NUMERICAL SIMULATIONS OF NON-EQUILIBRIUM TURBULENT BOUNDARY LAYER FLOWING OVER A BUMP

Largeeddy simulation (LES) and Reynolds-averaged Navier-Stokes simulations (RANS) with
different turbulence models (including the standard k - E , the standard k - 0, the shear stress
transport k - 0 (SST- k - w model) and Spalart-Allmaras (S-A)-turbulence models) have been
employed to compute the turbulent flow of a two-dimensional turbulent boundary Iayer over an
unswept bump. The predictions of the simulations were compared to available experimental
measurements in the literature. The comparisons of the LES and the SST- k - w model including
the mean flow and turbulence stresses are in satisfied agreements with the available measurements.
Though the flow experiences a strong adverse pressure gradient along the rear surface, the
boundary layer is unique in that intennittent detachment occurring near the wall. The numerical
results indicate that the boundary layer is not followed by mean-flow separation or incipient
separation as that shown from the numerical results. The resolved turbulent shear stress is in a
reasonable agreement with the experimental data, though the computational result of LES shows
that its peak is over-predicted near the trailing edge of the bump, whiIe the other used turbulence
models, except the standard k - & , under-predicts it. Analysis of the numerical results from LES
confirms the experimental data, in which the existence of intemal layers over the bump surface
upstream of the summit and along the downstream flat plate. It also demonstrates that the quasi￾step increase in skin fiiction is due to perturbations in pressure gradient. The surface curvature
enhances the near-wall shear production of turbulent stresses and is responsible for the formation
of the internal layers.
The present investigation also explains the capability of the used RANS turbulence models to
capture the driving mechanism for the surprisingly rapid return to equilibrium boundary layer over
the trailing flat plate found in the measurements.