Airflow Patterns through Roof Openings of a Naturally Ventilated Greenhouse and their Effect on Insect Penetration

Experiments were carried out to study the airflow through continuous vertical roof openings of a naturally ventilated greenhouse. The major objectives of this work were to investigate the effect of ambient wind speed and direction, with respect to the greenhouse, on the mean and turbulent characteristics of the airflow. The air velocity was measured simultaneously at the two edges of each of the greenhouse openings by one-dimensional sonic anemometers and at the mid-span of the openings by a three-dimensional sonic anemometer. It is shown that when the wind is not perpendicular to the plane of the openings there are outflow and inflow, at the windward and leeward edges of the openings, respectively. A wind blowing from the back of the openings and nearly perpendicular to them reduced the mean air velocity at the two edges but did not change the turbulent velocity much. Over a considerable part of the day the mean and turbulent air velocities scale with the ambient wind. Monitoring whitefly population in the roof openings and around the greenhouse indicated a main daily activity period between 08:00 and 12:00. The number of whiteflies trapped under the eastern roof opening was significantly higher than the number trapped under the other roof openings. This was in agreement with the higher probability to have inflow in the eastern opening than in the western one as measured by the sonic anemometers.