Effect of Large-Scale Flow on the Boundary Layer Velocity Field in Turbulent Convection

TLDR: In this article, the effect of shear on the boundary layers on the hot plate in Rayleigh-Benard convection for a range of Prandtl numbers 4.69 ≤ Pr ≤ 5.88 and Rayleigh numbers 105 ≤ Ra ≤ 109 was studied.

Abstract: We study the effect of shear by the inherent large-scale flow in the bulk on the boundary layers on the hot plate in Rayleigh–Benard convection for a range of Prandtl numbers 4.69 ≤ Pr ≤ 5.88 and Rayleigh numbers 105 ≤ Ra ≤ 109. We observe that, at each Ra, at an instant, the distribution of horizontal velocities within the boundary layer in a horizontal plane is either of a unimodal nature or of a bimodal nature. Unimodal distributions occur either at low Ra or at high Ra while the bimodal distributions occur more at intermediate Ra. The peak of the unimodal distribution at low Ra occurs at values at around Vbl, the natural convection boundary layer velocity while the peak of the unimodal PDFs at high Ra occur at values greater than or equal to Vsh, the large-scale flow velocity. In the case of bimodal distributions, the first peak occurs in between Vbl and Vsh while the second peak occurs after Vsh. We show that the second peak of the bimodal distribution and the unimodal peak which occur at ≥Vsh scales as Vsh scales with Ra. The first peak of bimodal distribution and the single peak occurring at around Vbl scales with Ra as the natural convection boundary layer velocity forced by shear.