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In this paper, an experimental study on the stability of hypersonic plate boundary layer is carried out using a spanwise plasma actuation array. The characteristics and evolution of different kinds of unstable waves in the hypersonic plate boundary layer are analyzed based on the results of linear stability theory and high-frequency pulsation sensors. The typical morphological characteristics of the boundary layer and the macro-control effect of the plasma actuation array are explored through a high-speed schlieren method. Finally, based on grayscale mode extraction and proper orthogonal decomposition (POD), the influences of three different actuation frequencies on the instability waves and characteristic structure of the boundary layer are studied, including the dominant frequency of the first mode wave, the half-frequency of the dominant frequency of the first mode wave, and the dominant frequency of the second mode wave, the change of characteristic structures under the regulation of plasma actuation is further discussed. The corresponding regulation rules and mechanisms are summarized. The results show that the plasma actuation array can advance the starting position of laminar discontinuities and that the induced coherent structure can excite instabilities at an earlier flow-direction position. The actuation can be used to control the stability of the boundary layer by acting on the first mode wave to break the original unstable wave spectrum characteristics. This verifies the ability of extensional array plasma actuation to regulate the stability of the hypersonic plate boundary layer and suggests it has great potential in the promotion of hypersonic boundary layer transition.

Boundary layer transition has always been a frontier in the field of fluid mechanics, and hypersonic boundary layer stability experiments can help reveal the physical mechanisms behind such transitions. In particular, the regulation of unstable waves in boundary layers is critical for transition prediction and control. Plasma actuation is a popular flow control method that has made progress in moderating the stability of supersonic boundary layer. However, there have been few studies on regulating the stability of hypersonic boundary layers with plasma actuation. In this paper, wind tunnel experiments are carried out under Mach 6 flow to study the stability regulation of a hypersonic sharp cone boundary layer with nanosecond pulsed plasma actuation. First, the typical characteristic structure of the rope-like structure is captured by the high-speed schlieren method. Then, combining the sensor results and theoretical analysis, the rope-like structure and the dominant instability wave resolved by the schlieren power spectrum density method are determined to correspond to the second-mode wave. The characteristic unstable quasi-ordered structure of the boundary layer under actuation is then extracted, and the impact effect and modulation effect of this structure on the second-mode wave are analyzed. Finally, the mechanism by which actuation influences the boundary layer instability is studied using proper orthogonal decomposition. The results show that actuation can enhance boundary layer pulsation, and the coupling effect between the actuation and boundary layer can produce regular unstable quasi-ordered structures. The intrinsic mechanism works by impinging on and modulating the second-mode waves, and there are characteristic modes of the rope-like structure and the unstable structures distributed over the whole flow-direction range. This verifies the ability of plasma actuation to stimulate the instability of hypersonic cone boundary layers and provides technical support for the further development of transition control methods.

Swept shock wave/boundary layer interactions occur widely in the internal and external flows of supersonic and hypersonic aircraft, which can seriously affect aircraft stability and engine intake efficiency. Based on the conventional S–A turbulence model, this study investigates surface arc plasma actuation for regulating swept shock wave/boundary layer interactions at Mach 2.95 to explore the ability and the three-dimensional shock wave/boundary layer interactions control method of plasma actuation. First, the flow control effect is explored in terms of indirect control by applying actuation in the upstream boundary layer or in front of the separation line, and in terms of direct control by applying actuation in the separation region. These three methods all achieve clear control effects. Control results show that the first method is more effective in regulating the wall pressure and friction coefficient and can improve the friction and heat transfer of the wall in a wide range of flow direction and cone direction. The second method is more effective in regulating separated shock waves. The third aspect is more effective in regulating the reattachment region. The associated control mechanisms are then refined. The control effects of the first control method depend on the transmission of vortices, those of the second are based on the virtual surface generated by actuation, and those of the third rely on energy injection. Finally, the application scenarios of the different control methods are determined according to the flow control requirements of aircraft and the corresponding control mechanisms. This study provides a reference method for solving more complex three-dimensional shock boundary layer interaction problems.

An experimental study on supersonic cavity flow control using a spanwise pulsed spark discharge array (SP-PSDA) is performed in this paper. High-speed schlieren imaging at a frame rate of 50 kHz is deployed for flow visualization. The schlieren snapshots, as well as their statistics, are analyzed to reveal the supersonic cavity flow control effect and its underlying mechanism. Results show that the shear layer presents a wave-like oscillation due to thermal bulbs induced by SP-PSDA. Specifically, the shear layer structure in the baseline case resembles an incomplete hairpin structure, which becomes complete after plasma actuation. SP-PSDA actuation at 5 kHz has a better control effect, which enhances the IRMS of the whole hairpin structure and produces several channels within it—these aid momentum transport within the shear layer. According to the results of proper orthogonal decomposition, the thermal bulbs couple with the shear layer to form large-scale coherent structures. These structures excite the Kelvin–Helmholtz instability, converting the oscillation frequency of the shear layer to an actuation frequency.