Jeff Kastner

Active control of high-speed and high-Reynolds-number jets using plasma actuators

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 1.1×10 6 . 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 ( St DF 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.

Active Control of a Mach 0.9 Jet for Noise Mitigation Using Plasma Actuators

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 thejetisexcitedaroundthejet’spreferred-modeinstabilityStrouhalnumber(StDF 0:2–0:5),inagreementwiththe 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 StDF 1:5 to 2.0 at a 30-deg angle, but around StDF 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 angleandlowerStrouhalnumbers,theeffectisnotasclearathigherforcingStrouhalnumbersandata90-degangle.

Development and use of localized arc filament plasma actuators for high-speed flow control

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.

Exploring High-Speed Axisymmetric Jet Noise Control Using Hartmann Tube Fluidic Actuators

This work is part of an ongoing effort to better understand the influence of frequency and amplitude when controlling high Reynolds number, high-speed jets using a Hartmann Tube Fluidic Actuator (HTFA). In addition to HTFA forcing, a steady mass injector (SMI) with the same mass flow rate as the HTFA was used. Forcing was applied with two actuators separated by 180° and four actuators separated by 90° in the azimuthal direction. The jet under study was axisymmetric with Mach numbers between 0.6 and 0.98 and Reynolds number between 5.7 x 10 5 and 7.6 x 10 5 . To understand the influence of forcing on far-field noise, acoustic measurements were acquired at 30° and 60° with reference to the jet axis. With two actuators, the jet noise increased when the actuators operated at a frequency between St D = 0.2 - 0.5. Forcing with two or four actuators at higher Strouhal numbers (St D > 0.75) or with steady mass injection slightly reduced the peak broadband jet noise and increased the high frequency noise. The slight reduction was only observed when the total mass injection was greater than 1%. The actuators (SMI or HTFA) had their largest impact when they were injected at an angle between 45 and 60° with respect to the jet axis.

A study of the correlation of large-scale structure dynamics and far-field radiated noise in an excited Mach 0.9 jet

The main goal of the present work is to excite various instabilities of an axisymmetric Mach 0.9 jet with a ReD of 0.76 × 106, track the ensuing large-scale structures/instability waves, and investigate relations between the dynamics of these structures and the far-field sound. The jet was excited over a large range of Strouhal numbers and several azimuthal modes by eight localized arc filament plasma actuators, equally spaced around the circumference of the nozzle, near the nozzle exit. The flow field and far-field noise were investigated using particle image velocimetry and a three-dimensional array of 12 microphones at 30° polar angle to the downstream jet axis. The microphone array results show that the high amplitude noise radiated to 30° polar angle is originated just downstream of the end of the potential core, in agreement with our previous results and the results in the literature. The streamwise noise source distribution was only sensitive to azimuthal modes around the jet preferred mode. Otherwise, the general trend was that forcing the jet at low Strouhal numbers moves the distribution upstream compared to the baseline jet, and at high Strouhal numbers results in a source distribution similar to the baseline jet. ***Conditionally-averaged PIV data were used to relate the flow dynamics and noise sources. The growth, saturation, and decay of the conditionally-averaged velocity fluctuations along the jet centerline correlate well with the far-field noise and the noise source distribution estimated using the microphone array. For m = 0 mode excitation around the jet column Strouhal number, the conditionally-averaged streamwise velocity fluctuations correlate well with the noise source distribution. While for m = 1, the correlation is best with the conditionally-averaged cross-stream fluctuations.

