Flow Visualization of the Velocity and Vorticity Fields of a Synthetic Jet Using Molecular Tagging Velocimetry Laura A. Bear Honors Thesis Proposal Advisors: Professor Piergiovanni Marzocca and Professor Douglas Bohl Abstract: The purpose of this thesis project is to determine the behaviors of the velocity fields created by a Synthetic Jet Actuator (SJA). Synthetic jets are one method of flow control for an aircraft and when added to an airfoil, the flow around it can be altered to increase lift and decrease the amount of drag experienced by the aircraft in order to improve its aerodynamic performance. A synthetic jet is comprised of a vibrating piezoelectric membrane that induces flow in and out of a cavity through a small opening. The amplitude of the vibration is controlled by the applied voltage. For this project a sine function is used to drive the SJA, inducing into and out of the cavity of the SJA. One goal of the project is to determine and characterize the performance of the SJA to modify the flow field in the proximity of the orifice; this will be achieved by using an experimental apparatus where a data acquisition system and a labview setup will enable a sweep of frequency of actuation of the SJA. Results from this testing will also show changes in the flow field and associated changes in the characteristics of the vortices formed at the edges of the jet. In addition, the flow measurements will help identifying the overall dimensions of the jet flow, defining the distance from the slot that would be affected by the jet. Analysis would be done in order to correlate the jet frequency with this length. Finally, numerical investigations through a series of CFD simulations are also contemplated for comparison purposes. Introduction: With increasing costs of fuel and environmental awareness, the demand for more efficient aircraft as well as the research needed to gain this efficiency is becoming ever more important. Increased efficiency can be gained through the reduction of drag, control of the boundary layers, or changing the surface properties of the body of the aircraft . All of these changes are dependent on the control of the fluid flow around the body. Altering the flow around a body from its natural course to a more desired path is called active flow control (AFC)  and can be accomplished through open-loop control, or closed-loop control. In open-loop control, a mechanism is designed based on the original state of the flow and remains at that state throughout operation. Closed-loop control, on the other hand, is designed based on the original state of the flow, but continues to take measurements throughout operation to adapt the control to the transforming flow. This strategy is designed to continue to take measurements and alter until the desired flow is reached . Either method requires knowledge of the original, non-preferred flow state which will be modified. In order to modify this flow, a flow actuator is applied to the body that will interact with the boundary layer. The chart below describes the process of active flow control: Figure 1: Block diagram of an active flow control process . An additional challenge, however, to flow control is to establish control mechanisms that will “ensure that the control itself does not initiate new, undesirable flow.” [1, pg253] Most flow in which aircraft travel is highly turbulent, nonlinear, and unpredictable create a state that is highly difficult to model. Nearly all earlier models have been based on steady state flows, due to its simplicity. Analyzing flow at its actual conditions will be highly beneficial in increasing effectiveness of the control method. Active flow control has this capability and may offer a decrease in the cost of production of an aircraft, the drag caused during flight, as well as the overall weight . Background: One method of Active Flow Control (AFC) is through the use of Synthetic Jet Actuators (SJA). When SJA’s are applied to an airfoil it will allow the plane to carry increased loads without the increased drag effects and flow separation . SJA’s operate through the addition of momentum to the flow without adding mass flux. The change in momentum causes the natural streamlines to rearrange around the body and modify the aerodynamic forces on the body. This is called “dynamic virtual shaping” . The displacement of the streamlines induce a “virtual” change in the shape of the body in which the flow surrounds, resulting in little or no boundary layer separation. In order to produce a synthetic jet only the working fluid in the system around the body is used, no additional fluid mass is added . Instead, a small slot is cut in the top of an enclosed hollow space and a thin covering over the bottom, creating a diaphragm made of a piezoelectric material. The diaphragm is forced to oscillate up and down with an amplitude proportional to the applied voltage, which controls the flow. When the diaphragm moves downward toward the bottom of the opening air is brought into it through the slot non-directionally. The upward movement of the diaphragm pushes the air out through the same slot directionally, creating a jet and vortex pair for a rectangular slot or a vortex ring for a round opening . These vortices are created at a distance far enough away from the slot that the top of the hollow area does not affect the flow coming out of it . The vortices then move away from the slot before the diaphragm moves again and pulls in more fluid. This ensures that the created vortices are unaffected by the SJA inflow . There is no supply of air