INSIGHT INTO RAYLEIGH NUMBER EFFECTS ON TURBULENCE CHARACTERISTICS OF A THERMALLY DRIVEN FLOW ADJACENT TO UPWARD-FACING HORIZONTAL HEATED ROUND PLATE BY LARGE-EDDY SIMULATION

摘要

The Rayleigh number effects on the thermally-driven flow adjacent to an upward-facing isothermally-heated round plate, which is horizontally placed at a floor in air, are investigated by using a well-resolved large-eddy simulation. The special attention is paid to the turbulence characteristics, including developing process, in the vicinity of the heated plate. The simulation is performed with an open-source code, OpenFOAM; the Cartesian coordinate grid system with a large computational domain are employed to avoid the decrease in a grid resolution near the edge of heated plate and to reduce the disturbance of ambient fluid. The Rayleigh number based on the radius of the heated plate is set to 2.1×10~7 or 4.2×10~7; both yield transition of the boundary layer from the laminar to turbulence. The visualized structures and statistics of flow and thermal fields reveal that the developing process with the transition includes vertical wavy motions. The vertical wavy motions might be related to the vortex rings periodically generated by the discontinuity of thermal conditions at the edge of heated plate. The increase in Rayleigh number results in weakening of the coherency of the vortex ring, but this impact is limited to the near the fringe of the heated plate. Thus, the turbulence characteristics of fully developing region is maintained regardless of Rayleigh number: of both the simulations performed, the cell-like structures are observed and the vertical profiles of turbulence intensity of temperature and vertical velocity fluctuations are expressed by the similarity of λ-layer.