Session L20: Flow Control: Plasmas and Actuators

On the pulsating electric wind of a Single Dielectric Barrier Discharge (SDBD) plasma actuator

An experimental study is conducted on the electric wind produced by a Single Dielectric Barrier Discharge (SDBD) plasma actuator placed at the top of a half cylinder. Laser Doppler Velocimetry (LDV) measurements were performed and results show that increasing the driving voltage (6-16 kV peak-to-peak) and frequency (0.5-2 kHz) of the actuator increases the induced jet velocity (up to 4 m/s) and thus the momentum added by the actuator. The focus of the present study is on the phase-resolved behavior of the electric wind, in particular, its two strokes. Phase-averaged LDV data reveals that while the velocity during both strokes remains positive, there is nearly a factor of two in amplitude. The difference of behavior between the two strokes and its downstream and wall-normal evolution are mapped for various driving voltages. Results indicate that this difference is restricted to the vicinity of the actuator, thereby justifying the assumption of a steady force in simulations to model the induced force. The study is part of a larger investigation aiming at separation control on the A-pillar of a truck cabin.   

DBD Actuated Flow Control of Wall-Jet and Cross-Flow Interaction for Film Cooling Applications

In this work, we use surface DBD actuators to control the interaction between a wall jet and mainstream flow in film cooling applications. The intention of the study is to improve the contact of the jet with the wall and enhance the convective heat transfer coefficient downstream of the jet exit. A 2D wall jet (10 mm height) is injected into the mainstream flow at an angle of 30$^{\circ}$. With an injected jet velocity (U$_{i})$ of 5 m/s, two blowing ratios M ($= \quad \rho_{i}$U$_{i}$ / $\rho_{\infty }$U$_{\infty })$ of 1.0 and 0.5 are studied corresponding to the mainstream flow velocity (U$_{\infty })$ of 5 m/s and 10 m/s respectively. Different configurations of the DBD actuator are studied, positioned both inside the jet and on the downstream side. PIV measurements are conducted to investigate the flow field of the interaction between the jet and cross flow. Streamwise velocity profiles at different downstream locations are compared to analyze the efficacy of the plasma actuator in improving the contact between the injected jet stream and the wall surface. Reynolds shear stress measurements are also conducted to study the mixing regions in the plasma-jet-mainstream flow interaction.   

In-Flight Infrared Measurements for Quantification of Transition Delay with DBD Plasma Actuators

Active flow control with a single DBD plasma actuator is performed in flight on wing of a motorized in order to delay laminar-turbulent transition at $Re_c=3\cdot 10^6$. While earlier experiments measured transition delay with point wise sensors such as microphones or surface hot wires, these dynamic sensors are now simultaneously applied with the infrared measurement technique. This allows a more accurate spatial quantification of the flow control impact. The miniature high resolution IR camera is mounted below the wing as the experiments are conducted on the pressure side. Two control strategies, boundary layer stabilization and active wave cancelation of Tollmien Schlichting (TS) waves, are performed in flight experiments, showing significant advantages of the IR measurement technique. Spanwise and streamwise effects on the transition delay are measured and evaluated with novel post processing strategies. This allows a detailed view on the correlation of TS wave damping and transition delay for different plasma actuator operation modes and flight conditions.   

An Out-of-Plane Velocity Component in Dielectric Barrier Discharge Actuator Flow

The performance of an array of two, two-dimensional dielectric barrier discharge actuators was studied, including both power dissipation measurements and flow visualization using stereo particle image velocimetry. The power dissipation over a range of operating conditions was characterized, showing a relationship between power dissipation, frequency, and voltage such that $P_{diss} \propto fV^{3.5}$. Additionally, the flow induced by plasma generation was measured in quiescent air using PIV, with the driving voltage and frequency being varied. Kinetic energy within a control volume was calculated to quantify the effect of each driving condition, both during the transient start-up flow (lasting up to 600 ms) and at steady state. The induced flow was found to have a non-negligible velocity component in the out-of-plane direction. This component of the kinetic energy, as compared with in-plane kinetic energy, is studied as a function of voltage, frequency, PIV particle size, and actuator design. Potential causes of this velocity component are discussed, considering both electrophoretic forces on the PIV particles, as well as the possibility that it is inherent to the plasma induced flow.   

