Structure of a low-momentum elevated jet in a cross-flow

Abstract

An elevated jet in a cross-flow is a free jet issuing orthogonally into a dominant cross-wind from a pipe extending above a ground-induced boundary layer. The present thesis is concerned with jets having low momentum-flux relative to the cross-flow. It presents flow visualization and velocity measurements conducted in a variable-speed, closed-circuit water channel. Four major types of coherent structures were identified: von Karman vortices, a pipe-end vortex, shear-layer vortices, and tendrils. The von Karman vortices are similar to those shed by a finite cylinder with a free end. The pipe-end vortex is a stationary vortex forming immediately downstream of the pipe-end. The shear-layer vortices are formed by the Kelvin-Helmholtz instability in the mixing layer between the cross-flow and the jet. Finally, the tendrils are evolutions of sections of the shear-layer vortices, formed under certain conditions as the latter are stretched by the pipe-end vortex and move downstream. The size of the pipe-end vortex is insensitive to the momentum-flux ratio as long as the latter is less than a critical value. For higher momentum-flux ratios, the pipe-end vortex grows in size and shifts beyond the free-end of the pipe. The Strouhal numbers of the shear-layer vortices and the tendrils increase drastically as the momentum-flux ratio decreases. The shear-layer vortices, marking the jet fluid, move away from the pipe exit upon their generation, then reverse direction as they pass over the pipe-end vortex, while accelerating in the streamwise direction until their velocities approach the cross-flow speed. A vorticity balance for each type of the vortices has been attempted. The von Karman vortices obtain their vorticity from the pressure gradient along the outer surface of the pipe. The pipe-end vortex acquires its vorticity from the shear-layer vortices and generates some vorticity in a pressure gradient on the outer surface of the pipe as well. The shear-layer vortices acquire their vorticity from the cross-flow fluid flowing over the edge of the upstream pipe tip as well as from the vorticity inside the pipe from the jet flow. The vorticity in the tendrils originates in the shear-layer vortices.