Critical Phenomena of a Drop Through a Stratified Fluid - Selected Topics of Computational and Experimental Fluid Mechanics - page 472

structures can be trapped inside the stratification for long periods of time and the
mixing process is altered (Camassa et al. 2013 ).
The behavior of a vortical structure of a miscible fluid as it traverses a stratified
medium is not well understood. When a drop enters a fluid of lower or equal den-
sity, an annular ring is formed. The structure falls due to gravity. In this study, the
interaction of such a structure with the interface between two layers of water with
different concentrations of salt is observed.
In general, for this process, eight parameters have to be considered: the density
of the drop, its radius, the densities of the two fluids, the height at which the drop is
released, the thicknesses of the two layers and the thickness of the interface. In the
present work, only the density of the drop, the height at which the drop is released
and the thickness of the upper layer are varied.
There is a maximum length of penetration before the vortex becomes unstable
and breaks. This length depends on initial and boundary conditions. The viscosity
decelerates the structure as it travels through the fluid, a Rayleigh-Taylor instability
appears and entails a collapse of the core (Camassa et al. 2013 ).
Four different behaviors were observed confirming results reported previously by
Camassa et al. ( 2011 ): Settling, Chandelier, Bounce and Core Fallout. They will be
described in the following section.
The shadowgraphs presented in this paper show better images than the ones that
appear in the literature.

2 Experimental Set-Up

A large glass container was filled with the two fluids. Salt water with a density of
1

cm 3 cm 3 .A

. 027 g / is the bottom fluid; the upper fluid has a density of 0 . 99 g /

cm 3 was used. The drop has a radius of
2mm and was always released at a height of 2 cm from the free surface (Fig. 1 ). All
lengths were measured from the images with a precision of 0.01mm. As soon as
the drop touches the fluid, a toroidal vortex was formed. The thickness of the top
layer was varied from 10 to 40mm. The interface has a thickness between one and
two centimeters because a density gradient was created between the two fluids. The
container was filled up very carefully to avoid any mixing between the two fluids.
Two images of the phenomena were obtained simultaneously. A high speed black
and white video camera was pointed directly to the container. A color video camera
recorded images through a shadowgraph (Settles 2001 ). Figure 2 shows the position
of the cameras.

densimeter with an uncertainty of 0 . 005 g /