Novel methodologies for the characterization of airflow properties of shading screens by means of wind-tunnel experiments and CFD numerical modeling

摘要

Shading screens are broadly used in the protected cultivation sector, since they allow both to reduce the heat load and thus to control temperature, and to have lower and uniform levels of light intensity inside greenhouses. Various types of shading screens are available on the market, with different colors, material and textures. The choice of the best screen depends on the specific application and needs of the grower. Despite this, some screens can negatively affect ventilation and indoor climate, since their porosity can generate extra mass, heat and momentum transfer resistance. Most studies have evaluated the screen-related parameters, such as permeability and porosity, and the screen effect on ventilation referring to screens with simple and regular textures. In this paper, these parameters have been measured for three screen types available on the market using different approaches for their characterization. A novel approach based on image analysis together with wind tunnel tests has been set up to yield the permeability and inertial coefficient. On the other hand, a computational methodology based on CFD modeling has been carried out in order to obtain the relation between air velocity through a screen and the relative pressure drop, avoiding any experiment. The CFD methodology has been developed and validated, with the aim to possibly derive the parameters of different screens through simulations rather than more demanding experiments feasible only with specific equipment. In particular, a portion of the whole screen has been chosen for CFD simulations and the numerical results have been validated by a comparison with Particle Image Velocimetry (PIV) data. This has allowed both to improve the model and to evaluate its effectiveness in simulating this specific fluid dynamics domain. By these novel approaches, the basis for extending the knowledge about the characterization of the screens used in agriculture have been laid.