Borui Zheng

Asymmetric vortices over slender bodies at high angles of attack can generate large side forces and degrade flight stability. Here, a reinforcement-learning-based closed-loop plasma control method is proposed and validated online in wind tunnel tests. A wide-sector dielectric-barrier-discharge plasma actuator generates oblique wall jets, while pressure-derived side-force feedback is used to adapt the actuation parameters autonomously in real time. A centerline paired-difference method is further introduced to quantify leeward pressure imbalance from circumferential pressure measurements. Under the present conditions, closed-loop control outperforms both steady and open-loop actuation by suppressing the dominant leeward positive-vorticity structure, reducing the mean pressure difference from 0.807 to 0.241 and decreasing the side-force coefficient from 0.57 to 0.16. These results demonstrate the feasibility of real-time pressure-feedback plasma control for adaptive suppression of slender-body asymmetric vortices at high angles of attack.

This study experimentally characterizes the unsteady induced-flow response of a wide-sector dielectric barrier discharge plasma actuator (WS-DBD-PA) designed to extend the flow-control capability of conventional geometric DBD actuators, which predominantly impart streamwise momentum. Planar two-dimensional particle image velocimetry is employed to characterize the evolution of the near-wall jet under unsteady actuation. Owing to the coupling between a streamwise near-wall jet and a wall-normal vortex response, the WS-DBD-PA produces a broad near-wall induced flow within the sector region. The transient evolution of this induced flow can be classified into four stages: vortex-core formation, vortex growth, jet expansion, and stabilized jet development. Vorticity–velocity correlation analysis indicates that duty cycle and pulse frequency jointly regulate near-wall momentum injection and downstream diffusion. Reduced-order analyses based on proper orthogonal decomposition and dynamic mode decomposition further show that the duty cycle mainly affects the energy envelope, whereas the pulse frequency modulates the discreteness of momentum input, together influencing the transition between jet- and vortex-dominated flow responses. Under the tested unsteady-actuation conditions, the nondimensionalized velocity profiles collapse onto a self-similar trend, which can be empirically described by a quadruple-Gaussian correlation with good agreement (R2 ≈ 0.95). Overall, the results provide experimental evidence, within the present planar measurement framework, for interpreting the coupled streamwise-jet and wall-normal vortex response of the WS-DBD-PA under unsteady actuation.

Background Oriented Schlieren (BOS) technology, known for its simple setup, low cost, and applicability to visualize complex flow fields over large areas, has become a crucial method for flow field visualization. However, current BOS technology has several limitations, such as inadequate real-time performance, challenges in quantitative analysis, and high computational resource requirements. To address these limitations, this study presents the Real-time Gradient-based BOS (RTG-BOS) method, which enhances optical flow performance by substituting the traditional constant brightness assumption with a constant gradient assumption. This gradient assumption is applied to both the Dense Inverse Search (DIS) and Farneback algorithms, leveraging the principle that local image structures remain stable over short time intervals, despite variations in brightness. Additionally, the preprocessing pipeline includes sub-pixel interpolation and a three-layer progressive temporal smoothing to enhance image quality and retain schlieren image details. To achieve real-time processing, a GPU-based parallel architecture is employed with an optimized data pipeline. This design allows for concurrent module execution and full GPU resource utilization, thereby accelerating the entire BOS workflow. Experimental validation shows that the proposed method achieves 30 frames per second in most scenarios, fulfilling the real-time requirements for industrial applications.

Low-temperature plasma technology has demonstrated significant potential for applications such as gas treatment and energy conversion. This study investigates the optimization of a three-electrode discharge system, combining dielectric barrier discharge (DBD) and direct current (DC) discharge at ambient conditions. By experiment and simulation, the effects of electrode geometry, location, and discharge parameters on plasma stability are analyzed. Results reveal that DC-induced pre-ionization plays a crucial role in transitioning from streamer to diffuse discharge, requiring a minimum pre-ionization density of 2×1017 m−3. Smaller electrode diameters enhance the electric field and reactive species densities, while optimized discharge gaps ensure stable and spatial discharge. A minimum voltage of −6 kV (DC) and 3.5 kV (DBD) are necessary for breakdown, with breakdown time decreasing as voltage increases. Power analysis shows dominant energy consumption by the DBD component, emphasizing the need for DBD voltage minimization to improve energy efficiency. These findings provide valuable insights into achieving stable and energy-efficient plasma discharges, offering a foundation for scalable applications in gas treatment and related fields.

