Computational Fluid Dynamics Modeling of Synethic Jet Actuators on Airfoils
Author: Christopher Mundy
Adviser: Pier Marzocca
7 March 2005
Goal
To develop a model that can accurately represent the aerodynamics and vibration control
capabilities of an airfoil equipped with optimized synthetic jet technology.
Introduction and Pertinent Background Information
The synthetic jet has emerged as a useful fluidic device with increasing potential for
practical applications, such as thrust vectoring of engines, mixing enhancement, and
active control of separation and turbulence in boundary layers3. Figure 1 illustrates a
typical synthetic jet actuator. A vibrating membrane oscillating at a frequency, ω, forces
air out or sucks air in through a small orifice opposite the membrane. Synthetic jets are
considered to be zero net mass flux devices2.
Figure 1: Diagram of a synthetic jet actuator.
The performance of the synthetic jet actuator is dependent on various physical, structural,
and geometrical parameters. Cavity size, orifice diameter, and frequency of oscillation
are all controllable parameters that affect the performance of the actuator3.
Over the past decade, there has been a significant amount of experimental research
conducted to demonstrate the effectiveness and potential capabilities of the synthetic jet
with respect to boundary layer separation on conventional airfoils1. Much of this research
effort has been conducted by experimental means; that is, airfoils equipped with synthetic
jet technology have been tested in laboratory wind tunnels to empirically determine the
properties and characteristics of the airfoil that are altered by the addition of a synthetic
jet.
Smith and Glezer5 studied the airflow behavior of a rectangular synthetic jet actuator with
an exit orifice that is 0.5 mm by 75 mm. This actuator contained a piezo-electric
diaphragm oscillating at a given frequency. Smith and Glezer demonstrated that when the
jet is blowing air out of the orifice, a shear layer is formed between the ejected fluid and
the surrounding fluid, caused by the velocity difference between the fluids. At the end of
the blowing stroke, this layer rolls up and forms a vortex pair. During the suction stroke,
the fluid is brought into the actuator cavity, but the vortex pair generated during the
blowing stroke has moved far enough away from the orifice and is not brought back into
the cavity. Thus, there is no mass flux out of the cavity in the time-average sense, but
there is a momentum impulsed into the external fluid6.
In laboratory experiments, these actuators have been able to increase lift and pitching
moment and delay boundary layer separation of the airflow over the wing of an aircraft,
reducing turbulent flow and drag. Figures 2(a) and 2(b) illustrate the results of
experiments conducted by Traub, Miller, and Rediniotis. In these experiments, a
synthetic jet actuator was placed at 12% of the NACA 0015 airfoil chord, capable of
operating at oscillation frequencies up to 120 Hz. The experimental results show that both
the lift and pitching moment coefficients can be increased using a synthetic jet actuator6.
Figure 2 (a) and (b): Systematic effects of SJA Cµ on integrated lift and pitching moment coefficient6.
Modeling of synthetic jet actuators has also been an area of extensive research in recent
years. In March 2004, at a workshop hosted by the Langley Research Center in
Williamsburg, VA, over two dozen groups generated models for predicting the flow
effects induced by a synthetic jet for three different scenarios: a synthetic jet in quiescent
air, a synthetic jet in a cross flow, and synthetic jet flow over a hump model (actuator
control) 4. The models presented illustrate the general gaps in understanding of the
interactions between a synthetic jet and its environment. That is, there was a distinctly
high amount of variation between models.
Proposed Methods
In order to further study the effects of synthetic jet actuators on airflow over an airfoil, a
Computational Fluid Dynamics (CFD) model will be created using a combination of the
Gambit and Fluent software packages to assess the impact of a synthetic jet on the flow
over a flat plate. Once the jet/boundary layer interactions are validated and understood,
the model will be adapted to support the inclusion of multiple actuators, as well as an
optimization algorithm to help identify the most beneficial locations of the synthetic jet
actuators.
This model will supplement the design and testing of a test plate equipped with synthetic
jet actuators. Such testing will serve to validate the CFD model and to provide evidence
supporting the optimization generated by the model.
Abbreviations used in this document:
Cl
Coefficient of lift
Cm
Coefficient of pitching moment
Cµ
Coefficient of jet momentum
k
Non-dimensional ramp rate
ω
Frequency of membrane oscillation
Timeline
Date
Goals
April 2005
Complete CFD modeling of synthetic jet airflow over flat
plate geometry.
October 2005
Complete CFD modeling of synthetic jet orifice.
December 2005
Complete CFD modeling of plate and orifice together,
and begin optimization process.
Spring 2006
Complete thesis.
References
1. Amitay, Michael, Smith, Douglas R., Kibens, Valdis, Parekh, David E., Glezer, Ari.
(2001). Aerodynamic Flow Control over an Unconventional Airfoil Using
Synthetic Jet Actuators. AIAA Journal, Vol. 39, No. 3.
2. Guo, Dahai, Cary, Andrew W., Agarwal, Ramesh K. (2003). Numerical Simulation of
Vectoring of Primary Jet with a Synthetic Jet. AIAA Journal, Vol. 41, No. 12.
3. Mittal, R., Rampunggoon, P., Udaykumar, H.S. (2001). Interaction of a Synthetic Jet
with a Flat Plate Boundary Layer. AIAA 2001–2773.
4. NASA Langley Research Center Workshop on Synthetic Jets and Turbulent Separation
Control. (2004). < http://cfdval2004.larc.nasa.gov/>
5. Smith, B.L., Glezer A. (1997). Vectoring and Small–scale Motions Effected in free
Shear Flows Using Synthetic Jet Actuators. AIAA 97–0213.
6. Traub, Lance W., Miller, Adam, Rediniotis, Othon. (2004). Effects of Synthetic Jet
Actuation on a Ramping NACA 0015 Airfoil. Journal of Aircraft, Vol. 41, No. 5.