Airflow patterns through roof openings of a naturally ventilated greenhouse

In recent years there has been a trend to build large greenhouses equipped with roof openings since side openings are not very effective in the natural ventilation of large structures. The major objective of this work was to investigate the effect of ambient wind speed and azimuth on the airflow patterns through openings of a greenhouse. Experiments were done in a four-span polyethylene covered greenhouse with floor area of about 960 m(2) and gutters at 3.9m height that were oriented north-south. During the period of the experiments the height of the pepper plants grown in the greenhouse increased from 0.15 to 0.5 m. The greenhouse was equipped with continuous vertical roof openings, one in each span, and was ventilated by three of them that were opened to a height of about 0.8 m from the gutter. Measurements of wind speed through the openings were carried out using two sonic one-dimensional anemometers (CA27, Campbell Sci., USA) and one sonic three-dimensional anemometer (USAI, Metek, Germany). In each opening the two one-dimensional anemometers were placed about 5 m away from the northern and southern greenhouse sidewalls while the three-dimensional anemometer was placed at the centre of the opening. Results show that the directions of airflow at the two edges of each opening are generally opposite except at few short time periods during which the air velocity at the two edges is relatively small. Outflow at one edge is associated with inflow at the other and vice versa. Transitions of flow directions are mainly related to variations in the ambient wind azimuth. The flow patterns indicate that airflow through the openings is outwards at the windward edge and inwards at the leeward edge. This can be related to the wind-induced pressure field developed around the greenhouse. The normalized air speed through the openings, U-*, is changing with time during the early morning hours but during the rest of the day it remains almost constant. Constant normalized air speed implies on close correlation with the ambient wind. Results of the three-dimensional sonic anemometer show that when the ambient wind rotates clockwise from southeast to northwest, the horizontal airflow near the centre of the opening rotates anticlockwise from southeast to northeast. The flow patterns in the three openings were qualitatively similar.