Slender-body aircraft operating at high angles of attack often experience nonlinear, asymmetric multi-vortex flow structures that generate random, unsteady lateral forces, undermining stability, and maneuverability. Dielectric barrier discharge plasma actuators can eliminate these lateral forces. However, conventional open-loop plasma control method cannot adapt to dynamic flow fields in real time, limiting the overall effectiveness of active flow control. This study introduces a plasma control framework grounded in physical principles and develops plasma actuator design methods to regulate vortex interactions, stabilize flow dynamics, and optimize control efficiency. An intelligent closed-loop flow control strategy based on proximal policy optimization, a deep reinforcement learning algorithm, is utilized to enable real-time plasma parameter adjustments for suppressing lateral force at high angle of attack. The spatiotemporal interaction of plasma-induced and asymmetric vortices was investigated through synchronized pressure measurements and particle image velocimetry. The proximal policy optimization-based parameter optimization model was trained online in an educational open-return wind tunnel and subsequently deployed in a low-speed closed-return wind tunnel. Based on vortex stability analysis and comprehensive results, the closed-loop control algorithm significantly mitigates lateral forces, achieving a 68.5% reduction compared to steady plasma actuation, while improving energy efficiency by 70% over conventional methods.

Reducing turbulent skin friction drag is a fundamental goal for aircraft transportation to conserve energy and decrease emissions. We introduce an annular dielectric barrier discharge plasma actuator (A-DBD-PA) that merges the advantages of near-wall micro-blowing with pulsed plasma flow control to reduce turbulence drag. Wind tunnel experiments on a flat plate assessing the performance of A-DBD-PA revealed that the wall-normal jet on the symmetry plane is critical for turbulent drag reduction in an unsteady flow field. As the duty cycle of plasma actuation increases, it steadies the wall-normal jet, which diminishes shear stress and velocity fluctuations in the boundary layer. This enhanced steadiness fosters induced vortices' formation and evolution, directly impacting the drag reduction rate. Duty cycles below 50% yield a limited drag reduction rate because the airflow's viscous effects predominate over the influence of plasma actuation. Conversely, duty cycles above 50% enhance the interaction of induced vortices, contributing to a stronger disturbance and more effective control, optimizing drag reduction rate up to a maximum of 5.197%.

Reduction of turbulent drag force is one of important works in the design of airplanes and hypersonic aircrafts. The annular plasma synthetic jet (APSJ) has become an interesting and popular flow control method in reducing the drag of turbulent boundary layers. In this paper, a comprehensive experimental study is carried out on the turbulent drag reduction regulated by an array of annular plasma synthetic jet actuators. The effects of the operating parameters such as the actuation voltage, the pulse frequency and the incoming wind speed on the drag reduction rate are studied and discussed in detail. The performances of the plasma actuator array are evaluated and summarized at multiple working conditions. Finally, the coherent structures of turbulence and the hairpin-like vortices are depicted and discussed. The results show that the optimal drag reduction rate is achieved, as the actuation voltage, pulse frequency and incoming wind speed are Vpp = 7 kV, fp= 50 Hz, and U∞ = 7 m/s, respectively. A resonant coupling phenomenon is observed when the pulse frequency of the actuators approaches the characteristic frequency of the coherent structure. The upward sweep flow induced by APSJ actuators may lead to a reduction of the turbulent drag force, but the downward wash flow leads to an increase in the drag. Present study could provide solid experimental data and a helpful guidance for the drag reduction of an airplane.

There are numerous scientific and engineering fields where the surface dielectric barrier discharge driven by nanosecond pulses (NS-SDBD) has important applications. To improve its performance, more research is still needed on the effects of electrical parameters on the NS-SDBD actuator’s discharge characteristics. In this study, a two-dimensional numerical model based on 13 discharge particle chemical processes was constructed using a numerical simulation approach, producing findings for the NS-SDBD actuator’s voltage–current (V-A) characteristics, discharge profile, and spectrum analysis. Additionally, a comprehensive investigation into the trends and underlying mechanisms of the effects of the voltage amplitude, pulse width, rise time, and fall time parameters on the discharge behavior of the NS-SDBD actuator was carried out. The results show that higher voltage amplitudes increase the maximum current and electron density, which enhances the plasma excitation effect. The peak power deposition during the second discharge is also raised by longer pulse widths and rise times, whereas the total power deposition during the second discharge is decreased by longer fall times.

The plasma flow control devices include plasma actuators (PAs) and plasma power supplies. Based on the principle of capacitive charging and discharging, this article independently developed a microsecond pulse plasma power supply (MPPS). At the same time, based on the traditional dielectric barrier discharge PA (DBD-PA) configuration, an exposed electrode was added to form a three-electrode sliding discharge PA (TSD-PA), which can effectively increase the plasma discharge area and improve the effect of flow control. In this article, three aspects of electrical, temperature, and aerodynamic characteristics are studied experimentally. The experimental results show that the maximum time-averaged power of the microsecond pulse sliding discharge is less than 1 W, but the instantaneous power can reach up to 14.16 kW. When the actuator is working, due to the thermal effect caused by the collision between plasma particles, the temperature of the discharge area increases sharply, and the maximum working temperature reaches 51.4 $^{\circ}$ C. In addition, the microsecond pulse superimposed on dc voltage of different polarities generates different induced flow structures. A “cavity” structure is also found in the flow field, and it may be due to the strong adsorption effect of the dc electrode on the charge, resulting in a large amount of charge accumulation.