A unified multiscale framework for shock-boundary-layer interaction in transonic buffet

The mechanisms of shock–boundary-layer interaction in transonic buffet have long been a focal point of academic research. However, a unified analytical framework that consistently describes shear, dilatation, and thermodynamic effects is still lacking. To address this, we perform a delayed detached-eddy simulation of transonic buffet over the OAT15A supercritical airfoil. Building on these results, we develop a unified multiscale coupling framework that integrates longitudinal (dilatation), transverse (shear), and thermodynamic processes to relate flow structures to irreversible entropy behavior. By analyzing the kinematic source term and the dynamics of entropy fluctuations and diffusion, the framework offers a consistent interpretation of transonic-flow structure formation and evolution. The divergence of the generalized Lamb vector serves as a key diagnostic for regions where compressibility and thermodynamic irreversibility dominate. Results show that the downstream buffet flow is characterized by the coupled evolution of vortical, entropy, and pressure waves, which, along with trailing-edge acoustic waves, form a multimodal wave system modulating buffet dynamics. Shock-induced separation and shear-related dissipation determine the primary energy loss pathway. Near-wall analysis shows that the wall acts as a conduit for thermal, pressure, and entropy disturbances into the outer flow. Consistently, the near-wall-dominated negative signature downstream of the shock indicates that surface-normal deformation introduces irreversible dissipation, playing a key role in buffet-amplitude saturation. Thus, transonic buffet can be seen as a dynamically maintained equilibrium sustained by the coupling of dilatation, shear, and entropy processes.