Experimental investigation into a turbulent jet with negative buoyancy

Abstract

An experimental investigation of a steady-state negatively buoyant jet has been performed using a hot water jet projected vertically downwards from a 7.5 mm radius inlet pipe into a cold ambient. The Richardson number was 0.1 based on inlet pipe radius and the Reynolds number was 5000. Mean and fluctuating velocities and temperatures, triple velocity correlations and velocity–temperature correlations were measured for the jet in a constant temperature ambient. Correlations between orthogonal velocity components were also measured. Velocity measurements were performed using laser Doppler anemometry (LDA), and errors associated with beam movement (due to refractive index fluctuations) were experimentally quantified. Temperatures were measured with fast response thermocouples. The two techniques were used in conjunction to provide velocity–temperature correlations. The intermittency factors around the edge of the flow were measured optically, employing refractive index as a tracer. The normal stresses were found to be high in the shear region at the jet-flow/reverse-flow interface and in the region of large-scale fluid reversal. The contribution from the intermittency at the boundary of the flow was evident in the velocity and temperature fluctuations. The triple velocity products were interpreted as fluxes of the Reynolds stresses and in general exhibited net fluxes away from the regions of high stress intensity. Balances of the terms in the axial momentum and turbulent kinetic energy equations showed that the main contribution from buoyancy was in the mean motion, with very little direct input to the turbulence field.