the way how disturbances spread sidewise in a wedge-shaped reg ion inside of the shear layer. (C) 2000 American Institute of Physics. [S10 70-6631(00)50309-2].