  A linear combination with constant in space amplitudes of a pair of dual
anisotropic decaying Beltrami flows (the Trkal solutions)with the same
eigenvalue of the curl operator and of an orthogonal constant velocity vector
to the Beltrami pair,yields a triplet solution of the force-free Navier-Stokes
equation. Slight space variation of the amplitudes (large scale perturbation)
yields the emergence of the time depending phase between the dual Beltrami
flows and of the upward velocity,which are unstable at large values of the
Reynolds number as well as the formation of the large scale curved prisms of
streamlines with edges being the strings of singular vorticity.
