Second-mode waves produce hypersonic boundary layer heating

Aerodynamic heating is a key issue in hypersonics and is crucial to vehicle design and performance. Specifically, a major question about heat transfer is in the transition from laminar to turbulent flow and how second- and higher-instability modes affect the phenomenon. With a combination of experimental, theoretical and numerical methods, authors confirm in Physics of Fluids that a heat-transfer peak at the second mode transition occurs. They also present the first demonstration that it is due to viscous dissipation induced by shear and dilatation as the second mode instability progresses.

At the Mach 6 quiet wind tunnel at Peking University, the authors studied flow over an instability-enhanced flared cone model at three different Reynolds numbers. A host of measurement tools, including pressure sensors, temperature-sensitive paint (TSP), and particle image velocimetry (PIV) provided data for their analysis using parabolic stability equations (PSE) and direct numerical simulations (DNS).

Previous work demonstrated that a surface temperature peak forms before turbulence sets in, growing rapidly and then decaying as second-mode waves begin. A second fast temperature increase then occurs at the transition to turbulence. The first pre-transition temperature peak results from periodic dilatational heating. This heating accompanies the second-mode transition to shear-induced viscous dissipation. The results provide a new understanding of boundary layer hypersonic heating phenomena, vital to the design of hypersonic vehicles, and thus, to the realization of hypersonic travel.

Source: “Aerodynamic heating in transitional hypersonic boundary layers: Role of second-mode instability,” by Yiding Zhu, Xi Chen, Jiezhi Wu, Shiyi Chen, Cunbiao Lee, and Mohamed Gad-el-Hak, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5005529 .