Timothy Jukes

Turbulent boundary-layer control with plasma actuators.

This paper reviews turbulent boundary-layer control strategies for skin-friction reduction of aerodynamic bodies. The focus is placed on the drag-reduction mechanisms by two flow control techniques—spanwise oscillation and spanwise travelling wave, which were demonstrated to give up to 45 per cent skin-friction reductions. We show that these techniques can be implemented by dielectric-barrier discharge plasma actuators, which are electric devices that do not require any moving parts or complicated ducting. The experimental results show different modifications to the near-wall structures depending on the control technique.

Flow control around a circular cylinder using pulsed dielectric barrier discharge surface plasma

Dielectric barrier discharge (DBD) plasma actuators have been used to control the flow around a circular cylinder at Re=15 000, where the near-wake structure was studied using time-resolved particle image velocimetry with simultaneous measurements of the dynamic lift and drag forces. It was shown that the vortex shedding was suppressed when the surface plasma placed near the natural separation point was activated in a pulsed mode at nondimensional frequency, fp+, above 0.6 with a force coefficient, Cp, greater than 0.05%. Plasma actuator performance on flow control was summarized by mapping the changes in drag and lift fluctuations as a function of the forcing frequency and the force coefficient. They showed that more than 70% reduction in lift fluctuations was obtained with up to 32% drag reduction at fp+=2.0 and Cp=0.32%. Here, narrowing of the wake was observed as the plasma promoted shear-layer roll-ups at the forcing frequency. This, however, did not affect the shear layer on the opposite side of the...

Characterization of Surface Plasma-Induced Wall Flows Through Velocity and Temperature Measurements

An investigation into the induced airflow around weakly ionized surface glow discharge plasma has been performed in initially static air to study radio-frequency plasma actuators designed for the use of boundary-layer control. Hot-wire and cold-wire anemometry have been used to study the velocity and temperature distribution around a symmetric and an asymmetric electrode configuration. The plasma appears to couple momentum into the ambient air such that it drives a laminar wall jet away from the electrode centerline. Temperature measurements indicate an air temperature rise of 2°C at 1 mm from the plasma, and the flow does not appear to be buoyancy driven. A maximum velocity of 2.5 m/s was observed, indicating the induced flow will be strong enough for flow control applications in low-speed test facilities.

Turbulent Drag Reduction by Surface Plasma Through Spanwise Flow Oscillation

An experimental study has been undertaken to reduce skin-friction drag by applying a spanwise oscillation in the near-wall region of a turbulent boundary layer using RF glowdischarge surface plasma. Measurements were taken using hot-wire anemometry at Reτ = 380. The oscillatory plasma forcing was created using two opposing sets of asymmetric plasma actuators. By alternately activating the electrodes, skin-friction drag was reduced by up to 45% in the downstream of the actuators. The plasma action creates a tangential force very close to the wall surface, which creates alternating, co-rotating streamwise vortices in the inner region of the boundary layer. A 40% reduction in mean streamwise velocity and a 30% reduction in turbulent intensity were observed for y < 30. This was accompanied by over 50% reduction in sweep duration and a 30% reduction in sweep intensity. It is suggested that in addition to the spanwise oscillation drag reduction mechanism, the plasmainduced streamwise vortices may be interacting with the naturally occurring quasistreamwise vortices, thus disrupting the turbulence production cycle and leading to weakened near wall events.

Control of unsteady flow separation over a circular cylinder using dielectric-barrier-discharge surface plasma

Dielectric-barrier-discharge plasma actuators have been used to control the unsteady flow separation over a circular cylinder at Re=15 000 with a view to maximizing the lift with a minimum drag increase. The flow structure was studied using time-resolved particle image velocimetry and flow visualization, while the dynamic lift and drag were simultaneously measured with a two-component force balance. Our results show that the lift can be dramatically increased by a single, short-duration pulse of plasma at carefully chosen location and timing. A peak lift coefficient of 1.24 was observed, showing that the plasma can increase lift by over 300%, with less than 25% increase in drag. Optimal conditions occurred when the actuator was located at 75° from the streamwise axis, with plasma forcing timed to coincide with the retreat of the separation point. This corresponds to the actuator being located 7° upstream of the natural separation point. The peak lift was relatively insensitive to the duration of the plasma.

