Measured local-velocity fluctuations in turbulent convection

Light-Activated Active Colloid Ribbons

We report a dynamic self-organization of self-propelled peanut-shaped hematite motors from non-equilibrium driving forces where the propulsion can be triggered by blue light. They result in one-dimensional, active colloid ribbons with a positive phototactic characteristic. The motion of colloid motors is ascribed to the diffusion-osmotic flow in a chemical gradient by the photocatalytic decomposition of hydrogen peroxide fuel. We show that self-propelled peanut-shaped colloids readily form one-dimensional, slithering ribbon structures under the out-of-equilibrium collisions. This self-organization intrinsically results from the competition among the osmotically driven motion, the phoretic attraction and the inherent magnetic moments. The giant size number fluctuation in colloid ribbons is observed above a critical point 4.1 % of the surface density of colloid motors. Such phototactic colloid ribbons may provide a model system to understand the emergence of function in biological systems and have potential to construct bioinspired active materials based on different active building blocks.

Large-scale velocity structures in turbulent thermal convection

A systematic study of large-scale velocity structures in turbulent thermal convection is carried out in three different aspect-ratio cells filled with water. Laser Doppler velocimetry is used to measure the velocity profiles and statistics over varying Rayleigh numbers Ra and at various spatial positions across the whole convection cell. Large velocity fluctuations are found both in the central region and near the cell boundary. Despite the large velocity fluctuations, the flow field still maintains a large-scale quasi-two-dimensional structure, which rotates in a coherent manner. This coherent single-roll structure scales with Ra and can be divided into three regions in the rotation plane: (1) a thin viscous boundary layer, (2) a fully mixed central core region with a constant mean velocity gradient, and (3) an intermediate plume-dominated buffer region. The experiment reveals a unique driving mechanism for the large-scale coherent rotation in turbulent convection.

Temperature fluctuations in a convection cell with rough upper and lower surfaces

A turbulent convection experiment is conducted in a cell with rough upper and lower surfaces. Temperature statistics, frequency power spectrum, and thermal dissipation are measured over varying Rayleigh numbers in the central region of the cell. The temperature histogram in the rough cell is found to have the same exponential shape as that in the smooth cell, but the width of the distribution is increased by approximately 25%. The measured power spectrum shows that temperature fluctuations in the rough cell are increased uniformly across the whole frequency range. The cutoff frequency f(c) of the power spectrum and the time averaged square temperature time derivative <( partial differentialT/ partial differentialt)(2)> are used to characterize the thermal dissipation in turbulent convection. It is found that the normalized f(c) as well as <( partial differentialT/ partial differentialt)(2)> in the smooth and rough cells with different aspect ratios can all be superposed onto a single curve, indicating that the thermal dissipation in these cells is determined by the same mechanism. The experiment suggests that the enhanced heat transport observed in the rough cell is determined primarily by the local dynamics near the upper and lower boundaries.

Large-scale coherent rotation and oscillation in turbulent thermal convection

Laser Doppler velocimetry is used to measure the velocity profile of turbulent thermal convection in an aspect-ratio-one cell filled with water. Velocity fluctuations are found to be homogeneous and isotropic in the turbulent bulk region. Despite the large velocity fluctuations, the mean flow field maintains a large-scale structure, which rotates and oscillates in a coherent manner. The experiment suggests a unique driving mechanism for the large-scale coherent rotation and oscillation in turbulent convection.