An experimental investigation on stability and transition at Mach 3

This dissertation presents a study of stability and transition at Mach 3. A supersonic wind tunnel of the blowdown type has been developed and operated at low stagnation pressures (4-5 psia) with a corresponding range of Reynolds number from Re/m = 2.1 × 10⁶ to Re/m = 5.5 × 10⁶. At the lowest stagnation pressure of the investigation (P₀ ~ 4 psia) with laminar boundary layers on the nozzle walls, the freestream disturbance level was found to be .11%. The mean flow over a flat plate has been mapped in detail with pitot-static probes and comparisons to CFD numerical simulations performed by Dr. Martinelli (Mechanical and Aerospace Engineering Department, Princeton University) made. Spectra, wave angles, wave speeds, growth rates and amplitude distributions of naturally occurring boundary layer waves and the freestream fluctuating field have been measured using single and dual hot wire configurations. The results have been compared with the numerical calculations from linear stability theory which have been provided by Dr. Balakumar (Aerospace Engineering Department, Old Dominion University). Similarities and discrepancies between predictions and measurements are discussed, and receptivity and instability issues are addressed. The efficiency of the freestream acoustic disturbances in initiating boundary layer oscillations is highlighted. Also, issues concerning the calibration of the sensor have been addressed in the context of the determination of the y- distribution of the mass flux disturbance amplitude in the boundary layer. Late stages of transition at a stagnation pressure of 5 psia under a higher initial freestream disturbance level, have also been explored. Qualitative comparisons with the incompressible case suggest, despite the strong evidence of different receptivity and linear instability mechanisms, similar local nonlinear mechanisms leading to the development of turbulent spots between the Mach 0 and the Mach 3 cases. The design of a glow discharge actuator and measurements of an artificially generated spot-like wavepacket are finally described and comparisons with predictions (Balakumar) regarding amplitude distributions, wave speed and shape of the wavefront are presented.