Constraints, limitations and achievements in greenhouse natural ventilation

Natural ventilation occurs as a result of the interaction of pressure differences created across an open ventilator and the flow characteristics of the opening. The significant characteristics of air flow through ventilation openings are outlined. The current understanding of the air flow produced by differences between the internal and external temperatures, by the mean dynamic pressure field created over the greenhouse surface by the wind and by the fluctuating pressure at a ventilator opening due to wind turbulence is summarised, and the current models for predicting natural ventilation are outlined. The methods used to measure air exchange rates are critically reviewed and the underlying assumptions identified. The characteristics of leeward ventilation of greenhouses, including the influences of wind speed, ventilator opening and ventilator aspect ratio are reviewed. The limited information on windward ventilation and combined leeward and windward Ventilation is discussed. The performance of ventilation. models in estimating greenhouse air exchange is assessed. The state of knowledge on the air flows created inside greenhouses by natural ventilation is outlined and the implications for Ventilation measurement and greenhouse climate considered.