Passive acoustic diagnostics of an atmospheric pressure linear field jet including analysis in the time-frequency domain

This study presents the results of passive acoustic diagnostics of an atmospheric pressure linear field jet source operating inflowing helium. Variations of the electrical, optical and acoustic parameters of the source are monitored with respect to changing gas flow rate [0.5–7 liters per minute (lpm)] and applied voltage amplitude (3.5–7 kV). It was found that the jet length and coupled power were maximized when the jet flow was laminar. Flow mode transitions (buoyant, laminar and turbulent) associated with the jet Richardson number could easily be distinguished acoustically through their unique wavelet scalograms. Each scalogram can also be explained through qualitative correlation with the discharge electrical parameters. As the jet became turbulent, low frequency oscillation (c. 180 Hz) bursts were present in the time-frequency trace which were compared to an empirical relation for the Richardson number in the case of flow induced oscillations. It was found that the frequency value correlated well with the frequency of 200 Hz determined from literature. Anomalous sparking events were detected which manifested as almost constant magnitude, broadband acoustic transients in the time-frequency domain. Comparisons between the average acoustic output power and the average dissipated power from the discharge at two different flow rates (4 lpm and 2 lpm) reveal an approximately equal linear trend for a fixed microphone-discharge placement while in the laminar flow regime. Due to the increased turbulence induced noise, however, no such linear correlation could be drawn. Finally, optical emission spectra from the discharge at a point 8 mm downstream of the plume exit were taken and correlations drawn for each flow regime.