Comparison of finite element and finite volume methods for simulation of natural ventilation in greenhouses

International audienceThis article aims to contribute to the discussion on the efficiency of two different discretization methods used as computational fluid dynamics (CFD) solvers for the simulation of natural ventilation in greenhouses. The focus is not on a general use of CFD, but rather on its specific application to simulate airflow in naturally ventilated greenhouses. After a short review of the basic model and its extensions, we compare the accuracy and computational efficiency of two simulation codes based on the Finite Element Method (FEM) and the Finite Volume Method (FVM) for two-dimensional incompressible turbulent flow in naturally ventilated greenhouses. FVM software (ANSYS/FLUENT v 6.3.) is the most frequently used CFD code in ventilation research, but few papers using FEM software (ANSYS/FLOTRAN v. 11.0) have been published. CFD simulations have been compared to experimental data for 12 cases corresponding to three greenhouse types. The experimental greenhouses were chosen to represent a large range of ventilation situations: buoyancy effect in a mono-span greenhouse with adiabatic walls, buoyancy and wind effect in a multi-span greenhouse and ventilation in an Almería-type greenhouse under conditions of large temperature gradient and high wind speeds. The data from simulations and field experiments were compared using different parameters to analyze the effectiveness of experimental data in the validations of CFD models. The possibility of repeating simulations with different discretization methods and commercial software has been tested, as well as the type of experimental data needed to ensure correct validation of CFD models for use in greenhouse ventilation studies. To this end, temperature distribution measurements are preferable to set-point measurements and the use of visualization techniques (laser sheets) or the measurement of velocity vectors (anemometer) are more indicative than ventilation rates. The computational capacity of these approaches has also been analyzed, comparing their performance in terms of the overall database space necessary to store the numerical models and the necessary CPU time to compute one step of the convergence process. On average, the FEM required twice as much computing time per cell and step as FVM, and the amount of required memory storage was approximately 10 times greater for the FEM

Low Tunnels inside Mediterranean Greenhouses: Effects on Air/Soil Temperature and Humidity

The main objective of this work was to analyze the microclimate generated inside a low tunnel (floating row cover) installed in an Almería-type greenhouse. Low tunnels are commonly used in the open field to protect plants against insect attack and to improve the production of muskmelon and strawberry. Floating row covers can also be used inside greenhouses during the first few weeks after the transplantation of muskmelon and watermelon crops in spring-summer cycles. This work was carried out during the first weeks of a watermelon culture (Citrullus lanatus Thunb.) growing with a polyethylene row cover inside an Almería-type greenhouse (2115 m2). Air temperature and humidity, plant temperature and soil temperature and humidity were measured in the greenhouse inside and outside the row covers. During the three days of measurement, all greenhouse vent openings were closed. The use of the low tunnels increased average air temperature around plants from 24.0 ± 9.0 °C to 26.9 ± 9.7 °C. A maximum difference in air temperature of about 5.9 °C was observed at noon. The average daily temperature of the crop was 28.2 ± 11.8 °C inside the row cover and 24.6 ± 8.9 °C without it. Similarly, the absolute humidity of air was clearly higher inside the low tunnel (0.0201 ± 0.0098 g/g) than around the plant rows without floating cover (0.0131 ± 0.0048 g/g). The soil temperature was also higher inside the low tunnel compared to the area without this second plastic cover. The effect of the tunnel decreased with depth, with average temperature differences of 1.2 ± 0.5 °C on the soil surface and 0.6 ± 0.5 °C at 20 cm depth