Jets deflected in a crossflow

It is well known that jets or plumes emitted at right angles to a cross flow, either through a circular orifice in a plane wall or from a pipe at right angles to the stream, are bent downstream and become progressively more closely aligned with the outer flow. A persistent feature is that embedded contrarotating vortex pairs form in such deflected jets and plumes. A number of fundamental questions have yet to be resolved, in particular relating to the source of vorticity for and the mechanism of formation of the embedded vortices, their role in entrainment into the curved jet, and the circumstances under which such deflected jets possess wakes in the form of lee regions of flow with reduced momentum flux. These questions are of considerable practical importance as very large quantities of gaseous, particulate, and liquid wastes are discharged to the atmosphere and oceans on the assumption that undesirable materials convected with the stream will suffer rapid dilution and transport away from the neighborhood of discharge. In this paper earlier results are extended and supplemented with laboratory observations not previously described of appropriately visualized jets in a water channel. It is argued that deflected jets must necessarily contain embedded pairs of vortices of modest strength with vortex axes approximately parallel to the curved jet axis and occupying most of the jet cross section. Conceptual models are described for the production of these embedded vortex pairs by interaction of the jet and cross-flow boundary layer vorticity fields and for the dominant role in entrainment of these vortex pairs, both in the strongly curved regions where the jet bends to the cross-stream and beyond where the slope of the jet is small but the embedded vortex pair remains a dominant feature. It is argued and supported by observations that deflected jets do not directly produce wakes except in and close to levels through which the approaching upstream flow is sheared with cross-stream vorticity, for example in the boundary layer on the wall through which the jet is discharged. However, viscously retarded fluid is extracted from this boundary layer and channelled out from the boundary through the “wake vortices” that are observed in the lee of the jet. The key to an understanding of the physics of deflected jets lies in the recognition that their structure results from the interaction of two shear layers, the cross-flow boundary layer and the cylindrical sheath discharged from the jet orifice. The primary interaction occurs rapidly and almost entirely within a small neighborhood of the leading edge of the orifice encompassing the thickness of the two shear layers. The consequences of that interaction, however, are dramatic and extend far downstream. The authors aim to clarify the role of vorticity in the behavior of deflected jets, thereby resolving some of the misconceptions that have existed in the literature.