During the atmospheric reentry of reusable launch vehicles, the thermal protection system undergoes high-temperature ablation, leading to the formation of surface roughness. Concurrently, the flow exhibits complex thermochemical nonequilibrium phenomena, including internal energy excitation and molecular-level chemical reactions. Under hypersonic conditions, the local flow gradients induced by such roughness elements significantly complicate the evolution of disturbances within the boundary layer, posing challenges for transition prediction. In this study, the direct simulation Monte Carlo (DSMC) method is employed to perform unsteady, high-fidelity numerical simulations of hypersonic boundary-layer flows over a flat plate containing local roughness elements. By introducing specific-frequency disturbances into the freestream, the evolution of disturbance structures in the roughness wake is characterized. Furthermore, the impact of high-temperature nonequilibrium effects on boundary-layer flow characteristics is analyzed through a comparison of numerical results obtained from real-gas and calorically perfect gas models. The results indicate that when the characteristic size of a roughness element is large relative to the boundary-layer thickness, flow separation is induced, accompanied by the formation of new compression and expansion waves, which significantly amplify thermal nonequilibrium effects. Under perturbed freestream conditions, both the upstream thermodynamic state and the geometric parameters (size and location) of the roughness elements considerably influence the development of downstream disturbance waves. A systematic analysis of 25 test cases reveals that optimal disturbance suppression is achieved when the roughness element is positioned at x = 0.0125 m with a relative height H = 0.5δ, yielding a 72% increase in the disturbance attenuation rate compared to a smooth flat plate. As the roughness height increases, the suppression effect exhibits a non-monotonic trend, initially weakening and then strengthening, whereas as the roughness element moves downstream, the suppression effect first strengthens and then weakens. Moreover, three-dimensional roughness elements induce additional spanwise disturbance components and enhance wake mixing. This modulating effect on disturbance evolution cannot be neglected. This study elucidates the physical mechanisms underlying the boundary-layer disturbance response induced by roughness elements under hypersonic conditions, providing valuable insights for the aerodynamic and thermal protection system design of next-generation reusable spacecraft.

High-temperature airflow interacts with the surface material of the aircraft through multiple physical and chemical processes, significantly altering the aircraft's surface morphology, which consequently affects the evolution of flow structures as well as the aerodynamic and thermal characteristics of the vehicle. Accurate prediction of the ablation process during re-entry is crucial for designing thermal protection systems. Existing numerical simulations of aerodynamic ablation primarily focused on flow fields under fixed wall temperature conditions, neglecting the influences of complex chemical reactions and material property differences on the heating process and ablation morphology during ablation. This study employed the Direct Simulation Monte Carlo (DSMC) method, coupled with the wall energy conservation equation, and utilized the open-source program SPARTA to conduct a decoupled analysis of the aerodynamic heating process during vehicle re-entry. Using a cylindrical model as an example, corresponding governing equations were established separately for the decoupled processes of wall heating and model ablation. By integrating gas-gas and gas-solid chemical reactions behind the shock wave, the thermochemical reaction and aerodynamic ablation mechanisms under two-dimensional conditions were analyzed. The results indicate that the developed computable ablation model not only improves the accuracy of internal energy sampling for gas molecules behind the shock wave but also successfully reproduces ablation morphologies documented in existing literature. This model not only reproduces the difference in erosion morphology between the front and rear edges of a cylinder but also, after incorporating the amplification effect of surface roughness on aerodynamic heating, controls the relative error between the predicted retreat distance of the ball cone erosion and experimental data within 5%. This method represents the first comprehensive integration of wall material properties, variable wall-temperature effects, and surface roughness into the physical modeling of the ablation process within a DSMC framework, providing a theoretical basis and data support for deepening the understanding complex thermochemical non-equilibrium phenomena under variable wall temperatures.