Humps/steps influence on stability characteristics of two-dimensional laminar boundary layer

The influence of two-dimensional humps and steps on the stability characteristics of a two-dimensional laminar boundary layer is investigated by means of direct numerical simulations (DNS). The localized surface irregularity is hereby modeled within the Cartesian grid by assigning body forces over surfaces that need not coincide with grid lines. Compared to the use of a body-fitted coordinate system, this method saves memory and computation time. The method is validated by grid refinement tests as well as by a comparison with water channel experiments. The DNS results for the steady flow over a rectangular hump, as well as for an instability wave traveling over a hump, show good agreement with the experimental ones. Simulation results show that a localized hump destabilizes the laminar boundary layer, whereas a forward facing step stabilizes it. The destabilization is stronger when the height or the width of the localized hump are increased. A rounded shape of the hump is less destabilizing than a rectangular shape with sharp corners. The parameter that shows the strongest influence on the stability characteristics of the boundary layer is clearly the height of the localized hump.