 
Summary: J. Fluid Mech. (2008), vol. 607, pp. 119139. c 2008 Cambridge University Press
doi:10.1017/S0022112008001882 Printed in the United Kingdom
119
Torsional oscillations of the largescale
circulation in turbulent RayleighB´enard
convection
DENIS FUNFSCHILLING, ERIC BROWN
AND GUENTER AHLERS
Department of Physics and iQCD, University of California, Santa Barbara, CA 93106, USA
(Received 8 May 2007 and in revised form 28 March 2008)
Measurements over the Rayleighnumber range 108 < R < 1011
and Prandtlnumber
range 4.4 < < 29 that determine the torsional nature and amplitude of the oscillatory
mode of the largescale circulation (LSC) of turbulent RayleighB´enard convection
are presented. For cylindrical samples of aspect ratio = 1 the mode consists of
an azimuthal twist of the nearvertical LSC circulation plane, with the top and
bottom halves of the plane oscillating out of phase by half a cycle. The data for
= 1 and = 4.4 showed that the oscillation amplitude varied irregularly in time,
yielding a Gaussian probability distribution centred at zero for the displacement
angle. This result can be described well by the equation of motion of a stochastically
