Plasma Actuator Research Articles

A computational study of a laminar methane–air flame assisted by nanosecond repetitively pulsed discharges

Nanosecond repetitively pulsed (NRP) discharges have been considered a promising technique for enhancing combustion efficiency and control. For successful implementation, it is necessary to understand the complex plasma–combustion interactions involving chemical, thermal, and hydrodynamic pathways. This paper aims to investigate the mechanisms enhancing a laminar methane–air flame assisted by NRP discharges by high fidelity simulations of the jet-wall burner employed in a previous experimental study. A phenomenological plasma model is used to represent the plasma energy deposition in two channels: (1) the ultrafast heating and dissociation of O2 resulting from the relaxation of electronically excited N2 , and (2) slow gas heating stemming from the relaxation of N2 vibrational states. The flame displacement, key radical distribution and flame response under plasma actuation are compared with experimental results in good agreement. The computational model allows a systematic investigation of the dominant physical mechanism by isolating different pathways. It is found that the kinetic effect from atomic O production dominates the flame dynamics, while the thermal effect plays a minor role. Hydrodynamic perturbations arising from weak shock wave propagation appear to be sensitive to burner geometry and is found to be less significant in the case under study.

Plasma Actuators Based on Alumina Ceramics for Active Flow Control Applications

Plasma actuators have demonstrated great potential for active flow control applications, including boundary layer control, flow separation delay, turbulence control, and aircraft noise reduction. In particular, the material used as a dielectric barrier is crucial for the proper operation of the device. Currently, the variety of dielectrics reported in the literature is still quite restricted to polymers including Kapton, Teflon, poly(methyl methacrylate) (PMMA), Cirlex, polyisobutylene (PIB) rubber, or polystyrene. Nevertheless, several studies have highlighted the fragilities of polymeric dielectric layers when actuators operate at significantly high-voltage and -frequency levels or for long periods. In the current study, we propose the use of alumina-based ceramic composites as alternative materials for plasma actuator dielectric layers. The alumina composite samples were fabricated and characterized in terms of microstructure, electrical parameters, and plasma-induced flow velocity and compared with a conventional Kapton-based actuator. It was concluded that alumina-based dielectrics are suitable materials for plasma actuator applications, being able to generate plasma-induced flow velocities of approximately 4.5 m/s. In addition, it was verified that alumina-based ceramic actuators can provide similar fluid mechanical efficiencies to Kapton actuators. Furthermore, the ceramic dielectrics present additional characteristics, such as high-temperature resistance, which are not encompassed by conventional Kapton actuators, which makes them suitable for high-temperature applications such as turbine blade film cooling enhancement and plasma-assisted combustion. The high porosity of the ceramic results in lower plasma-induced flow velocity and lower fluid mechanical efficiency, but by minimizing the porosity, the fluid mechanical efficiency is increased.

Investigation of Transition Flow on Suction Surface of Highly Loaded Compressor Cascade Controlled by Plasma Actuator

A practical method for improving the performance of highly loaded compressor cascades is to delay the onset of transition. Plasma actuators are used on the suction surface to manage the transition from laminar to turbulent by injecting momentum into the boundary layer. An experimental study was conducted in a low-speed wind tunnel to validate the actuator's control capability and the numerical simulation model. Both experimental and numerical results show that actuators located at various positions can improve the performance of the cascade and reduce flow loss to varying degrees. The most effective method is to place the actuator upstream of the separation bubble to delay the onset of the transition from laminar to turbulent flow, which results in a 6.32% decrease in total pressure loss and a 2.5% increase in static pressure rise. The control scheme at the transition position restores laminar flow locally, causing a secondary transition. However, it has the lowest control capability due to the dissipation of the actuator momentum. The downstream control scheme only suppresses the degree of turbulent separation without delaying laminar separation, resulting in weaker control effectiveness compared to the upstream laminar separation scheme. The study also examined the interaction between the plasma actuator on the suction surface and the flow in the endwall region, which showed that the accumulation of low-energy fluid in the corner region increases, resulting in a slight elevation of shear losses between the midspan boundary layer and the corner region.

