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.

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

http://absimage.aps.org/image/DFD05/MWS_DFD05-2005-000785.pdf

Active Control of High Speed and High Reynolds Number Jets via Plasma Actuators$^{\ast }$

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.

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

The Entropy Wave Generator (EWG): A Reference Case on Entropy Noise

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. 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.

Active Control of High Speed and High Reynolds Number Free Jets Using Plasma Actuators

Coaxial model jets, including those of high bypass ratio engine exhaust hot gas conditions, were excited internally by tone and broadband noise. Acoustic excitation in the secondary (outer) duct was found to be most effective in jet noise amplification due to the sensitivity of the outer shear layer. Jet noise amplification at the subharmonic of the excitation frequency occurred in a number of cases. An acoustic elliptic mirror was used to observe the noise sources along the jet. It revealed local noise source characteristics in different shear layer regions and noise source location changes from unexcited to excited jets.

Effect of excitation on coaxial jet noise

Noise and flow characteristics of model swirling jets were measured. A swirling jet from a plug-nozzle generated a strong vortex flow and a broadband noise higher than that of a nonswirling jet. Exit guide vanes were found to be able to decrease the swirl and the noise simultaneously. A swirling jet with a nonswirling jet core flow was tested. The nonswirling core flow prevented the formation of strong vortex and kept the swirling jet noise at a relatively lower level. Noise attenuation for this flow only occurred at small angles from jet axis. The noise increase of the swirling jet over the nonswirling jet was found to depend on swirl angle, pressure ratio, and total temperature of the jet.

Noise of Swirling Exhaust Jets

The effects of a fuselage and its boundary layer on sound propagation to the fuselage surface and on sound scattering in the farfield were analyzed. A hard-wall infinite cylinder with a boundary layer of both velocity and temperature variations was modeled to simulate the fuselage of an aircraft in flight. Examples for a monopole noise source outside the boundary layer showed considerable noise attenuation on the cylindrical surface forward of the source and much less effect on the downstream side. Data from a transonic wind tunnel test showed the same trends. For enroute and airport community noise, the boundary layer alters the interference pattern caused by the fuselage.

Fuselage boundary-layer effects on sound propagation and scattering

Noise of swirling exhaust jets

An analytical model was developed for the evaluation of entropy noise generated in a low Mach number nozzle flow. The acoustic intensity radiated from the nozzle exit was obtained in closed form. Correlations among upstream temperature, pressure, and velocity fluctuations are required for calculation of radiated noise. The mean flow and the flow inhomogeneities were assumed to be quasi-one-dimensional, and an exponential nozzle was selected to simplify the analysis. Results show that the upstream fluctuation of temperature is an important source of nozzle entropy noise and the noise intensity is roughly proportional to the nozzle contraction rate.

H. Y. Lu

Papers

Effect of excitation on coaxial jet noise

Noise of Swirling Exhaust Jets

Fuselage boundary-layer effects on sound propagation and scattering

Noise of swirling exhaust jets

An Analytical Model for Entropy Noise of Subsonic Nozzle Flow