G. Falkovich et al., INSTANTONS AND INTERMITTENCY, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 54(5), 1996, pp. 4896-4907
We describe the method for finding the non-Gaussian tails of the proba
bility distribution function (PDF) for solutions of a stochastic diffe
rential equation, such as the convection equation for a passive scalar
, the random driven Navier-Stokes equation, etc. The existence of such
ails is generally regarded as a manifestation of the intermittency ph
enomenon. Our formalism is based on the WKB approximation in the funct
ional integral for the conditional probability of large fluctuation. W
e argue that the main contribution to the functional integral is given
by a coupled field-force configuration-the instanton. As an example,
we examine the correlation functions of the passive scalar u advected
by a large-scale velocity field delta correlated in time. We find the
instanton determining the tails of the generating functional, and show
that it is different from the instanton that determines the probabili
ty distribution function of high powers of u. We discuss the simplest
instantons for the Navier-Stokes equation.