Scalar dissipation rate measurements in a starting jet

TL;DR

This study measures the scalar dissipation rate in an impulsively started gas jet using planar laser induced fluorescence, applying Wiener filtering to mitigate noise and analyzing the accuracy and limitations of the measurements.

Contribution

It introduces a method combining PLIF and Wiener filtering for accurate scalar dissipation rate measurements in inhomogeneous flows, with detailed error analysis.

Findings

Scalar dissipation rate measured within 20% accuracy.

Wiener filter effectively reduces noise, with residuals resembling white noise.

Error in dissipation rate estimates is about 30% at Reynolds numbers up to 1000.

Abstract

Measurements of the scalar dissipation rate are performed in an impulsively started gas jet, using planar laser induced fluorescence. The measurements are well resolved spatially. The deteriorating effect of experimental noise on this experiment is treated with a Wiener filter, which is shown to be applicable to this large-scale inhomogeneous flow. The accuracy of the scalar dissipation rate is within $20\%$, as determined from an explicit calculation of the filtering errors. The residual fields that remain after the filtering are analysed in detail and their statistical properties show that these resemble white noise to a good approximation. The level of corrections is minimal for the scalar field but it is of the order of $40\%$ for the scalar dissipation rate. An examination of the filtering operation using modeled spectra and the measured spatial resolution shows that the Wiener filter produces errors in the estimate of the scalar dissipation rate $\sim30\%$, for Taylor-scale Reynolds number up to 1000. The implications of this modelling are discussed with respect to common experimental situations and point out the relative merits of improving the spatial resolution as compared to improvements in the signal to noise ratio.