Enhancement of large scale structures in supersonic axisymmetric jets using laser excitation

A creative method of controlling and forcing supersonic mixing layers has been demonstrated. A laser beam from a frequency-doubled pulsed Nd:YAG laser is focused at the nozzle exit of an axisymmetric jet. Pulsing the laser causes a thermal peak at the jet surface, forcing the shear layer where it is formed. Preliminary measurements show that this is an effective means to enhance and control the large scale structures formed at the exit of perfectly expanded jets with Mach numbers of 1.36, 1.5, and 2. The convective Mach numbers of these jets are 0.63, 0.68, and 0.85, respectively. Two laser pulses are used: the first excites the flow, and the second, delayed in time and formed into a sheet, interrogates the flow for visualization. The convective velocity of the large scale structures was found to be slightly higher than predicted theoretically. Since the formation of the large scale structures can be controlled, high frequency pressure measurements were made simultaneously with each instantaneous image. The pressure trace indicates the lower and higher pressures associated with the vortex core and braid regions of the large scale structures, respectively. The laser excitation technique provides a unique opportunity to study the spatially stable large scale structures and a novel method for enhancing the growth rate of a supersonic shear layer.