A novel convection experiment is conducted in a cell with rough upper and lower surfaces. The measured heat transport in the rough cell is found to be increased by more than 76%. Flow visualization and near-wall temperature measurements reveal new dynamics for the emission of thermal plumes. The experiment shows that the interaction between the horizontal shear flow due to the large-scale circulation and the ordered rough surface creates a secondary flow (eddies) in the groove region. The secondary flow together with the large-scale circulation enhance the detachment of the thermal boundary layer from the tip of the rough elements. These extra thermal plumes are responsible for the enhanced heat transport in the rough cell. The discovery of the enhanced heat transport has important applications in engineering for more efficient heat transfer.

A novel convection experiment is conducted in a cell with rough upper and lower surfaces. The heat transport across the rough cell is found to be increased by more than 76%. Flow visualization and nearwall temperature measurements reveal new dynamics for the emission of thermal plumes. The discovery of the enhanced heat transport has important applications in engineering and atmospheric convection.[ S0031-9007(98) PACS numbers: 47.27. Te, 05.60. + w, 44.25. + f, 47.27.Nz Turbulent flows over a rough surface are ubiquitous in nature. An example is convection in the atmosphere and oceans, where the underlying surfaces are almost always rough. The study of turbulence over a rough surface is of fundamental interest for understanding the structure and dynamics of turbulent boundary layers and is also relevant to many practical applications, ranging from effective heat transfer for a reentry vehicle to turbulent drag reduction of a commercial aircraft. Our current knowledge about the roughness effect on turbulent flows comes largely from experiments in wind tunnels and other open systems [1], where the disturbance flow produced by a rough wall is confined in the near-wall region and is quickly discharged to the downstream. Because of these reasons the surface roughness usually does not perturb the turbulent bulk region very much, and its effect can often be described by rescaling the relevant parameters with the surface roughness height k [2]. This situation is changed completely for flows in a closed cell, in which the disturbances produced by the boundaries are inevitably mixed into the turbulent bulk region.Turbulent Rayleigh-Bénard convection is an example of such a closed system, which has attracted much attention in recent years [3]. Intermittent bursts of thermal plumes from the thermal boundary layers and the coherent largescale circulation, which modifies the boundary layer via its shear, are found to coexist in the convection cell. These salient features are directly related to the heat transport across the cell, and have been adopted in several theoretical models [3][4][5] to explain the observed scaling laws in the heat flux and temperature statistics [4,6]. In this Letter, we describe a novel convection experiment in a closed cell with rough upper and lower surfaces. It is found that when k becomes much larger than the thermal boundary layer thickness d, the heat transport across the rough cell is increased by more than 76%. Flow visualization and near-wall temperature measurements reveal that the large-scale circulation interacts with the surface roughness and produces more thermal plumes from the tip of the rough elements. The striking effect of the surface roughness provides new insights into the nature of convective turbulence, and also has a myriad of applications in engineering, geography, and meteorology.The experiment is conducted in a cylindrical cell filled with water. Details about the apparatus have been described elsewhere [7], and here we mention only some key points. The rough upper and...

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.

In this work we study the convection flow driven by a heat-releasing concentration field which itself is stably stratified. The heat-releasing rate is linearly proportional to concentration. Linear stability analysis is conducted to determine the critical heat-releasing rate for given fluid properties and concentration differences. The most unstable mode associated with the critical heat-releasing rate can be oscillatory for a large concentration Rayleigh number, i.e. the non-dimensionalized concentration difference and large Schmidt number, i.e. the ratio of viscosity to diffusivity of the concentration component. Fully developed flows are then investigated by direct numerical simulations. Flow structures near the bottom plate have larger horizontal scales than those near the top plate. The concentration in the bulk is almost constant and takes a similar value for all the explored parameters, which results in the convective flux increasing linearly with height. To explain the dependences of the global transport properties, we extend the unifying theory of the Rayleigh–Bénard convection to the current system and develop scaling laws for the global fluxes. The numerical results can be described accurately by the theoretical model.

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