411 Wind Pressure Distribution on Plastic-Covered Low-Tunnels Used in Protected Cultivation Antonis Mistriotis and Demetrios Briassoulis Laboratory of Farm Structures Agricultural University of Athens Iera Odos 75, 11855 Athens Greece Keywords: wind load, protected cultivation, low-rise structures, CFD Abstract Wind loads on a typical low-tunnel structure used in short-term (less than one year) protected cultivations, were studied experimentally and numerically. Wind pressures were measured by a full-scale field experiment on a cross section of the structure in the case of a transverse wind. Two-dimensional Computational Fluid Dynamics (CFD) simulations were performed for analysing and interpreting the measurements for the same case of transverse wind. The boundary conditions were defined to simulate the measured wind characteristics. The numerically calculated wind pressure distribution is qualitatively similar to the experimental one. However the negative pressure measured at the tunnel ridge is lower compared to the computa- tional value. INTRODUCTION Plastic films are used in a constantly growing range of agricultural applications. Beyond, the well known and extensively studied plastic-covered greenhouses, agricultural plastic films are also used for protecting crops under very light, low-rise temporary structures, called low-tunnels (height below 1m) (Briassoulis, 2004). Although such protected cultivation techniques are in use for almost two decades, the protecting system is poorly studied with respect to loading due to limited requirements with respect to safety. However, the extensive use of plastic in agriculture and the tedious work required for the collection of the agricultural plastics after use resulted in dangerous environmental pollution. Therefore, the mechanical behaviour of films used in low-tunnel cultivation has to be optimised in order to avoid excessive consumption and use of plastics in rural areas. Even more, the possible use of biodegradable low-tunnel films requires a very careful design of the very thin films to avoid premature failure and allow for a fast bio- degradation (Briassoulis, 2004). For these reasons, the loads on these low-rise structures have to be studied and the film-arches structural system to be optimised. In this work the pressure distribution on a typical low-tunnel (50 cm high) is studied experimentally and numerically. The analysis of the wind characteristics in a lowest surface layer, 1 m above the ground, was a necessary first step for studying wind loads on very-low-rise structures lying below this height. For this reason, the characteristics of the wind in this layer have been studied. The wind pressure distribution along a middle cross section of a totally closed low-tunnel, which is 50 cm high, 60 cm wide and 6 m long, was experimentally studied. A set of 10 pressure sensors was installed along the cross section. The measurements were correlated to the wind velocity and direction measured at a reference height of 10 m and also at 1 m height at the location of the experiment. In the case of a transverse wind, the measurements indicated a negative (outward) pressure at the top segment and positive (inward) at the lower windward segment of the tunnel. At the lower leeward part of the cover, the pressure was weakly negative (outward). Two-dimensional Computational Fluid Dynamics (CFD) simulations were per- formed for analysing and interpreting the measurements in the case of the transverse Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005