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Dual-beam intensity cross-correlation spectroscopy is used to measure the viscous boundary layer over a rough conducting surface in turbulent convection in water. It is found that the velocity boundary layer near the rough surface can be characterized by three quantities: the maximum velocity v(m), the shear rate gamma(v) and the total boundary layer thickness d + delta(v); all of them obey power laws of Rayleigh number Ra. The maximum velocity vm is located outside the rough surface and its numerical value remains the same as that in the smooth cell. The Ra-dependence of d + delta(v) is found to be the same as that in the smooth cell, but the power-law amplitude is increased by a factor close to 2. Because of the formation of small eddies inside the roughening grooves, the measured gamma(v) is reduced considerably in the groove region. It is also found that the Nusselt number Nu similar to Ra-0.35, which is larger in both magnitude and scaling exponent as compared to the smooth case.

We analyse simultaneous velocity and temperature measurements in turbulent thermal convection. Our results show the existence of a cross-scaling between the normalized velocity and temperature structure functions, as implied by the Bolgiano-Obukhov scaling, at the centre of the convection cell. We find that the cross-scaling exponents are, however, different from the values implied by the Bolgiano-Obukhov scaling.

A dual-beam dynamic light-scattering arrangement is devised to measure the time-dependent mean squared relative displacement of a pair of tracer particles with a small separation of micrometers. The technique is tested by the measurement of the relative diffusion of polymer latex spheres suspended in a simple viscous fluid. The experiment verifies the theory and demonstrates its applications. The dual-beam dynamic light-scattering technique, when combined with an optical microscope, provides a powerful tool for the study of two-particle microrheology of soft materials. The advantages of the new technique are its high statistical accuracy, faster temporal response, and ease of use. (C) 2004 Optical Society of America.

A systematic study of velocity oscillations in turbulent thermal convection is carried out in small aspect-ratio cells filled with water. Local velocity fluctuations and temperature-velocity cross-correlation functions are measured over varying Rayleigh numbers and spatial positions across the entire convection cell. These structural measurements reveal how the thermal plumes interact with the bulk fluid in a closed cell and provide an interesting physical picture for the dynamics of the temperature and velocity oscillations in turbulent convection. (C) 2004 American Institute of Physics.

Local convective heat flux in turbulent thermal convection is obtained from simultaneous velocity and temperature measurements in an aspect-ratio-one convection cell filled with water. It is found that fluctuations of the vertical heat flux are highly intermittent and are determined primarily by the thermal plumes in the system. The experiment reveals a unique mechanism for the heat transport in turbulent convection.

We analyze velocity fluctuations in turbulent Rayleigh-Benard convection. The velocity measurements were taken at the center of an aspect-ratio-one convection cell filled with water. The measured probability density functions of the velocity difference over a time interval tau are found to change with tau, indicating that the velocity fluctuations are intermittent. The velocity intermittency can be well characterized by the She-Leveque hierarchical structure. Our analyses further show that the vertical velocity component has distinct statistical features from the horizontal components. This result indicates that the vertical direction is special and buoyancy is important even at the center of the convection cell.

A systematic study of temperature oscillations in turbulent thermal convection was carried out in two aspect-ratio-one convection cells filled with water. Temperature correlation functions and local velocity fluctuations were measured over varying Rayleigh numbers and spatial positions across the entire cell. These measurements fully characterize the spatial structure of the temperature oscillation and reveal the mixing and emission dynamics of the thermal plumes near the conducting surface. A sharp transition from a random chaotic state to a correlated turbulent state of finite coherence time is found when the Rayleigh number becomes larger than a critical value Rac≃5×107. Above Rac the measured temperature correlation functions show a well-defined oscillation with a finite coherence time. The oscillation period is found to be twice as large as the cell crossing time. The experiment demonstrates how the thermal plumes in a closed cell organize themselves both in space and time and generate coherent oscillations in a turbulent environment.

We report temperature cross correlation and velocity profile measurements in the aspect-ratio-one convection cell filled with water. A sharp transition from a random chaotic state to a correlated turbulent state of finite coherence time is found when the Rayleigh number becomes larger than a critical value Rac≃5×107. The experiment reveals a unique mechanism for the onset of coherent oscillations in turbulent Rayleigh-Bénard 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.

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.