You are here : Home > The DSBT > Experimental evidence of the statistical intermittency in a cryogenic turbulent jet of normal and superfluid Helium

Davide Duri

Experimental evidence of the statistical intermittency in a cryogenic turbulent jet of normal and superfluid Helium

Published on 30 November 2012
Thesis presented November 30, 2012

This experimental work is focused on the the statistical study of the high Reynolds number turbulent velocity field in an inertially driven liquid helium axis-symmetric round jet at temperatures above and below the lambda transition (between 2.3 K and 1.78 K) in a cryogenic wind tunnel. The possibility to finely tune the fluid temperature allows us to perform a comparative study of the quantum HeII turbulence within the classical framework of the Kolmogorov turbulent cascade in order to have a better understanding of the energy cascade process in a superfluid. In particular we focused our attention on the intermittency phenomena, in both He I and He II phases, by measuring the high order statistics of the longitudinal velocity increments by means of the flatness and the skewness statistical estimators. A first phase consisted in developing the cryogenic facility, a closed loop pressurized and temperature regulated wind tunnel, and adapting the classic hot-wire anemometry technique in order to be able to work in such a challenging low temperature environment. A detailed calibration procedure of the fully developed turbulent flow was the carried out at 2.3 K at Reynolds numbers based on the Taylor length scale up to 2600 in order to qualify our testing set-up and to identify possible facility-related spurious phenomena. This procedure showed that the statistical properties of the longitudinal velocity increments are in good agreement with respect to previous results. By further reducing the temperature of the working fluid (at a constant pressure) below the lambda point down to 1.78K local velocity measurements were performed at different superfluid density fractions. The results show a classic behaviour of the He II energy cascade at large scales while, at smaller scales, a deviation has been observed. The occurence of this phenomenon, which requires further investigation and modelling, is highlighted by the observed changing sign of the third order structure function of the longitudinal velocity increments. The intermittency phenomena is also observed and a quantitative analysis is carried out by measuring the scaling behaviour of the velocity increments flatness which is consistent with results obtained in Navier-Stokes fluids. This Ph.D. thesis has been carried out at the LEGI (Laboratoire des Écoulement Géophysiques et Industriels) laboratory in Grenoble and at the CEA Low Temperature Department (SBT) in Grenoble.

On-line thesis.