Insect-proof screens constitute a physical means of protecting crops and their use has become widespread in recent years. There is no doubt as to their efficiency in controlling insects, but they do have a negative influence on greenhouse ventilation as they obstruct air-flow. It is therefore necessary not only to evaluate their efficiency as a means of protecting crops, but also to estimate the degree to which they obstruct airflow. To this end the present work analyses the aerodynamic characteristics of these screens, carrying out experiments with two devices which force a flow of air through them, thus providing data of the pressure drop as a function of air velocity. The analysis of these data has provided simple ratios of the permeability and the inertial factor as a single function of porosity.

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The aim of the present study was to evaluate geometrical characteristics and airflow resistance of eleven different insect-proof screens by three different experimental procedures: equipment based in water-flow suction, low-speed wind tunnel, and CFD simulations. The two first arrangements had the same principle, in that air was forced through the test samples in order to create a pressure drop. Last analyses were carried out by numerical simulations of airflow through insect-proof screens using a commercial fluid dynamics code based in Finite Element method (ANSYS/ FLOTRAN v8.0). Previously, an analysis images system, called EUCLIDES v1.1, was designed with MS Visual Basic 6.0 running under MS Windows, for the analysis of the screens samples images captured with a microscope. A geometrical characterization of the eleven screens materials was carried out using this software tool. The software allows to determine all the geometric parameters that characterize the screens, as thread diameter and distances between two adjacent threads in two directions, from the four coordinates that defined each pore. The results obtained in this work show that the eleven screens can be classed in three groups, corresponding with the fibre density, with similar porosity and airflow properties (permeability and inertial factor). However, sample 8 has a small thread diameter and screen thickness that decreased the pressure drop coefficient. The results suggest that equations based on the porosity of the screen and the Reynolds number can be used to calculate the pressure drop coefficient.

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