Boundary Layer Receptivity to Unsteady Free-Stream Pressure Gradients
It has been speculated that boundary layer receptivity to free-stream disturbance fields can be the result of distributed mechanisms. These distributed mechanisms cause the receptivity to take place over streamwise distances of several Tollmien-Schlichting wavelengths, due to the interaction of the streamwise modulation of the free-stream disturbance and the otherwise parallel boundary layer flow. This is distinct to the usual localized receptivity mechanism which result from the interaction of the long wave-length free-stream disturbance with a short scale (local) surface irregular. Linear triple-deck theory is used to model the recent experiments of Nishioka and Morkovin (1986) and Kendall (1987) which are often viewed as supporting the distributed receptivity concepts. These experiments are modeled by introducing unsteady sources into the upper deck of the triple-deck structure to appropriately account for the effects of the unsteady pressure gradients in the free-stream. It is shown that this analysis reproduces the essential features of the experimental results. The structure of the solution suggests that distributed mechanisms can only cause exponentially small receptivity levels.
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