Dielectric Barrier Discharge Actuator Research Articles

Control of diffuser jet flow: turbulent kinetic energy and jet spreading enhancements assisted by a non-thermal plasma discharge

An axisymmetric air jet exhausting from a 22-degree-angle diffuser is investigated experimentally by particle image velocimetry (PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied (Reynolds numbers ranging from 3.2 to 12.8 × 104), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to 1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s.

Detachment and attachment of an axisymmetric non-reactive jet with turbulent shear layer: Control by plasma actuator

The control of an axisymmetric air jet equipped with a small-angle diffuser exhaust using a single non-thermal plasma actuator is studied with a stereoscopic PIV system. A single Dielectric Barrier Discharge (DBD) actuator is used to separate a naturally attached flow (12° angle diffuser) or to reattach a naturally separated one (22° angle diffuser). The experimental set-up allows us to measure the flow modifications occurring at the bevel of the diffuser. The actuator can generate a local flow (electric wind) of 6 m s −1 used to modify an axisymmetric air jet with centreline velocities of 10, 20 and 30 m s −1. The flow modifications under actuation are investigated in terms of time-averaged, phased-averaged fields as well as eddy localization. The analysis is mainly focused on the lower primary jet velocity at which the actuator presents a high efficiency. The results highlight that a plasma actuator can force the flow separation or attachment in a few dozen of milliseconds and that this device can be effective to manipulate the coherent structures surrounding the potential core. The global conclusion of the present study is that a single DBD actuator could be an effective device to promote the vortex shedding at the diffuser lip of an axisymmetric air jet. This may results in an improvement of the mixing characteristics, at least in the near-field region.

Cross-talk in multiple dielectric barrier discharge actuators

Phase locked particle image velocimetry (PIV) is used to study flow alteration by dielectric barrier discharge actuators mounted on a bluff body. Images from these PIV experiments confirm the importance of negative ions in the actuation process and demonstrate cross-talk interactions between adjacent actuators.

On the role of oxygen in dielectric barrier discharge actuation of aerodynamic flows

Phase-locked particle image velocimetry is used to study the mechanism of induced flow in the near field of a rf dielectric barrier discharge actuator mounted in the separated flow region of a bluff body. Flow actuation is found to be asymmetric, with suction toward the buried downstream electrode when it is biased positively relative to the upstream exposed electrode. Lesser flow is seen on the reverse voltage swing, where the buried electrode should attract positive ions. This phenomenon is enhanced when oxygen is added to the flow, suggesting that oxygen negative ions, possibly O2−, play a dominant role in plasma actuation.

Optimization of a dielectric barrier discharge actuator by stationary and non-stationary measurements of the induced flow velocity: application to airflow control

Several studies have shown that a surface dielectric barrier discharge (DBD) may be used as an electrohydrodynamic (EHD) actuator in order to control airflows. In this paper, a parametric study has been performed in order to increase the velocity of the ionic wind induced by such actuators. The results show that an optimization of geometrical and electrical parameters allows us to obtain a time-averaged ionic wind velocity up to 8 m/s at 0.5 mm from the wall. Moreover, non-stationary measurements of the induced wind have been performed with synchronized records of current and voltage signals. These experiments show that the DBD actuator seems to generate a pulsed velocity at the same frequency than the applied high voltage.

Control of an axisymmetric subsonic air jet by plasma actuator

It is known that surface non-thermal plasma actuators have proved their efficiency for aerodynamics flow control. In this study, a dielectric barrier discharge (DBD) is mounted on the diffuser of an axisymmetric turbulent air jet in order to control the flow separation along a 12-degree diffuser bevel. The momentum created by the actuator is applied to separate an air flow naturally attached to the diffuser for air flow velocity up to 40 m s−1. Laser sheet visualizations and LDV measurements are achieved to characterize the unforced and forced air jet. The flow separation, the induced velocity fluctuations, the jet mixing improvement and vectoring are investigated. The main results of this study demonstrate that DBD actuators are suitable to fully detach the air flow along the bevel for a velocity of 20 m s−1 and that a jet vectoring between 13.5° and 5.5° could be achieved for velocity ranging between 20 and 40 m s−1. Considerations about a potential improvement of the jet mixing are also introduced and the laser sheet visualization attests that induced flow perturbations are highly 3D.

