Experimental investigation into turbulent negatively buoyant jets using combined PIV and PLIF measurements

20 Dec 2021

Turbulent negatively buoyant jets occur when the buoyancy of a jet opposes its source momentum. In these flows, the fluid will rise until it reaches a stagnation point and a return flow is established, forming a fountain (Hunt and Burridge, 2015). This study looks at both the initial negatively buoyant jet stage of this flow, before the return flow has established, and the fully developed fountain stage. Two-dimensional particle image velocimetry (PIV) and planar laser in- duced fluorescence (PLIF) are used to simultaneously measure the velocity and scalar concentration fields. An experimental and image processing procedure for the PLIF is introduced that accounts for pulse-to-pulse variations in laser power and beam profile for an Nd:YAG laser, which has been demonstrated to reduce the error in scalar concentration measurements. The flow is investigated experimentally using a 1m3 tank of salt-water ambient with freshwater+ethanol negatively buoyant jets, allowing for measurements to be taken at F ro = 30 and Reo = 5900. The entrainment coefficient for a negatively buoyant jet has been estimated as α ∼= 0.054, lower than a neutral jet at α ∼= 0.058. A finding con- sistent with existing literature (Bloomfield and Kerr, 2000; McDougall, 1981).