Noise Mitigation in High Speed and High Reynolds Number Jets Using Plasma Actuators

*† ‡ § This work is part of our ongoing research in the development and application of localized arc filament plasma actuators (LAFPA) to control high-speed and high Reynolds number jets for noise mitigation. The jet in the current work is an axisymmetric Mach 0.9 jet with a Reynolds number based on the nozzle exit diameter of 7.6x10 5 . We increased the number of actuators, distributed azimuthally inside the nozzle, near the nozzle exit, from 8 to 12, enabling us to excite higher azimuthal modes of the jet (m=0 to 5 and m=±1, ±2, ±3, and ±6) over a large frequency range (StDF of 0.1 to 5.0). Time-resolved pressure measurements, PIV measurements along with the Galilean decomposition of the velocity field, and far-field acoustic measurements were used to investigate the growth and decay of perturbations and instability waves in the flow, to visualize large-scale structures and the effects of control on the turbulence field, and to evaluate the control effects on the radiated noise field. Excitations with lower frequencies coupled to the flow, and the instability waves grew slowly, saturated farther downstream, stayed saturated for a longer time, and then gradually decayed, similar to the reported results in the literature using other techniques. The saturation and decay of instability waves moved upstream as the excitation frequency increased. Instability waves with higher azimuthal modes exhibited faster decay. Excitation of the jet around the jet preferred mode Strouhal number produced robust and coherent large-scale structures, similar to those in much lower speed and lower Reynolds number jets. The structures became smaller and less coherent in higher Strouhal number excitation. Far-field acoustic results show a significant noise increase when the jet is excited around the jet preferred mode Strouhal number (StDF=0.2-0.5), with larger increase from the shallower angle to the jet axis (30°) to the larger angle (90°), in agreement with the results in the literature. Noise reduction is observed over a large excitation frequency range - this reduction seems to peak around StDF =1.0 to 2.0 at 30°, but around StDF =3.0 to 3.5 at 90°. While forcing the jet with higher azimuthal modes (limited to m=0 to 5 in the current experiments, extended from m=0 to 3 in our previous work) is advantageous for noise mitigation at 30° and lower frequencies, the effect is not as clear in higher forcing frequencies and at 90°.

Impact of Heat on the Pressure Skewness and Kurtosis in Supersonic Jets

Mach wave radiation and crackle are dominant noise components from high-speed jets, found in both high-power engines and scale nozzles. The statistics of the pressure signal and its time derivative (dP/dt) have been widely studied to identify and quantify crackle. In this paper, we investigate the impact of operating condition on the overall sound pressure level, skewness, and kurtosis of the pressure and dP/dt signals of a jet issuing from an Md=1.5 converging–diverging conical nozzle. The effect of temperature and nonideal expansion were independently investigated. An increase in convective Mach number Mc, achieved by increasing either jet temperature or nozzle pressure ratio, proved to be related to elevated values of overall sound pressure level, skewness, and kurtosis, in both the near and far fields. The peak values of overall sound pressure level, skewness levels, and kurtosis levels were found to propagate at different angles for cold jets, but at elevated temperature, the directivity was more sim...

Comparison of Noise Sources in High and Low Reynolds Number High Speed Jets

Results from low and high Reynolds number jets are compared to investigate how the Reynolds number (Re) of a jet influences the mechanisms generating jet noise. Direct numerical simulations (DNS) and experimental results of the authors were used for this purpose. The DNS results are for a Mach 0.9, low Re (ReD~3600) axisymmetric jet (Freund, 2001) and the experimental results are for an ideally expanded, Mach 1.3, high Re (ReD~1.06 x 10 6 ) axisymmetric jet (Hileman et al. 2004b). These two cases will be referred to as LReJ and HReJ, respectively, hereafter. Previous experimental work on the HReJ using a three- dimensional microphone array, located at 30° with respect to the jet axis, with a novel beamforming technique estimated the source location and the time of generation of each large amplitude sound wave that reached the far-field. Simultaneous with the pressure measurements, the flowfield, the source region, was visualized using a MHz rate imaging system, and was analyzed using reconstructions via Proper Orthogonal Decomposition (POD). These results were compared to periods involving no large amplitude far-field noise events and revealed that the growth and decay of large structures in the mixing layer (a wave-like series) are the dominant flow feature during the emission of sound waves that travel in at the aft quadrant (Hileman et al., 2004b). In the current work, this technique is used with the LReJ and the results are compared. There are many similarities between the two including the far-field acoustic spectrum, coherence, average waveform and mean noise source location. The few differences observed are believed to be due to the limited extent of turbulence scales and/or because the initial shear layer was laminar in LReJ, both related to the Re. The main conclusion is that the rapid breakdown of the large-scale structure appears to be an important, and perhaps the main mechanism of jet noise. Right before the breakdown, the structures seem to contract in size, tilt and eventually disintegrate. To offer a possible explanation to the observed noise mechanism, a simple one-dimensional instability wave model is shown to create more noise in the far-field when the wave is subject to truncation simulating a breakdown.

