S. Murakami’s research while affiliated with Tohoku University and other places

Large eddy simulation (LES) of the flow field over a surface-mounted prism under two different conditions, namely smooth and turbulent inflows, are carried out. A higher-order scheme, in which artificial dissipation is controllable, is used. The simulations are validated through comparison with experimental measurements. The results present new physical aspects and details regarding the development of the primary separation flow on the upwind face of the prism, the development of the horseshoe vortex flow, the primary and secondary separations of the reversed flow on the roof, and finally the locations of the highest suction pressures (in the mean) on the roof and the sides of the prism with respect to the secondary separation lines. The effects of the incident turbulence are determined by pointing out differences in the flow topologies between the two incident flow cases. Based on the validation with the experimental results, the compact upwind difference fifth-order scheme, used here, is recommended for performing LESs of complex flows.

In this study, velocity, temperature and humidity fields in urban area were analyzed using a new three dimensional plant canopy model developed by the authors. This model for the plant canopy developed in this study considers the following three effects: (1) drag force of the planted trees, (2) shading effects on shortwave and longwave radiations, (3) production of latent heat from the plant canopy. Three cases were carried out here in order to evaluate the relaxation effect of plants on the outdoor thermal environment. In case1, green area ratio of ground surface was assumed 10%, while the green area ratio of ground surface was changed to 100% in case2. Furthermore plant canopies were set on the ground surface of case2 in case3. By comparing the distributions of scalar wind velocity, ground surface temperature, MRT (Mean Radiant Temperature) and SET* (Standard Effective Temperature) in these three cases, effects of the plant canopy were clarified.

When predicting unsteady flow and pressure fields around a structure in a turbulent boundary layer by Large Eddy Simulation (LES), velocity fluctuations of turbulence (inflow turbulence), which reproduce statistical characteristics of the turbulent boundary layer, must be given at the inflow boundary. However, research has just started on development of a method for generating inflow turbulence that satisfies the prescribed turbulence statistics, and many issues still remain to be resolved. In our previous study, we proposed a method for generating inflow turbulence and confirmed its applicability by LES of an isotropic turbulence. In this study, the generation method was applied to a turbulent boundary layer developed over a flat plate, and the reproducibility of turbulence statistics predicted by LES computation was examined. Statistical characteristics of a turbulent boundary layer developed over a flat plate were investigated by a wind tunnel test for modeling the cross-spectral density matrix for use as targets of inflow turbulence generation for LES computation. Furthermore, we investigated how the degree of correspondence of the cross-spectral density matrix of the generated inflow turbulence with the target cross-spectral density matrix estimated by the wind tunnel test influenced the LES results for the turbulent boundary layer. The results of this study confirmed that the reproduction of cross-spectra of the normal components of the inflow turbulence generation is very important in reproducing power spectra, spatial correlation and turbulence statistics of wind velocity in LES.

Recently, the prediction of wind environment around a building using Computational Fluid Dynamics (CFD) technique comes to be carried out at the practical design stage. However, there have been very few studies which examined the accuracy of CFD prediction of flow around a high-rise building including the velocity distribution at pedestrian level. The working group for CFD prediction of wind environment around building, which consists of researchers from several universities and private companies, was organized in the Architectural Institute of Japan (AIJ) considering such a background. At the first stage of the project, the working group planned to carry out the cross comparison of CFD results of flow around a high rise building by various numerical methods, in order to clarify the major factors which affect prediction accuracy. This paper presents the results of this comparison.

A hybrid cooling system using radiational panel cooling with wind-induced cross ventilation in an office setting is investigated. The characteristics of the indoor environment are examined using CFD (Computational Fluid Dynamics) simulation, which is coupled with radiation heat transfer simulation and with HVAC control in which the PMV value for the human model in the center of the room is controlled to be at the set-point (target) value. The system is devised on an energy-saving strategy that utilizes stratified room air with a vertical temperature gradient. The air cooled by the radiational panel settles down within the lower part of the room, while the hot and humid outdoor air passes through the upper region of the room and sweeps out the heat and contaminants generated in the room.