Stochastic Modelling in Fluid Dynamics: Itô vs Stratonovich

Darryl D. Holm

Published 2019-11-21Version 1

Suppose the observations of Lagrangian trajectories for fluid flow in some physical situation may be modelled sufficiently accurately by a spatially correlated It\^o stochastic process obtained from data which is taken in fixed Eulerian space. Suppose we also want to apply Hamilton's principle to derive the stochastic fluid equations for this situation. Now, the variational calculus for applying Hamilton's principle requires the Stratonovich process, so we must transform from It\^o noise in the \emph{data frame} to the equivalent Stratonovich noise. However, the transformation from the It\^o process in the data frame to the corresponding Stratonovich process shifts the drift velocity of the transformed Lagrangian fluid trajectory out of the data frame into a non-inertial frame obtained from the It\^o correction. The question is, "Will fictitious forces arising from this transformation of reference frames make a difference in the interpretation of the solution behaviour of the resulting stochastic equations?" This issue will be resolved by elementary considerations.