Flow Visualization of the Velocity and Vorticity Fields of a Synthetic

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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 [1]. 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) [1] 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 [1]. 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 [1].
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 [1].
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 [2]. 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” [3]. 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 [3]. 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 [4]. 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 [4]. 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 [5]. 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 [4]. 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 [6]. 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 [6].
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 [7]. 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 [7]. 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 [7]. Overall, the pressure
gradients remained unchanged which suggests that synthetic jets did not improve flow
conditions for three-dimensional conditions [7].
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 [8].
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 [8]. 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 [8].
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 [8]. 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 [9]. This creates a two dimensional
velocity vector in the directions of the laser beams [10].
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 [11]. 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 [9]. 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 [9].
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:
[1] 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.
[2] D. Patel, P. Marzocca, R. De Breuker, M. Abdalla, and M. Stolk “Lifting Surface
Aerodynamic Modeling Accounting for Synthetic Jet Actuators,” Proceedings of
ICAST2007, 18th International Conference on Adaptive Structures and Technologies,
Oct. 3-5, 2007, Ottawa, Canada, pp. 1-8.
[3] R. De Breuker, M. Abdalla, P. Marzocca, Z. Gürdal, “Flutter Suppression using
Synthetic Jet Actuators: the Typical Section,” Proceedings of ICAST2006, 17th
International Conference on Adaptive Structures and Technologies, Oct. 16-19, 2006,
Taipei, Taiwan, pp. 1-8.
[4] Hong, G., Mallinson, S.G., and Reizes, J.A., 1999 “Some Characteristics of Synthetic
Jets,” 30th AIAA Fluid Dynamics Conference, June 28 – 1, 1999, Norfolk,
Virginia, A99-33673.
[5] Guo, D. and Kral, L.D., 1999, “Characterization of Jet Actuators For Active Flow
Control,” 30th AIAA Fluid Dynamics Conference, June 28 – 1, 1999, Norfolk,
Virginia, A99-33637.
[6] Bales, K., Jefferies, R., and Khoo, P., 2003, “Flow Control of a NACA 0015 Airfoil a
Chordwise Array of Synthetic Jets,” 41st Aerospace Sciences Meeting and Exhibit,
January 6–9, 2003, Reno, Nevada, AIAA 2003-0061.
[7] Bernal, L.P., Miska, P.K., Muller, M.O., and Washabaugh, P.D., 2001, “Flow
Structure and Performance of Axisymmetric Synthetic Jets,” 39th Aerospace Sciences
Meeting and Exhibit, January 8 – 11, 2001, Reno, Nevada, A01-16802.
[8] Amitay, M. and Cannelle, F., 2005, “Synthetic Jets: Spatial Evolution and Transitory
Behavior,” 43rd AIAA Aerospace Sciences Meeting and Exhibit, January 10-13, 2005,
Reno, Nevada, AIAA-2005-102
[9] Genrich, C.P. and Koochesfahani, M.M., 1996, “A Spatial Correlation Technique For
Estimating Velocity Fields Using Molecular Tagging Velocimetry (MTV),”
Experiments In Fluids, 22(1996), pp.67-77.
[10] Koochesfahini, M.M. and Nocera, D.G., 2007, “Molecular Tagging Velocimetry,”
Handbook of Experimental Fluid Dynamics, Springer-Verlag, Chapter 5.4.
[11] Stier,B. and Koochesfahini, M.M., 1999, “Molecular Taging Velocimetry (MTV)
Measurements in Gas Phase Flows,” Experiments in Fluids, 26 (1999), pp.297-304.
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