Sean Dungan | Searcharxiv

arXiv2026

Transitional Fluid-Ablation Interactions on a Compression Ramp at Mach 8

Direct numerical simulations (DNS) are performed for a Mach 8 flow over a 15 degree compression ramp undergoing ablation to analyze fluid-ablation interaction (FAI) effects on instability growth and transition of the high-speed boundary layer. Wind-tunnel experiments at these conditions show characteristic heating streaks which are first replicated for laminar flow over an inert ramp. Perturbations superimposed on the laminar baseflow are informed by stability calculations. The ramp is then replaced by a low-temperature ablator in our DNS and the interaction of the streaks with the ablating surface is examined. Ablation models implemented in two independently-developed fluid dynamics solvers are used to study this problem. Overall, both solvers have shown an excellent agreement regarding heat fluxes, mass blowing rates, and recession patterns. Numerical experiments are performed to assess the impact of accelerating recession and of higher amplitude perturbations. We demonstrate accurate predictions of ablative recession and present novel findings regarding its influence on the perturbation evolution for this configuration.

physics.flu-dyn↗

arXiv2026

Distributed Roughness-Induced Transition on a Blunt Body at Mach 6: a Numerical Investigation

Surface roughness significantly impacts transition to turbulence, especially over high-speed, blunt geometries where surface ablation is necessary to mitigate heat loads during atmospheric entry. Inspired by sand-grain roughness experiments performed by Hollis (2017), we perform the first direct numerical simulation (DNS) of a blunt cylinder in Mach 6 cross-flow with roughness elements distributed along the entire surface. Such simulations aimed to uncover the precise means by which laminar-turbulent transition occurs given the limited measurements attainable from experiments and non-existent high-fidelity simulations. Element heights were held fixed at approximately 35% boundary layer thickness, while the relative phasing between streamwise rows was varied. All configurations exhibited convective instabilities driving the transition process, with the mode type being set by the roughness configuration. A fundamental sinuous streak mode dominated the aligned roughness element case, whereas both the staggered and randomly phased cases saw 2D T-S waves dominating. These instability waves, when grown to sufficient amplitude, triggered the steady streaks seeded by the underlying roughness pattern to begin forming hairpin vortices and breakdown occurred soon thereafter. The roughness arrangement was found to dramatically influence the degree to which the waves were destabilised, as well as the strength of the underlying steady streaks, thereby combining to dictate the position along the surface where LTT occurred. Finally, exogeneous forcing was not required for the T-S dominated cases as the acoustics generated by the turbulence in the subsonic flow excited T-S waves on the other side - a feedback mechanism hitherto unknown.

physics.flu-dyn↗