We use numerical and asymptotic techniques to study the stability ofatwophase air/water owabovea at porous plate. This flow is a model of the boundary layer which forms on a yawed cylinder and can be used as a useful approximation to the air flow over swept wings during heavy rainfall. We show that the interface between the water and air layers can significantly destabilize the flow, leading to traveling wave disturbances whichmovealong the attachment line. This instability occurs for lower Reynolds numbers than is the case in the absence of a water layer. We also investigate the instability ofinviscid stationary modes. We calculate the effective wavenumber and orientation of the stationary disturbance when the fluids have identical physical properties. Using perturbation methods we obtain corrections due to a small stratification in viscosity, thus quantifying the interfacial effects. Our analytical results are in agreement with the numerical solution which we obtain for arbitrary uid properties
