Helical turbulence: turbulent viscosity and instability of the second moments

The evolution of weak large-scale disturbances in the helical turbulence is considered. High-order statistical moments are taken into account in the frames of two-scale analog of Orszag diffusion approximation. The appearance of the instability of second moments is demonstrated for k < alpha/nu (k is wave vector, alpha and nu are proportional to the mean helicity and mean energy viscosity, respectively). Turbulent viscosity diminishing in comparison with the non-helical case is also demonstrated. These phenomenona qualitatively agree with previous results of other authors on the slow-down of energy transfer along the spectrum, from large to small scales at non-zero helicity. (C) 1998 Published by Elsevier Science B.V. All rights reserved.