Influence of Reynolds number on the modes of unsteadiness on the double-wedge configurations

This study investigates the influence of the Reynolds number on two double-wedge configurations with wedge length ratios L1/L2=1.5 and 1, under low-enthalpy, high-Mach-number free-stream conditions representative of Hypervelocity Expansion Tube experiments using laminar computations. A new OpenFOAM solver, machXFoam, is developed using the AUSM+-up scheme, operator splitting, and third-order Runge–Kutta time integration. Simulations are performed for three Reynolds numbers: the experimental value (x−Re), half (0.5x−Re), and double (2x−Re). The L1/L2=1.5 case exhibits a persistent oscillatory unsteady mode across all Reynolds numbers, with the separation bubble at the compression corner varying with the Reynolds number. This bubble governs the excursion length of the transmitted shock (Edney type-V interaction), leading to more oscillations at lower Reynolds numbers over the same simulation time. The Strouhal number shows an indirect dependence on the Reynolds number between 0.5x − Re and x − Re but becomes insensitive at higher values, suggesting a critical Reynolds number in between. Modal decomposition techniques (Proper Orthogonal Decomposition, Dynamic Mode Decomposition, Spectral Proper Orthogonal Decomposition) consistently capture coherent structures. A new unsteady mode, resembling “free oscillations,” is identified for the L1/L2=1 case at the lowest Reynolds number, featuring high-frequency shear layer disturbances and low-frequency shock wave cycles. In contrast, higher Reynolds numbers exhibit a pulsating mode where the separation bubble size dictates the spectral content. The Strouhal number scales with the Reynolds number and is closely tied to the bubble size. Spectra show a fundamental frequency linked to shock–bubble interactions and a first harmonic related to escaping vortices.