Formation of head-tail structure in a two-dimensional uniform straining flow

The formation of head-tail structure, which is commonly observed in the interaction of vortex tubes, is investigated by direct numerical simulation of a two-dimensional Navier-Stokes flow. A head-tail structure is created in the course of evolution of the Batchelor dipole in a uniform straining flow. By noting that the Batchelor dipole is steady if there is no external flow and that the dipole is deformed before viscous effect appears, it is concluded that the relative strength of the straining flow plays an important role in the head-tail formation.