The streamwise pulsed arc discharge array (S-PADA), in which five actuators are connected in series with adjustable frequency, is employed to control the shock wave/boundary layer interaction (SWBLI) at a 24° compression ramp in a M = 2.0 flow and under two Reynolds numbers based on boundary layer thickness (Re1 = 46 800 and Re2 = 11 700). High-speed schlieren imaging at 50 000 fps is used for flow visualization. The schlieren snapshots, as well as the statistics of the image sequence, namely, mean and root-mean-square, are examined to reveal the control outcome. The results show that the separated wave foot gradually presents bifurcation and partial disappearance under Re1 with the increasing pulse number of the S-PADA, indicating the decline in the shock intensity. The increase in frequency does affect the control outcome remarkably because shock weakening effect can be achieved under Re1 through 10 kHz and 20 kHz actuations, while no obvious change can be observed by the 5 kHz actuation. The experiments under Re2, where little control effect is exerted by the same methods, are also discussed. It is believed that the separated wave under a lower Reynolds number of Re2 presents the poorly developed turbulent boundary layer; hence, the effective SWBLI control is difficult to be ensured.
Plasma synthetic jet actuator (PSJA), which produces pulsed jets, is used to control the shock wave boundary layer interaction at a compression ramp at Ma=2.0. The flow topology of the wall jets from the PSJA is first visualised through particle laser scattering (PLS) photography. The PSJA aperture effect is also examined by comparing the jets out of the apertures of 1,2 mm and 2 mm respectively. The control effect is later investigated by both PLS and particle image velocimetry (PIV). , which was erupted from different jet apertures of 1.2mm and 2mm, were compared experimentally in a wind tunnel of Mach 2. Further, the interaction between the TPJ and the ramp induced separation was explored. The phase-locked two-component particle image velocimetry (PIV) and particle laser scattering (PLS) were used for flow visualizations. The K-H vortices and hairpin vortices due to the shear stress between the jet plume and high-speed mainstream were identified.The results show that the TPJ in supersonic flow is characterized by two typical parts: the attached jet plume (AJP) and the detached jet plume (DJP). The penetration height of the jet plume, which is closely related to the jet aperture, plays a dominant role in the proportion of the two parts. The higher jet penetration height leads to the more detached jet plume. As for the interaction between the jet plume and separation zone, the attached jet plume was blocked by the separation zone, which formed a recirculation zone and contributed to an expansion of the separation. In contrast, the detached jet plume transited along the shear layer and then enhanced the velocity exchange between the shear layer and mainstream. Ultimately, the reduction of the separation zone was revealed with the overall shear layer reduced. Furthermore, a conceptual model based on two typical morphological features was suggested to reveal the interaction mechanism.
Hypersonic boundary layer transition is a hot yet challenging problem restricting the development and breakthrough of hypersonic aerodynamics. In recent years, despite great progress made by wind tunnel experiment, transition mechanism and transition prediction, only partial knowledge has been gained so far. In this paper, firstly, the specific scenarios of hypersonic boundary layer transition control are clarified. Secondly, the experimental research progress and mechanism of passive control and active control methods under different hypersonic transition control demands are summarized, with their advantages and disadvantages being analyzed separately. Plasma actuation is easy to produce controllable broadband aerodynamic actuation, which has potential in the field of boundary layer transition control. Hence, the following part of the paper focuses on plasma flow control. The feasibility of plasma actuation to control the hypersonic boundary layer transition is demonstrated and the research ideas are presented. Finally, hypersonic boundary layer transition control methods are summarized and the direction of future research is prospected.
A streamwise array of pulsed spark discharge operating at a high frequency of 10 kHz is used to control the shock wave/boundary layer interaction (SWBLI) induced by a 24° compression ramp (CR) in a Mach 2.0 flow. High-speed schlieren imaging at 50 000 frames/s is deployed for flow visualization. The schlieren snapshots, as well as statistics of the schlieren sequence, show that the intensity weakening effect on the shock wave can be achieved for a long time up to 20 ms, which is the first experimental demonstration of stable control on the CR-SWBLI with plasma actuators. Then, based on fast Fourier transform analyses, the low-frequency unsteadiness of the shock wave is also significantly modified and the dominant oscillation frequency is switched to the high frequency St = 0.32. The mechanism of the encouraging control outcome is finally discussed that the strong separation wave induced by a CR is replaced by a multistage weak compression wave produced through a high-frequency actuator array.
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