Three-Dimensional Nature of Shock Trains in Rectangular Scramjet Isolators

Abstract

The dual-mode scramjet isolator flow field is simulated experimentally and numerically in rectangular aspect ratio 1.0, 3.0, and 6.0 ducts with inflow Mach numbers ranging from 2.4 to 2.7. Often neglected in past research efforts, the focus of this work is on resolving the three-dimensional nature of the shock train front. Previous work utilizing multiplane shadowgraphy and focusing schlieren deflectometry revealed that the shock train front is hybrid oblique/normal in nature, with the low momentum corner flow separating up to one duct height upstream of the center flow field. This initial separation spawns oblique shock planes that transform into stronger, more normal shock structures near the center flow region. This study provides supplemental analysis of the three-dimensional isolator flow features through steady-state k − ω RANS simulation, experimental and numerical boundary-layer profile analysis, and quantitative global density gradient magnitude calculations by way of the Background Oriented Schlieren (BOS) method. Validation and verification of the fully started isolator simulations show good agreement to experimental data, while the unstarted isolator simulations match the qualitative features of the three-dimensional shock train front observed in the experimental aspect ratios tested. Experimental and numerical boundary-layer measurements indicate that the major axis (side wall) boundary-layer is more susceptible to separation than the minor axis (nozzle-bounded) boundary-layer due to a lower momentum thickness, influencing the formation of the shock train along the wall center lines. The application of the BOS method indicates that the shock train structure strength doubles between the outboard and inboard regions of the preliminary shock train element. A continued discussion regarding the importance of accounting for the often neglected three-dimensionality of the isolator shock train is provided.