Isotropic turbulence in compact space

Abstract

Isotropic turbulence is typically studied numerically through the direct numerical simulations (DNS). The DNS flows are described by the Navier-Stokes equation in a 'box', defined through periodic boundary conditions. The DNS flows live in a compact space and they are not isotropic in their large scales. The investigation of important phenomena of isotropic turbulence, such as anomalous scaling, through the DNS is affected by large scale effects. In this work we put isotropic turbulence - or better, the associated formal theory - in a 'box', through imposing periodicity at the level of the correlations functions. We offer a framework where one may investigate isotropic theories/models through the data of DNS in a formally consistent manner. We work at the level of the Karman-Howarth equation. Unlike the Navier-Stokes equation, infinitely smooth periodicity is obstructed in this theory, a fact expressed by a sequence of relations obeyed by the normal modes of the Karman-Howarth equation. Similar relations are imparted to the two-point functions under the condition that the energy spectrum and energy transfer function are realizable. Naturally constructed closures scheme for the Karman-Howarth equation do not conform to such relations, thereby destroying realizability. A closure can be made to conform to a finite number of them by adding corrective terms, in a procedure which possesses certain analogies with the renormalization of quantum field theory. The spectrum becomes unphysical (through sign-changing oscillations) for infinitely large wavenumbers, but we can controllably extend the regime where the spectrum remains physical deep enough in the dissipation range so that to be realistically adequate. We show that one or two such 'regularity relations' are needed at most for comparisons of the predictions of the theory with the current resolution level results of the DNS.