Effect of Mach number on the acoustic field of 2:1 elliptic-slot jet

The shock-structure and the related acoustic field of underexpanded jets undergoes significant changes as the Mach number M-j is increased. The present investigation is carried out to study the effect of Mach number on an underexpanded 2:1 elliptic-slot jet. Experimental data are presented for fully expanded Mach numbers ranging from 1.3 to 2.0. It is observed that the 'cross-over' point at the end of the first cell at low Mach numbers gets replaced by a normal shock at a highly underexpanded condition resulting in the formation of a 'barrel shock along the minor-axis side with a 'bulb' shock formed along the major-axis side. The above change in shock structure is accompanied by a related change in the acoustic field. The amplitude of fundamental frequency along the minor-axis side grows with M-j but falls beyond M-j = 1.75. Along the major-axis side, however, the fundamental frequency does not exist at low Mach numbers. It appears at M-j = 1.75 but then falls at M-j = 2.0. The related azimuthal directivity of overall noise levels (OASPL) shows significant changes with M-j.