Assessing effects of wind speed and wind direction on discharge coefficient of sidewall opening in a dairy building model – A numerical study

Abstract

The discharge coefficient (Cd) of an opening is an important parameter in the application of the orifice equation to calculate the airflow rate of naturally-ventilated buildings. It is questionable to regard Cd as a constant under varied external airflows for large openings that are often applied in dairy buildings. This study investigated the impact of external wind speed and wind direction on the discharge coefficient of the windward sidewall opening with various opening sizes for a scaled naturally-ventilated dairy building model. The investigations were performed with Computational Fluid Dynamics (CFD) simulations. A set of data from wind tunnel experiments with a building model was used for the CFD model validation. The simulation results indicated that the discharge coefficient was almost unaffected by outdoor wind speed but varied considerably with the wind direction. Lower Cd values were observed for smaller wind angles. The pressure distributions on sidewalls were non-uniform. It is concluded that in the application of the orifice equation in oblique wind directions and large openings, the discharge coefficient is required to be specified instead of using the constant value, and it is necessary to distribute enough pressure taps along the horizontal direction to obtain the representative pressure difference.