General density gradients in general domains: the "two-tank" method revisited

An experimental technique for the creation of any statically stable density profile is presented. This technique is essentially a generalized, unsteady version of the "two-tank" method that is well known to the stratified flow community. It involves specifying the desired density profile and then solving an inverse problem in order to determine the necessary flow rates of light and dense fluid into the test-section tank. In addition to creating nonlinear density profiles, this technique is also useful for creating linear profiles in tanks whose planform areas vary with the vertical coordinate. The execution of this technique is carried out with computer-controlled peristaltic pumps. Several tests of the method are presented. The first consists of creating a hyperbolic tangent density profile in a rectangular tank. The second consists of creating, again in a rectangular tank, a density profile that is representative of those found in oceans and lakes. Finally, the third test involves creating a linear profile in a tank whose planform area is not constant. In all cases, the measured density profile shows excellent agreement with the requested profile.