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An essential feature of the interaction phenomena between the compression shock and the boundary layer is the coexistence of parallel streams of subsonic and supersonic velocities. The compression disturbance of the shock is propagated *'ahead" of the shock through the subsonic portion of the boundary layer, and "softening" of the shock takes place. It is the purpose of the present paper to demonstrate definitely this essential feature of boundary layer-shock interaction by using a simplified model. The boundary layer is replaced by a uniform subsonic stream of finite width, bounded on one side by the solid wall and on the other side by the interface with the uniform supersonic stream of infinite extent. The flow fields of two cases are analyzed in detail: (a) a compression wave in the supersonic stream incident upon the subsonic layer, and (b) outgoing compression waves generated by a sudden change in the slope of the solid wall. Both types of disturbances are assumed to be small, so that linearization of the differential equations is possible. A general result is that the distance of upstream propagation is directly proportional to the width of the subsonic layer. Therefore, when the boundary layer is turbulent and the subsonic layer is extremely thin, there is negligible softening of the shock.

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