required and the only power supply needed is an electrical supply. Non-synthetic jets require compressors to supply air as well as piping and additional equipment, which add weight to the aircraft . Synthetic jets are more efficient due to the fact that less weight is required. Below is a picture of an actual synthetic jet and a diagram illustrating how the diaphragm works. Figure 2: Image of a typical synthetic jet. Figure 3: Schematic diagram of the vibrating diaphragm of a synthetic jet. Previous Research: Research has been done in the past to determine the feasibility and effectiveness of the use of synthetic jets. From various reports and articles some known characteristics have been determined. Studies have been conducted on the flow formations from a synthetic jet and how these formations can affect thrust. When looking at the jets in two dimensions it was found that the flow ejected from a slot was first observed to create a laminar vortex pair that convects into the flow field. At a far enough distance from the body, approximately seven slot widths, the vortices became turbulent . It was also discovered that in two dimensions the flow structure of a synthetic jet is comparable to that of a turbulent jet. The main differences being the rate at which the vortices form and then die out. This is said to be due to a lower static pressure at the slot of the synthetic jet . Research in determining the effects of three dimensional flows on the effectiveness of synthetic jets was done by the USAF academy. The goals were to minimize any losses cause by 3-D flow as well as increase the outward velocity of the jets . For this, an array of synthetic jets were attached to an airfoil. Results showed that accelerating the flow over the airfoil would reduce the boundary layer and minimize or even eliminate separation . Results also showed that controlling the separation on the leading edge or trialing edge of the wing gave an decreased turning radius and reduction in drag. However, when both used simultaneously, the results appeared to cancel each other out. Their results showed that the synthetic jets were capable of altering the stream lines around the body in order to change the flow patterns, however, these results were only proven for areas close to the airfoil . Overall, the pressure gradients remained unchanged which suggests that synthetic jets did not improve flow conditions for three-dimensional conditions . Additional synthetic jet characteristics not fully understood include: span-wise consistency as well as the optimal frequency and amplitude at which the diaphragm should oscillate. Experiments were run at Rensselaer Polytechnic Institute using Particle Image Velocimetry (PIV) in order to observe the velocity fields of the flow . Measurements were taken along two planes, the x-y plane defined as running perpendicular to the synthetic jet slot and the x-z plane as running parallel to the slot. It was discovered the varying the frequencies at which the diaphragm oscillated changed the strength of the jet. Stronger jets run at higher frequencies causes a longer wavelength in the flow ejected from the jet . The experiments further illustrated in the x-y plane that two dimensionally the flow acts as two vortices coming out of the slot. These vortices eventually dissipate and convert to turbulent flow as illustrated by measurements taken from the x-z plane which demonstrated flow uniformity and minimal three dimensional effects near the slot. However, these vortices, once multiple vortex lengths from the slot began to move at different rates and showing affects of three dimensionality and turbulence . The effects of altering the frequency at which the jet is run were also reported. Results were given in terms of the ratio of stroke length to length of the slot. For small ratios of nominally 16 at a frequency of 300Hz, the vortices did not fully develop before the next oscillation of the diaphragm, causing air from the vortex to be pulled back in through the slot. Therefore, many oscillations were needed before the flow was able to form a vortex and become fully developed. At larger ratios, about 50, the flow was able to break away from the slot before it was sucked back in through the slot during the next oscillation . These experiments show the importance of the effects of the driving frequency of the diaphragm and the geometry of the synthetic jet. Previous research concludes that the synthetic jet method of AFC will reduce the weight needed for flow control while using less power and equipment than traditional blowing jets. Experimentation has shown a vortex pair flowing from the slot of a synthetic jet that will alter the flow around the body, however, the velocity fields of these flows are still inconclusive and need further experimentation to fully determine the effectiveness of a synthetic jet. Thesis Project: For this project, research will be done to analyze the velocity fields of a single synthetic jet. In addition, further analysis will be done to determine differences in the characteristics of flow between the suction and blowing oscillations of the jet. The goal is to quantify the velocity field at all phases of throughout the SJA cycle. The method of Molecular Tagging Velocimetry (MTV) will be utilized to measure the flow fields. The expected result of this experimental process is to be able to visualize and map the velocity and vorticity fields induced by a synthetic jet and the resulting effects of these vortices on the surrounding air. Theory of Molecular Tagging Velocimetry: The purpose