風速変動を考慮した自然換気・通風性能評価手法の提案

Abstract

INTRODUCTION Natural cross-ventilation is a complex phenomenon, influenced by the distribution of wind velocity, direction, and total pressure on walls. In recent studies using computational fluid dynamics (CFD), steady-state flow fields in a cross-ventilated house model have been studied. Cross-ventilation is not observed using steady-state simulations (e.g., Raynolds-Averaged Navier-Stokes Simulation); nevertheless it is an actual phenomenon. In practice, a flow field varies depending on the fluctuation of outdoor wind, and cross-ventilation is affected by the wind turbulence around a house. Therefore, replicating actual phenomena using CFD is very important to evaluate the unsteady cross-ventilation performance of a house. Recently, unsteady fluid phenomena has been analyzed using large-eddy simulation (LES). In this study, a new evaluation method for natural cross-ventilation that takes flow fluctuation into consideration is proposed. In this method, particles are released at openings and the ratio of particles arriving at the evaluation area within the house model is calculated. Ratio of particles entering into the model, ratio of particles arriving at the evaluation area, effective ventilation rate and effective ventilation ratio are analyzed for the model. Evaluation results concerning unsteady natural cross-ventilation performance are reported. METHOD In this study, particles are released based on the results of an LES analysis reported previously. The evaluation area (i.e., the inner half part of the house) is set over the central plane of the model to exclude short circuit phenomena. The ratio of particles arriving at the evaluation area is calculated. 100 particles are released from the opening per second. The time-history of the flow field obtained for 10 s are used repeatedly until a steady state is achieved. Next, only the particles that reached the evaluation area are counted as contributing to the natural cross-ventilation. The ratio of particles arriving at the evaluation area is computed from Equation (1) and the effective ventilation rate is computed from Equation (2) by the amounts of inflow on the opening and the ratio of particles arriving at the evaluation area. Equation (3) shows the effective ventilation ratio based on the results of Case 1 and the other cases. RESULTS AND DISCUSSION The results are as follows; (1) The ratio of particles arriving at the evaluation area in Cases 1, 2, 3, 4, and 5 are 97%, 42%, 58%, 49%, 57% on an average, respectively. (2) The effective ventilation rates in Cases 1, 2, 3, 4, and 5 are 10.43 m3/h, 0.98 m3/h, 1.76 m3/h, 1.20 m3/h, and 2.12 m3/h on average, respectively. The effective ventilation ratio is given by Equation (3) based on the effective ventilation rate of Case 1. (3) In Cases 2, 3, 4, and 5 the average effective ventilation ratios are given by 0.0937, 0.1685, 0.1154, and 0.2030, respectively. CONCLUSION In this study, a new evaluation method for natural cross-ventilation with consideration of flow fluctuation has been proposed and evaluation results are reported. Consequently, in Cases 2 to 4 wherein no ventilation occurs in a time-averaged flow field, there are unsteady ventilation phenomena. In cases 2 to 4, the effective ventilation rates under unsteady flow are approximately 10% to 20% of those in Case1. The particle animation for this study can be found at the URL below: http://tkkankyo.eng.niigata-u.ac.jp/dougainfo/journal/les2015_2/les2015_2.html