Observations of screech of jets emerging from the 90° corner between plane reflectors, pertinent to valve noise.

Abstract

The screech frequency of the free choked jet (nozzle 17.8×3.7 mm) was systematically lower (≊16%) than expected; first cell length and jet instability convection speed were even lower (≊23%, 30%). The sound intensity rose rapidly until pressure ratio R=3.0 (vice 2.3 expected), then more slowly; the jet spread angle (included) increased from 17° at R=2.2, to 35° at 3.0, then decreased to 20° at R=4.6. Pairs of reflectors (hard or soft) influenced the frequency negligibly, although cell length and convection speed changed. The intensity hardly changed (hard) yet the spread angle increased to 50° max (hard), or decreased, 22° max (soft). A third reflector, parallel to one and abutting the other, did not change the frequency but increased the intensity by 3–5 dB, increasing the spread angle only for R≳4. In all cases, a downward frequency jump occurred at R=2.16: the new mode is symmetric. [Work supported by Texas Advanced Research Program and Hitachi Ltd., Japan.]