The 3D structure of a dipole in a shallow two-layer fluid

The canonical laboratory setup to study two-dimensional (2D) turbulence is the electromagnetically driven shallow fluid layer. The argument used here is that whenever the vertical length scale is much smaller than the horizontal length scale the flow is presumed to behave in a 2D fashion. However, this assumption disregards the presence of a strong non-uniform magnetic field used to electromagnetically force the flow, a vertical component of the Lorentz force, and, most importantly, it oversimplifies the structure of 3D recirculating flows [1, 2]. The limitations of the single fluid layer setup have been recognized and now the two-layer fluid is commonly used for experimental 2D turbulence research. In this contribution we will study experimentally and numerically the 3D motion inside a two-layer fluid. We will discuss the behaviour of the flow when decreasing the upper fluid layer thickness in steps down from 9 mm to 3.5 mm, the latter being the commonly used upper fluid layer thickness for 2D turbulence experiments. Surprisingly, the flow structures and evolution seen in the two-layer flow are qualitatively the same as in the single-layer flow.