Recent progresses on hypersonic boundary-layer transition

Abstract

Boundary-layer transition (BLT) is one of the key problems in developing hypersonic aircrafts. Major aerospace countries all over the world have been heavily supporting relevant researches, and the project funding keeps increasing in recent years. In this paper, wind tunnel experiments, numerical simulations and theoretical analysis are combined to study some hot issues in this area, which include effects of unit Reynolds number, angle of attack and nose bluntness, nonlinear interactions during breakdown, secondary instability of crossflow vortices, etc. Some of our views on understanding hypersonic BLT are summarized. The wind tunnel tests are conducted in the F1 m hypersonic wind tunnel of China Aerodynamics Research and Development Center, and the relation between transition location and different unit Reynolds numbers, angles of attack and nose bluntness is obtained. It is well known that hypersonic BLT undergoes an important phenomenon called "nose-bluntness inversal", but a worldwide accepted reasonable explanation for this phenomenon is still missing. To investigate the "nose-bluntness reversal", we suggest to consider the influence of front shock waves because free-stream disturbances are changed by shock waves. Meanwhile, we should also consider the following two aspects: blunt-nose receptivity and entropy-layer effect. Some premiliary but encouraging results are presented on these two aspects via direct numerical simulations (DNS) and stablity analysis. During the late stage of transition, a "quiet zone" may appear when the second mode waves reach large amplitude, and this is followed by lowfrequency modes. DNS and the dynamic mode decomposition are applied to study this problem. The results indicate that the relation between the second modes and the low-frequency modes is possibly mode transformation. In the end of this paper, a newly discovered secondary unstable mode of saturated crossflow vortices is introduced. This mode has the same level of growth rate as Z mode, but is related to the wall-normal shear stress, thus it is named Y' mode.