Structure in the Near Field of the Transverse Jet
Abstract
Photographs of an axisymmetric turbulent jet issuing from a wall into a crossflow display 4 types of vortical structures which exist in the near field: jet shear layer vortices, nascent far field vortex pair, near wall horseshoe vortices, and a system of vortices in the jet wake. Results of hot- wire measurements in the wake of the transverse jet are presented. Among these results are characteristic wake Strouhal frequencies which vary with the jet/ crossflow velocity ratio, and wake velocity profiles. It is found that the wake vorticity is not 'shed' from the jet but is formed from vorticity which originated in the wall boundary layer. Thus analogies between the wakes of transverse jets and the wakes of solid cylinders are incorrect. Since the jet is not a solid obstacle to crossflow, as a cylinder is, new vorticity is not generated at the interface between the jet and the crossflow. Instead, the boundary layer on the wall from which the jet issues separates near the downstream side of the jet because it cannot negotiate the adverse pressure gradient imposed on it by the flow around the jet, which is not 'separated' as it is for a cylinder. Wake vortices subsequently formed are found to be most coherent near a jet/crossflow velocity ratio of 4. Near field development of the counterrotating vortex pair, which is the dominant structure of the far field jet, is also addressed. It is argued that the source of vorticity for the vortex pair is the vorticity from the boundary layer within the jet nozzle. Estimates for the strength of these vortices are obtained by considering the flux of vorticity emanating from the nozzle. Possible implications for mixing are briefly discussed. (EDC)
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 13, 1990
- Accession Number
- ADA224502
Entities
People
- Thomas F. Fric
Organizations
- California Institute of Technology