Airfoil friction drag reduction based on grid-type and super-dense array plasma actuators

To improve the cruise flight performance of aircraft, two new configurations of plasma actuators (grid-type and super-dense array) were investigated to reduce the turbulent skin friction drag of a low-speed airfoil. The induced jet characteristics of the two actuators in quiescent air were diagnosed with high-speed particle image velocimetry (PIV), and their drag reduction efficiencies were examined under different operating conditions in a wind tunnel. The results showed that the grid-type plasma actuator was capable of producing a wall-normal jet array (peak magnitude: 1.07 m/s) similar to that generated in a micro-blowing technique, while the super-dense array plasma actuator created a wavy wall-parallel jet (magnitude: 0.94 m/s) due to the discrete spanwise electrostatic forces. Under a comparable electrical power consumption level, the super-dense array plasma actuator array significantly outperformed the grid-type configuration, reducing the total airfoil friction drag by approximately 22% at a free-stream velocity of 20 m/s. The magnitude of drag reduction was proportional to the dimensionless jet velocity ratio (r), and a threshold r = 0.014 existed under which little impact on airfoil drag could be discerned.

Flow Control Over a Finite Wall-Mounted Square Cylinder by Using Multiple Plasma Actuators

Abstract The present study aims to investigate the effectiveness of plasma actuators in controlling the flow around a finite wall-mounted square cylinder (FWMSC) with a longitudinal aspect ratio of 4. The test is conducted in a small-scale closed return-type wind tunnel. The Reynolds number of the experiments, Red, is 500 based on the width of the bluff body and the freestream velocity. The plasma actuators are installed on the top surface and the rear surface of the square cylinder. The induced flow velocities of the plasma actuators are modulated by adjusting the operating voltage and frequency of the high-voltage generator. In this work, particle image velocimetry (PIV) is used to obtain the velocity fields. Furthermore, force calculations are conducted to investigate the effect of using plasma actuators with different driving voltages on the drag force. Our results show that the plasma actuators can successfully suppress flow separation and reduce the turbulent kinetic energy (TKE) in the wake. A correlation between the drag coefficient and the operating voltage of the power generator is also revealed, and the mean drag coefficient is found to decrease with increasing imposing voltage. The plasma actuators can enhance the momentum exchange and the interactive behavior between the shear layer and the flow separation region, resulting in flow reattachment at the free end and shrinkage of the recirculation zone in the near-wake region of the bluff body. Overall, the present study demonstrates the practical effectiveness of using plasma actuators for flow control around FWMSC.

Advances and opportunities in wind energy harvesting using plasma actuators: a review

Abstract The dielectric barrier discharge plasma actuator has been recognized as a leading technology for controlling fluid flow and has found remarkable applications in wind energy harvesting over the past decade. Wind turbine aerodynamics are critical in this concept and performance is mainly determined by flow controllers, although significant technical progress is still required. This paper examines all the critical studies to investigate the potential application of plasma actuators for airflow control over wind turbines. This approach has been divided into three categories: wind turbine airfoils, horizontal-axis wind turbines and vertical-axis wind turbines aerodynamic performance and generated power. Finally, the potential functions of plasma actuators in current and future wind turbine generators are discussed. These actuators offer promising solutions to increasing power output, minimizing torque fluctuations and enabling self-starting capabilities, particularly in vertical-axis wind turbines. By adjusting blade pitch angles in conjunction with plasma actuators, significant improvements in airflow optimization and power extraction have been demonstrated. Despite the advancements, challenges persist, such as determining optimal actuator placement and overcoming structural limitations, especially concerning 3D effects and high Reynolds numbers. While plasma actuators enhance aerodynamic efficiency, their complexity needs to be balanced against marginal gains in power production, especially in high-megawatt turbines, for which controlling flow at low wind speeds is challenging. Future research must focus on the sustainable integration of plasma actuators, pitch angle adjustments and active control mechanisms to fully exploit the potential of wind energy for a sustainable future.

