ORCID

0009-0009-4930-9213

Keywords

Propulsion; Airbreathing engines; Isolators; Unstart; Pseudo-Shocks; Shock-Trains; Turbulence; Modal analysis; Experimental

Subject Categories

Aeronautical Vehicles | Aerospace Engineering | Propulsion and Power

Abstract

The isolator of a scramjet engine houses a pseudo-shock, a compressive flow structure that raises the static pressure and decelerates the incoming supersonic flow for efficient combustion while shielding the inlet from the elevated pressure downstream. Under sufficient back pressure the leading shock bifurcates and forms a shock train followed by a shockless mixing region, and the resulting system oscillates even under nominally steady conditions; this unsteadiness distorts the flow, can drive the pseudo-shock out of the isolator and unstart the inlet. This thesis experimentally examines how a pseudo-shock in a constant-area isolator at a measured entrance Mach number of 2.90 responds to controlled back pressure. An adjustable wedge at the isolator exit imposed geometric blockage ratios from 0 to 18.73 percent, producing steady, repeatable back-pressure ratios up to 6.75 before inlet unstart occurred at the highest setting. Wall-mounted pressure transducers recorded the mean wall-pressure distribution, from which the pressure recovery and shock-train position were obtained, while high-speed schlieren imaging, captured in two streamwise sections, resolved the leading shock foot and the downstream breakdown region. Tracking the leading shock foot yielded broadband displacement spectra, and spectral proper orthogonal decomposition (SPOD) of the schlieren fields separated two coexisting unsteadiness mechanisms an order of magnitude apart in Strouhal number: a compact, low-frequency hydrodynamic shedding of the leading-shock foot at Strouhal numbers of 0.02 to 0.05, within the canonical shock wave/boundary layer interaction band, whose frequency decreases steadily as blockage rises, and a periodic, convecting train of cells in the breakdown region at Strouhal numbers of 0.19 to 0.22 that is largely insensitive to blockage. This differing response to blockage decouples the hydrodynamic shedding mechanism at the leading shock from the convective-acoustic mechanism downstream, informing the stable operating envelope of scramjet inlets.

Completion Date

2026

Semester

Summer

Committee Chair

Dr. Kareem Ahmed

Degree

Master of Science in Aerospace Engineering (M.S.A.E.)

College

College of Engineering and Computer Science

Department

Aerospace and Mechanical Engineering

Format

PDF

Document Type

Thesis

Language

English

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