Restricted by the cavity combustor configuration size and influenced by the high-speed airflow, fuel mixing efficiency within a scramjet combustor is low. To address this issue, based on a pulsed arc plasma actuator matrix, a synchronous actuation mode and two traveling wave actuation modes were designed. High-speed schlieren imaging was employed to conduct tests on plasma actuation for controlling the supersonic cavity flow field. The control effectiveness of the number of synchronously actuated actuator columns and the different actuation modes on cavity flow fields with three different aft-wall inclination angles were comparatively analyzed by synthesizing flow field evolution and statistical analysis of schlieren images. Plasma actuation can effectively excite density fluctuations in the cavity shear layer. Under baseline (no actuation) conditions, as the aft-wall inclination angle increases, the fluctuation intensity of the shear layer exhibits a monotonically increasing trend due to enhanced cavity resonance; moreover, the larger the aft-wall inclination angle, the smaller the increment in density fluctuations induced by plasma actuation. In the synchronous actuation mode, as the number of actuated columns increases, the disturbance range to the flow field expands, leading to a monotonic increase in the amplitude of shear layer density fluctuations. Both traveling wave actuation modes outperform the synchronous mode. The upstream traveling wave mode distributes disturbances evenly over one actuation cycle, aiming to increase the equivalent actuation frequency. Conversely, the downstream traveling wave mode superimposes the thermal bulbs induced by plasma actuation along the streamwise direction, enhancing the degree of interaction with the shear layer and causing a significant increase in its fluctuation amplitude. Physically, the shock waves and thermal bulbs induced by plasma actuation are the root causes of the disturbances. These disturbances thicken the boundary layer and excite instabilities in the shear layer, thereby leading to the generation of wavy oscillations.

Aircraft friction drag accounts for more than half of the total cruising drag; thus, turbulent friction drag reduction is vital to improving flight performance and reducing flight costs. Plasma actuation is a controllable disturbance of pressure, temperature, and physical property changes caused by discharging gas or the moving plasma subjected to electromagnetic force. Compared to other turbulent friction drag reduction methods, plasma actuation has many advantages, such as lightweight and intelligent control capability, so it has received widespread attention. This paper provides a detailed overview of the development history and research progress of the plasma-based turbulent drag reduction method. In flat-plate turbulent boundary layers, plasma actuation can reduce friction drag by more than 40% by inducing large-scale vortical structures that enhance transportation. For airfoils at cruising angles of attack, plasma actuation can reduce the total drag of airfoils by up to 13.7% at a freestream velocity of 20 m/s. Developing new efficient plasma drag reduction actuation methods, adapting to flows with high Reynolds numbers and strong pressure gradients, and developing from open-loop blind control to intelligent adaptive control combining artificial intelligence, are the future development trends of the plasma-based turbulent drag reduction method.

Dielectric Barrier Discharge (DBD) based turbulent drag reduction methods are used to reduce the total drag on a NACA 0012 airfoil at low angels of attack. The interaction of DBD with turbulent boundary layer was investigated, based on which the drag reduction experiments were conducted. The results show that unidirectional steady discharge is more effective than oscillating discharge in terms of drag reduction, while steady impinging discharge fails to finish the mission (i.e. drag increase). In the best scenario, a maximum relative drag reduction as high as 64 % is achieved at the freestream velocity of 5 m/s, and a drag reduction of 13.7 % keeps existing at the freestream velocity of 20 m/s. For unidirectional discharge, the jet velocity ratio and the dimensionless actuator spacing are the two key parameters affecting the effectiveness. The drag reduction magnitude varies inversely with the dimensionless spacing, and a threshold value of the dimensionless actuator spacing of 540 (approximately five times of the low-speed streak spacing) exists, above which the drag increases. When the jet velocity ratio smaller than 0.05, marginal drag variation is observed. In contrast, when the jet velocity ratio larger than 0.05, the experimental data bifurcates, one into the drag increase zone and the other into the drag reduction zone, depending on the value of dimensionless actuator spacing. In both zones, the drag variation magnitude increases with the jet velocity ratio. The total drag reduction can be divided into the reduction in pressure drag and turbulent friction drag, as well as the increase in friction drag brought by transition promotion. The reduction in turbulent friction drag plays an important role in the total drag reduction.

Large-scale wind farms with multiple rows of horizontal-axis wind turbines suffer from significant power losses (30%-40%) due to wake interactions. To deal with this situation, the yaw-based active wake control (AWC) has been proposed. The principle of AWC is to yaw the upstream wind turbines so that the wake can be deflected away from the turbine row, which hopefully will lead to a net gain of the total wind farm power production. In this paper, the progress of the AWC technique in the past decade is reviewed from four aspects: wake models of single non-yawed wind turbine, wake models of single yawed wind turbine, wake superposition methods for multiple wind turbines, and wind farm power optimization. Meanwhile, issues needed to be addressed before being applied to engineering are summarized. Based on these reviews, it is fair to conclude that the AWC technique is more or less mature now, in the sense that earlier laboratory results from analytical modeling, numerical simulations, and wind tunnel studies have been successfully applied to field tests of commercial wind farms, and significant improvement of the net power gain has been obtained. In terms of theoretical progress, EPFL Gaussian wake models, primary and secondary wake deflection models based on the vortex-induced cross-wind velocities, momentum-conserving wake superposition laws are increasingly becoming the standard in the wind farm power prediction. Regarding practical engineering, it has been found that a whole bunch of parameters such as turbine rows, streamwise turbine spacing, turbulence intensity, thermal instability of atmospheric boundary layer, wind speed, and direction variability can affect the magnitude of net power gain in the active wake control. According to recent field tests performed by National Renewable Energy Laboratory (NREL, US) and Stanford University, AWC is able to improve the total wind farm power production by 5%-15% if the wind direction is aligned with turbine rows, and when these net power gains are averaged over all wind directions, an increase of 1%-3% in the wind farm efficiency is expected.