ns-DBD Plasma Actuator Research Articles

Thermal effects on the performance of a nanosecond dielectric barrier discharge plasma actuator at low air pressure

The thermal effects of a pulsed nanosecond dielectric barrier discharge plasma actuator (NSDBD) with varying pulse voltages and pulse repetitive frequencies under different air pressures ranging from 0.1 to 1 bar are studied experimentally. By observing discharge features with a charge-coupled device camera, the transition from a filamentary discharge mode to a diffuse mode with decreasing air pressure is described. The filamentary streamers extend along the radius direction, forming a thicker yet more stable and uniform plasma region due to the increasing ionized volume yielded by the decreasing air pressure to maintain the high values of the reduced electric field. The spatiotemporal temperature distribution on the surface is captured by an infrared camera, indicating that the heated surface can be divided into three typical regions with different features. Because gas heating is generated in the quenching process of excited molecules, the maximum temperature increase on the surface occurs in the plasma region and attenuates downstream. The surface temperature increase is primarily caused by heat convection from the residual heat in plasma and the heat generated by the dielectric losses. The results of heat flux on the surface suggest that the rising applied voltage may not increase the heat flux in a moderate air pressure ranging from 0.6 to 0.8 bar. Different discharge modes and discharge parameters exhibit markedly different thermal performances. Also, the Schlieren technique and the pressure sensor are used to visualize the induced shock wave, estimate the thermal expansion region, and measure the overpressure strength. The results of the overpressure strength at different air pressures are similar to the thermal features, which highlights the strong influence of the discharge mode on the thermal effect of NSDBD plasma actuators.

Flow separation control in S-shaped ∼inlet with a nanosecond pulsed surface dielectric barrier discharge plasma actuator

A nanosecond pulsed surface dielectric barrier discharge (nSDBD) plasma actuator is proposed to eliminate the complex 3D flow separation in an S-shaped ∼engine inlet. A set of wind tunnel experiments based on pressure measurements are conducted to verify the flow control effectiveness and obtain the influence laws of actuation voltage, pulse frequency, actuator layout and coverage area. The results show that nSDBD can effectively eliminate flow separation, leading to a noticeable improvement in the total pressure distribution at the outlet, manifested as a reduction in the low-pressure zone and an extension of the high-pressure zone. The spanwise layout nSDBD delivers a better flow control effect compared to the streamwise layout. For the same geometrical layout, the flow control effect improves with increasing voltage, and the optimal actuation frequency corresponds to a Strouhal number of approximately F + = 0.5. In particular, with a spanwise array of nSDBD plasma actuators operating at V p-p = 10 kV and F + = 0.5, the flow separation region is completely suppressed. The mechanism of flow separation with nSDBD can be ascribed to the mixing enhancement between the boundary layer and the main flow, which improves the boundary layer’s ability to resist the adverse pressure gradient.

A comparative study of Ac-dielectric barrier discharge versus Ns-dielectric barrier discharge plasma actuators in an incompressible turbulent mixing layer

A comparison between ac-dielectric barrier discharge (DBD) and ns-DBD plasma actuators is performed in a low-speed turbulent mixing layer to assess differences in their control mechanisms (momentum versus thermal). The mean flow response to actuation is matched for the two cases at a single downstream location to gauge scaling of relevant parameters (carrier frequency, energy per pulse, etc). The similarity between local behavior downstream is found to extend to the global flow, both in the mean and fluctuating components. However, the ns-DBD requires approximately six times more electrical energy to achieve the same control as the ac-DBD in this specific flow. An analysis of the flow field very near the splitter plate trailing edge sheds light on the difference in momentum versus thermal actuation mechanisms. A velocity deficit is observed for both actuators, but a thermal bump effect is evident in the ns-DBD case while a near surface jet is observed in the ac-DBD case.

