Transitions and Probes in Turbulent Helium

Abstract

Previous analysis of a Paris turbulence experiment \cite{zoc94,tab95} shows a transition at the Taylor Reynolds number $\rel \approx 700$. Here correlation function data is analyzed which gives further evidence for this transition. It is seen in both the power spectrum and in structure function measurements. Two possible explanations may be offered for this observed transition: that it is intrinsic to the turbulence flow in this closed box experiment or that it is an effect of a change in the flow around the anemometer. We particularly examine a pair of ``probe effects''. The first is a thermal boundary layer which does exist about the probe and does limit the probe response, particularly at high frequencies. Arguments based on simulations of the response and upon observations of dissipation suggests that this effect is only crucial beyond $\rel\approx 2000$. The second effect is produced by vortex shedding behind the probe. This has been seen to produce a large modification in some of the power spectra for large $\rel$. It might also complicate the interpretation of the experimental results. However, there seems to be a remaining range of data for $\rel < 1300$ uncomplicated by these effects, and which are thus suggestive of an intrinsic transition.