Effect of Scale on the Far-Field Pressure Skewness and Kurtosis of Heated Supersonic Jets

In heated supersonic jets, Mach wave radiation and crackle have been identified as dominant noise components that propagate to the downstream region of the jet, in a direction noted as the Mach wave angle. At certain conditions, the Mach waves coalesce in the near field causing steepening of the wavefront, which exceeding a certain level produces a noise feature called “crackle.” The skewness and kurtosis of the pressure and its time derivative (dP/dt) have been widely studied as a measurement of crackle. In this paper, we investigate the impact of different test conditions and different nozzle exit diameters on the far-field high-order statistics of the pressure and dP/dt signals of three converging-diverging conical nozzles, with a design Mach number of 1.5 and jet exit diameters of 0.542, 0.813, and 1.085. Results are compared to a smooth contoured nozzle designed by the Method of Characteristics, with the same design Mach number. For all nozzles, cold and heated jets, TR=1.0 to 3.0, are tested at over, design, and under-expanded conditions. Second, third, and fourth order statistics are examined in three far-field arrays positioned at a nondimensionalized constant radial distance of r=40De. The OASPL, skewness, and kurtosis magnitudes and their propagation angles are proportional to the jet temperature and the NPR, and have peak amplitudes near the Mach wave angle. The pressure skewness and kurtosis plots collapsed for all three scaled nozzles when the pressure signals were not filtered. The dP/dt statistics collapsed when the signals were downsampled proportional to the nozzle exit diameters, applying beforehand a low-pass filter at a proportional cutoff frequency, to avoid aliasing effects.

Toward Noise Mitigation in High Speed and High Reynolds Number Jets Using Plasma Actuators

5 . Eight actuators, distributed azimuthally inside the nozzle, near the nozzle exit, were used to excite various azimuthal modes of the jet over a large frequency range (StDF of 0.1 to 5.0). Dynamic pressure measurements were used to investigate the growth and decay of perturbations and instability waves in the flow, PIV measurements were used to evaluate the effects of control on turbulence field, and far field acoustic was measured to evaluate the control effect on the radiated noise field. The jet responded to the forcing over the entire range of excitation frequencies, but with varying degrees. The growth and decay of perturbations imparted to the flow by the actuators and the ensuing instability waves were found to be similar to the reported results in the literature using other techniques. Excitations with lower frequencies coupled to the flow, and the instability waves grew slowly, saturated farther downstream, stayed saturated for longer time, and then decayed gradually. The saturation and decay of instability waves moved farther upstream as the excitation frequency increased. Instability waves with higher azimuthal modes exhibited faster decay. Preliminary far field acoustic results show a significant noise increase (2 to 4 dB) when the jet is excited around the jet preferred mode instability frequency (StDF=0.2-0.5), which becomes larger from the shallower angle to the jet axis (30°) to the larger angle (90°), in agreement with the results in the literature. Noise reduction of 0.5 to over 1.0 dB is observed over a large excitation frequency range - this reduction seems to peak around StDF =1.5 to 2.0 at 30° angle, but around StDF =3.0 to 3.5 at 90° angle. While forcing the jet with higher azimuthal modes, with azimuthal modes limited to m=0 to 3 in the current experiments, is advantageous for noise mitigation at 30° angle and lower frequencies, the effect is not as clear in higher forcing frequencies and at 90° angle. These preliminary results are quite encouraging and show the potential of the technique. However, a great deal more work must be done to better understand the effects and to improve the technique.