Whole-field velocity measurements in a turbulent free surface jet.

Abstract

An experimental investigation of a round, turbulent, free-surface jet was carried out. The jet issued parallel to, and one diameter below, the surface. The jet was operated in a coflow of varying velocity and was, at times, pulsed in its preferred mode. Jet exit Reynolds numbers were 12,700 and 25,400 with corresponding Froude numbers of 1.0 and 2.0. Hot-film anemometry, PIV, and a newly developed technique (HPIV) were used to measure the velocity. Holographic Particle Image Velocimetry (HPIV), was developed to make whole-field instantaneous three component velocity measurements. It uses in-line holographic recordings to measure the displacement of small particles within a seeded flow volume. The spatial dynamic range of the system is greater than 100 in each direction, providing resolution similar to planar PIV. The system was demonstrated by making phase-locked measurements of the velocity field in the pulsed jet. The mean flow behavior of the jet was studied using cross-stream velocity profiles at various distances from the jet nozzle. It was determined that the proper velocity scale in the near field of the jet is the exit excess velocity and the proper length scale is the excess momentum thickness, based on exit excess velocity. The jet exhibited an expected decreased centerline velocity decay due to the presence of the free surface. The effect of coflow was to increase the initial decay rate. Pulsing of the jet at 10% RMS amplitude in its preferred mode did not measurably change its decay rate. A surface current existed for all flow conditions. Averaged PIV velocity fields in planes near the surface showed the spreading angle at the surface to be about 22$\sp\circ$, compared to 6$\sp\circ$ at the centerline. The results are consistent with previous investigations. Surface-normal vorticity exists in pairs indicating the breaking and reconnection of subsurface vortices. These structures first appear two jet depths downstream of the nozzle. The translational velocity of the reconnected vortex pair is estimated to be initially 150% that of the bulk flow rapidly decaying to about 50%. There is a net reduction in surface normal vorticity at the free surface due to compressional straining but there are local regions of intense production.