Abstract:
Hypersonic vehicles are likely to undergo laminar/turbulent boundary layer transition(BLT). Laminar boundary layers and turbulent boundary layers show essential differences in skin friction, heat transfer, mixing, and noise. Hypersonic BLT is one of basic scientific problems, which restrict the development of advanced hypersonic technology. Nowdays, hypersonic BLT is a hot and difficult topic. Worldwide studies on hypersonic BLT are briefly summarized in this paper. The phenomena and regularities of hypersonic BLT are classified into three categories: what we know the main reasons, what we know parts of the reasons, and what we don't know or are inconsistency. Wall temperature effects, Mach number effects and wind tunnel noise effects are which we know the main reasons. Nose bluntness effects, angle-of-attack effects and entropy layer interference are which we only know parts of the reasons. Unit Reynolds number effects, length of transitional zones, surface friction and heat distributions in transitional zones are which we know little. Factors affecting hypersonic BLT, physical mechanisms of hypersonic BLT, methods and models of predicting hypersonic BLT, and and control methods of delaying/accelerating hypersonic BLT, are reviewed, together with unclassified hypersonic flight tests. In order to avoid interactions of different factors, it is suggested to detach individual factors from composite transition clauses and study these individual factors separately. In future study on hypersonic boundary layer instability, special efforts shall be focused on crossflow instability, entropy layer, and mode interactions. Future prediction methods and models need to account for three-dimensional flows and free-stream disturbances. Future control methods shall be high effective, and low additional drag and surface heating. Flight test is very important for the study of hypersonic BLT. Flight test and quiet wind tunnel are becoming more and more useful and fruitful. The past more than 60 years of studies on hypersonic BLT indicate the importance of the combination of different research methods, such as numerical simulations, wind tunnel experiments, flight tests, and stability analysis.