Experimental investigation of acoustically forced helium jet in crossflow using shadowgraphy and modal analysis

This study presents an experimental investigation of helium jet in a crossflow of air, with an objective to understand the influence of acoustic forcing on jet behavior and mixing. Unforced and sinusoidally forced jets are studied. High-speed shadowgraphy is used to capture instantaneous jet images. These images are further processed using the Proper Orthogonal Decomposition (POD) algorithm to provide insights into the spatiotemporal behavior. Schlieren Imaging Velocimetry (SIV) is also used to understand jet entrainment and identify regions of low and high velocities. Moreover, an interface tracking method is used to obtain interface location oscillations for near nozzle and far field locations. The energy spectrum of these oscillations is obtained through a Fast Fourier Transform (FFT) analysis. The unforced jet is observed for different jet-to-crossflow velocity ratios (R), and it is observed that the jet penetration increases with R. Unforced cases show a broadband spectrum, indicating the absence of any dominant frequency, except for the lowest velocity ratio case. The instability in unforced cases is limited to shear layer oscillations. For the forced cases, a clear dominant frequency corresponding to the forcing frequency (and occasionally their harmonics) is recorded. It is observed that the impact of forcing function is not the same for all the frequencies, and the jet response is observed to be much more pronounced for a higher frequency as compared to a lower frequency forcing. Two typical cases, highlighting the effect of frequency response, are compared in detail using image analysis, interface tracking, POD, and SIV.