Sub-Filter-Scale Shear Stress Deficit in Hypersonic Wall-Bounded Turbulence and Its Implications for Large-Eddy Simulation

Speaker: Takahiko Toki, Purdue University
April 2, 2026

Presentation

Abstract

Turbulent boundary layers on hypersonic vehicles entail significant skin-friction and heat-transfer loads. Accurate prediction of the boundary-layer state is therefore of paramount importance for the design of efficient thermal protection systems. While Reynolds–averaged Navier–Stokes (RANS) simulations are widely used in engineering applications due to their relatively low computational cost, mid- to high-fidelity approaches such as large-eddy simulation (LES) are required to capture the dynamics of turbulent fluctuations. However, current LES models often exhibit strong grid dependence in the prediction of skin friction and heat transfer, highlighting the need for a better understanding of near-wall behavior in hypersonic wall-bounded turbulence.

In this talk, the limitations of current LES models are first illustrated through simulations of hypersonic conical boundary layers. The focus then shifts to sub-filter-scale (SFS) turbulent behavior, investigated using direct numerical simulations (DNS) of hypersonic Couette flow with particular attention to shear-stress components. Semi-local scaling laws are shown to collapse mean profiles and second-order turbulence statistics despite strong wall-normal gradients of temperature and density. In contrast, the SFS shear stress exhibits an unexpected profile characterized by a region of pronounced stress deficit. The underlying mechanism of this deficit is elucidated through instantaneous flow visualization and statistical quadrant analysis, providing insight into near-wall turbulence dynamics in hypersonic flows.

Biography

Takahiko Toki is a Visiting Assistant Professor in the School of Mechanical Engineering at Purdue University. His research focuses on high-fidelity computational fluid dynamics, with an emphasis on compressible turbulence and hypersonic flows, as well as related topics such as aerothermochemistry. His work combines direct numerical simulation and large-eddy simulation to investigate fundamental turbulent processes and heat transfer mechanisms in extreme flow environments. He received his Ph.D. in Aeronautics and Astronautics from The University of Tokyo in 2018. Before joining Purdue, he was a postdoctoral scholar at the California Institute of Technology and later a postdoctoral research associate at Purdue University.

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