Electro-Fluid Dynamic Jets

The success of dielectric barrier discharge (DBD) plasma actuators as flow control devices in transducing electrical energy directly into near instantaneous fluid motion has been limited due to momentum loss near the wall. To increase the feasibility of these devices, they have been used to drive a channel flow, creating a jet under quiescent conditions. Electrostatic Fluid Accelerators (EFA) have also been shown to drive internal gas flows. The present work draws on the success of the DBD driven plasma channels, while exploring a new electrode configuration that stems from EFA designs, in order to actuate more of the bulk fluid. Major parameters, applied voltage and operating frequency as well as electrode gap and choice of electrode (material, shape, size); were experimentally investigated using Particle Image Velocimetry to obtain time averaged, 2D velocity fields. Results indicate significant variation of performance with these parameters and suggest that in comparison to surface DBD actuators an order magnitude improvement in efficiency is possible. Furthermore, the qualitative aspect of an electro-fluid dynamic jet shows greater versatility in application for use as both boundary layer flow control and driving internal gas flows.~   

Numerical analysis of ion wind flow using space charge for optimal design

Ion wind flow has been widly studied for its advantages of a micro fluidic device. However, it is very difficult to predict the performance of the ion wind flow for various conditions because of its complicated electrohydrodynamic phenomena. Thus, a reliable numerical modeling is required to design an otimal ion wind generator and calculate velocity of the ion wind for the proper performance. In this study, the numerical modeling of the ion wind has been modified and newly defined to calculate the veloctiy of the ion wind flow by combining three basic models such as electrostatics, electrodynamics and fluid dynamics. The model has included presence of initial space charges to calculate transfer energy between space charges and air gas molecules using a developed space charge correlation. The simulation has been performed for a geometry of a pin to parallel plate electrode. Finally, the results of the simulation have been compared with the experimental data for the ion wind velocity to confirm the accuracy of the modified numerical modeling and to obtain the optimal design of the ion wind generator.   

Influence of Cavity Shape on Synthetic Jet Performance

A synthetic jet is a fluidic actuator that transfers linear momentum to the surroundings by alternately ingesting and expelling fluid from a cavity containing an oscillating diaphragm. This work presents the first experimental effort to validate the limited number of numerical investigations that have postulated synthetic jets are insensitive to cavity shape. Three axisymmetric synthetic jets with different cavity shapes were used to examine jet performance while keeping other parameters constant such as cavity volume, nozzle length and orifice diameter. Cylindrical, conical and contraction shaped cavities were considered. The cavity pressure and velocity at the orifice exit plane were measured using a microphone and hot-wire, respectively. The results demonstrated that for several operating conditions near Helmholtz resonance of the cavity, noticeable differences were observed in the radial velocity profiles between the three geometries. The Reynolds number decreased sequentially from the cylindrical to conical to contraction cavity. The momentum flux, which is relevant in flow control applications, followed the same trend. In general, the experimental results showed that synthetic jet performance is, to some degree, dependent on cavity shape.   

Interactions of a Cross-flow with Dynamic Discrete Roughness Elements

Flow over passive discrete roughness elements was studied in the past to understand their role in controlling transition from laminar to turbulent boundary layers. In the present study, the use of dynamic roughness elements (i.e., low aspect ratio circular cylinders) was explored. These roughness elements were actuated using piezoelectric strips and were designed to have a frequency range of up to 300 Hz. The interaction of a laminar boundary layer over a flat plate with the dynamic roughness elements was studied with both a single and a spanwise array of roughness elements. The discrete elements were 4 mm in diameter, and with a free-stream velocity of 10 m/s, the Reynolds number based on diameter was about 2500. Simultaneous measurements of the performance of the surface roughness elements and the flow field around them were accomplished using a laser displacement sensor and Stereoscopic Particle Image Velocimetry, respectively. Data were collected along spanwise planes at multiple streamwise locations downstream of the roughness elements for the static and dynamic cases at different protrusions into the boundary layer. For the dynamic cases, the effects of amplitude and frequencies were also explored. Flow structures in the wake of these roughness elements will be discussed.   