Aerodynamic performance improvement of an airfoil using the Gurney flap and fast on–off DBD plasma actuator

Aerodynamic performance improvement of an airfoil using the Gurney flap and fast on–off DBD plasma actuator

On durable materials for dielectric-barrier discharge plasma actuators

In the current experimental investigation various electrode and dielectric materials for dielectric-barrier discharge plasma actuators have been studied in quiescent air under consideration of actuator degradation during long-term operation. The performance variation of the different actuators was initially monitored via alteration of the electrical power consumption P̄a during 6-hour continuous operation. While some material combinations led to premature failure, certain dielectrics such as quartz-glass and aluminum oxide maintained constant performance. The latter was selected for screen-printing of electrodes, so as to obtain reproducible actuator geometries. These actuators were deployed in 10-hour continuous operation. Besides P̄a, the cold capacitance C0 was tracked for each actuator, in order to assess the degradation process of the actuator. Among the tested metals for the screen-printed electrodes, copper showed the best endurance characteristics and, thus, is recommended for both comparable laboratory experiments and durability in AFC application. Admixtures of platinum in the electrode material are to be avoided because of heavy oxidation under ozone exposition. The quantitative outcomes supported by the P̄a and C0 measurements were qualitatively supported by visual inspection of the actuators and of the discharge light emission. On a final note, the screen-printed copper-aluminum-oxide actuator configuration, featuring both good durability and reproducibility, is a recommended combination.

Experimental study on Q-learning control of airfoil trailing-edge flow separation using plasma synthetic jets

In this paper, a continuous Q-learning algorithm is deployed to optimize the control strategy of a trailing-edge airfoil flow separation at a chord-based Reynolds number of 2×105. With plasma synthetic jets issued at the middle chord and a hot wire placed in the separated shear layer acting as the actuator and sensor, respectively, a high-speed reinforcement learning control at an interaction frequency of 500 Hz is realized by a field-programmable gate array. The results show that in the Q-learning control, the controller only needs several seconds to elevate the instantaneous reward to a level close to the final mean reward, and convergence of the control law typically takes less than 100 s. Although the relative drag reduction achieved by Q-learning control (10.2%) is only slightly higher than the best open-loop periodical control at F∗=4 (9.6%), the maximum power saving ratio is improved noticeably by 62.5%. Physically, Q-learning control creates more turbulent fluctuations, earning more rewards by increasing the transition possibilities toward high-value states. With increasing penalty strength of plasma actuation, the final control laws obtained from Q-learning exhibit a decreasing number of active states. Detailed comparisons between the open-loop and Q-learning control strategies show that the statistics of the controlled velocity fields remain similar, yet the turbulent fluctuations contributed by the vortex shedding mode are reduced by constant-frequency plasma actuation.

Closed-loop plasma flow control of a turbulent cylinder wake flow using machine learning at Reynolds number of 28 000

Machine learning is increasingly used for active flow control. In this experimental study, alternating-current dielectric barrier discharge plasma actuators are deployed for the closed-loop intelligent control of the flow around a cylinder at a Reynolds number of 28 000 based on the velocity feedback from two hot-wire sensors placed in the wake. Variations in the cylinder drag are monitored by a load cell, and the temporal response of the wake flow field is visualized by a high-speed particle image velocimetry system working at 1 kHz. The high-speed control law is operated using a field programmable gate array optimized by genetic programing (GP). The results show that the peak drag reduction achieved by machine learning is of similar magnitude to that of conventional steady actuation (∼15%), while the power saving ratio is 35% higher than with conventional techniques because of the reduced power consumption. Analysis of the best GP control laws shows that the intensity of plasma actuation should be kept at a medium level to maximize the power-saving ratio. When compared with the baseline uncontrolled flow, the best controlled cases constrain the meandering motion of the cylinder wake, resulting in a narrow stabilized velocity deficit zone in the time-averaged sense. According to the results of proper orthogonal decomposition and dynamic mode decomposition, Karman vortex shedding is promoted under the best GP control.

