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: ͑1͒ a thin viscous boundary layer, ͑2͒ a fully mixed central core region with a constant mean velocity gradient, and ͑3͒ an intermediate plume-dominated buffer region. The experiment reveals a unique driving mechanism for the large-scale coherent rotation in turbulent convection.

We studied simultaneously the 4 He(e, e p), 4 He(e, e pp), and 4 He(e, e pn) reactions at Q 2 = 2 (GeV/c) 2 and xB > 1, for an (e, e p) missing-momentum range of 400 to 830 MeV/c. The knocked-out proton was detected in coincidence with a proton or neutron recoiling almost back to back to the missing momentum, leaving the residual A = 2 system at low excitation energy. These data were used to identify two-nucleon short-range correlated pairs and to deduce their isospin structure as a function of missing momentum, in a region where the nucleon-nucleon (N N ) force is expected to change from predominantly tensor to repulsive. The abundance of neutron-proton pairs is reduced as the nucleon momentum increases beyond ∼500 MeV/c. The extracted fraction of proton-proton pairs is small and almost independent of the missing momentum. Our data are compared with calculations of two-nucleon momentum distributions in 4 He and discussed in the context of probing the elusive repulsive N N force.

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. DOI: 10.1103/PhysRevLett.90.074501 PACS numbers: 47.27.Te, 44.25.+f An important issue in the study of turbulent RayleighBénard convection is to understand how heat is transported vertically through a convection cell [1][2][3]. A large number of global heat transport measurements have been carried out in various convecting fluids and under different experimental conditions. Some of the measurements were conducted with wide parameter range and great precession [4 -9]. These measurements have stimulated considerable theoretical efforts, aimed at explaining the functional form of the measured Nusselt number (normalized heat flux), Nu(Ra,Pr), as a function of the two experimental control parameters: the Rayleigh number Ra and the Prandtl number Pr. Like many transport phenomena in condensed matter physics, the measured macroscopic transport properties can often be explained by theories with different microscopic mechanisms [1][2][3][4]. A main issue of an unresolved theoretical debate is whether the heat transport in turbulent convection is determined primarily by thermal plumes, which erupt from the upper and lower thermal boundary layers, or by the large-scale circulation (LSC) that spans the height of the convection cell. Direct measurements of the local convective heat flux, therefore, become essential to the understanding of the heat transport mechanism in turbulent convection.In this Letter, we report direct measurements of the normalized local convective heat flux, Jr hvr; tTr; ti t H=T, over varying Rayleigh numbers and spatial positions r across the entire cell. Here is the thermal diffusivity of the convecting fluid, T is the temperature difference across the cell height H ( 20:5 cm), and h. . .i t represents an average over time t. In the experiment, the local temperature fluctuation Tr;t Tr;t ÿ T 0 and the flow velocity vr;t are measured simultaneously. The mean temperature T 0 of the bulk fluid is kept at 30 C and the corresponding Prandtl number, Pr =, is 5:4. The convection cell is an upright cylindrical cell of aspect ratio one and is filled with water. Details about the apparatus have been described elsewhere [10]. The upper and lower plates are made of brass and the sidewall is a transparent Plexiglas ring with a narrow and long rectangular flat window for velocity measurement. Two silicon rubber film heaters connected in parallel are sandwiched on the backside of the lower plate to provide constant and uniform heating. The upper plate is in contact with a cooling chamber, whose temperature is maintained constant by circulating cold water from a temperature bath.Local velocity meas...

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 Ra(c) approximately equal to 5 x 10(7). The experiment reveals a unique mechanism for the onset of coherent oscillations in turbulent Rayleigh-Bénard convection.

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.