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.
