HELICAL TURBULENCE - TURBULENT VISCOSITY AND INSTABILITY OF THE 2ND-MOMENTS

The evolution of weak large-scale disturbances in the helical turbulen ce is considered. High-order statistical moments are taken into accoun t in the frames of two-scale analog of Orszag diffusion approximation. The appearance of the instability of second moments is demonstrated f or k < alpha/nu (k is wave vector, alpha and nu are proportional to th e mean helicity and mean energy viscosity, respectively). Turbulent vi scosity diminishing in comparison with the non-helical case is also de monstrated. These phenomenona qualitatively agree with previous result s of other authors on the slow-down of energy transfer along the spect rum, from large to small scales at non-zero helicity. (C) 1998 Publish ed by Elsevier Science B.V. All rights reserved.