Date of Award
Master of Science
John D. Schmisseur
James G. Coder, Trevor M. Moeller, Zhili Zhang
Two optical techniques are examined for their feasibility to measure laser-induced disturbances and freestream disturbances in supersonic flow. The two techniques examined are laser differential interferometry and time-resolved Schlieren. They provide unique capabilities for measuring these disturbances, and may provide much insight when used simultaneously. Laser differential interferometry can measure broadband noise in a wind tunnel in the < 100kHz range. Focusing laser differential interferometry may be a more appropriate technique to measure laser-induced disturbances, as it can spatially filter regions of little interest. Laser differential interferometry is path-integrated, and measurements can be obscured by dynamic content present in the regions of little interest along the measurement volume; however, it was used to make mean measurements of laser-induced disturbances.Schlieren can make qualitative observations of freestream disturbance levels for a global field of view. Schlieren allows observation of many characteristics of a laser-induced disturbance such as the growth rate and internal spatial frequencies in quiescent air. It was determined that the growth rate of laser-induced disturbances is highly non-linear, of the form D(t) = a(t)b where t is time in seconds and D is diameter in meters. Mean values of a = 0.0271 and b = 0.171 are observed. Spatial frequencies of the thermal disturbance are typically found to vary between 7 and 15 cm-1. Turbulent structures are found to evolve within the first 500 us, and diffusion causes the disturbance to continue to expand for the next 10 ms where the disturbances would typically become too diffuse to observe.Internal spatial frequencies are found to be proportional to the diameter of the disturbance by normalizing spectra by the instantaneous diameter. This shows that after the evolution of turbulent structures, diffusion of the forced disturbance becomes the dominant mechanism in the absence of a driving force. The disturbance behaves very similarly in a Mach 2 flow as it does in quiescent air, though the mean diameter is observed to be larger by a ratio of 1.71. This is likely due to the lower density environment.
Harris, Adam Joel, "Optical Diagnostic Techniques as an Approach to Characterizing Forced and Freestream Disturbances. " Master's Thesis, University of Tennessee, 2018.