Ultrasonic-driven pulsating supersonic jet actuator: Generation of high-frequency, large amplitude velocity fluctuation in jet

In this study, a new actuator for generating a supersonic jet pulsating at an ultrasonic frequency (several tens of kHz), which is called ultrasonic-driven pulsating supersonic jet (USDPSJ) actuator, is proposed. This actuator consists of a flat wall with a 1.0-mm-diameter orifice and a top surface of an ultrasonic oscillator with a sub-millimeter gap between them. A supersonic jet pulsating at an ultrasonic frequency is successfully generated by driving the oscillator under the conditions that the ratio of the pressure upstream to downstream of the orifice is higher than the ratio at which choking occurs in the flow passing through the orifice. The jet is visualized by a schlieren method using a high-speed video camera. The visualization results reveal that a pulsating supersonic jet with a shock cell structure is generated. Furthermore, it is found that the jet is generated intermittently under specific conditions regarding the gap height, oscillator amplitude, and pressure ratio. The time-varying jet Mach numbers are calculated from the periodically fluctuating pressures measured by a pressure sensor placed at the orifice location under the same oscillator driving conditions as when the jets are generated from the orifice. The calculated jet Mach numbers are confirmed to agree reasonably well with the jet Mach numbers estimated from the shock cell lengths of the visualized jets. Based on this confirmation, the time-varying jet velocities are estimated. The estimation results reveal that the jet pulsates over its velocity fluctuation range from 0 to above 500 m/s under specific conditions regarding the gap height and oscillator amplitude, when the pressure ratio exceeds 2, demonstrating that the proposed actuator can achieve unprecedentedly large velocity fluctuations at an ultrasonic frequency. In addition, based on the experimental results, the maximum values for the actuator's energy conversion efficiency, jet penetration distance, and jet spreading rate are estimated to be 45%, 15.1 times the orifice diameter, and 0.35, respectively. • A new actuator is proposed that can create a pulsating supersonic jet at an ultrasonic frequency. • The proposed actuator is driven by an ultrasonic transducer. • The proposed actuator can generate a pulsating jet over its velocity fluctuation range of 0 to above 500 m/s. • The proposed actuator requires the pressure only 2 times higher than the back pressure to achieve the range of 0 to above 500 m/s.