LARGE-SCALE FLOW DRIVEN BY THE ANISOTROPIC KINETIC ALPHA-EFFECT

Three-dimensional incompressible flows lacking parity-invariance (reversal of coordinates) can display a large-scale instability analogous to the alpha -effect of magnetohydrodynamics, but essentially anisotropic. At low Reynolds number R, the unstable modes have wavenumbers 0(R**2). A specific example is given which leads to the growth of a very strong large-scale Beltrami flow. Nonlinear saturation of the instability is obtained by feed-back on the small-scale flow. All the results are illustrated by full simulations of the three-dimensional Navier-Stokes equations.