Representation of a vortical velocity streamline. The velocity field at two points, x₀ and x₁, is shown by green arrows and denoted as v₀ and v₁, respectively. The two points are relatively close to each other and the vector joining them is labeled ds. The point x₀ is at distance r from the vortex center that is marked with a star. The purple unit vector, e_r, represents the radial direction.