Fluid actuators for high speed flow control

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Crittenden, Thomas M.
Glezer, Ari
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In order to extend fluid-based flow control techniques that have been demonstrated at low subsonic speeds to high speed flows, it is necessary to develop actuators having sufficient momentum to control and manipulate high speed flows. Two fluidic actuation approaches are developed where the control jet may reach supersonic velocities and their performance is characterized. The first actuator is a compressible synthetic (zero net mass flux) jet. This is an extension of previous work on synthetic jets with an increase in driver power yielding substantial pressurization of the cavity such that the flow is compressible. The jet is generated using a piston/cylinder actuator, and the effects of variation of the orifice diameter, actuation frequency, and compression ratio are investigated. Operation in the compressible regime uniquely affects the time-dependent cylinder pressure in that the duty cycle of the system shifts such that the suction phase is longer than the blowing phase. The structure of the jet in the near-field is documented using particle image velocimetry and Schlieren flow visualization. In the range investigated, the stroke length is sufficiently long that the jet flow is dominated by a starting jet rather than a starting vortex (which is typical of low-speed synthetic jets). A simple, quasi-static numerical model of the cylinder pressure is developed and is in generally good agreement with the experimental results. This model is used to assess system parameters which could not be measured directly (e.g., the dynamic gas temperature and mass within the cylinder) and for predictions of the actuator performance beyond the current experimental range. Finally, an experiment is described with self-actuated valves mounted into the cylinder head which effectively icrease the orifice area in suction and 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 (nominally 1 cc) combustion chamber which is filled with premixed fuel and oxidizer. The mixture is ignited using an integrated spark gap, creating a momentary high pressure burst within the combustor that drives a high-speed jet from an exhaust orifice. At these scales, the entire combustion process is complete within several milliseconds and the cycle resumes when fresh fuel/oxidizer is fed into the chamber and displaces the remaining combustion products. The actuator performance is characterized by using dynamaic measurements of the combustor pressure along with Schlieren flow visualization, limited dynamic thrust measurements, and flame photography. The effects of variation in the following system parameters are investigated: 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 resulting performance trends are documented and the basis for each discussed. Finally, a proof-of-concept experiment demonstrates the utility of teh combustion-driven jet actuators at low-speed for transitory reattachment of a separated flow over an airfoil at high angles of attack.
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