Synthetic Jet Actuator Performance Enhancement

Over the last 20 years synthetic jets have been studied as a means for aerodynamic flow control. Specifically, synthetic jets provide momentum transfer with zero-net mass flux, which has been proven to be effective for controlling flow fields. A synthetic jet is created by the periodic formation of vortex rings at its orifice due to the periodic motion of a piezoelectric disk(s). The present study seeks to optimize the performance of a synthetic jet actuator by utilizing different geometrical parameters such as disk thickness, orifice width and length, cavity height and cavity diameter, and different input parameters such as voltage and frequency. Experiments were conducted using a synthetic jet apparatus designed for various geometrical parameters utilizing a dual disk configuration. Velocity and temperature measurements were acquired at the center of the synthetic jet orifice using a temperature compensated hotwire and thermocouple probe. The disk displacement was measured at the center of the disk with a laser displacement sensor. It was shown that the synthetic jet actuators are capable of exceeding peak velocities of 200 m/s with a relatively large orifice. Data suggests that jet velocities greater than 200 m/s are attainable.   

Interaction of a Dynamic Vortex Generator with a laminar Boundary Layer

The effectiveness in delaying boundary layer separation by vortex generators (VGs) is well established. However, there could be a drag penalty when the flow it attached. Therefore, in this study, a piezo-based dynamic vortex generator was developed with the goal of mitigating any additional drag that might occur when not in use. The dynamic VG (DVG) was driven by bimorph piezoelectric motor and was designed to operate at frequencies up to 300 Hz. Experimental studies were performed on the interaction of the laminar boundary layer over a flat plate with a DVG placed at a skew angle of 18$^{\circ}$ with respect to the free-stream direction. The experiments were conducted for different height of the DVG, where the Reynolds number based on the local boundary layer thickness was about 2000. In addition, the DVG was oscillating at different frequencies and amplitudes and its effect of the flow field was compared to a steady VG. Simultaneous measurements of the DVG performance and the flow field behind it were accomplished using a laser displacement sensor and Stereoscopic Particle Image Velocimetry (SPIV), respectively. The SPIV data were taken at multiple downstream locations and the flow structures formed in the wake of the DVG will be discussed.   

Experiments on the Thrust of a Synthetic Jet in Crossflow

A set of water tunnel experiments were conducted to investigate the effect of crossflow on the thrust of a synthetic jet. This research was motivated by the desire to generate significant turning moments on a fully-submerged, supercavitating vehicle without using control fins or canards. The water tunnel model was a sting-mounted, 3-inch diameter cylindrical body interfaced to a 6-axis waterproof load cell. The synthetic jet actuator was contained within the model and the jet orifice located near the aft end of the model was oriented perpendicular to the mean flow. The actuator consisted of an externally controlled solenoid driving a piston into the cavity. The jet thrust was measured over a broad range of synthetic jet operating parameters, including the actuation frequency and duty cycle, as well as the jet-to-crossflow velocity ratios. Previous work which is based on the slug flow model of an individual vortex ring predicts the time-averaged thrust scales with the square of actuation frequency and the stroke length. The measurements will be compared with the theoretical predictions, and the results will be used to assess the effect of crossflow on the thrust of synthetic jet.   

High-magnification velocity field measurements on high-frequency, supersonic microactuators

The Resonance-Enhanced Microjet (REM) actuator developed at our laboratory produces pulsed, supersonic microjets by utilizing a number of microscale, flow-acoustic resonance phenomena. The microactuator used in this study consists of an underexpanded source jet flowing into a cylindrical cavity with a single orifice through which an unsteady, supersonic jet issues at a resonant frequency of 7 kHz. The flowfields of a 1 mm underexpanded free jet and the microactuator are studied in detail using high-magnification, phase-locked flow visualizations (microschlieren) and 2-component particle image velocimetry. The challenges of these measurements at such small scales and supersonic velocities are discussed. The results clearly show that the microactuator produces supersonic pulsed jets with velocities exceeding 400 m/s. This is the first direct measurement of the velocity field and its temporal evolution produced by such actuators. Comparisons are made between the flow visualizations, velocity field measurements, and simulations using Implicit LES for a similar microactuator. With high, unsteady momentum output, this type of microactuator has potential in a range of flow control applications.