Effect of external magnetic field applied to a DBD plasma actuator

Effect of external magnetic field applied to a DBD plasma actuator

Experimental study on the interaction between induced flow by DBD plasma actuator and jet flow

Experimental study on the interaction between induced flow by DBD plasma actuator and jet flow

Effect of Plasma Actuators on Film Cooling Performance with Forward Injection Slots: A Numerical Approach

Effect of Plasma Actuators on Film Cooling Performance with Forward Injection Slots: A Numerical Approach

Understanding the coupling between nanosecond repetitively pulsed discharges and the thermoacoustic behavior of a swirl flame at 2 bar

This study experimentally investigates the coupling between nanosecond repetitively pulsed (NRP) discharges and the thermoacoustic behavior of a methane-air swirl flame at 2 bar for different power ratios (ratio of plasma power to flame thermal power), at pulse repetition frequencies of 10 kHz, 30 kHz, and 70 kHz, and applied voltages in the range 7 to 14 kV. The NRP discharges are created in the annular gap between a bluff body and the metal ring located at the tip of the injection tube. The results show that when NRP discharges are applied, the thermoacoustic behavior and the discharges are strongly coupled. At each pulse repetition frequency studied, for low power ratios, a strong thermoacoustic instability (TAI) is induced by the discharges until a critical power ratio beyond which reduction in amplitude of turbulent induced noise (TIN) of the flame is observed. This nonlinear behavior can be attributed to different levels of coupling between the discharge regimes and the acoustic fluctuations. At low power ratios, the discharge regimes oscillate between no discharges and glows/sparks at the frequency of the TAI. These oscillations are in phase with the pressure fluctuations leading to the strong TAI. However, this modulation of the discharges transits to continuous NRP sparks when the power ratio is increased. This helps in reducing the amplitude of TIN. By explaining how different plasma strategies can be beneficial or detrimental for the control of flame dynamics, best practices for the development of plasma actuators are also discussed.

Large Eddy Simulation of Pulsed Film Cooling with a Dielectric Barrier Discharge Plasma Actuator

The effects of the coolant pulsation and the plasma aerodynamic actuation (PAA) on the film cooling are herein explored via large eddy simulations. The electrohydrodynamic force derived from the PAA was solved through the phenomenological plasma model. The Strouhal number of the sinusoidal coolant pulsation and the averaged pulsation blowing ratio were 0.25 and 1.0, respectively. Comprehensive analyses were carried out on the time-averaged flow fields, and the results reveal that the pulsed cooling jet might cause a deeper penetration into the crossflow, and this phenomenon could be remarkably mitigated by the downward force of the PAA. Comparing steady film cooling to pulsed film cooling revealed a modest 15.1% reduction in efficiency, while the application of the dielectric barrier discharge plasma actuator (DBDPA) substantially enhanced the pulsed film cooling efficiency by 42.1%. Moreover, the counter-rotating vortex pair (CRVP) was enlarged and lifted off from the wall more poorly due to the coolant pulsation, and the PAA weakened the detrimental lift-off effect and entrainment of the CRVP. Then, the spatial–temporal development of the coherent structures was figured out by the alterations in the centerline temperature, reflecting the formation of the intermittent coherent structures rather than hairpin vortices due to the coolant pulsation, and their size and upcast behaviors were reduced by the PAA; thus, the turbulent integration of the coolant with the crossflow was suppressed fundamentally. Finally, the three-dimensional streamlines confirmed that the coherent structure dynamic behaviors were significantly regulated by the PAA for alleviating the adverse influences of the coolant pulsation. In summary, the PAA can effectively improve the pulsed film cooling efficiency by controlling the spatial–temporal development of the dominant coherent structures.

Numerical study of exhaust chemical composition in a methane DBD plasma actuator under different operating conditions

In this paper, numerical simulations of a Dielectric-Barrier Discharge (DBD) plasma actuator are performed to predict its output chemical composition. The plasma actuator is cylindrical, with two coaxial electrodes and a quartz dielectric. The electrical discharge occurs in a pulsed manner around the quartz wall. This device is utilized for methane conversion in plasma-assisted combustion applications. The effects of voltage, frequency, temperature, and relative permeability parameters on methane conversion and some selected species concentrations were investigated. The results show that plasma discharging converts methane to hydrogen and heavier hydrocarbons such as C2H3, C2H4,C2H5, C2H6, and C3H8. Moreover, increasing the relative permeability coefficient and voltage had the greatest effect on methane conversion. Increasing the relative permeability from 2 to 6 increases the methane conversion by 43 % and also increases the H2 density by 57 %. The conversion rate of methane and the H2 density increase by 18 % with increasing the voltage from 6 to 8 kV. The partial conversion of methane to hydrogen and radical species benefits combustion applications in terms of flame stability and emissions reduction.

