Transition Downstream of an Isolated Cylindrical Roughness Element on a Flat Plate Boundary Layer

In this experimental study, the effects of an isolated, cylindrical roughness element on laminar-turbulent transition is investigated and compared to three-dimensional, linear stability theory. Experiments have been done at both sub- and supercritical Reynolds numbers. In the subcritical case, controlled forcing was applied to show that the roughness amplifies unsteady disturbances which can grow large enough to trip the boundary layer. At a higher, but still subcritical Reynolds number, natural disturbances suffice to trip the boundary layer intermittently. Further increase of the Reynolds numbers leads to permanent transition. This transition Reynolds number and dominant frequencies are compared to the critical Reynolds number from theory to provide experimental evidence of a global instability.