Schlieren imaging has been widely used in science and technology to investigate phenomena occurring in transparent media. In particular, it has proven to be a powerful tool in fundamental studies and process optimization for atmospheric pressure plasma diagnostics, providing qualitative and (in some cases) also quantitative information on the fluid-dynamic characteristics of plasmas generated by many different types of sources. However, obtaining significant and reliable results by schlieren imaging can be challenging, especially when considering the variety of geometries and applications of atmospheric pressure plasma sources. Therefore, it is necessary to adopt solutions that can address the specific issues of different plasma-assisted processes. In this paper, an overview on the use of the schlieren imaging technique for atmospheric pressure plasma characterization is presented. In the first part, the physical principles behind this technique and the different setups that can be adopted to perform it are presented. In the second part, examples of schlieren imaging applied to different kinds of atmospheric pressure plasmas (non-equilibrium plasma jets, plasma actuators for flow control and thermal plasma sources) are presented, showing how it was used to characterize the fluid-dynamic behavior of plasma-assisted processes and reporting best practices in performing this diagnostic technique.

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Schlieren imaging: a powerful tool for atmospheric plasma diagnostic

Unified mechanisms for separation control around airfoil using plasma actuator with burst actuation over Reynolds number range of 10 3 -10 6

Photocatalytic removal of elemental mercury from flue gas using multi-walled carbon nanotubes impregnated with titanium dioxide

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%.

Turbulent skin-friction drag reduction by annular dielectric barrier discharge plasma actuator

Aerodynamic Enhancement and Improving the Performance of a Six-Megawatt DOWEC Wind Turbine by Micro-Plasma Actuator

Dielectric Barrier Discharge (DBD) plasma actuators can generate trains of vortices under burst mode actuation, which is very different from the wall jet flow under steady operation. To clarify the mechanism of DBD under burst mode actuation on separation control, the improved simulation model of burst-mode plasma actuation was developed and validated, and then, the flow field of NACA0015 under 12° and 14° of stall was numerically simulated and compared with the experimental result simultaneously. The results show that the DBD plasma actuator under burst mode actuation is very efficient for separation flow control, and the ability of separation flow control under F+ = 10 is more efficient than that under F+ = 0.3–5 investigated. The mechanism of burst mode actuation proposed that trains of vortices shed by burst mode actuation could enhance the momentum transfer between the boundary layer and the main flow, and the vortex of large scale shed from the leading edge could merge with the vortices of small scale shed by the DBD plasma actuator with burst mode actuation, which results in the reattachment of separated flow.

Simulation of NACA0015 flow separation control by burst-mode plasma actuation

A tri-electrode dielectric barrier discharge (DBD) plasma actuator, which consists of a traditional DBD plasma actuator and an additional direct current (dc) electrode, is proposed to achieve a deflected synthetic jet ranging from 0 deg to 180 deg by only changing the polarity and magnitude of the dc component. Depending on the polarity of dc voltage, the tri-electrode plasma actuator can be operating in three discharge modes: traditional DBD discharge, extended discharge, and sliding discharge. The characteristics of three discharge modes are revealed and analyzed. The evolution of vortices induced by the tri-electrode plasma actuator under burst-mode actuation is investigated by particle image velocimetry (PIV) measurement and schlieren visualization. Results show that in DBD discharge mode, the vortices induced by the tri-electrode plasma actuator is symmetrical and a vertical synthetic jet forms. However, a deflected synthetic jet forms due to the asymmetrical vortices induced by extended discharge and sliding discharge. The deflection mechanism is analyzed from the viewpoint of body force induced by the tri-electrode plasma actuator. The deflected angle of the synthetic jet is identified from PIV measurement and schlieren visualization and found to be nearly linear with , which probably will be a promising result for further flow control applications. 

Deflected Synthetic Jet due to Vortices Induced by a Tri-Electrode Plasma Actuator

The pressure field of plasma flow induced by surface-dielectric-barrier-discharge plasma actuators in quiescent air is difficult to measure accurately due to the very slight variations in pressure ...

Body Force Calculation of Steady-State Plasma Flow with Accurate Pressure Measurement

To alleviate the deterioration in wind turbine performance caused by dynamic stall, the flow control of a pitching NACA0012 airfoil is investigated through numerical simulation of an alternating current dielectric barrier discharge (AC-DBD) plasma actuator at a Reynolds number Re = 135 000. To avoid the harmonic oscillations of aerodynamic force caused by unsteady DBD actuation, this work focuses on improving the control potential for steady actuation. The control mechanisms of actuators at various positions are investigated using five groups of actuators mounted at 0%, 3%, 10%, 45%, and 80% chord lengths c above the upper surface of an airfoil. The actuator at 80% c performs more efficiently in terms of lift enhancement in the initial upstroke and the final downstroke. The actuator at 0% c suppresses the growth of the leading-edge vortex and maintains the suction of the dynamic stall vortex (DSV). After the shedding of the DSV, it suppresses the secondary separation to delay the onset of dynamic stall. At the flow reattachment stage, the actuators at 3% c and 10% c accelerate the boundary layer reattachment by momentum injection. From these results, a multi-DBD control strategy is proposed. The scheme selects the optimal actuator in operation at a certain stage of dynamic stall and takes advantage of actuators at different positions to enhance the average and maximum aerodynamic force, delay the onset of dynamic stall, accelerate flow reattachment, and avoid excessive energy consumption.

Numerical simulation of dynamic stall flow control using a multi-dielectric barrier discharge plasma actuation strategy

Using single and multiple dielectric barrier discharge plasma actuators, the velocity fields induced by steady, pulsed, asynchronous, and duty cycle plasma actuation are studied based on Particle Image Velocimetry (PIV). First, the flow fields induced by a single dielectric barrier discharge with different discharge models and actuation parameters are investigated. According to the velocity distributions, a parametric study on the influence of excitation voltages, carrier frequencies, pulse frequencies, and duty ratios on plasma actuation is presented and the optimum actuation parameters are provided. Emphatically, through double actuators alternating discharge, the peristaltic acceleration plasma actuation, a new plasma actuation mode, has been proposed and studied. The efficiency of the plasma actuators in steady and pulsed discharge modes is explored. The velocity field induced by plasma actuation with different phase angles and duty cycle ratios is acquired and the momentum transfer characteristics of the flow field were discussed. Finally, based on the above-mentioned results, the technical superiority and mechanism of the peristaltic acceleration induced by multiple dielectric barrier discharge plasma actuators are analyzed.

Parametric Study of Paraelectric and Peristaltic Pulsed DBD Plasma Actuation for Aerodynamic Flow Control Based on PIV

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