Peak Strouhal frequency of subsonic jet noise as a function of Reynolds number

The narrowband spectral characteristics of the acoustic field generated by subsonic jets at low Reynolds numbers were studied in terms of a dimensionless power spectral density. It was found that the variation of the sound magnitude of the spectrum is an increasing function of the Reynolds number. The maximum level of the spectrum is not a linear function of the Reynolds number. The spectra are insensitive to Reynolds number above a critical value of about 100,000. Up to Reynolds number of this magnitude, the peak Strouhal frequency observed at 30 deg emission angle is identical with that at 90 deg for any exit Mach number. At higher Reynolds number the peak Strouhal number increases with increasing emission angle. The peak Strouhal number was found to be about 0.2, which is in agreement with previous findings that a few discrete modes at a Strouhal number of around 0.2 in low Reynolds number supersonic jets are powerful noise generators.