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.
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