Abstract
In recent years, cross-ventilation is actively used in the mid-season and at night and in the early morning in summer as a countermeasure toward global warming. This aims to gain energy-saving effect of buildings' cooling down and thermal environmental improvements by using of cross-ventilation. Lately, some studies calculated energy-saving effects of cross-ventilation.
In the calculation of cross-ventilation rate, discharge coefficient is known to change with the wind direction as the same as the wind pressure coefficient is. However, most of the studies calculating the energy-saving effects fixed the discharge coefficient. This means that the precision of these results has still room of examination.
To this problem, the authors offered the local dynamic similarity model based on the dynamic conditions of inflow openings, and successfully calculated the discharge coefficient changes through the pressure conditions at the openings. This model uses a ventilation driving force, which is the difference between the wind pressure and the indoor pressure, and a dynamic pressure in tangential direction at the opening. The proportion of these two parameters is expressed as dimensionless indoor pressure (PR*), and the discharge coefficient and the inflow angle are decided by PR*.
This study firstly examined the discharge coefficient changing mechanism on different PR* conditions with CFD (Large Eddy Simulation). The results show that total pressure loss coefficient is steady over all regardless of PR* differences, and that the static pressures increase widely by airflow crashes to window flames when dynamic pressure in tangential direction at the opening excels. The results also show that the cross-ventilation rate and the discharge coefficient decrease because airflows counteract from the static pressure increases to go through the openings.
Secondary, this study conducted an examination with virtual stream tube to reveal the ventilation energy variation and the pressure loss mechanism after airflow passing through the openings. In the results, the dynamic pressure in tangential direction converted to the dynamic pressure in normal direction and the static pressure decrease appeared in the area where the dynamic pressure in tangential direction at the opening is relatively large (around the area where PR* is close to zero), and the dynamic pressure increase by vena contracta and static pressure decrease also appeared. The results shows that the turbulent kinetic energy production is one of the considerable factors of total pressure decrease.
