To handle moving boundaries in some fluid flow problems you can apply the arbitrary Lagrangian-Eulerian (ALE ) formulation . The method covers situations in which the topology of the geometry does not change during the simulation and a suitable PDE for the mesh velocity exists. In the animation below is a model of a circular domain (radius 0.7) with a fluid about it. A "gravity like force" fx=−10x,fy=−10y. keeps the fluid on top of the circular domain. The motion of the fluid is triggered by an initial velocity.