Both fluid and gas exhibits density profile variations or stratification in nature, e.g., the atmosphere and the ocean. Experimentation has played a key role in understanding solid and fluid interactions, as with a seed falling, uncontrolled space re-entry, etc. Here we look at an object falling in a stratified fluid.
The two-tank method is used to create the linear density profile in a holding or test tank. The Two-Tank or Double-Tank method is illustrated in the figure below, Figure 1. Originally proposed by Oster in 1965, which used a forced-drained approach by using mechanical pumps to control the fluid flow. A more recent revisted by Hill in 2002. Here we used a free-drained approach, which rely on gravity and not on pumps. One similarly described by Economidou and Hunt in 2009.
For the experiment we wish the density profile to be linear where the higher density is toward the bottom and the lower density toward the top of the tank. Thus, Tank A has the higher density salt-water solution and Tank B is filled with water. The mixer ensure that solution in Tank A is thoroughly mixed. For salt solution one can observe that the salt is thoroughly mixed when the solution is clear, i.e., the salt dissolved. When value connected Tank A and Tank B is open fluid is allowed to flow between the Tanks. As one opens the value to the Main Tank, fluid from Tank B will flow to Tank A and then into the Main Tank. The solutions will be mixed and as it fills the Main Tank, the density profile will be higher initially, matching the density of Tank A, and then linearly changes to that of Tank B’s. As shown in the Figure 1, the tank is filled from the top. Another option is to reverse the order of the Tanks and fill from the bottom.
Figure 1: Two-Tank Stratification Setup.
The value controlling the flow to the main tank can also be used to control the flow rate. This becomes important when one needs to adjust the flow while creating the linear stratification. In our current setup, the two tanks drains to a reservoir and the water is then pumped into the main tank. To reduce mixing the flow rate into the main tank is approximately 40 gallons-per-hour.
Below are some details of the setup.