Parametric study of Ns-DBD plasma actuators in a turbulent mixing layer

Excitation of a low-speed turbulent mixing layer using nanosecond dielectric barrier discharge (ns-DBD) plasma actuators is demonstrated here under various forcing conditions. The mixing layer behavior downstream is examined in the context of observations near the point of actuation (splitter plate trailing edge). Parameters explored include carrier/burst frequency, pulse energy, and duty cycle. Control authority is found to increase with carrier frequency, but a reduction in efficacy is observed for high pulse energy and lower duty cycle. During actuation, a streamwise velocity deficit is found near the trailing edge relative to the baseline. This causes velocity fluctuations that are amplified and lead to the formation of large-scale structures. The results indicate ns-DBD plasma actuators in burst mode acts as an obstruction giving rise to cross-stream perturbations that cause mixing layer growth.

Pressure dependency on a nanosecond pulsed dielectric barrier discharge plasma actuator

The behavior of a nanosecond pulsed dielectric barrier discharge (ns-DBD) plasma actuator with the ambient pressure from 30 to 100 kPa was characterized with Schlieren images. Shock wave propagation speed and strength were recorded, showing clear trends with decreasing ambient pressure. Higher ambient pressures result in stronger shock waves; this has been observed irrespective of the actuator thickness. This might be explained with fewer air molecules to ionize at lower ambient pressures and hence a lower temperature from the exothermal recombination reactions. The thickness of the dielectric barrier also influences the shock strength. In accordance with previous findings, it was confirmed that a thinner dielectric barrier results in a greater shock strength. NS-DBD shock waves were modeled numerically using OpenFOAM through a source term added to the energy equation, which controls the amount of thermal energy added to the near-wall deposition region. The compressible, unsteady sonicFoam solver was used with second order schemes. A mesh sensitivity study gives confidence that the solution is grid independent. The overall shock wave structure and propagation speed match well with experimental data. The heat addition required to reproduce experimental results varied with ambient pressure. Less heating of the near-wall region was needed with lower ambient pressures.

Post-stall flow control using nanosecond pulse driven dielectric barrier discharge plasma actuators

The efficacy of thermal perturbations generated by nanosecond pulse driven dielectric barrier discharge (ns-DBD) plasma actuators for post-stall flow control is explored on a NACA 0012 airfoil. Baseline and controlled flow fields are studied using static pressure measurements, particle image velocimetry and constant temperature anemometry. Experiments are primarily performed at Re = 0.74 × 106 (U∞ = 40 m s−1) and α = 18°. Three regimes of forcing are identified corresponding to separation control (0.92 < F+ < 1.52), bluff body shedding (0.23 < F+ < 0.92) and an impulse-like response (F+ < 0.23). The response of the flow to a single high voltage pulse is also examined and compared to other studies of transient separation control. Results show that the global structure of the controlled flow is not specific to ns-DBD plasma actuator forcing. Rather, it is the local behavior of the ns-DBD that results in control authority at conditions that are challenging for the majority of active flow control actuators that rely on zero net mass flux momentum addition. The implications of these findings as well as open questions and suggestions for future work are discussed.

An experimental study on the thermal characteristics of NS-DBD plasma actuation and application for aircraft icing mitigation

A comprehensive study was performed to explore the thermal characteristics of NS-dielectric-barrier discharge (DBD) plasma actuation over an airfoil/wing surface and evaluate the anti-/de-icing performance of NS-DBD plasma actuators for aircraft in-flight icing mitigation. While the fundamentals of thermal energy generation and heat transfer in NS-DBD plasma actuation over the airfoil/wing model were described in great details, a series of experiments were conducted to evaluate the effects of different environmental parameters on the heating efficiency of NS-DBD plasma actuators over the airfoil/wing surface. With the temporally-synchronized-and-spatially-resolved high-speed visualization and infrared imaging system, not only the transient thermal characteristics of NS-DBD plasma actuation over the airfoil/wing surface were revealed, but also the anti-icing performances of the NS-DBD plasma actuators were evaluated under different icing conditions, i.e. rime, mixed, and glaze. The impacts of incoming airflow velocity, air temperature, and angle of attack of the airfoil/wing model on the thermal characteristics of NS-DBD plasma actuation over the airfoil/wing surface were systematically investigated based on the measurement results. It was found that the thermal characteristics of NS-DBD plasma actuation over the airfoil/wing surface are closely coupled with the boundary layer airflow and the unsteady heat transfer process over the airfoil/wing model exposed in the frozen-cold airflows. The anti-icing performances of the NS-DBD plasma actuators under the different icing conditions were found to be varying significantly due to the variations of surface heating efficiency of the NS-DBD plasma actuators. The anti-/de-icing performance of the NS-DBD plasma actuators was found to be improved dramatically by increasing the operating frequency of the plasma actuators. The findings derived from the present study are very helpful to explore/optimize design paradigms for the development of novel plasma-based anti-/de-icing strategies tailored specifically for aircraft inflight icing mitigation to ensure safer and more efficient aircraft operation in atmospheric icing conditions.

