New similarity reductions and exact solutions for helically symmetric viscous flows

Abstract

In the present paper, we derive exact solutions for the helically invariant Navier–Stokes equations. The approach is based on an invariant solution ansatz emerging from the Galilean group in helical coordinates, which leads to linear functions in the helical coordinate ξ = az + bφ for the two helical velocity components uξ and uη. The variables z and φ are the usual cylinder coordinates. Starting from this approach, we derive a new equation for the radial velocity component ur in the helical frame, for which we found two special solutions. Moreover, we present an exact linearization of the Navier–Stokes equations by seeking exact solutions in the form of Beltrami flows. Using separation of variables, we found exponentially decaying time-dependent solutions, which consist of trigonometric functions in the helical coordinate ξ and of confluent Heun-type functions in the radial direction.