Prandtl number dependence of the small-scale properties in turbulent Rayleigh-Bénard convection

We analyze the Prandtl number (Pr) dependence of spectra and fluxes of kinetic energy, as well as the energy injection rates and dissipation rates, of turbulent thermal convection using numerical data. As expected, for a flow with Pr 1, the inertial-range kineticenergy flux is constant, and the kinetic-energy spectrum is Kolmogorov-like (k−5/3). More importantly, we show that the amplitudes of the kinetic-energy fluxes and spectra and those of structure functions increase with the decrease of Pr, thus exhibiting stronger nonlinearity for flows with small Prandtl numbers. Consistent with these observations, both the kinetic-energy injection rates and the dissipation rates increase with the decrease of Pr. Our results are in agreement with earlier studies that report the Reynolds number to be a decreasing function of Prandtl number in turbulent convection. On the other hand, the tail of the probability distributions of the local heat flux grows with the increase of Pr, indicating increased fluctuations in the local heat flux with Pr.