Temporal variation of the spatial density distribution above a nanosecond pulsed dielectric barrier discharge plasma actuator in quiescent air

The thermal perturbation caused by a nanosecond pulsed dielectric barrier discharge (ns-DBD) plasma actuator may lead to boundary layer transition. Hence, understanding of the thermal flow induced by the ns-DBD plasma actuator will contribute to the development of an efficient flow control device for various engineering applications. In this study, the spatial density distribution related to the thermal flow was experimentally investigated using both qualitative and quantitative schlieren techniques. The focus of this study is to understand the initial temporal variation of the spatial density distribution above the ns-DBD plasma actuator in quiescent air. The quantitative visualisation showed that a hot plume is generated from the edge of the exposed electrode and moves slightly towards the ground electrode. A possible explanation is that an ionic wind and/or an induced jet leads to the movement of the hot plume. However, the plasma-induced flow (the ionic wind and the induced jet) is generated after the primary plasma discharges; namely, the hot plume does not move immediately after the first plasma discharge. At almost the same time as the movement of the hot plume, consecutive plasma discharges enhance the density of the hot plume; thereafter, the density reaches almost a steady state.

Experimental Study on Anti-Icing Performance of NS-DBD Plasma Actuator

An experimental study was conducted to evaluate the anti-icing performance of NS-DBD plasma actuator under the conditions of airflow speed U = 65 m/s, ambient temperature T = −10 °C, liquid water content LWC = 0.5 g/m3, mean-volume diameter MVD = 25 μm, mainly to clarify the effect of pulse frequency and voltage amplitude of actuation on anti-icing performance. A NACA0012 airfoil model with a chord length of c = 280 mm was used in the tests. The NS-DBD plasma actuator was mounted at the front part of the airfoil. A FLIR infrared (IR) imager and CCD camera were used to record the anti-icing process of the NS-DBD plasma actuator. Two typical discharge conditions were selected for the anti-icing experiments. The first was HV-LF discharge, corresponding to discharge under higher voltage amplitude with lower pulse frequency; the second was LV-HF discharge, corresponding to discharge under lower voltage amplitude with higher pulse frequency. Results reveal that NS-DBD is a very promising method for anti-icing. With the same power consumption, the LV-HF discharge shows a better anti-icing performance compared to HV-LF discharge under the same icing conditions. In view of pulse duration and duty circle, combined with heat dissipation, it is suggested that there is a threshold frequency, corresponding to the voltage amplitude of electric actuation signal and the incoming flow condition, to achieve effective anti-icing performance.

Analysis of flow separation control using nanosecond-pulse discharge plasma actuators on a flying wing

Dielectric barrier discharge (DBD) plasma is one of most promising flow control method for its several advantages. The present work investigates the control authority of nanosecond pulse DBD plasma actuators on a flying wing model’s aerodynamic characteristics. The aerodynamic forces and moments are studied by means of experiment and numerical simulation. The numerical simulation results are in good agreement with experiment results. Both results indicate that the NS-DBD plasma actuators have negligible effect on aerodynamic forces and moment at the angles of attack smaller than 16°. However, significant changes can be achieved with actuation when the model’s angle of attack is larger than 16° where the flow separation occurs. The spatial flow field structure results from numerical simulation suggest that the volumetric heat produced by NS-DBD plasma actuator changes the local temperature and density and induces several vortex structures, which strengthen the mixing of the shear layer with the main flow and delay separation or even reattach the separated flow.

