Aerodynamic characterisation of two large screenhouse roofs

Large screenhouses are popular in Mediterranean horticulture, individual houses having often horizontal dimensions of over 100 m. As the size of these structures increases, the effect of ventilation through the side-walls diminishes and most of the momentum, mass and energy exchange with the atmosphere takes then place through the roof. In the limiting case of an 'infinite screenhouse', the transfer becomes one dimensional in the vertical direction. The traditional 'mixing length' analysis of vertical flux profiles over 'infinite' fields has been adapted here to flat-roof, 'infinite' screenhouses. The analysis of this configuration indicates that the aerodynamic effect of the screen is determined by just two parameters. One quantifying the horizontal shear force on the roof, and another − the effective mixing through the screen. The shear parameter can be determined from momentum flux measurements above and below the roof, and the mixing parameter can be determined from the velocity profile above the roof. Data collected in two large screenhouses, one covered with a coarse 'shade' screen and another with a fine insect-proof screen, were used to compare their aerodynamic characteristics. The shade screen absorbed 80% of the atmospheric momentum flux, while the insect-proof screen absorbed 95%. The shade screen's mixing length is estimated at 0.40 m, while that of the insect-proof screen is only 0.14 m.