Nonstationary vortices in a water glass

Who has not experienced the formation of a vortex or a whirlpool in a glass of milk when trying to dissolve cocoa in it by stirring the liquid with a spoon? In this paper, we analyse the mechanical aspects of the unsteady vortex formed in swirling water that was stirred with a spoon around a central axis in a cylindrical glass. We show that the formed unsteady vortex is of forced type and we compare the fluid dynamics of this unsteady vortex to that of the well-known steady free and forced vortices and with the dynamics of the unsteady free isolated vortex (also known as Oseen-Lambís vortex). We show that the observation of the generation and decay of the unsteady vortex in a cylindrical glass of rotating water is a simple experiment that may constitute a very fruitful experience for understanding the kinematics, dynamics, and static aspects of the axisymmetric, rotational, or torsional flow of an homogeneous and incompressible fluid.