Superdirective Acoustic Radiation by Vortex Pairing in Subsonic Excited Jets

Abstract

This paper is concerned with acoustic radiation of the first subharmonic of the forcing frequency in a low-Mach-number excited jet. Two jets with laminar and transitional initial conditions are experimentally studied in the same facility. An acoustic directivity with an extinction angle in the sideline direction is observed for the laminar case (ReD = 6.7 × 10 4 ), that can be interpreted as the radiation of a compact axisymmetric quadrupolar source. A superdirective radiation is measured for the transitional jet (ReD = 1.3 × 10 5 ), that is characterized by an exponential angular variation providing a larger range of the sound pressure level. A wave extrapolation method is developed to interpret the measured near and far pressure fields of these two configurations.