Aeroacoustics of Opposing Planar Air Jets

Abstract

The self-excited response of two opposing planar air jets is experimentally examined as a function of both flow velocity of the jets, as well as the impingement distance, i.e. the distance z between them. This initial study is performed for a single jet thickness of h = 2 mm and a spanwise aspect ratio of 50. Two sets of experiments have been performed, with the first set of measurements having been obtained over a range of impingement ratios from z/h = 7 to z/h = 130 for several moderate Mach numbers (M = 0.44, 0.58 & 0.73), whereas the second set of measurements has been performed by varying the jet Mach number at fixed impingement ratios. The opposing planar jets are found to produce intense acoustic tones over the entire range of impingement ratio and flow velocity examined in this study, showing robust oscillation behavior even for very large distances between the jets. Acoustic tone frequency has been found to increase linearly with the jet exit velocity at constant impingement ratio and to decrease with the impingement distance at constant jet velocity. The flow field over the oscillation cycle has been investigated by means of phase-averaged PIV measurements technique, which has shown that the high noise levels and acoustic tones are produced by a pronounced flapping oscillation of the opposing jet columns.