Doctoral Dissertations
Date of Award
12-2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Aerospace Engineering
Major Professor
Trevor M. Moeller
Committee Members
John D. Schmisseur, Phillip A. Kreth, S. Michael Spottswood
Abstract
The operational viability of dual-mode scramjet (DMSJ) engines across a wide Mach range depends on the isolator's ability to condition the flow for combustion while maintaining sufficient margin against inlet unstart. Experimental simulations of isolator back-pressure rise due to combustion often rely on methods that decouple the shock train response from the back-pressuring device itself. While useful for simulating gross pressure rise due to combustion heat release, these approaches fail to fully capture the coupled nature of three-dimensional, shock interaction effects that would be generated by a fuel injection flow blockage. The work presented here aims to address this knowledge gap by quantitatively assessing the formation process, propagation characteristics, and inherent unsteadiness of a shock train using a three-dimensional back-pressuring device.
Experimental testing of a shock train was conducted in the Mach 2.2 UTSI direct-connect isolator facility. Back-pressure was generated using a floor-mounted cylindrical rod inserted perpendicular to the flow at the exit plane of the isolator. The choice of a cylindrical rod was motivated by its known shock-feature similarity to a flush-wall fuel injector. Floor surface pressure measurements and optical diagnostic techniques including high-speed background-oriented schlieren and fast-response pressure sensitive paint were performed, and quantitative time-histories of shock features were extracted from the optical imagery using a custom tracking algorithm. Shock train unsteadiness and identification of the underlying propagation mechanism was analyzed using results from zero-crossing frequency estimates and cross-correlation of the tracked shock signals.
This work reveals the strong coupling between shock train behavior and three-dimensional back-pressuring, evidenced by distinct shock train propagation phases within the isolator that originate from cylinder-induced boundary layer separation events. The intentionally simple method of using a floor-mounted cylinder as a back-pressuring device was designed to integrate multiple, and previously isolated, research areas. Specifically, the cylindrical-rod representation of flow blockage caused by flush-wall fuel injection, the unsteady physics of cylinder-induced shock boundary layer interactions, and the complex sensitivity of shock train propagation to back-pressure effects all converge in this work, providing crucial insights into shock train stability and control strategies for the future development of robust DMSJ propulsion systems.
Recommended Citation
Stevens, William, "Experimental Investigation of Cylinder Back-Pressuring Effects on Isolator Shock Train Dynamics. " PhD diss., University of Tennessee, 2025.
https://trace.tennessee.edu/utk_graddiss/13641