Turbulent Boundary-Layer Control for Drag Reduction Using Surface Plasma

An investigation into the induced airflow around surface discharge plasma has been performed as a preliminary to skin-friction drag reduction studies. The device studied is a single, symmetric, weakly-ionized RF surface glow discharge plasma actuator designed for the use of boundary layer control. Hot-wire and cold-wire anemometry have been used to study the velocity and temperature distribution around the electrode in initially static air. The plasma appears to couple momentum into the ambient air such that it drives a laminar wall jet away from the electrode centerline. Temperature measurements indicate an ambient air temperature rise of 2°C at 1mm from the plasma and therefore it is unlikely that the flow is buoyancy driven. A maximum velocity of 2.1m/s was observed, indicating the induced flow is large enough for flow control in low speed test facilities.

Jet flow induced by a surface plasma actuator

The induced flow field in the vicinity of a single, symmetric, AC plasma actuator has been studied in initially static air at atmospheric pressure. Hot-wire and cold-wire anemometry, together with flow visualization, were used to observe the temporal and spatial structure of the induced flow. The plasma discharge is pulsed on a millisecond scale and the flow forms a series of pulsed wall jets, which can be maintained indefinitely, similar to a synthetic jet. It was observed that the plasma actuator initiates a pair of vortices at the instant of plasma creation, moving at 25° to the surface. After an initiation period, the plasma develops into a laminar wall jet. Thermal imagery has been used to estimate the surface temperature of the dielectric sheet during plasma operation.

Feedback Control of Flow Separation on NACA0024 Airfoil under Periodic Wall Oscillation by Means of DBD Plasma Actuator and FBG Sensor

Feedback control of periodic flow separation on a NACA0024 airfoil has been investigated by using dielectric barrier discharge (DBD) plasma actuators and fiber Bragg grating (FBG) sensors under oscillation of the test section wall. A tangential jet is generated from sheet-type DBD plasma actuators placed at the leading edge of the airfoil. An FBG flow sensor is mounted at the root of a cantilever beam, which detects the vibrations of the cantilever tip by wake flow around the trailing edge. Flow separation can be detected by the standard deviation of Bragg wavelength (B’). A demonstration of flow separation control was conducted successfully under periodic flow separation by a feedback algorithm using a threshold level of B’ = 0.006 at Re = 5.3×10 4 and showed that the flow separation can be mitigated by more than 20%. In addition, string-type DBD plasma actuators were developed to apply DBD plasma actuators on metallic and three dimensional surfaces and their induced flow properties were investigated by PIV.

Wall Normal Jet Produced by DBD Plasma Actuator With Doughnut-Shaped Electrode

Properties of coaxial annular jets produced by a dielectric barrier discharge (DBD) plasma actuator with a doughnut shaped electrodes were investigated under atmospheric pressure and room temperature. The actuator consists of two circular electrodes sandwiching a thin dielectric layer. By applying 0 – ±3.3 kV between the electrodes at radio frequencies, the plasma jet is formed near the inner edge of the top electrode. The radial jet runs toward the center of the electrode and then impinges at the center to generate a wall normal annular jet. The evolution of the wall normal jet was observed precisely using particle image velocimetry (PIV) system. It was found that characteristic velocities increase in proportion to the bursting frequency and inversely proportional to the inner diameter of the electrode at the surging time of the voltage at 5.0 × 10−6 sec.Copyright

Feedback Control of Flow Separation Using Plasma Actuator and FBG Sensor

A feedback control system for mitigating flow separation was developed by using a string-type dielectric-barrier-discharge (DBD) plasma actuator and a fiber Bragg grating (FBG) sensor. Tangential jets were induced from the string-type DBD plasma actuator, which was located at 5% chord from the leading edge of an NACA0024 airfoil. The FBG sensor was attached to the interior surface near the root of the cantilever beam modeled on the pressure surface of the airfoil. The strain at the cantilever root was reflected in the form of Bragg wavelengths ( ) detected by the FBG sensor when the cantilever tip was vibrated by the flow near the trailing edge of the airfoil. It was found that calculating running standard deviations in the Bragg wavelength ( ) detected by the sensor was valuable for judging flow separation in real time. The feedback control of flow separation on the NACA0024 airfoil was successfully demonstrated by setting with periodic flow separations generated in a wind tunnel by oscillating a side wall of the test section with frequency  Hz. It was confirmed that the appearance probability of flow separation tends to decrease with a decrease in the duration for calculating and with an increase in the duration of jet injection.