Ph.D. Thesis Defense by Thomas M. Crittenden
Monday, April 21, 2003
(Dr. Ari Glezer, advisor)
"Fluidic Actuators for High Speed Flow Control"
To extend previously demonstrated flow control techniques to higher velocity regimes, it is necessary to develop actuators with sufficient momentum to successfully interact with and manipulate high speed flows. Two such actuation schemes are developed and their performance with variation in the relevant system parameters is described. The first of these is the compressible synthetic jet. This is an extension of previous synthetic (zero net mass flux) jet research with an increase in driver power such that substantial pressurization of the cavity is possible, and the system operates in the compressible flow regime. Variation in three fundamental system parameters is investigated: orifice diameter, actuation frequency, and compression ratio. The effects of these parameters on the time-dependent cylinder pressure are detailed, highlighting the unique effects of compressibility on the pressure curve. The near-field jet structure is documented using PIV measurements and Schlieren flow visualization. A quasi-static numerical model of the cylinder pressure is also developed and used to verify the dimensionless parameters, investigate variables which could not be directly measured, and make predictions beyond the current experimental range. Finally, an experiment is described with self-actuated valves mounted in the cylinder head which overcome some of the limitations inherent to compressible operation.
The second actuation concept is the combustion-driven jet actuator. This device consists of a small-scale (O~1 cc) combustion chamber into which premixed fuel and oxidizer is fed. The mixture is ignited, creating a momentary high pressure burst within the combustor and a subsequent jet emanating from the exhaust orifice. At the scales tested, the entire combustion process is complete within several milliseconds and the cycle resumes with fresh fuel/oxidizer mixture entering the chamber and displacing remaining combustion products. The actuator performance is characterized based on dynamic combustor pressure measurements with additional analysis via Schlieren flow visualization, limited dynamic thrust measurements, and flame photography. The variable system parameters include fuel type and mixture ratio, exhaust orifice diameter, chamber aspect ratio, chamber volume, fuel/air flow rate, ignition/combustion frequency, and spark ignition energy. The performance trends for variation in each of these are documented and the theoretical basis for each proposed and discussed. Additionally, a proof-of-concept experiment demonstrates the utility of the combustion-driven jet actuators for low-speed flow control, specifically for transient reattachment of separated flow over an airfoil at high angle of attack.