Generalized Batchelor functions of isotropic turbulence

TL;DR

This paper extends Batchelor's autocorrelation functions for isotropic turbulence by introducing multiple small scales, improving the approximation of energy spectra in the universal equilibrium range.

Contribution

It proposes a generalized Batchelor function with multiple small scales, enhancing the accuracy of spectral approximations for isotropic turbulence.

Findings

The generalized function accurately approximates actual spectra.

It captures the bottleneck effect and dissipation spectra well.

Comparison with DNS data validates the model.

Abstract

We generalize Batchelor's parameterization of the autocorrelation functions of isotropic turbulence in a form involving a product expansion with multiple small scales. The richer small scale structure acquired this way, compared to the usual Batchelor function, is necessary so that the associated energy spectrum approximate well actual spectra in the universal equilibrium range. We propose that the generalized function provides an approximation of arbitrary accuracy for actual spectra of isotropic turbulence over the universal equilibrium range. The degree of accuracy depends on the number of higher moments which are determinable and it is reflected in the number of small scales involved. The energy spectrum of the generalized function is derived, and for the case of two small scales is compared with data from high-resolution direct numerical simulations. We show that the compensated spectra (which illustrate the bottleneck effect) and dissipation spectra are encapsulated excellently, in accordance with our proposal.