Instanton calculus in shell models of turbulence

It has been shown recently that the intermittency of the Gledzer-Ohkitani-Yamada (GOY) shell model of turbulence has to be related to singular structures whose dynamics in the inertial range includes interactions with a background of fluctuations. In this paper we propose a statistical theory of these objects by modeling the incoherent background as a Gaussian white-noise forcing of small strength Gamma. A general scheme is developed for constructing instantons in spatially discrete dynamical systems and the Cramer function governing the probability distribution of effective singularities of exponent z is computed up to first order in a semiclassical expansion in powers of Gamma. The resulting predictions are compared with the statistics of coherent structures deduced from full simulations of the GOY model at very high Reynolds numbers.