My good friend Thomas D. Norum recently passed away. He worked as a researcher at NASA Langley over most of his career. I knew him starting in 2009 through 2016 while I was working there. He worked as an experimentalist in the jet noise lab of NASA Langley. As an experimentalist, he worked to understand the physics of jet turbulence and noise, while also reducing aircraft noise for the benefit of communities. I often see people referencing his publication on screech tones. Two major publications have been made on screech that are often cited (in my opinion). The first is by C. K. W. Tam, and the second is by T. D. Norum. Dr. Norum’s seems to be more predictive and based in measurement while Tam’s is based more purely on theory. Even today, there is no closed-form well accepted model for screech tones. I use to hang out with Dr. Norum at Afterburner’s at Langley. He always was positive (with a bit of snark) and am appreciative of our time together. God’s speed Dr. Norum.
Read more of his work through AIAA –
Screech suppression in supersonic jets by T. D. Norum – https://doi.org/10.2514/3.8059 Abstract – Screech from underexpanded supersonic jets has been investigated experimentally. Multiple screech modes, or stages, are found to be present at most jet operating conditions. The fundamental screech tone of each mode attains a maximum amplitude at about 20 deg from the inlet axis, with higher harmonics exhibiting multiple lobes. The directivity of each harmonic is predicted quite well from a stationary array of acoustic monopoles, with phasing between consecutive monopoles determined by the shock cell spacing and eddy convection velocity. Large reduction of screech amplitude can be obtained from modifications to the jet exit geometry, although the extent of this suppression is mode dependent.