The basic problems of transition in both incompressible and compressible boundary layers are reviewed. Flow structures in low-speed transitional and developed turbulent boundary layers are presented, together with almost all of the physical mechanisms that have been proposed for their formation. Comparisons of different descriptions of the same flow structures are discussed as objectively as possible. The importance of basic structure such as solitonlike coherent structure is addressed. For compressible flows, the receptivity and instability of boundary layer are reviewed, including the effect of different parameters on the transition. Finally, the principle of aerodynamic heating of hypersonic boundary layer is presented.

The evolution of the first- and second-mode instabilities in a hypersonic flat plate boundary layer is investigated. Experiments are conducted in a Mach 6.5 quiet wind tunnel using particle image velocimetry, Rayleigh-scattering flow visualization, and schlieren methods. Glow discharge is introduced as an artificial disturbance. The results show that an artificially introduced disturbance in the first-mode frequency range can excite a specific second-mode wave that is one of the high-order harmonics of the added disturbance. For the first time, we find a clear harmonic relationship between the first- and second-mode waves, as well as the phase lock phenomenon between them.

The hypersonic boundary layer transition over a concave wall is investigated in a Mach 6.5 quiet wind tunnel using temperature sensitive paint (TSP), CO2-enhanced filtered Rayleigh scattering flow visualization, PCB fast-response pressure sensors, and a high-frequency schlieren technique. The TSP shows that low- and high-temperature streaks are distributed in the spanwise direction. The wavelengths of naturally developing Görtler streaks are randomly distributed, with an average of approximately 7 mm, and change little as the unit Reynolds number increases. More importantly, three-dimensional waves are clearly visualized and quantitatively measured inside the Görtler streaks. This is the first time that the entire evolution of the Görtler instability has been visualized using the Rayleigh-scattering flow visualization in hypersonic flow. The results demonstrate that three-dimensional waves are amplified as a result of the Görtler instability, resulting in a localized high-shear layer around the interface of the three-dimensional waves, which contributes to the formation of hairpin vortices and mushroom-like structures. The three-dimensional waves grow and play an important role in Görtler instability-induced boundary layer transitions.

We study turbulence production at the end of a second-mode-induced transition in a Mach 6 boundary layer based on both experiments and numerical simulations. By using ultra-fast visualization and particle image velocimetry, we succeed in capturing a soliton-like wave packet that plays a determining role in turbulence production. We reproduce the experiment by direct numerical simulation, revealing that this wave packet arises from the vortical first Mack mode (the counterpart of Tollmien–Schlichting waves in low-speed boundary layers), rather than the rapidly growing dilatational second Mack mode, which triggers a Λ-vortex and consequent turbulent structure. These results reveal the existence of a transition mechanism in hypersonic boundary layers similar to that in low-speed boundary layers.

This experimental study provides striking examples of the complex flow and turbulence structures resulting from blade–wake and wake–wake interactions in a multistage turbomachine. Particle image velocimetry measurements were performed within the second-stage rotors of a two-stage compressor. The first-stage stator wake is distorted and produces a kink structure in the second-stage rotor blades passage. This kink, also called a turbulent hot spot, with concentrated vorticity, high turbulence levels, and high turbulence kinetic energy, is caused by the interaction between the first-stage rotor wake and the stator wake. A high-speed region and a low-speed region are observed around the turbulent hot spot. The perturbation velocity is counterclockwise around the turbulent hot spot, with a magnitude much larger than that in the wake. The turbulent hot spot is more unstable and active than the wake and, thus, might play a pivotal role in the passage flow. The high turbulence and the negative jet behavior of the wake dominate the interaction between the unsteady wake and the separated boundary layer on the suction surface of the blade. When the upstream wake impinges on the blade, the boundary layer thickness first increases owing to the presence of the negative jet, and a thickened boundary layer region in the form of a turbulent spot is formed because of the high turbulence intensity in the wake. Then, the boundary layer gradually becomes thinner because of the presence of a calmed region that follows the thickened boundary layer region. Finally, the boundary layer gradually thickens again and recovers to separation. Thus, the boundary layer thickness is periodic in a wake passing cycle.