A large component of the noise radiated by civilian and military aircraft is from the exhaust plumes of the gas turbine engines. For more than half a century, researchers have studied how to predict this noise from fundamental, physics-based principles (instead of using empiricism). A critical element in this effort is modeling the space-time correlation of the turbulent Reynolds stresses. So far, it has not been possible to directly measure this critical element from realistic high-velocity, high-temperature jets. In an effort to overcome this barrier, researchers at the NASA Glenn Research Center improved the molecular Rayleigh scattering technique to measure the space-time correlation of air-density fluctuations. This is the first step toward measuring these fluctuations in terms of density × velocity × velocity, which makes the full Reynolds stress term. The technique has been applied to unheated free jets from mach 0.9 to 1.8.
Rayleigh scattering setup around a single-stream supersonic jet facility.
The optical setup consists of two laser beams crossing the jet plume in a vertical and a horizontal plane. Laser light scattered from each 1.06-mm-long beam was collected using two separate sets of lenses oriented 90° to the laser paths and was measured with photomultiplier tubes. The intensity of the scattered light was directly proportional to the air density. The beam path and the collection direction were designed for the next goal of simultaneously measuring density and velocity fluctuations from two different points in the jet.
Magnitude of the frequency-wave number (k-ω) spectrum measured from the lip shear layer (left column) and the centerline (right column) of a subsonic (mach 0.95, Mj = 0.95) and a supersonic (Mj = 1.8) jet. The magnitude of each spectrum was normalized by the square of the difference between the jet density ρj and the ambient density ρa. Here x is the distance downstream of the nozzle, D is the nozzle diameter, r is the radial distance, and S is the spectrum. The staircase-like appearance of the plots is reflective of the coarse wave number resolution achievable from the small jet. (a) Mj = 0.95; fixed probe: x/D = 3, r/D = 0.5. (b) Mj = 0.95; fixed probe: x/D = 7, r/D = 0.0. (c) Mj = 1.8; fixed probe: x/D = 3, r/D = 0.5. (d) Mj = 1.8; fixed probe: x/D = 10, r/D = 0.0.
The k-ω spectrum identifies the energy levels and the range of convective velocities associated with different frequency fluctuations. At any point, the ω/k ratio is a measure of this velocity. The turbulent fluctuations are convected by the jet flow; the overall inclination angle of each plot shows the average velocity. However, the spread at a given frequency ω shows the dispersion of convection velocities. For the density fluctuations in the jet to become sound waves, they need to reach a velocity equal to or above the ambient speed of sound c (shown by a diagonal line in each plot). Therefore, the component of the k-ω spectrum lying left of the diagonal line is capable of radiating sound at different observer angles. Although most of the fluctuations in the subsonic mach 0.95 jet are incapable of radiating to the far field, the supersonic mach 1.8 jet shows a different scenario. With an average supersonic convective velocity, fluctuations in both the lip shear layer and the centerline of the jet are expected to radiate strongly, with peak radiation 25° to the jet axis. The low-frequency part of the spectra, more prominently from around the centerline, is found to radiate at all mach numbers. These data are expected to be valuable in validating various computational aeroacoustics codes.
Panda, Jayanta: Two Point Space-Time Correlation of Density Fluctuations Measured in High Velocity Free Jets. NASA/CR--2006-214222 (AIAA-2006-0006), 2006. http://gltrs.grc.nasa.gov/Citations.aspx?id=95
Ohio Aerospace Institute (OAI) contact: Dr. Jayanta Panda, 216-433-8891, Jayanta.Panda-1@nasa.govLast updated: December 14, 2007
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