Effect of boundary conditions on axial flow in a concentrated vortex core

The character of axial flow in the core of a concentrated line vortex is studied using flow visualization. Results show that axial flow can be initiated when the no-slip boundary condition is imposed on the vortex core over a spatial extent comparable to the core diameter. It is further demonstrated that no-slip is not a necessary condition to generate the axial flow; a region of spanwise shear (i.e., a slip boundary) is sufficient. Two distinct axial flow velocity profiles are observed; they are either jetlike or spatially undulated corresponding to a maximum axial speed occurring at the core center or outer edge, respectively.