A two-dimensional subsonic air flow past a block mounted on a flat plate is investigated experimentally. The block is equivalent to a forwardbackward facing step pair. It is shown that although the length-to-height ratio is high (L/h=lQ), the flow separation at the backward facing step is strongly influenced by the oncoming perturbations of the forward facing one: the reattachment occurs 3.5 step heights downstream of the edge and the wall pressure field is influenced by eddies generated by the forward facing step separation. The latter also creates the strongest flow perturbations, resulting in a dominant contribution to sound radiation. The experiment is carried out in the large anechoic room of the Ecole Centrale de Lyon. The h= 0.05 m high block is placed in an acoustically transparent channel and the corresponding Reynolds number based on the step height is 1.7 10 5 . The boundary layer thickness of the incoming flow is about 0.7 h. Measurements include a detailed Laser Doppler Anemometry analysis of the mean and fluctuating velocity field, in-depth measurements of the wall pressure fluctuations around the two steps, and streamwise source localisations obtained with a near field acoustic array.
Sound radiation by a backward facing step under a plane wall jet is examined. The investigation is based on an experiment where the mean velocity, the step heights, and the cross-stream extent of the jet are varied. The comparison between the backward-facing-step ow and the corresponding wall jet shows an acoustic source near the step. Both near-and far-eld measurements indicate how the step changes the radiation pattern of this source. The sound level is signi cantly increased into the upstream directions and accompanied by strong low-frequency peaks in the spectra. Aerodynamic results show that the reattachment length is much shorter for a step placed under a wall jet than for a step placed in a channel and that the most turbulent ow regions, namely, the jet mixing layer and the step reattachment region, coincide with the acoustic source locations.
NomenclatureC = far-eld array center c 0 = speed of sound e = nozzle cross-stream width h = step height k = acoustic wave number l = jet half-width O = coordinate origin R = observation distance from O R 0 = observation distance from far eld array center C Re h = Reynolds number based step height h and maximal streamwise velocity at x D 0, U ref ; U ref h=º Re j = Reynolds number based on nozzle cross-stream width e and jet exhaust velocity U j ; U j e=º U; V = mean velocity in the x and y directions, respectively U j = jet outlet velocity U m = maximal streamwise velocity at a xed streamwise location U ref = maximal streamwise velocity at x D 0; U m .x D 0/ u; v = streamwise and cross-stream turbulent velocity uctuations u 0 ; v 0 = rms values of u and v X = streamwise distance from nozzle x = streamwise coordinate x r = x coordinate of the reattachment point y = cross ow coordinate y m = y location where U m is reached z = spanwise coordinate µ = observation angle with respect to (O; x )
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