An experimental investigation for the aerodynamic characteristics of the incompressilbe two-dimensional turbulent separating and reattacting flow downstream of a backward-facing step with a rough bottom wall is presented. The distributions of boundary layer parameters, forward-flow fraction and turbulent stresses are measured and the reattachment position is determined for different rough wall conditions.
For accurate and reliable measurements, the newly developed split film sensor is employed.
It is demonstrated that the streamwise distribution of forward-flow fraction in the recirculation and reattachment region is given by a similarity carve independent of the roughness. The reattachment length is slightly affected by the roughness. It increases with increasing roughness. The mean velocity profiles and the integral parameters on the redeveloping boundary layer indicate that the redeveloping zone downstream of the reattachment can be divided into two regions: In the upstream recovery region the Clauser's shape factor is larger than that of the equilibrium boundary layer and the flow perturbation in the outer region of the redeveloping boundary layer is reduced rapidly, but in the downstream region the Clauser's shape factor is smaller and the recovery takes place slowly. The roughness has influence on the recovery process from the perturbed state through the reattaching shear layer in both recovery regions. On the whole, the turbulent structure on the rough surfaces approaches to that of the equilibrium turbulent boundary layer faster than that on the smooth surface, in the redeveloping region.