Gap ratio effect on the nearstream behavior of a confined jet flow

This work investigated the near stream behavior of a confined jet flow when passive rings were placed at the nozzle exit plane at different gap ratios (β). β is the gap (da) between the ring and the inner nozzle wall, normalized by the ring thickness (drc) and ranged from 0.2≤da/drc≤1.0. The results show that the shedding of vortices behind the ring depends strongly on β. The suppression of the vortices was complete and partially at β=0.2 and 0.6, respectively, but negligible at β=1.0. The vortex suppression was associated with flow deflection due to delayed separation. The normal Reynolds stress Uu at the center of the gap was 1.255m2s−2 at β=0.2 and 3.083e−5m2s−2 at β=1.0. The increase in stress across the gap at small β values enhanced the momentum transfer between the fluid layers. This sustained the flow on the ring surface against an adverse pressure gradient and delayed the flow separation. The ring generally suppressed the jet spread. However, the mean velocity spread suppression only correlated strongly with β at the nozzle exit because the negative forcing on the shear layer coherent structure by the wake vortices was local. At 0.3 nozzle diameter downstream of the origin, the velocity spread became independent of β because of the decay of the wake vortices.