The Influence of Specific Heat Ratio on the Flow Characteristics of Over-Expanded Single Expansion Ramp Nozzles

Abstract

The flow field in the over-expanded single expansion ramp nozzle (SERN) demonstrates pronounced flow separation, intricate interactions involving intersecting and reflecting shocks, and mutual interference between shocks and boundary layers. Discrepancies arising from cold flow experiments vs. actual operational conditions introduce a notable influence of the specific heat ratio (γ ) on shock angles and shock/boundary layer interactions, thereby impacting the entire flow field. In this study, the differences between the cold flow conditions inside the nozzle and the actual operating conditions are found through numerical simulations, and the effects of γ on the shock structure and the shock/boundary layer interactions in an over-expanded SERN are investigated. Results indicated that cold flow experiments inadequately represented the authentic flow dynamics within the nozzle. Under varying nozzle pressure ratios (NPR) and γ conditions, the two-dimensional (2D) flow field structure of the over-expanded SERN exhibited diverse configurations of separated shock structures, encompassing Mach reflections, regular reflections, and triangular shock waves. Furthermore, alterations in γ influenced the movement of separation and reattachment points, induced pressure variations, and potentially triggered a transition in flow separation patterns. The changes in NPR and γ also modified the position and scale of the separation region, with an augmented NPR intensifying this effect. Moreover, the smaller values of γ were likely to lead to flow separation, separation pattern transitions, and changes in the structure of the separated shock waves. Compared with the 2D case, the three-dimensional outcomes indicated that the separated shock structure on the symmetry plane remained consistent, whereas that away from the symmetry plane contracted inward. The interaction patterns of the separated shocks on the ramp and flap also changed.OPEN ACCESS Received: 23/07/2024 Accepted: 07/11/2024 Published: 07/04/2025

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