Regensburg 2004 – scientific programme

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TT: Tiefe Temperaturen

TT 4: Quantenflüssigkeiten, Bose-Einstein Kondensate, ultrakalte Atome, ...

TT 4.2: Talk

Monday, March 8, 2004, 10:30–10:45, H19

Fluctuations and stability of superfluid turbulence in He-4 at mK temperatures — •Wilfried Schoepe — Fakultaet fuer Physik, Universitaet Regensburg

Turbulent flow of superfluid helium-4 at mK temperatures around an oscillating microsphere is known to be unstable at low driving forces, switching intermittently between turbulent and laminar phases. The lifetimes of the turbulent phases are exponentially distributed and the mean lifetimes grow exponentially with the square of the driving force. These experimental results are attributed to statistical fluctuations of the vorticity. As a model it is suggested that the turbulent state breaks down when a fluctuation crosses a threshold level. Applying Rice’s formula for the average level crossing per unit time of a stationary normal process gives information on the probability density function and the spectral bandwidth of the fluctuations. As a result a normal distribution is found and the standard deviation as well as the spectral bandwidth are determined quantitatively. In addition to superfluid turbulence this model may be applicable also to other instabilities which result from random fluctuations exceeding a certain threshold.