Ultra-High Speed Rainbow Schlieren Deflectometry for whole field acoustic measurements in supersonic jets
The goal of this research is to advance the field of noise measurement techniques to better understand the fundamental guiding principles of noise generation. This is accomplished in this study by demonstrating the capabilities of Ultra-High Speed Rainbow Schlieren Deflectometry (UHS-RSD) technique to visualize and quantify, in real-time, sound waves propagating from a supersonic cold air jet. Basic optical theory states that light rays passing through varying density transparent medium undergo deviations from their original paths because of refraction. An experimental setup was developed to direct parallel white light rays through a supersonic air jet. UHS-RSD technique employs aforementioned phenomena and enables mapping the light deflection angles, a measure of deviation of a light ray from its original path. Ray deflection angle mapping process is realized through variation in color (hue) between a schlieren image with and without test medium. Since all information in the field of view can be acquired in real time, this technique provides a means to determine full field of view scalar properties of any transparent flow. The current experiment captures sound waves emanating from a supersonic cold air jet at high spatial and temporal resolution, while still maintaining the hue sensitivity needed to detect the small pressure fluctuations characteristic of sound waves. The sound probe data showing general maximum sound generation could be employed to support the visual UHS-RSD data where pressure gradient waves are seen propagating from the same jet noise source. Initial analysis reveals the UHS-RSD system to capture the acoustic field properties matching previous studies. Mach wave fronts are generated in small packets of 2 to 10 waves with small intermittency window. This research has shown that the UHS-RSD technique has the capability of capturing acoustic waves emanating from supersonic jets. A detailed discussion is presented on how the RSD system has been optimized to significantly improve sensitivity and the signal to noise ratio.