Localized arc filament plasma actuators are used to control an axisymmetric Mach 1.3 ideally expanded jet of 2.54 cm exit diameter and a Reynolds number based on the nozzle exit diameter of about 1.1×106. Measurements of growth and decay of perturbations seeded in the flow by the actuators, laser-based planar flow visualizations, and particle imaging velocimetry measurements are used to evaluate the effects of control. Eight actuators distributed azimuthally inside the nozzle, approximately 1 mm upstream of the nozzle exit, are used to force various azimuthal modes over a large frequency range (StDF of 0.13 to 1.3). The jet responded to the forcing over the entire range of frequencies, but the response was optimum (in terms of the development of large coherent structures and mixing enhancement) around the jet preferred Strouhal number of 0.33 (f = 5 kHz), in good agreement with the results in the literature for low-speed and low-Reynolds-number jets. The jet (with a thin boundary layer, D/θ ∼ 250) also responded to forcing with various azimuthal modes (m = 0 to 3 and m = ±1, ±2, ±4), again in agreement with instability analysis and experimental results in the literature for low-speed and low-Reynolds-number jets. Forcing the jet with the azimuthal mode m = ±1 at the jet preferred-mode frequency provided the maximum mixing enhancement, with a significant reduction in the jet potential core length and a significant increase in the jet centreline velocity decay rate beyond the end of the potential core.

The paper discusses recent results on the development of localized arc filament plasma actuators and their use in controlling high-speed and high Reynolds number jet flows. Multiple plasma actuators (up to 8) are controlled using a custom-built 8-channel high-voltage pulsed plasma generator. The plasma generator independently controls pulse repetition rate (0-200 kHz), duty cycle and phase for each individual actuator. Current and voltage measurements demonstrated the power consumption of each actuator to be quite low (20 W at 20% duty cycle). Emission spectroscopy temperature measurements in the pulsed arc filament showed rapid temperature increase over the first 10-20 µs of arc operation, from below 1000 • C to up to about 2000 • C. At longer discharge pulse durations, 20-100 µs, the plasma temperature levels off at approximately 2000 • C. Modelling calculations using an unsteady, quasi-one-dimensional arc filament model showed that rapid localized heating in the arc filament on a microsecond time scale generates strong compression waves. The results of the calculations also suggest that flow forcing is most efficient at low actuator duty cycles, with short heating periods and sufficiently long delays between the pulses to allow for convective cooling of high-temperature filaments. The model predictions are consistent with laser sheet scattering flow visualization results and particle imaging velocimetry measurements. These measurements show large-scale coherent structure formation and considerable mixing enhancement in an ideally expanded Mach 1.3 jet forced by eight repetitively pulsed plasma actuators. The effects of forcing are most significant near the jet preferred mode frequency (ν = 5 kHz). The results also show a substantial reduction in the jet potential core length and a significant increase in the jet Mach number decay rate beyond the end of potential core, especially at low actuator duty cycles.

Localized arc filament plasma actuators were used to control an axisymmetric Mach 0.9 jet with a Reynolds number based on the nozzle exit diameter of 7:6 10 5. Eight actuators, distributed azimuthally inside the nozzle, near the nozzle exit, were used to excite various instabilities and azimuthal modes of the jet over a large Strouhal number range (St DF of 0.1 to 5.0). Time-resolved pressure measurements were used to investigate the development of actuation perturbations in the jet, particle image velocimetry measurements were used to evaluate the control effects on the turbulence field, and far-field sound was measured to evaluate the control effects on the radiated acoustic field. The jet responded to the forcing over a large range of excitation Strouhal numbers with varying degrees. As expected, the low Strouhal number seeded disturbances grew slowly, saturated farther downstream, and stayed saturated for a longer time before decaying gradually. The saturation and decay of the seeded perturbations moved farther upstream as their Strouhal number was increased. Seeded perturbations with higher azimuthal modes exhibited faster decay. Particle image velocimetry results showed that when exciting the jet's preferred-mode instability at lower azimuthal modes, the jet potential core was shortened and the turbulent kinetic energy was increased significantly. At higher Strouhal numbers and higher azimuthal modes, forcing had less of an impact on the mean velocity and turbulent kinetic energy. Far-field acoustic results showed a significant noise increase (2 to 4 dB) when the jet is excited around the jet's preferred-mode instability Strouhal number (St DF 0:2-0:5), in agreement with the results in the literature. Noise reduction of 0.5 to over 1.0 dB is observed over a large excitation Strouhal number range; this reduction seems to peak around St DF 1:5 to 2.0 at a 30-deg angle, but around St DF 3:0 to 3.5 at a 90deg angle. Although forcing the jet with higher azimuthal modes is advantageous for noise mitigation at a 30-deg angle and lower Strouhal numbers, the effect is not as clear at higher forcing Strouhal numbers and at a 90-deg angle.

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