Effects of cavity shapes and sizes on rarefied hypersonic flows

Abstract

Cavity structures are frequently encountered on hypersonic vehicle surfaces, which change the aerodynamic heat flux and force (pressure) compared with flat vehicle surfaces. The effects of the cavity length-to-depth ratio, cavity rounded corners, Maxwellian accommodation coefficient, and free-stream Knudsen number on the heat flux and pressure loaded on the vehicle surface with rectangular cavities have been studied extensively. Still, the influence of the cavity shape and size remains. In this study, the effects of cavity shape and size on the flow field characteristics and aerodynamic surface quantities on the surface are analyzed using the direct simulation Monte Carlo method. The rectangular cavities with sidewall angles (α) of 0°, 15°, 30°, and 45° are selected to investigate the shape influence, and the semicircular cavities with radii (R) of 3, 6, 9, and 12 mm are chosen for size effect. The results indicate that the gas flows over the cavity configurations specified in our study hold the “open-cavity” characteristics. The vortexes formed inside the cavity are shrunk as α increases from 0° and 45°, but they are similar under the R-range explored. The pressure, skin friction, and heat transfer coefficients over the surface display complex trends as varying α and R. In addition, the effect of cavities on the overall aerodynamic heat flux and force, which are vital for practical applications, are explored. The results reveal that the aerodynamic heat flux is dramatically influenced by α and R, while the aerodynamic force is mildly affected. Overall, the larger the α and R, the greater the effect of the cavity on the rarefied hypersonic flows. Our research is beneficial to the protection and optimal design of hypersonic vehicles.