Low frequency sound generated by flow separation in a conical diffuser

The sound field in a circular pipe generated by a concentric jet flow enter ing the pipe is studied. In the first case the air how enters the pipe thro ugh a convergent nozzle only. In the second case a short diffuser is attach ed to the nozzle. When the diffuser half angle is small enough to ensure at tached how conditions, the sound pressure level in the duct is reduced over the entire frequency range measured. When the diffuser angle is increased up to the point where flow separation occurs, an increase in the duct sound pressure level is observed. It is shown by means of cross-correlation meas urements involving the unsteady wall pressures in the diffuser and the soun d pressure in the duct that the increased sound levels are in fact caused b y the flow separation in the diffuser. (C) 2000 Editions scientifiques et m edicales Elsevier SAS.