Abstract
The ine uence 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 ishereby modeled within theCartesian grid by assigning body forces oversurfacesthat need notcoincidewith grid lines.Comparedtotheuseofabody-e ttedcoordinatesystem,thismethodsavesmemoryandcomputationtime.The method is validated by grid ree nement tests as well as by a comparison with water channel experiments. The DNS results for the steady e ow over a rectangular hump, as well as for an instability wave traveling over a hump, show good agreementwith theexperimental ones. Simulation resultsshow thata localized hump destabilizesthelaminar 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 ine uence on the stability characteristics of the boundary layer is clearly the height of the localized hump.
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