Response of a Turbulent Boundary Layer to an Imposed Synthetic Large-Scale Structure

The dynamic response of a zero-pressure gradient turbulent boundary layer (TBL) to an active flow control actuator was experimentally investigated. The experimental TBL had a sufficiently low momentum thickness Reynolds number, such that there were no energetic organized large-scale structures in the outer region, as evidenced from measurements of the premultiplied wavenumber–frequency spectra. The periodically pulsed plasma actuator, placed inside the outer region of TBL, introduced a synthetic large-scale structure, and the boundary-layer response to this synthetic structure downstream of the actuator at select wall-normal and streamwise locations was investigated. The turbulence amplitude modulating effect of the synthetic large-scale structure, within the inner and log-linear regions of the boundary layer, was isolated and analyzed using a phase-locked analysis. The dynamic interaction of the synthetic large-scale structure and smaller-scale turbulent motions was quantified using a modulation coefficient, and a strong positive correlation within the inner and log regions of the boundary layer was measured. The streamwise development of the synthetic large-scale structure and its modulating effect on the near-wall turbulence across several streamwise locations are described. Profiles of the phase speed at these streamwise locations were extracted and were found to be constant within the log region, further confirming a strong coupling between the near-wall fluctuations in turbulence intensity and the synthetic large-scale motions introduced in the outer region. Overall, the turbulence amplitude modulation effect induced by the synthetic large-scale structure was found to be dynamically similar to the large-scale modulation measured in canonical TBLs at higher Reynolds numbers.

Effect of Plasma Actuator Layout on the Passage Vortex Reduction in a Linear Turbine Cascade for a Wide Range of Reynolds Numbers

This study examined how various plasma actuator (PA) configurations affect the passage vortex (PV) reduction in a linear turbine cascade (LTC) utilizing dielectric barrier discharge PAs. The experiments were carried out under three specific layout conditions: axial placement of the PA, slanted placement at the blade inlet, and slanted placement inside the blade. Particle image velocimetry was employed to measure the velocity distribution of the secondary flow at the LTC exit, followed by an analysis of the streamline patterns, turbulence intensity distribution, and vorticity distribution. At a Reynolds number of 3.7 × 104, the PA with an oblique orientation at the blade inlet provided the most effective PV suppression. The average value of the secondary flow velocity and the peak vorticity value at the LTC exit decreased by 59.0% and 68.8%, respectively, compared to the no-control case. Furthermore, the wind tunnel blower’s rotation speed was modified, adjustments were made to the LTC’s mainstream velocity, and the Reynolds number transitioned from 1.0 × 104 to 9.9 × 104, approximately 10 times. When the slanted PA was used at the blade inlet, the PV suppression effect was the highest. The peak vorticity value owing to the PV at the LTC exit decreased by 62.9% at the lowest Reynolds number of 1.0 × 104. The Reynolds number increased with a higher mainstream velocity and decreased flow induced by the PA, consequently reducing the PV suppression effect. However, the drive of the PA was effective even under the most severe conditions (9.9 × 104), and the peak vorticity value was reduced by 20.2%.

Optimizing flow control with deep reinforcement learning: Plasma actuator placement around a square cylinder

This study introduces a deep reinforcement learning-based flow control approach to enhance the efficiency of multiple plasma actuators on a square cylinder. The research seeks to adjust the control inputs of these actuators to diminish both drag and lift forces on the cylinder, ensuring flow stability in the process. The proposed model uses a two-dimensional direct numerical simulation of flow past a square cylinder to represent the environment. The control approach involves adjusting the AC voltage across three specific configurations of the plasma actuators. Initially tested at a Reynolds number (ReD) of 100, this strategy was later applied at ReD of 180. We observed a 97% reduction in the mean drag coefficient at ReD = 100 and a 99% reduction at ReD = 180. Furthermore, the findings suggest that increasing the Reynolds number makes it harder to mitigate vortex shedding using plasma actuators on just the cylinder's rear surface. However, an optimized configuration of these actuators can fully suppress vortex shedding under the proposed control scheme.

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

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%.