Surface roughness effects on noise from pipe jets

Experiments are conducted to study the influence of surface roughness on the noise emitted from underexpanded pipe jets. Two sets of circular pipes of different orders of surface roughness are considered, and the pipe lengths are varied between 1 and 6 diameters. The blowdown studies are performed in the range of pressure ratios varying from 2 to 7. Acoustic measurements at various underexpansion levels indicate that although screech frequencies remain unaltered with surface roughness, the tonal sound pressure levels differ by almost 10 dB.