# Long's vortex revisited

Hewitt, R.E. and Duck, P.W. (2009) Long's vortex revisited. Journal of Fluid Mechanics, 634. pp. 91-111.

We reconsider exact solutions to the Navier--Stokes equations that describe a vortex in a viscous, incompressible fluid. This type of solution was first introduced by Long (1958) and is par ameterised by an inverse Reynolds number $\epsilon$. Long's attention (and that of many subsequent investigators) was centred upon the `quasi-cylindrical' (QC) case corresponding to $\epsilon = 0$. We show that the limit $\epsilon \to 0$ is not straightforward, and that it reveals other solutions to this fundamental exact reduction of the Navier--Stokes system (which are not of QC form). Through careful numerical investigation, supported by asymptotic descriptions, we identify new solutions and describe the full parameter space that is spanned by $\epsilon$ and the pressure at the vortex core. Some erroneous results that exist in the literature are corrected.