Symmetrical mode of rectangular choked jet screech

Abstract

The screech and edge tones of rectangular jets had always been considered to be due to asymmetric (sinuous) jet instabilities, with out-of-phase sound fields across the jet plane. Kozu’s 1991 screech measurements showed a discontinuity at 2.12 ≤Rd<2.16, R=pressure ratio, the wavelengths λ below this being about 25% less than the empirical λ/h =k√(R−Rc), h=small dimension of rectangular nozzle, k=constant=5.2 (Powell),=5.0 (Krothapali), Rc=critical value. However, Lin’s 1992 schlieren photographs showed a symmetric (varicose) mode for edge tones at a pressure ratio R=2.36 and small nozzle-to-edge distances for the same nozzle, aspect ratio 4.8, with wavelength about half that for (asymmetrical) screech. Lin’s 1992 investigation of screech for R<Rd showed unambiguous symmetry of the sound field, implying symmetrical (varicose) jet instabilities. The evidence consisted of signals from microphones symmetrically placed in the nozzle plane that showed unambiguous symmetry below the discontinuity and asymmetry above; correlation measurements were consistent with this (though of low coherence). Powell’s phase criterion λ/h=(1+Mcon)/Mcon is satisfied within 10% for 2.5≤R<3.6, with significant discrepancies elsewhere.