Centerline Focusing of Disturbances From Nozzle Wall Discontinuities in a Mach 6 Quiet Tunnel

Controlled experiments were performed in the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT) to quantify the effects of centerline focusing of disturbances generated by wall discontinuities. Forward-facing steps of varying height were applied to the nozzle wall using Kapton tape to simulate geometric imperfections that may occur during wind tunnel nozzle fabrication and assembly. Mean and fluctuating pitot-stagnation pressure levels were measured using a rake containing Kulite XCE-062-15A transducers positioned at multiple radial and streamwise locations. Results demonstrate that disturbances generated by the wall steps focus sharply along the centerline of the tunnel, with disturbance magnitude increasing with step height. Off-centerline mean pitot-stagnation measurements remained unaffected. Freestream fluctuation levels remained below the quiet threshold of 0.05%, except at the focus of the strongest disturbance. High-fidelity computational simulations using Navier-Stokes and parabolized Navier-Stokes solvers captured the magnitude of the disturbance to within 10\% of the experimental values, but slight discrepancies in the peak location and spatial extent of the disturbance occurred. The experimental data represent an initial step toward validating computational models of nozzle imperfections which could be used in the future to inform design tolerances for hypersonic nozzles.