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A laser Doppler velocimeter was used to measure the stream wise velocity component in unsteady separated flowfields in a two-dimensional transonic diffuser with and without externally induced periodic fluctuations. Results are also presented for fully attached, unsteady, unexcited flow. The time-dependent perturbation flowfield associated with the external excitation was determined throughout the subsonic region for a case with shock-induced separation. The results indicate that a transverse, wavelike motion of the boundary-layer/coreflow interface is an important aspect of the overall oscillations and overwhelms effects associated with acousticwave propagation. NomenclatureA = amplitude coefficient in Fourier series / = excitation frequency h = channel height M = Mach number TV = number of velocity samples T = temperature u = mean streamwise velocity u = rms of streamwise fluctuating velocity component u = ensemble-averaged streamwise velocity u = periodic component of ensemble-averaged streamwise velocity x = dimensionless streamwise coordinate; x/h*, x-Q at throat y = dimensionless vertical coordinate; y/h*, y = 0 at lower wall a. = perturbation phase angle 5* = dimensionless displacement thickness, d*/h * c/> == phase angle in Fourier series § = dimensionless momentum thickness, B/h * Subscripts e -exit ( -lower wall m = midstream (y = 0.432) n -Fourier harmonic o = plenum r -reference s = static t = total u = upper wall o =shock * =throat

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