Progress in flight tests of hypersonic boundary layer transition

Boundary layer transition (BLT) can cause a sharp rise in heat flux and skin friction, which can seriously affect the flight performance and safety of hypersonic flight vehicles. Therefore, the mechanism, prediction and control of transition have become important issues that must be dealt with for the development of advanced flight vehicles, and it is also a research hotspot of particular interest to major aerospace countries. Compared to other transition research approaches, model flight tests can better present the transition problems under real flight conditions, thus have been carried out extensively over the past 30 years. The United States, Germany, France, Australia, and other countries have carried out transition research based on flight tests, such as the Pegasus wing-glove crossflow transition and the Hypersonic Boundary Layer Transition (HyBOLT) transition control flight test of the United States, the joint research project of the Hypersonic International Flight Research and Experimentation-1 (HIFiRE-1) circular cone and the HIFiRE-5 elliptic cone transition flight tests between the United States and Australia, the flight test of compression surface transition of the scramjet forebody (LEA) in France and so on. Although these flight tests suffered various setbacks, they still obtained valuable transition data. Recently, the United States is carrying out the concave-surface transition flight tests of Hypersonic Boundary Layer Transition (BOLT) and BOLT-II. Since its first model flight test mission for verification purpose launched successfully in 2015, several hypersonic BLT flight tests have been conducted by China Aerodynamics Research and Development Center (CARDC). The flight tests have measured valid transition data under flight conditions, obtained the transition front and its dynamical variation on blunt cones at various angles of attack and a lifting body Hypersonic Transition Research Vehicle (HyTRV). The crossflow traveling waves in high-altitude flight were measured for the first time, and our understanding of hypersonic BLT has been greatly improved. Graphical abstract