A typical turbulent flow produces skin-friction and heat-transfer levels that are an order of magnitude higher than that corresponding to laminar flow. In flight, control of skin friction is important for energy-efficient operation. In gas turbine engines, prediction of the onset of turbulent flow is critical for prediction of the heat transfer on turbine blades. Gas-turbine flows are characterized by high-amplitude freestream noise and turbulence in contrast to the flight environment, which typically has very low turbulence levels. It is not clear how to define these disturbances except to say that they can be decomposed to a vortical part (turbulence) and an irrotational part (noise). Disturbances in the freestream enter the boundary layer as small fluctuations of the basic state and excite unstable modes. The receptivity stage of the transition process is the least understood, but is extremely important because it provides the initial condition on the disturbance amplitude. Here we consider disturbances consisting of only plane acoustic waves. We ignore freestream turbulence and artificial disturbances within the boundary layer. Moreover, the main objective of this experiment is to isolate the influence of the leading edge on the initial amplitudes of T-S waves and to determine the limit of linear receptivity for 2-D roughness. The goal is to establish the framework for the active control of such fluid motions and to provide the initial conditions for computational and analytical modeling.
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