The withdrawal of layered fluids from an open tank through a hole centred on the bottom is investigated numerically under the assumption of potential flow. A fully cubic-spline nonlinear axisymmetric boundary-integral-method scheme with the built-in boundary conditions, which effectively reduces the numerical errors at the intersection lines where the tank wall and the density interfaces meet, is used. Two cases are studied: (i) the tank contains only one fluid with a free surface; (ii) the tank contains two fluids having different densities with a distinct interface and a free surface
The numerical results show two different phenomena, depending upon the drain rate and initial conditions. When the tank is rapidly drained, a dip forms at the centre of the lower interface and extends into the hole very quickly, as observed by Lubin & Springer (1967). For a slowly draining tank, a jet forms in the centre of the depression region. This jet can either shoot up or move down, depending on the initial conditions.