SOUND GENERATION IN A MIXING LAYER

The sound generated by vortex pairing in a two-dimensional compressibl e mixing layer is investigated. Direct numerical simulations (DNS) of the Navier-Stokes equations are used to compute both the near-field re gion and a portion of the acoustic field. The acoustic analogy due to Lilley (1974) is also solved with acoustic sources determined from the near-held data of the DNS. It is shown that several commonly made sim plifications to the acoustic sources can lead to erroneous predictions for the acoustic field, Predictions based on the quadrupole form of t he source terms derived by Goldstein (1976a, 1984) are in excellent ag reement with the acoustic field from the DNS. However, despite the low Mach number of the flow, the acoustic far field generated by the vort ex pairings cannot be described by considering compact quadrupole sour ces. The acoustic sources have the form of modulated wave packets and the acoustic far held is described by a superdirective model (Crighton & Huerre 1990). The presence of flow-acoustic interactions in the com puted source terms causes the acoustic field predicted by the acoustic analogy to be very sensitive to small changes in the description of t he source.