Experimental method to quantify the efficiency of the first two operational stages of nanosecond dielectric barrier discharge plasma actuators

A method to quantify the efficiency of the first two operational stages of a nanosecond dielectric barrier discharge (ns-DBD) plasma actuator is proposed. The method is based on the independent measurements of the energy of electrical pulses and the useful part of the energy which heats up the gas in the discharge region. Energy input is calculated via a back current shunt technique as the difference between the energy given and the energy reflected back. The ratio of the difference of the latter two quantities and the energy input gives the electrical efficiency (ηE) of a ns-DBD. The extent of the energy deposited is estimated via Schlieren visualizations and infrared thermography measurements. Then, the ideal power flux obtained if all the inputted energy was converted into heat is calculated. Transient surface temperature was measured via infrared thermography and used to solve a 1D inverse heat transfer problem in a direction normal to the surface. It gives as output the actual power flux. The estimated ratio between the two power fluxes represents a quantification of the mechanical fluid efficiency (ηFM) of a ns-DBD plasma actuator. Results show an inverse proportionality between ηE, and ηFM, and the thickness of the barrier. The efficiency of the first two operational stages of a ns-DBD is further defined as η = ηE · ηFM.

Effect of nanosecond-pulsed plasma actuation on a separated laminar flow

An experimental investigation was carried out on the effect of unsteady periodic control on a separated laminar shear layer. Time resolved Particle Image Velocimetry (tr-PIV) was used to characterize a backward-facing step (BFS) flow (Reh=3600), periodically perturbed by a nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuator. Ensemble averaged vector fields indicate a decrease of reattachment length with increasing actuation non-dimensional frequency, reaching a minimum at a Sth=0.32. Further increase of forcing non-dimensional frequency, up to 0.4, resulted into an increase of the reattachment length, which nevertheless remained shorter than the non-actuated case. Spectral analysis of the fluctuating fields revealed a change of the amplified frequency range for the actuated cases with respect to the base flow. Proper Orthogonal Decomposition (POD) analysis showed that actuation leads to a redistribution of energy among coherent spatial modes. Stability diagrams were calculated from mean velocity field data for each case via Linear Stability Theory (LST). Results indicate that stability of each actuated case changes with respect to the non-actuated case. Moreover, looking into the calculated growth rate for all the cases a more stable flow regime is observed for the cases of most successful reduction in terms of reattachment length. The effect of a pulsed periodic perturbation on the control of a laminar shear layer promotes the development of large structures, i.e. K-H vortices, due to inviscid-viscous interaction. These convect downstream resulting in a mean flow deformation (MFD) which causes a change of stability. New unstable frequencies are excited and promote the redistribution of energy among modes. This ultimately affects the efficiency of actuation in promoting transition from laminar to turbulent flow.

Energy deposition characteristics of nanosecond dielectric barrier discharge plasma actuators: Influence of dielectric material

An experimental study aimed at the characterization of energy deposition of nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuators was carried out. Special attention was given on the effect of the thickness and material used for dielectric barrier. The selected materials for this study were polyimide film (Kapton), polyamide based nylon (PA2200), and silicone rubber. Schlieren measurements were carried out in quiescent air conditions in order to observe density gradients induced by energy deposited. Size of heated area was used to qualify the energy deposition coupled with electrical power measurements performed using the back-current shunt technique. Additionally, light intensity measurements showed a different nature of discharge based upon the material used for barrier, for a fixed thickness and frequency of discharge. Finally, a characterisation study was performed for the three tested materials. Dielectric constant, volume resistivity, and thermal conductivity were measured. Strong trends between the control parameters and the energy deposited into the fluid during the discharge were observed. Results indicate that efficiency of energy deposition mechanism relative to the thickness of the barrier strongly depends upon the material used for the dielectric barrier itself. In general, a high dielectric strength and a low volumetric resistivity are preferred for a barrier, together with a high heat capacitance and a low thermal conductivity coefficient in order to maximize the efficiency of the thermal energy deposition induced by an ns-DBD plasma actuator.

