Effect of nozzle asymmetry on jet-edge oscillations

Abstract

Strongly coherent oscillations of a jet impinging upon an edge persist when one lip of the nozzle is axially displaced with respect to the other, even by as much as one wavelength of the jet instability. This persistence of the oscillation is due to rapid stream wise adjustment of the flow in the region between the two nozzle lips, forced by upstream influence from the downstream edge; immediately downstream of the nozzle, cross-stream distributions of fluctuation amplitude and phase are very similar to those occurring in an initially symmetrical jet, as evidenced by comparison with linear theory. In the region between the upstream and downstream nozzle lips, “pre-amplification” of the disturbance occurs in the form of exponential disturbance growth, followed by another (slower) exponential growth in the free jet portion of the flow. These features of the flow structure occur in conjunction with the dipole-like upstream influence from the jet-edge interaction, the strength of which is determined by the pressure loading at the edge. In fact, the pressure fluctuations at the tip of the downstream impingement edge can have amplitudes approximately double those of the corresponding jet from a symmetrical nozzle. This increase in magnitude of the source of upstream influence is due to a slight transverse shift of the minimum amplitude and phase jump of the fluctuating velocity field approaching the edge, relative to those of the jet from a symmetrical nozzle. In the event that the nozzle lip axial ofiset is very large, coherent oscillations still persist, but in a different mode; the frequency is much higher, dictated by the most unstable frequency of the wall jet that forms between the nozzle lips.