Experimental observation of spontaneous temperature fluctuations in turbulent flows

TL;DR

This study experimentally investigates spontaneous temperature fluctuations in turbulent flows, revealing their correlation with pressure drops and vorticity filaments, and highlights the significance of compressible effects even at low Mach numbers.

Contribution

It provides the first detailed experimental analysis of spontaneous temperature fluctuations in low Mach number turbulent flows, linking them to vorticity and pressure variations.

Findings

Temperature fluctuations correlate with pressure drops.

Vorticity filaments cause intermittent temperature peaks.

Compressible effects are significant despite low Mach number.

Abstract

Even in the absence of externally applied temperature gradients, spontaneously generated temperature fluctuations arise in turbulent flows. We experimentally study these fluctuations in a closed von Karman swirling flow of air at Mach number of order $10^{-3}$, whose boundaries are maintained at a constant temperature. We observe intermittent peaks of low temperature correlated with pressure drops within the flow and show that they are caused by vorticity filaments. The measured ratio of temperature to pressure fluctuation agrees with the prediction based on adiabatic cooling within vortex cores. This experimental study shows that although the Mach number of the flow is small, there exist regions within the flow where compressible effects cannot be discarded in the equation for temperature and locally dominate the effect of viscous dissipation.