Jets In Cross-flow

This experimental study explores and quantifies mixing characteristics associated with a gaseous round jet injected perpendicularly into cross-flow for a range of flow and injection conditions. The study utilizes acetone planar laser-induced fluorescence imaging to determine mixing metrics in both centreplane and cross-sectional planes of the jet, for a range of jet-to-cross-flow momentum flux ratios (2 J 41), density ratios (0.35 S 1.0) and injector configurations (flush nozzle, flush pipe and elevated nozzle), all at a fixed jet Reynolds number of 1900. For the majority of conditions explored, there is a direct correspondence between the nature of the jet's upstream shear layer instabilities and structure, as documented in detail in Getsinger et al. (J. Fluid Mech., vol. 760, 2014, pp. 342–367), and the jet's mixing characteristics, consistent with diffusion-dominated processes, but with a few notable exceptions. When quantified as a function of distance along the jet trajectory, mixing metrics for jets in cross-flow with an absolutely unstable upstream shear layer and relatively symmetric counter-rotating vortex pair cross-sectional structure tend to show better local molecular mixing than for jets with convectively unstable upstream shear layers and generally asymmetric cross-sectional structures. Yet the spatial evolution of mixing with downstream distance can be greater for a few specific convectively unstable conditions, apparently associated with the initiation and nature of shear layer rollup as a trigger for improved mixing. A notable exception to these trends concerns conditions where the equidensity jet in cross-flow has an upstream shear layer that is already absolutely unstable, and the jet density is then reduced in comparison with that of the cross-flow. Here, density ratios below unity tend to mix less well than for equidensity conditions, demonstrated to result from differences in the nature of higher-density cross-flow entrainment into lower-density shear layer vortices.

Synthetic jet is a train of vortex rings generated by repetitive ejection and suction of fluid through an orifice. One complete cycle comprises an ejection stroke followed by a suction stroke of diaphragm. The net mass flux is zero across the orifice exit but it imparts a positive momentum to surrounding fluid. The shear layer zone of ejected fluid rolls at the orifice exit and propagates downstream due to both, momentum supplied by forward stroke and the self-induced velocity of vortex ring. In last two decades, it has gain a significant interest in range of applications like combustion, heat transfer enhancement, propulsion & manoeuvring, drag reduction, jet control and boundary layer control. This study is related to the formation and characteristics of synthetic jet in cross-flow. The synthetic jet used in the present study is circular and flush mounted with the curved surface of model. The torpedo shaped model has 60mm diameter and is placed inside the horizontal and recirculating water tunnel with test section cross sectional area equal to 400mm × 400mm. At flow velocities 0.072 m/s , the behaviour of synthetic is investigated at three different actuation frequencies 2 Hz, 4 Hz and 6 Hz in the range of velocity ratio equal to 1.69 to 4.77. The bulk flow visualization using color dye is shown in two orthogonal planes (side and top view) parallel to the crossstream flow direction. A two-component Laser Doppler Velocimetry (LDV) technique is used for velocity measurement of synthetic jet in cross flow.