Dispersion and upwind-side shear-layer characteristics of forward-inclined transverse jet in crossflow

Abstract

The effects of the forward inclination of a jet in crossflow on the jet fluid dispersion and upwind-side shear-layer characteristics were studied experimentally in an open-loop wind tunnel. The jet forward-inclination angle θ was varied from 0° to 50°, and the jet-to-crossflow momentum–flux ratio was adjusted between 1 and 2. Short- and long-exposure flow patterns were examined using the laser-assisted smoke flow visualization technique. The binary edge-detection technique was applied to the long-exposure flow images to obtain the jet penetration height. The instantaneous velocities in the upwind-side shear layer of the bent jet were captured by a hot-wire anemometer through a high-speed data acquisition system. The fast Fourier transform method was used to convert the instantaneous velocities from time to the frequency domain. Transverse dispersion of the jet fluids and jet trajectories described by the maximum jet-fluid concentrations were determined using the tracer-gas concentration detection method. The results revealed that at the same jet-to-crossflow momentum–flux ratio, a forward inclination did not significantly influence the flow characteristics and dispersion properties when the inclination angle was smaller than about 12°. At moderate forward inclinations (about 12°–42°), the upstream-side coherent flow structures appeared periodically. The penetration height, transverse width, and dispersion of jet fluids were greater than those of the elevated transverse jet and the slightly forward-inclined transverse jet. At forward inclinations larger than about 42°, the upstream-side coherent flow structures, penetration height, transverse width, and dispersion of jet fluids became the largest when compared with those of the elevated transverse jet as well as the slight and moderate inclination ranges. The jet trajectories in the moderate and large inclination ranges were located at transverse levels higher than those in the slight inclination range and elevated transverse jet due to the appearance of the large upwind-side shear-layer coherent flow structures. The effects of the forward inclinations on the time and length scales of the upwind-side coherent flow structures and the time and length scales of the turbulent eddies were presented and discussed.