in molecular tagging velocimetry is to observe the flow of a fluid through space. This is done by adding a chemical to the flow and allowing the molecules to evenly disperse themselves through the fluid. A laser is then used to illuminate the chemical within the fluid and make the fluid traceable. To make tracing easier, the laser beam takes the form of a pattern, in this case a 2 dimensional grid, and imposes this pattern in the molecules of the chemical within the flow. A photograph is taken at the instant the molecules are tagged and then again about a microsecond later. The distance and direction can then be measured and when taken as the ratio over the nanosecond time difference results in the velocity of the molecules . This creates a two dimensional velocity vector in the directions of the laser beams . In order to measure the velocity field a laser is used to “write” a grid of lines into the flow field premixed with a chemical lumophore. This grid can be constructed by a laser beam going through a splitter to divide it into two equal beams. Both beams can then be sent through a beam blocker which will result in series of laser lines orthogonal to each other, generating a grid . Two images of the grid are taken a know time apart. Image correlation software is used to find the displacement of each grid intersection. The velocity is found by dividing the displacement by the time between images . In order to be as accurate as possible, measured displacement vectors are often kept less than one half of a grid length to eliminate confusion of which grid corresponds to another grid when analyzing photos . MTV is preferred over previously used tagging methods because of its increased accuracy due to the fact that traces fluid motion rather than seed particle motion as is used in PIV. In flows with high velocity gradients seed particles may not accurately follow the flow due to lifting effects on the particles. With MTV there are no such issues due to the fact that the fluid itself is tracked. The major drawback to MTV is that it requires oxygen free surrounding, which will have to be taken into account in the experimental apparatus Experimental Apparatus and Procedure: Figure 4: Actual experimental setup including the synthetic jet, the sealed chamber, the camera, laser, and timer controller Hood Aluminum Cylinder Synthetic Jet/ Cubic Container Laser Timer Nitrogen Tank Camera Computer/ Labview DAQ Figure 5: Block diagram of the experimental setup. Figure 4 shows a photograph of the experimental set-up. A block diagram of the experimental set-up is shown in Figure 5. At the beginning of each experiment the nitrogen tank is used to run nitrogen through the synthetic jet and into the sealed chamber for about forty minutes in order to remove all oxygen from the container. The biacetyl used for the experiment is stored in the aluminum cylinder, but must also be free of oxygen. Therefore, before the experiment is started the nitrogen tank is also connected to the aluminum cylinder with hoses and placed in the hood during the oxygen removal. While the experiment is taking place the aluminum cylinder is connected to the chamber and nitrogen seeded with biacetyl is allowed to enter the container. Once the chamber is filled to slightly above atmospheric pressure a valve is shut, discontinuing the flow of the nitrogen/biacetyl mix into the container. Stopping this flow before starting the flow from the synthetic jets will prevent the two flows and their velocity fields from affecting each other. The container is also connected to the hood with the use of another hose in order to ventilate the chemicals after the experiment. The experiment is recorded with the use of a gated intensified CCD camera (Cooke, DiCam Pro) and a pulsed Nd:YAG laser (266 nnm). A digital delay generator (DDG) is connected to the camera and the laser to control the laser pulse and camera timing. The laser sheet and grids are formed using optical a variety of optics. After the grid is written into the flow the DDG controls the time at which the first and second images are be taken. The camera is attached to a PC for image storage and later analysis. A Labview® program is used to operate the SJA provide reference timing for the images. In particular, the Labview® program is used to record the time at which each picture is taken and, therefore, the cycle phase. It also provides the sinusoidal function to the SJA power source and allows the user to know the location of the experiment on that sine curve at any given time. Research Timeline: March 2008 Develop experimental setup Acquire geometries and parameters of experimental setup Debug and improve experimental setup Finish thesis proposal (due 3/14) April 2008 Continue development and debugging of experimental setup Begin testing and data collection Setup Fluent for CFD modeling CFD modeling May 2008-July 2008 Data analysis Additional testing and data collection Possible help from undergrad researcher for summer data collection September 2008 Testing, data collection, comparison of results CFD Analysis October 2008 – November 2008 Final data collection Continued data analysis Comparing CFD modeling to experimental results December 2008 – February 2009 Final data analysis Writing of thesis paper References:  Collis, S.S., Joslin, R.D., Seifert, A., and Theofilis, V., 2004, “Issues in Active Flow Control: Theory, Control, Simulation, and Experiment,” Progress in Aerospace Sciences, 40(2004), pp.237-239.  D. Patel, P. Marzocca, R. De Breuker, M. 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