Recent Performance-Based Advances in SparkJet Actuator Design for Supersonic Flow Applications

Practical application of active supersonic flow control is dependent upon the development of robust actuators that have the requisite control authority (in amplitude and frequency), are reliable and low cost and consume minimal power. The Johns Hopkins University Applied Physics Laboratory and Florida State University/Florida Center for Advanced Aero-Propulsion are continuing the investigation of a device intended for highspeed flow control called the SparkJet actuator, which manipulates high-speed flows without active, mechanical components. Previous experimental and computational efforts demonstrate peak cavity pressure efficiency of approximately 30%, while experience has shown difficulty in constructing a reliable device. This body of work addresses SparkJet design modifications to improve performance measures such as device reliability, efficiency and lifetime. Success toward improving these performance-based characteristics has been accomplished by focusing attention to the modification of the trigger mechanism and electrode design. The trigger mechanism has been changed from an external trigger to a pseudo-series trigger which allows for larger electrode gaps up to 4.0 mm corresponding to peak cavity pressure efficiency up to 75%. Electrode design considerations have included parametric studies to observe the effect of varying tip shape, electrode material and electrode design. Increasing electrode diameter improves survivability under high thermal loads; consistent tip shape design maintains control of spark location near the center of the cavity; and proper electrode material selection impacts survivability. Results confirmed that larger electrode diameters up to 3.175 mm improve survivability under the high thermal loads associated with the spark-discharge environment. Preliminary results also indicate that the length-to-diameter ratio of the electrodes should exceed approximately 24 for proper impedance matching at frequencies above 1 kHz. This paper summarizes these findings through presentation of experimental results including microschlieren imagery, cavity pressure measurements and qualitative observations.