Micro Synthetic Jets as Effective Actuator

MICRO SYNTHETIC JETS AS EFFECTIVE ACTUATORS Mehti Koklu Old Dominion University, 2007 Director: Dr. Oktay Baysal Synthetic jets have previously been studied as actuators for external macroflow control and recently been proposed for internal microflow applications. Despite the wide variety of the potential applications of synthetic jet actuators, the majority of the studies have been done at macro scales. Furthermore, there has not been any design methodology that addresses the effectiveness of the micro synthetic jet actuators. Bearing these needs in mind, a micro synthetic jet configuration is considered in a microscale environment where Kn number is less than 0.1 and more than 0.001. Flowfields are simulated by solving the compressible Navier-Stokes equations. The wall boundary conditions have been modified to accommodate the slip velocity and the temperature jump conditions encountered for this specific range of the Knudsen numbers. The membrane motion is modeled in a realistic manner as a moving boundary in order to accurately compute the flow inside the actuator cavity. Due to lack of experimental studies on micro synthetic jets, validation of the current method is accomplished in two steps. In the first step, capabilities of the methodology are tested successfully by computing flowfields inside a microchannel, microfilter, and micro backward facing step. In the second step, a realistic modeling of a synthetic jet in macro flow conditions is validated with experimental results. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. As the main contribution of this study, a detailed parametric study is presented that covers a large design space of synthetic jet actuation and design variables. In this study, both the synthetic jets in quiescent environment and in cross flow conditions are considered. The design variables for the parametric study are the membrane oscillation frequency, the membrane oscillation amplitude, the orifice width, the orifice height, the cavity height, and the cavity width. Studying the characteristic length allows an understanding of a synthetic jet for different Knudsen and Reynolds numbers. The momentum flux, jet velocity, vortex formation and shedding, the area and the circulation of the vortex, are the metrics considered to determine the effectiveness of a synthetic jet. The final phase of the present study is on developing and demonstrating a design optimization methodology. This is accomplished in two steps. First, each design variable is considered one at a time as and other design variables are kept constant. This approach yields an effective actuator when considering the possibility of the limits on any design variable to be constant. As compared to the baseline case, the optimization studies yield 2%, 15%, 15%, 200% increase in actuation efficiency when the single-variable is the orifice width, the orifice height, the cavity height, or the frequency, respectively. Then a multi variable optimization is performed to obtain the synthetic jet configuration that yields the best efficiency. This study includes shape optimization using shape parameters and Bezier polynomials. As compared to the baseline case, the shape optimization using shape parameters results in a 170% increase in the actuation efficiency while the shape optimization with Bezier polynomials results in more than 10 times increase. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.