Experimental Investigation of Cylinder Back-Pressuring Effects on Isolator Shock Train Dynamics

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.