|This we know—aircraft jet noise is caused by the intense turbulent mixing of hot high-speed exhaust gases and cool ambient air. By controlling the way a jet mixes with the surrounding air, we can potentially control the amount of noise it produces.
Easier said than done!
NASA Glenn is broadening our understanding of high energy flows through the development of advanced instrumentation systems like Particle Image Velocimetry (PIV). Systems like these allow us to gather new data that can be used to validate critical turbulence modeling quantities which are at the heart of many noise prediction schemes.
Particle Image Velocimetry is a technique used to measure unsteady velocities in hot jets. Particles are introduced to the stream(s) of interest which are illuminated by a sheet of laser light. Images captured by a pair of cross correlation cameras are processed, yielding turbulence data faster and in far more detail than previous techniques. More detailed measurements coupled with more accurate theories enable engineers to design quieter nozzles.
PIV is able to measure mean velocity, RMS velocity, and two-point correlation statistics which are all used to validate the latest aeroacoustic theories. And while there are a number of PIV systems are commercially available, researchers at NASA Glenn have recently deployed a new High Resolution Stereo Particle Imaging Velocimetry system that expanded the field of view to over four times that of the previous PIV system while dramatically reducing facility testing time and data acquisition system complexity.