CFD prediction of the natural ventilation in a tunnel-type greenhouse: influence of wind direction and sensibility to turbulence models.

摘要

A contribution to the development of a new model for the characterization of the climatic conditions in a tunnel-type greenhouse is presented, which takes into account the crop (tomato) like an active 3D region where both the momentum transfer (porous media) and the heat and humidity transfers between the crop and the inside air flow are considered. The effects of the wind direction on the climatic parameters inside the greenhouse are simulated together with the use of different turbulence models available in the computational fluid dynamics (CFD) code. The model consists in the determination for each node of the crop grid of the energy balance between the transpiration flux and the radiation flux. The heat and humidity transfer coefficients are deduced from the leaf laminar boundary layer characteristics which are calculated with the local velocity of air in the crop. This model is included in the FluentReg. CFD package in the form of a User Defined File (UDF) added to the main processing unit. The 3D geometry includes a tunnel-type greenhouse with five vents on each side and a five rows mature tomato crop and its external direct environment. The wind boundary conditions for the velocity distribution are deduced from experimental data and the direction of the wind, relative to the longitudinal axis of the greenhouse, can vary from 0 to 90 degrees. Three turbulence model are tested: the standard kappa - epsilon model, the renormalization group (RNG) model and the realizable kappa - epsilon model. The results of the simulations performed for 0, 45 and 90 degrees show clearly the influence of the wind direction on the velocity, temperature and humidity distributions inside the greenhouse. The computations of the velocity field using the different turbulence models also show noticeable difference in the velocity, temperature and humidity patterns inside the greenhouse and confirm the importance of the choice of the closure model for the modelling of turbulence..