  The turbulent boundary layer over a Gaussian shaped bump is computed by
direct numerical simulation (DNS) of the incompressible Navier-Stokes
equations. The two-dimensional bump causes a series of strong pressure
gradients alternating in rapid succession. At the inflow, the momentum
thickness Reynolds number is approximately 1,000 and the boundary layer
thickness is 1/8 of the bump height. DNS results show that the strong favorable
pressure gradient (FPG) causes the boundary layer to enter a relaminarization
process. The near-wall turbulence is significantly weakened and becomes
intermittent, however relaminarization does not complete. The streamwise
velocity profiles deviate above the standard logarithmic law and the Reynolds
shear stress is reduced. The strong acceleration also suppresses the wall-shear
normalized turbulent kinetic energy production rate. At the bump peak, where
the FPG switches to an adverse gradient (APG), the near-wall turbulence is
suddenly enhanced through a partial retransition process. This results in a new
highly energized internal layer which is more resilient to the strong APG and
only produces incipient flow separation on the downstream side. In the strong
FPG and APG regions, the inner and outer layers become largely independent of
each other. The near-wall region responds to the pressure gradients and
determines the skin friction. The outer layer behaves similarly to a free-shear
layer subject to pressure gradients and mean streamline curvature effects.
Results from a RANS simulation of the bump are also discussed and clearly show
the lack of predictive capacity of the near-wall pressure gradient effects on
the mean flow.
