Effects of ambient viscosity on the entrainment and dynamics of a buoyant jet

Abstract

A free-shear flow driven by both initial momentum and buoyancy is a buoyant jet. Turbulent buoyant jets spread in a direction normal to their primary-flow direction by incorporating irrotational ambient fluid into the turbulent jet-flow; this process is known as entrainment. Entrainment process and hence the dynamical behavior of the jet depend on several parameters such as ambient density stratification, axial-pressure gradient, cross-flow and off-source buoyancy addition, temperature and viscosity contrasts between the free-shear flow and the ambient medium. In this thesis, the effects of the viscosity of the ambient fluid on the entrairmient and dynamics of a buoyant jet are studied via experiments and 2-D simulations using vortex methods. Some of the applications of the above situation include the flow of lava into a magma chamber, where viscosity variation arises due to a change in temperature and/or constituents, and the process industry where polymers have to be blended with additives or with polymers having different physical properties. All the experiments are conducted in a glass tank of dimensions 30x30x45 cm^. A buoyant jet issuing into a fluid of viscosity different from that of the jet fluid (viscosity enhanced by addition of suitable amounts of Sodium carboxymethyl cellulose) is studied using flow-visualization and other entraiimient-quantification experiments. Experimental results indicate that the turbulent jet undergoes a reverse transition. Large scale eddies at the interface are suppressed, and the observed entrairmient rate also reduces dramatically for the jet in a higher-viscosity medium. Results from 2-D numerical simulations, using vortex methods are also presented. Issues concerning viscosity-stratification in vortex methods are addressed. Results from the numerical simulations have a reasonable agreement with the experimental results.