Anisotropy, inhomogeneity and inertial range scalings in turbulent convection

Abstract

This paper provides a detailed study of scale-by-scale budgets in turbulent Rayleigh-Bénard convection and aims at testing the applicability of Kolmogorov (1941) and Bolgiano (1959) theories for this flow. Particular emphasis is laid on anisotropic and inhomogeneous effects: the SO(3) decomposition of structure functions (Arad et al 1999) and a method of description of inhomogeneities proposed by Danaila et al (2001) are used to derive inhomogeneous and anisotropic generalizations of Kolmogorov and Yaglom equations applying to RB convection. The various terms in these equations are computed using data from a DNS of turbulent Boussinesq convection at $\rayleigh=10^6$ and $\prandtl=1$ with aspect ratio A=5. The analysis of the isotropic component demonstrates that the shape of the third-order velocity structure function is significantly influenced by buoyancy forcing and large-scale inhomogeneities, while the mixed third-order structure function appearing in Yaglom equation exhibits a clear scaling exponent 1 in a small range of scales. The magnitudes of the various low $\ell$ degree anisotropic components of the equations are also estimated and are shown to be comparable to their isotropic counterparts at moderate to large scales. Finally, a qualitative analysis shows that the influence of buoyancy forcing at scales smaller than the Bolgiano scale is likely to remain important up to $\rayleigh=10^9$, thus preventing Kolmogorov scalings from showing up in convective flows at lower Rayleigh numbers.