Numerical evidence for nonuniversality of the soft/hard turbulence classification for thermal convection

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Abstract
Direct numerical simulations of turbulent Rayleigh–Bénard convection have been conducted in large aspect ratio boxes to examine the characteristics of convection in the range of Rayleigh numbers corresponding to the soft turbulence regime in the Chicago group classification. A measured exponent in the power law dependence of Nusselt number on Rayleigh number is 0.274, closer to the value 2/7 found in a small aspect ratio cell at the University of Chicago for the hard turbulence regime than the 1/3 obtained there for soft turbulence. The distributions of temperature fluctuation T′ in the convective core do not exhibit a unique functional form. Gaussian, exponential, and mixed (Gaussian-exponential) forms are observed. Further, it is shown that a transition in the functional form of the T′ pdf can be produced by changing the aspect ratio of the computational box without changing Rayleigh number at all. These results are in agreement with other studies, which have indicated that the hard/soft characterization of turbulent convection may partially reflect as yet not fully understood distinctions between convective turbulence in large and small aspect ratio cells.