Abstract
Deformation and movement of a vortex in the decaying stage were investigated by measuring velocity fields around the vortex with tiny polystyrene particles dispersed in the fluid layer. The vortex was produced in a rotating water layer by withdrawing a fixed amount of water through a small hole at the top boundary. When the vortex was produced in a fluid layer at rest relative to the rotating tank, it spined down keeping the circular shape. When the vortex was produced in a parallel flow with a horizontal shear whose vorticity has the same sign of that of the vortex, the shape of the vortex was elongated in the direction of the parallel flow. When the sign of the vorticity of the shear flow is opposite with that of the vortex, however, the vortex was elongated in the normal direction to the shear flow and moved toward the center of the container at the same time. It was found that the essential dynamics of these phenomena can be explained by linear superposition of the forced vortex with the parallel flow with a horizontal shear.
