TRANSITIONS AND PROBES IN TURBULENT HELIUM

Previous analysis of a helium turbulence experiment by Zocchi et al. a nd Tabeling et al. [Phys. Rev. E 50, 3693 (1994); 53, 1613 (1996)] sho ws a transition al the Taylor Reynolds number Re-lambda approximate to 700. Here correlation function data are analyzed which gives further evidence for this transition. It is seen in both the power spectrum an d in structure function measurements. Two possible explanations may be offered for this observed transition: that it is intrinsic to the tur bulence Bow in this closed box experiment or that it is an effect of a change in the flow around the anemometer. We particularly examine a p air of ''probe effects.'' The first is a thermal boundary layer which does exist about the probe and does limit the probe response, particul arly at high frequencies. Arguments based on simulations of the respon se and upon observations of. dissipation suggest that this effect is o nly crucial beyond Re-lambda approximate to 2000. The second effect is produced by vortex shedding behind the probe. This has been seen to p roduce a large modification in some of the power spectra for large Re- lambda. It might also complicate the interpretation of the experimenta l results for higher values of the Reynolds number. However, there see ms to be a remaining range of data for Re-lambda < 1300 uncomplicated by these effects, and which are thus suggestive of an intrinsic transi tion.