Helical velocity fluctuations and large-scale circulation in turbulent thermal convection

Abstract: It is shown that helicity dynamics dominates spectral distribution of the velocity fluctuations in turbulent thermal convection at moderate and large values of the Rayleigh number (distributed chaos and scaling respectively). The large-scale circulation (wind) in the turbulent Rayleigh-B\'{e}nard convection is especially sensitive to the helicity dynamics and its chaotic reversals can be associated with the reversals of the mean helicity sign (the ergodic restoration of the reflectional symmetry in a long run). Results of the laboratory experiments and measurements in the free atmospheric convection have been used in order to support the theoretical considerations based on the adiabatic invariance of the second order moment of the helicity distribution. The geomagnetic field dipole variability and reversals have been also discussed in this context and good agreement with results of numerical simulations and observational data has been established.