Patterns of convective turbulence: an effect of Prandtl number

Abstract

Direct numerical modeling is used to study the influence of Prandtl number (Pr) on the style of high Rayleigh number thermal convection. The spline-characteristic method, an algorithm for direct numerical simulation of turbulent flows, is employed to simulate 3-D convection in the Boussinesq limit. The differences in plume dynamics reveal themselves in the spectra, and in the probability distributions of turbulent fluctuation. They indicate the growing dominance of large-scale temperature fluctuations with decreasing Pr. The boundary layer of infinite-Pr ]onvection forms ridges interconnected into cellular structures. These cells have a preferred size which decreases with increasing Rayleigh number. Well-shaped, strong plumes grow at the intersections of these ridges. The flowfield of a low-Pr convection exhibits ‘swarms’ of smaller plumes, and boundary-layer ridges join together to form large-scale coherent structures similar to the elongated ‘rivers’ found in homogeneous isotropic turbulence.