Mechanism of width effects on cavity-induced transitions at hypersonic speed using direct numerical simulation.

This article investigates the effects of cavity width on boundary layer transitions induced by three-dimensional (3-D) shallow cavities on hypersonic vehicles using direct numerical simulation (DNS). Six cavity widths, ranging from 0.25 to 1.5 times a baseline width (W), plus an infinite width case (InfW), were analyzed at Mach 9.575 to understand their influence on laminar-to-turbulent transition mechanisms. Results show three transition types: constant transition for 0.25 W and 1.0 W cavities caused primarily by adverse pressure gradients (APG) and vortical instabilities; intermittent transition for 0.5 W and 0.75 W cavities linked to oscillations in APG and recirculation "breathing"; and no transition for wider cavities (1.5 W and InfW) characterized by open cavity flow and laminar boundary layers. The study establishes a relationship between cavity geometry and transition frequency, highlighting the role of APG oscillations and streamwise vortices in transition onset, with implications for thermal protection system design on hypersonic vehicles.