Evolution and statistics of thermal plumes in tilted turbulent convection

We present a systematic experimental study of thermal plumes within tilted turbulent thermal convection using the shadowgraph technique. The measurements are performed in a rectangular cell (aspect ratio Gamma(x) =1 and Gamma(y) = 0.25) over a wide range of tilt angles (0 <= beta <= pi/2 rad) at constant Prandtl (Pr similar or equal to 10) and Rayleigh (Ra similar or equal to 6.80 x 10(10)) numbers. It is found that the plume width, lambda, is distributed log-normally in space and the time-averaged most probable plume width, <lambda(p)>, has similar scale with the thermal boundary layer thickness, lambda(T). <lambda(p)> and lambda(T) slowly increase as tilt angle, beta, increases from 0 to 1 rad and they broaden rapidly as beta > 1 rad. The average plume area density, (Phi), deduced from the image intensity, is proposed to represent the heat flux of thermal plumes. Its beta dependence is consistent with that of the Nusselt number, Nu, over the tilt angle range. lambda(p) and Phi exhibit oscillation of convective flow. The oscillation intensity and period strongly depend on beta, but are less affected by Ra and Pr. The plume velocity exhibits a different pattern from that of the two-dimensional convective flow measured at Pr similar or equal to 6.3 and Ra similar or equal to 4.42 x 10(9). The magnitude of plume velocity peak is weakly affected by the cell tilting. The position of the plume velocity peak increases linearly with increasing beta. (C) 2017 Elsevier Ltd. All rights reserved.