Impact of ns-DBD plasma actuation on the boundary layer transition using convective heat transfer measurements

This paper demonstrates that the impact of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators on the structure of the boundary layer can be investigated using quantitative convective heat transfer measurements. For the experiments, the flow over a flat plate with a C4 leading edge thickness distribution was examined at low speed incompressible flow (6.6–11.5 m s−1). An ns-DBD plasma actuator was mounted 5 mm downstream of the leading edge and several experiments were conducted giving particular emphasis on the effect of actuation frequency and the freestream velocity. Local heat transfer distributions were measured using the transient liquid crystal technique with and without plasma activated. As a result, any effect of plasma on the structure of the boundary layer is interpreted by local heat transfer coefficient distributions which are compared with laminar and turbulent boundary layer correlations. The heat transfer results, which are also confirmed by hot-wire measurements, show the considerable effect of the actuation frequency on the location of the transition point elucidating that liquid crystal thermography is a promising method for investigating plasma-flow interactions very close to the wall. Additionally, the hot-wire measurements indicate possible velocity oscillations in the near wall flow due to plasma activation.

Nanosecond-pulsed plasma actuation in quiescent air and laminar boundary layer

An experimental investigation of the working principles of a nanosecond-pulsed dielectric barrier discharge (ns-DBD) plasma actuator has been conducted. Special emphasis is given on the thermal effects accompanying the rapid deposition of energy associated with this kind of actuation. A ns-DBD plasma actuator has been operated in quiescent air conditions as well as in a flat plate laminar boundary layer, with external flow velocity of 5 and 10 m s−1. Schlieren imaging and particle image velocimetry have been used to characterize the actuation. Additionally, the back-current shunt technique has been used for current measurements, from which energy input (per pulse) is calculated. Cases of 10-, 20- and 50-pulse bursts are tested. Schlieren imaging in still air conditions shows the formation of a high-temperature region in the vicinity of the discharge volume. The spatial extent of the visible ‘hot spot’ depends upon the number of pulses within the burst, following a power law. Schlieren imaging of the span-wise effect of the plasma actuator reveals weak compression waves originating from the loci of discharge filaments. The thermal ‘hot spots’ exhibit significant three-dimensionality. Particle image velocimetry is used to measure the velocity field resulting from the ns-DBDs acting on a laminar boundary layer. The disturbance leads to formation of a Tollmien–Schlichting wave train, with spectral content in good agreement with linear stability theory. It is observed that the group length of the wave train is proportional to the number of pulses within the burst.

Characterization of nanosecond pulse driven dielectric barrier discharge plasma actuators for aerodynamic flow control

Positive polarity nanosecond pulse driven dielectric barrier discharge (ns-DBD) plasma actuators are studied experimentally in quiescent atmosphere. Pulse energy and instantaneous pulse power (hereafter referred to as energy and power) are calculated using simultaneous voltage and current measurements. Electrical characteristics are evaluated as a function of peak voltage, pulse frequency, discharge length, and dielectric thickness. Schlieren imaging is used to provide a relative estimate of discharge energy that is coupled to the near surface gas as heat for the same parameters. Characteristics of the DBD load have a substantial effect on the individual voltage and current traces which are reflected in the energy and power values. Power is mainly dependent on actuator length which is inconsistent with schlieren data as expected. Higher per unit length energy indicates a stronger compression wave for a given actuator geometry, but this is not universally true across different actuators suggesting some constructions more efficiently couple energy to the gas. Energy and compression wave strength are linearly related. Higher pulse frequency produces higher energy but is primarily attributed to heating of the actuator and power supply components and not to an optimal discharge frequency. Both energy and wave strength increase as peak voltage to the power of approximately 3.5 over a substantial range similar to ac-DBD plasma actuators.