Quasi-steady and unsteady actuation by surface non-thermal plasma discharge for control of a turbulent round air jet

The dielectric barrier discharge (DBD) is a nonmechanical device able to generate electrohydrodynamic forces close to a dielectric wall. The generated “electric wind” is actually used to control airflow over various bluff bodies. In this study, a DBD actuator is investigated for the control of a round turbulent air jet. The separation along the bevel of a small angle (12°) axisymmetric diffuser is studied using LDV measurements for a 20 m s− 1 airjet velocity. The results demonstrate that a DBD actuator is suitable to separate a turbulent airflow naturally attached. Under actuation, a jet deflection of 13° is observed and the coherent structures are energized. Quasi-steady and unsteady actuations are also compared for excitation frequencies up to 400 Hz and the analysis of the turbulent spectra demonstrates that a vortex shedding could be forced at the excitation frequency (up to 160 Hz). The investigation of different duty-cycle regimes allows us to define a threshold value inducing similar flow modifications than those obtain with quasi-steady actuation but with a significant power consumption reduction of 20%. The knowledge of the separation dynamics is essential for future reactive control devices. The last part of this study is dedicated to this point. The response times of the forced separation and the natural reattachment are evaluated. The results indicate that separation is achieved in 34.5 ms and that the natural reattachment time is about 45 ms. In conclusion, this study highlights the dynamics of the separation using a single DBD actuator, and demonstrates that a jet deflection and a mixing enhancement can be performed.

Contribution of Plasma Control Technology for Aerodynamic Applications

AbstractSummary: During these past ten years, plasma actuators have been envisaged to control air flows. Usually, two main kinds of plasma actuators are available: the DC discharge actuator and the dielectric barrier discharge (DBD) actuator, but also some combinations of both systems are now being studied. In this paper we present the results obtained with these different kinds of actuators. The control parameters, the electrical behaviors, and the mechanical efficiencies of the actuators are given. Aerodynamic experiments have been made in several wind tunnels using Particle Image Velocimetry (P.I.V.), visualization, Laser Dopler Velocimetry and special “Pitot” tube techniques. Advantages and disadvantages of each actuator are discussed, and finally some aerodynamic applications are shown.Effect of a discharge on the airflow around a NACA body.imageEffect of a discharge on the airflow around a NACA body.

Synchronous two-dimensional imaging of tissue under forced vibration: W.F. Walker, B.H. Friemel, G.E. Trahey, and L.N. Bohs, Department of Biomedical Engineering, Duke University, 136 Engineering Building, Durham, NC 27706

A proof-of-concept study is presented to understand the effect of continuous forcing by a dielectric-barrier-discharge (DBD) plasma actuator on the strength of the vortices formed in the tip-gap of a single NACA0065 airfoil. The airfoil was mounted with variable tip gaps in the test-section of a suction-type wind tunnel. Continuous forcing was generated by mounting a straight DBD actuator in the tip gap. The flow field was investigated experimentally by dynamic-pressure measurements, and stereoscopic-particle-image-velocimetry (SPIV). In addition, URANS was performed to investigate the flow field in the tip gap region, where SPIV could not be conducted. The results showed that the effectiveness of forcing decreased with increasing tip-gaps and free-stream velocity. The maximum cancellation of the vortex was observed when the blowing ratio was approximately 0.93, with a tip gap of 0.02c (c=chord) and free stream velocity of 2.7 m/s. Both experimental and numerical results showed that in this particular case, the DBD actuator created a strong reverse flow opposing the direction of tip flow. This reverse flow altered the pressure gradient in the tip gap region and canceled the vortex altogether.