Investigation of the role of transient growth in roughness-induced transition

Surface roughness can have a profound effect on boundary layer transition. However, the mechanisms responsible for transition with three-dimensional distributed roughness have been elusive. Various T-S based mechanisms have been investigated in the past but have been shown not to be applicable. During the past year, the applicability of transient growth theory to roughness induced transition has been studied. A model for roughness-induced transition is developed that makes use of computational results based on the spatial transient growth theory pioneered by the present authors. For zero pressure gradient flows, the resulting transition relation is reminiscent of the transition correlations found for slender hypersonic vehicles. For nosetip transition, the resulting transition relations reproduce the trends of the Reda and PANT data and carefully account for the separate roles of roughness and surface temperature level on the transition behavior.