Effect of the Prandtl number on the onset of turbulence in liquid 4He

2014 We have performed experiments on the onset of turbulence in liquid 4He, as a function of the Prandtl number (0.4 Pr 0.8), for a series of cells of confined geometry with two convective rolls. For a low value of Pr the dynamic behaviour is similar to our previous studies, i.e. two frequencies followed by lock-in and subharmonic bifurcations of the lowest frequency, before the onset of turbulence. For high values of Pr a new frequency appears before onset of the Busse oscillatory instability. We associate this new regime with a change of the convective roll structure, perhaps due to the skewed varicose instability predicted by Busse, which gives a three dimensional character to the roll geometry. Various approaches to turbulence are then observed. In some cases the onset of turbulence is intermittent in time; in some others a regime with three frequencies exists before the onset of turbulence. All the results are strongly dependent on the roll wavenumber, in qualitative agreement with the Busse theory. J. Physique LETTRES 41 (1980) L-515 L-518