Accurate estimation of third-order moments from turbulence measurements

TL;DR

This paper develops a method to accurately estimate third-order moments in turbulence data, crucial for measuring energy cascade rates, by analyzing statistical convergence and uncertainty, with applications to wind tunnel and solar wind data.

Contribution

It introduces a new technique for assessing the statistical convergence and uncertainty of third-order moments in turbulence measurements.

Findings

Third-order moments depend on sample size and correlation lengths.

A method for estimating statistical uncertainty of third-order moments is proposed.

The technique is validated using wind tunnel and solar wind data.

Abstract

Politano and Pouquet's law, a generalization of Kolmogorov's four-fifths law to incompressible MHD, makes it possible to measure the energy cascade rate in incompressible MHD turbulence by means of third-order moments. In hydrodynamics, accurate measurement of third-order moments requires large amounts of data because the probability distributions of velocity-differences are nearly symmetric and the third-order moments are relatively small. Measurements of the energy cascade rate in solar wind turbulence have recently been performed for the first time, but without careful consideration of the accuracy or statistical uncertainty of the required third-order moments. This paper investigates the statistical convergence of third-order moments as a function of the sample size N. It is shown that the accuracy of the third-moment depends on the number of correlation lengths spanned by the data set and a method of estimating the statistical uncertainty of the third-moment is developed. The technique is illustrated using both wind tunnel data and solar wind data.