JETS DEFLECTED IN A CROSS-FLOW

It is well known that jets or plumes emitted at right angles to a cros s flow, either through a circular orifice in a plane wall or from a pi pe at right angles to the stream, are bent downstream and become progr essively more closely aligned with the outer flow. A persistent featur e is that embedded contrarotating vortex pairs form in such deflected jets and plumes. A number of fundamental questions have yet to be reso lved, in particular relating to the source of vorticity for and the me chanism of formation of the embedded vortices, their role in entrainme nt into the curved jet, and the circumstances under which such deflect ed jets possess wakes in the form of lee regions of flow with reduced momentum flux. These questions are of considerable practical importanc e as very large quantities of gaseous, particulate, and liquid wastes are discharged to the atmosphere and oceans on the assumption that und esirable materials convected with the stream will suffer rapid dilutio n and transport away from the neighborhood of discharge. In this paper earlier results are extended and supplemented with laboratory observa tions not previously described of appropriately visualized jets in a w ater channel. It is argued that deflected jets must necessarily contai n embedded pairs of vortices of modest strength with vortex axes appro ximately 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 b oundary layer vorticity fields and for the dominant role in entrainmen t 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 i s small but the embedded vortex pair remains a dominant feature. It is argued and supported by observations that deflected jets do not direc tly produce wakes except in and close to levels through which the appr oaching upstream how is sheared with cross-stream vorticity, for examp le in the boundary layer on the wall through which the jet is discharg ed. 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 understandin g 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 w ithin a small neighborhood of the leading edge of the orifice encompas sing the thickness of the two shear layers. The consequences of that i nteraction, however, are dramatic and extend far downstream. The autho rs aim to clarify the role of vorticity in the behavior of deflected j ets, thereby resolving some of the misconceptions that have existed in the literature.