Formation of Vortices around a Sinkhole

The stability of the vortex formed around a sinkhole has been examined experimentally for various values of the amount of discharge Q. Two types of vortices have been found to appear depending on the amount of discharge. Below some transition region of Q, many weak vortices appear and fluctuate spatially and temporally; while above the transition region, a single stable vortex grows through a unification of several vortices. A typical example of the process of the unification of vortices has been presented and analyzed.