Enhanced Heat Transport in Turbulent Convection over a Rough Surface

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-Benard 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‐ 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 lower surfaces are made from identical brass plates and have woven V-shaped grooves on them. The grooves have a vertex angle of 90 ‐ and their spacing is such that a square lattice of pyramids is formed on the surface. The height of the pyramids is k › 9.0 mm and their spacing is 2k. The sidewall of the cell is a cylindrical ring made of transparent Plexiglas. Two cylindrical rings with the same inner diameter of 20 cm but having different heights of 20 and 40 cm, respectively, are used. The corresponding aspect ratios sA › diameteryheightd are 1.0 and 0.5. The upper plate is cooled by passing cold water through a cooling chamber fitted on the top of the plate. The lower plate is heated uniformly with an electric heating film attached on the back side of the plate. The temperature difference DT between the two plates is measured by two thermistors imbedded in the two plates. A small movable temperature probe is installed inside the cell in order to measure the local temperature T szd of the convecting fluid near the central axis of the cell as a function of distance z