Theory of Non-Degenerated Oscillatory Flows

Abstract

The aim of this paper is to derive the averaged governing equations for non-degenerated oscillatory flows, in which the magnitudes of mean velocity and oscillating velocity are similar. We derive the averaged equations for a scalar passive admixture, for a vectorial passive admixture (magnetic field in kinematic MHD), and for vortex dynamics. The small parameter of our asymptotic theory is the inverse dimensionless frequency $1/σ$. Our mathematical approach combines the two-timing method, distinguished limits, and the use of commutators to simplify calculations. This approach produces recurrent equations for both the averaged and oscillating parts of unknown fields. We do not use any physical or mathematical assumptions (except the most common ones) and present calculations for the first three (zeroth, first, and second) successive approximations. In all our examples the averaged equations exhibit the universal structure: the Reynolds-stress-type terms (or the cross-correlations) are transformed into drift velocities, pseudo-diffusion, and two other terms reminiscent of Moffatt's mean-fields in turbulence. In particular, the averaged motion of a passive scalar admixture is described only by a drift and pseudo-diffusion. The averaged equations for a passive vectorial admixture and for vortex dynamics possess two mean-field terms, additional to pseudo-diffusion. It is remarkable that all mean-field terms (including pseudo-diffusion) are expressed by invariant operators (Lie-derivatives) which measure the deviation of some tensors from their `frozen-in' values. Some physical assumptions and the results can be build upon obtained averaged equations. Our physical interpretation suggests purely kinematic nature of pseudo-diffusion.