Determining Discharge Coefficient of Large Openings of Wind-driven Cross Ventilation Barns by Wind Tunnel Tests

Abstract. Discharge coefficient of the opening is one of the most important parameters in accurately estimating ventilation rate of a building when applying orifice equation, which maybe not suitable to be taken as a constant value for wind-driven cross ventilated dairy barns. This study aims to investigate the impact of wall porosity and wind speed on discharge coefficient by means of wind tunnel tests and a 1:40 scaled building model. Air velocity at the windward opening and the pressure drop across it were measured by Laser Doppler Anemometer and differential pressure transducer respectively. The experimental result reveals that larger wall porosity and slower wind speed result in higher discharge coefficient. Its value was between 0.67 and 0.94 for wall porosity ranging from 0.22 to 1.61 under wind speed of 8 m/s, and decreased slightly from 0.75 to 0.6 for wall porosity of 0.33 when the wind speed increased from 6 m/s to 12 m/s. These findings indicate that the discharge coefficient of an opening cannot be generalized by a fixed value when predicting ventilation rate of a wind-driven cross ventilated building equipped with large openings.