Experimental Investigation on Frequency Characteristics of Plasma Synthetic Jets

The performance of a two–electrode plasma synthetic jet actuator (PSJA) is investigated for a wide range of dimensionless actuation frequencies (f*) using high-speed phase-locked Particle Imaging Velocimetry (PIV) measurements. The jet-induced velocity fields in the axisymmetric plane are measured during both transient and steady working stages of the PSJA. When f* increases, the jet duration time (Tjet) is reduced while the peak suction velocity (Us) increases consistently. Three integral parameters, including the total expelled gas mass,impulse and issued mechanical energy also decline considerably with increasing frequency, which is shown to relate to both the reduced cavity density and the decreasing jet duration. Theoretical analysis reveals that the mean cavity density decreases monotonically with the square root of discharge frequency.The decreasing rate is inversely proportional to a thermal cut-off frequency (fc, 210Hz for the current study), which scales with the convective heattransfer coefficient between the actuator cavity walls and the cavity gas, aswell as the area of the cavity internal surface. In the time-averaged velocity fields, the jet centreline velocity ( c U ) exhibits a local maximum in the axial coordinate. The nondimensional maximum centreline velocity reduces with increasing frequency of operation. The jet spreading rate of the plasmasynthetic jets (PSJ) decreases from 0.14 to 0.09 with increasing frequency.During the transient working stage of PSJ, the exit velocity trace elapses 20 successive actuation cycles to stabilize. In contrast to the exitvelocity, approximately 130cycles are needed for the mean cavity density/temperature to reach steady values.