Traditionally, researchers attributed this boundary layer transition to modal instabilities, particularly Mack modes. However, recent studies point to nonmodal mechanisms-especially in configurations with blunted leading edges-gaining prominence. A critical mystery remains unsolved: beyond a certain degree of nose bluntness, the flow unexpectedly becomes less stable, a phenomenon known as "transition reversal."

Now, a team of researchers has identified the interplay of two competing instability modes as a potential explanation. Their findings, published March 5, 2025, in the Chinese Journal of Aeronautics, show that at Mach 5.9, slow-growing disturbances in the entropy layer and fast-transient instabilities within the boundary layer can coexist and battle for dominance. This dual-mode interaction helps explain why experimental and computational data have previously appeared inconsistent.

Employing both resolvent analysis and classical stability equations, the team uncovered how these two mechanisms respond under varying flight conditions. Their analysis offers the most comprehensive view to date of how disturbances evolve on blunt hypersonic surfaces. "Our work finally provides a possible solution to explain observations that have puzzled the field for decades," stated the lead researcher.

These results arrive as global efforts to develop next-generation hypersonic systems intensify. Despite the idealized nature of the current study, the team plans to expand their research to include more complex flow fields, nonlinear dynamics, and realistic noise environments, such as those encountered in wind tunnels. Ultimately, these findings represent a vital step toward mastering turbulence control in high-speed aerospace vehicles.

Research Report:Optimal disturbances and growth patterns in hypersonic blunt-wedge flow

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