Airflow characteristics and patterns in screenhouses covered with fine-mesh screens with either roof or roof and side ventilation

Abstract

In many countries fine mesh screens are used to protect agricultural crops. The low porosity of such screens impedes the exchange of mass, heat and momentum between the air around the crop and that occur in the atmosphere, thereby modifying the crop microclimate. Experiments were carried out aimed at determining the internal airflow characteristics and patterns of screenhouses. The results showed that screenhouses reduced air velocity compared to that in an open field. For a given windspeed the air velocity inside the screenhouse increased with height. The internal air velocity increased with external windspeed but it increased to a greater extent near to the roof. Flow patterns and characteristics were affected by whether the screenhouse was ventilated only via the roof or via the roof and sidewalls. When the screenhouse was ventilated from the roof and side panels the air velocity was higher than under roof ventilation alone, and the airflow direction, both within the canopy and above it, was usually in a similar direction to that of the outside wind. However, under roof ventilation alone the internal airflow direction was generally opposite to that of the external wind, resulting in outflow through the windward section of the roof and inflow through the leeward section. In the central region of the screenhouse the mean vertical velocity within the canopy was near zero and its fluctuations with time were relatively small. Near the roof, there was a mean net inflow and the velocity fluctuations were much larger.