Active control of high Reynolds number and high-speed jets has been hampered due to the lack of suitable actuators. Some of the attributes that would make an actuator suitable for such flows are: high amplitude and bandwidth; small size for distribution around the jet; phase-locking ability for jet azimuthal mode forcing; and sufficient ruggedness for hot jets. We have been developing a class of actuators termed ‘localized arc filament plasma actuators,’ which possess such characteristics. In this paper, we present the development and characterization of these actuators as well as preliminary results on their applications in high Reynolds number Mach 0.9 and ideally expanded Mach 1.3 jets.

Development and characterization of plasma actuators for high-speed jet control

This investigation presents a unique and elaborate set of experiments relating the generation of noise to the evolution of large-scale turbulence structures within an ideally expanded, Mach 1.28, high-Reynolds-number jet. The results appear to indicate many similarities between the noise generation processes of high-speed low-Reynolds-number and high-speed high-Reynolds-number jets. Similar to the rapid changes observed in theregion of noise generation in low-Reynolds-number jets in previous experimental and computational work, a series of robust flow features formed approximately one convective time scale before noise emission and then rapidly disintegrated shortly before the estimated moment of noise emission. Coincident with the disintegration, a positive image intensity fluctuation formed at the jet centreline in a region that is immediately past the endof the potential core. This indicates mixed fluid had reached the jet core. These results are consistent with the formation of large-scale structures within the shear layer, which entrain ambient air into the jet, and their eventual interaction and disintegration apparently result in noise generation. These results are quite different from the evolution of the jet during prolonged periods that lacked significant sound emission. The observations presented in this work were made through the use of well-established technique that were brought together in an unconventional fashion. The sources of large-amplitude sound waves were estimated in time and three-dimensional space using a novel microphone array/beamforming algorithm while the noise-generation region of the mixing layer was simultaneously visualized on two orthogonal planes (one of which was temporally resolved). The flow images were conditionally sampled based on whether or not a sound wave wascreated within the region of the flow while it was being imaged and a series of images was compiled that was roughly phase-locked onto the moment of sound emission. Another set of images was gathered based on a lack of sound waves reaching the microphone array over several convective time scales. Proper orthogonal decomposition (POD) was then used tocreate a basis for the flow images and this basis was used to reconstruct the evolution of the jet.

http://web.mit.edu/hileman/www/Publications/JFM2005.pdf

Large-scale structure evolution and sound emission in high-speed jets: real-time visualization with simultaneous acoustic measurements

A real-time flow visualization system that produces 17 images over a time span of 150 μs is used to visualize the mixing layers of Mach 1.3 (Mc=0.59) and Mach 2.0 (Mc=0.87) ideally expanded high Reynolds number axisymmetric jets. These image sequences reveal details about the influence of compressibility on the dynamics of turbulence structures. In general, the behavior observed in axisymmetric jets is similar to that observed in planar shear layers at similar convective Mach numbers. In addition, large streamwise vortices are apparent in cross-stream images of the flow. Large-scale structures become more three-dimensional and less organized with increasing compressibility and more difficult to identify and track. Planar space–time correlations are used to track structures as they convect downstream. The histogram of the convective velocity for the Mach 1.3 jet revealed a broad distribution of convective velocities with an ensemble average of 266 m/s, which is much higher than the theoretical prediction of 206 m/s. The Mach 2.0 jet, however, exhibited a bimodal convective velocity distribution with an ensemble average of 402 m/s for the “fast” and 190 m/s for the “slow” mode. These modes are equally spaced from the theoretical convective velocity of 303 m/s. Approximately 2/3 of the measured velocities were in the slow mode.

http://www.mecheng.osu.edu/netl/files/netl/Compressibility%20Effects%20on%20Turbulence%20Structures.pdf

Compressibility effects on turbulence structures of axisymmetric mixing layers

Localized arc filament plasma actuators (LAFPAs) have been used in high Reynolds number, high-speed unheated jets to explore their potential for noise mitigation and mixing enhancement. These previous experiments, conducted at the Gas Dynamics and Turbulence Laboratory (GDTL) of the Ohio State University, included both subsonic and supersonic jets. Therefore, the next stage is to examine these actuators’ effectiveness in high speed jets over a wide range of temperatures. Recently, a study of the temperature effect was conducted at GDTL for a subsonic jet. The facility consists of an axisymmetric jet of exit diameter 2.54 cm with different nozzle blocks and variable jet temperature in an anechoic chamber. The focus of this paper is on a supersonic (Mj = 1.3) jet. The previous work with a Mach 0.9 jet has shown significant increases in mixing enhancement effectiveness with increasing temperature. A number of combinations of forcing azimuthal mode and forcing frequency at two temperature ratios are utilized to examine LAFPAs effectiveness for mixing enhancement in a heated supersonic jet. The preliminary results indicate that forcing effectiveness continues in this supersonic heated case, but the increase in effectiveness in higher temperature is not observed. It is concluded that data at additional temperatures is needed to fully describe the trends in forcing effectiveness.

Noise control of a high Reynolds number high speed heated jet using plasma actuators

A newly developed MHz rate imaging system that provides real-time flow visualization is described. The technique utilizes a custom-built Nd:YAG pulse burst laser and an ultra high-speed digital camera and is capable of capturing 17 images over 150 microseconds. The system was used to visualize a Mach 1.3 (M c =0.6) axisymmetric jet. Sample results indicate the potential of the technique to provide detailed information on the dynamic characteristics of large-scale structures. A two-dimensional cross-correlation technique was used to calculate the convective velocity of large-scale structures. Present results generally agree with the findings of earlier investigations that indicate a significant deviation of the convective velocity from theoretical predictions.

A technique for real-time visualization of flow structure in high-speed flows

Acoustic measurements of a Mach 2, rectangular nozzle with modified trailing edges were carried out using three microphones, placed 90 deg apart azimuthally in a plane normal to the jet axis. The measurements were obtained at three streamwise locations, with microphone angles of 90, 60, and 30 deg with respect to the jet centerline. The trailing-edge modified nozzles substantially reduced the turbulent mixing and broadband shock associated noise radiation by up to 12 dB for the underexpanded flow regime and up to 7 dB for the overexpanded condition. However, in some symmetric modifications, the very-high-frequency noise was increased for the overexpanded condition. Screech tones in the overexpanded flow condition were either reduced or eliminated for asymmetric modifications, but amplified for symmetric modifications. The trailing-edge modifications were found to not significantly alter the noise field for the ideally expanded flow condition.

http://www.angelfire.com/oh/JinhwaKim/papers/Kerechanin_Samimy_Kim.pdf

C. W. Kerechanin

Papers

Effects of Nozzle Trailing Edges on Acoustic Field of Supersonic Rectangular Jet