Dynamics of line plumes on horizontal surfaces in turbulent convection

We study the dynamics of line plumes on the bottom horizontal plate in turbulent convection over six decades of Rayleigh number .105 < Raw < 1011/ and two decades of Prandtl number or Schmidt number (0:7 < Pr < 5:3, Sc D 602). From the visualisations of these plumes in a horizontal plane close to the plate, we identify the main dynamics as (i) motion along the plumes, (ii) lateral merging of the plumes and (iii) initiation of the plumes; various other minor types of motion also occur along with these main dynamics. In quantifying the three main motions, we first find that the spatiotemporal mean velocity along the length of the plumes (Vsh) scales as the large-scale flow velocity (VLS), with the fraction of the length of the plumes affected by shear increasing with Raw as Lps=Lp ∼ Ra0:054wPr-0:12. The mean time of initiation of the plumes t∼, scales as the diffusive time scale near the plate, Z2w =≈, where Zw is the appropriate length scale near the plate, in agreement with Howard (Proc. 11th Int. Congress Applied Mechanics, Munich, 1964, pp. 1109-1115). Merging occurs in a large fraction of the area of the plate, with ∼70% of the length of the plumes undergoing merging at Raw ∼ 1011 and Sc D 602. The fraction of the length of the plumes that undergoes merging decreases with increase in Ra w as, Lpm=Lp ∼ Raw-0:054 Pr0:12; the exponents of Raw and Pr being of the same magnitude but of opposite sign as that in the relation for L ps=Lp. Measurements of the locational means of the velocities of merging of the plumes .Vm/ show that Vm is a constant during each merging cycle at any location. However, the values of these constant velocities depend on the location and the time of measurement, since the merging velocities are affected by the local shear, which is a function of space and time at any Raw -Pr combination. The merging velocities at all Raw and Pr have a common lognormal distribution, but their mean and variance increased with increasing Raw and decreasing Pr. Using mass and momentum balance of the region between two merging plumes, we show that the spatiotemporal mean merging velocities (Vm), which are an order lower than Vsh, scale as the entrainment velocity at the sides of the plumes, averaged over the height of the diffusive layer near the plate. This implies that Vm scales as the diffusive velocity scale near the plate ∼=Zw. The Reynolds number in terms of Vm and the layer height H(ReH) scales as Raw1=3, in the same way as the Nusselt number (Nu) scales approximately; therefore ReH ∼ Nu. These relations imply that Rew D VmZw=∼ a Reynolds number near the plate, is an invariant for a given fluid in turbulent convection. © 2014 Cambridge University Press.