The Boeing/AFOSR Mach-6 Quiet Tunnel achieved quiet flow to a stagnation pressure of 163 psia in Dec. 2008, the highest value observed so far. It remains quiet at pressures above 160 psia. Under noisy conditions, nozzle-wall boundary-layer separation and the associated tunnel shutdown appear to propagate slowly upstream, whereas under quiet conditions, the propagation is very rapid. A new diffuser insert has been designed, fabricated, and installed in the tunnel in order to start larger blunt models and increase run time. A flared cone with a circular-arc geometry was designed to generate large second-mode N factors under quiet flow conditions. When the computed N factor was 13, large instability waves were measured under quiet flow conditions using fast pressure sensors, but the flow remained laminar. Transition was observed only under noisy conditions. A laminar instability was detected in the wake of an isolated roughness element in the boundary layer on the nozzle wall; this appears to be the first such measurement at hypersonic speeds.

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AFOSR/NA Unclassified and Approved for Public Release M.S. thesis by Rodrigo Segura with development of LDI for Mach-6 quiet tunnel and measurements in Mach-4 quiet tunnel Laminar-turbulent transition is a pivotal factor for the design of hypersonic vehicles but the mechanisms that induce transition are not well understood. A laser differential interferometer (LDI) is a non-intrusive optical device that measures the optical path length difference between two laser beams. The LDI is a reliable calibrated instrument to assist the study of boundary layer instability-wave growth in hypersonic flow and has high sensitivity and frequency response. An LDI with a commercial balanced photodetector capable of detecting optical path length differences of lambda/21,000 from DC to 80 MHz was assembled and tested in the Purdue Quiet-Flow Ludwieg tube. Fluctuations in the subsonic region of a forward-facing cavity were measured with the LDI and compared to those detected with a Kulite pressure transducer at the base of the cavity. Predictions of self-resonating deep cavities were confirmed. The LDI was then adapted and transferred to the Boeing/AFOSR Mach-6 Quiet Tunnel. hypersonic boundary layer transition optical instrument supersonic cavity flow oscillations experiment Unclassified Unclassified Unclassified Unlimited 151 ii Este trabajo va dedicado a mi papá, mi mamá, mi hermano, y mi Dios. A ustedes les debo todo lo que soy. iii ACKNOWLEDGMENTS I want to thank my advisor, Professor Steven P. Schneider for building and managing the hypersonic wind tunnels at Purdue University and giving me the opportunity to work on this project. Had it not been for his vote of confidence, the LDI would sit in a drawer for years to come. Furthermore, his expertise and frankness helped me get through the difficulties encountered in this experimental research. I would also like to thank my committee members, Professors Steven H. Collicott and Anastasios S. Lyrintzis, their guidance and suggestions are appreciated. The machinists and technicians at the Purdue Aerospace Sciences Laboratory were extremely helpful during the course of the project. I thank them for their excellent craftsmanship and valuable technical assistance. I would like to express my special gratitude to Madeline Chadwell, Robin Snodgrass, and John Phillips all of whom went out of their way to assist me with unexpected experimental breakdowns on multiple occasions. Jerry Hahn and Jim Younts were also included in that crew. The office and information systems staff at the Aeronautical and Astronautical Engineering department at Purdue University also played an important role. Linda Flack's timely reminders made the logistics of my academic life easy. I would like to give special thanks to Joan Jackson; she made me laugh and was patient and helpful even when I demanded her help on rather short notice. Joe Kline's technical assistance is appreciated. A great deal of contribution came from my fellow researchers.

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Progress is reported for four of the experiments currently being conducted in the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT). Larger nozzle wall disturbances have been generated for use in the production of turbulent spots. The new design is currently undergoing refinement to produce more controlled disturbances. Two models were designed to maximize first mode instabilities in the BAM6QT. These models have been built and are currently being tested as a first step in developing a method for observing first mode instabilities in other hypersonic wind tunnels. The effect of discrete roughness elements on the stationary crossow instability was examined. Depending on the spacing of the roughness elements, the generation and breakdown of the stationary vortices was altered. A new 3-inch shock tube at Purdue has been tested and work on calibrating PCB-132 sensors has begun. Weak shocks with pressure rises less than 0.1 PSI have been created, and the PCB-132 sensors appear to have a linear response at small amplitudes, though results below 0.1 PSI are ambiguous. Nomenclature Re Reynolds number T temperature t time x axial coordinate Subscripts 0 stagnation condition Abbreviations TSP Temperature-Sensitive Paint SB Schmidt-Boelter heat transfer gauge