Experimental study of mixing of two fluids driven by buoyancy forces

Recently, mixing driven by buoyancy-induced flow has received considerable attention due to its contributions to industrial applications such as material processing. In this research we present an experimental concept by which on can control the buoyancy force by adjusting the density difference to study mixing. We studied experimentally the mixing characteristics of two miscible viscous fluids driven by buoyancy forces over a wide range of Grashof numbers, aspect ratios and impulsive Reynolds numbers. The interface behavior has been characterized by measuring the length stretch and striation thickness of the interface using image processing techniques, and the associated flow field using Particle Imaging Velocimetry (PIV).;The effects of the removal of the divider and the overwhelming buoyancy force cause an overturning motion which stretches and folds the interface to produce an internal breakwave. The structure of the breakwave is similar to the Rayleigh-Taylor instability morphology. The breakwave is dissipated either through internal or wall collision depending on the impulsive velocity of the divider as characterized by the impulsive Reynolds number. The decay of the collision event occurs through sloshing oscillations over a short time scale. The two fluids then become stably stratified with a diffusive band at the interface indicating local mass transport.;The bifurcation behavior of the interface is studied using the maximum length stretch as a metric. We show that the maximum length stretch characterizes the bifurcation behavior of the interface as a function of Gr (Grashof number), Ar (aspect ratio), and Re i (impulsive Reynolds number). We obtain the critical boundary which describes the quantitative changes from stretching to folding, asymmetry and its approach to symmetry in the planes of Gr and Ar, Gr and Rei, respectively. We constructed a predictive model which allows prediction of the length stretch of the interface over a continuous range of parameters (3.18 < Gr < 6.36 x 107, 0.04 < Ar < 1.0, 1000 < Rei < 2 x 104 and 500 < Sc < 8600).;Insight into the mechanism of folding is obtained from measurement of the flow field, which shows that during stretching the flow field has a single elliptic point dominated by a single vortex. The mechanism of folding is due to a hyperbolic point in the flow which serves as an organizing center for multiple vortex interactions. The flow field bifurcates from an elliptic point to a saddle point, which separates the stretching and folding events.;Calculation of the finite time scale topological entropy as well as construction of the horseshoe map were performed to analyze the likelihood of a chaotic mixing. We show that positive finite time scale topological entropy is a necessary but not a sufficient condition for chaos in a transient system. Experimentally we attempt to show that the horseshoe exists in our system, which indicates the existence of local transient chaos.