Tae Hattori

Study of unsteady natural convection induced by absorption of radiation based on a three-waveband attenuation model

The present study considers unsteady natural convection induced by the absorption of radiation for possible applications in the water quality management for the shallow regions of lakes and reservoirs. The direct absorption of the incoming radiation by the water body forms a stable thermal stratification, whilst residual radiation reaching the bottom bathymetry is re-emitted as a boundary flux, forming an unstable thermal stratification, which is a potential source for a Rayleigh-Benard type instability. The bottom boundary layer instability drives intermittent vertical convection in the form of rising plumes. The plume rise is, however, limited by the stable thermal stratification due to the direct absorption, which is controlled by the attenuation coefficient of water. The attenuation coefficient is therefore an important parameter in determining the plume rise and the associated vertical mixing. The wavelength dependency of the attenuation coefficient of water is accounted for by using a three-waveband model. A theoretical prediction is made for the plume rise distance, which represents the region of vigorous mixing. Two-dimensional numerical simulation provides verification for the accuracy of the theoretical prediction.

Two-dimensional numerical simulation of a steady-state buoyancy-driven flow in a semi-confined enclosure with a line heat source

A two-dimensional numerical simulation was conducted to study displacement ventilation flows inside a rectangular space with a single line heat source, with the working fluid connected to an external ambient fluid through upper and lower vents. Our preliminary aim was to benchmark our well-resolved DNS code for the prediction of steady-state flow behaviour against previously published numerical and experimental results. The steady-state height of the interface between upper buoyant and lower ambient layers and the steady-state buoyancy in the upper layer, predicted by a well-resolved DNS, were compared to experimental results and mathematical models. Our numerical results agreed reasonably well with experimental results and mathematical models, and showed a better agreement than the previous numerical results obtained using the standard k ? models. In most regions, the power spectra of velocity fluctuations conformed to the -3 power law, however there were some regions where the -5/3 Kolmogorov law gave a better fit. Transition phenomena from periodic to chaotic regimes were observed with increasing Reynolds number.