Influence of nozzle sharpness on the flow fields of V-notched nozzle jets

Abstract

This study reports on an experimental investigation on laminar jets discharging from V-notched nozzles of different relative sharpness. Each nozzle consists of two smooth peaks and two sharp troughs and its sharpness is defined by the aspect ratio (AR) of the half ellipses which make up each half of the V-notch. Two ARs of AR=2 and 4 are used here to look at how changes in relative sharpness affect the vortex dynamics and quantifiable flow characteristics. Flow visualization results reveal the formation of coherent streamwise vortices at both peaks and troughs, as well as the bending of the large-scale vortex roll-ups. Digital particle-image velocimetry measurements reveal that peaks produce vortex roll-ups which are stronger than those associated with the troughs and that their strength difference increases with nozzle sharpness. Correspondingly, flow stresses show that the peaks and troughs tend to confer more significant effects upon the Reynolds normal stresses when nozzle sharpness increases, with Reynolds shear stresses remaining relatively invariant. Comparisons with an earlier study using an AR=1.5 nozzle show that depending on the nozzle sharpness, the large-scale vortex-bending behavior linked to axis switching can reverse in direction. Unlike the earlier study which produced vortex bending toward the upstream direction, the present sharper nozzles resulted in vortex bending in the downstream direction. Earlier formation of streamwise vortices at the troughs and associated vortex disconnection/reconnection processes are deduced to produce such a discrepancy and flow interpretations are presented for further clarifications.