{"content":[{"text":"(PIV) systems for laboratory tests have allowed the airflow distribution to be \n quantitatively analyzed. In recent years, rapid progress in computational fluid dynamics \n (CFD) codes has taken and CFD techniques has been increasingly applied to study the \n airflows in greenhouses (Reichrath and Davies, 2002). In common CFD simulations, the \n computational domain is divided into cells and the discretized governing equations such \n as continuity equation and Navier-Stokes equation are solved across each cell. The \n advantages of CFD include the investigation of environmental parameters at many points \n of interest, the easy change in weather and structural conditions, the visualization of \n airflow, and the saving of time, labor and cost. The accuracy of CFD results has been \n improved by verification tests and improvement of related models. More recent concerns \n in the CFD studies have focused on the internal airflows in a greenhouse with plants and \n the interactions between the plant canopy and the ventilated air by incorporating the heat \n and mass balance models (Boulard and Wang, 2002). \n In this paper, the primary characteristics of the air movement caused by wind and \n stack effects in a single-span greenhouse are described. Recent studies on the air \n movement and the uniformity of environment under natural ventilation are reviewed and \n discussed with respect to the multi-span greenhouses, the effect of plants, the vent \n configurations, and the effect of wind direction.","bbox":{"left":0.133514,"top":0.061455,"width":0.72915,"height":0.28238500000000005}},{"text":"PRIMARY AIRFLOW CHARACTERISTICS FOR A SINGLE-SPAN \n GREENHOUSE \n The phenomenon of natural ventilation is complex and its design is more difficult \n than fan ventilation. Natural ventilation is induced by the pressure differences across the \n vent openings. The pressure differences are caused by external wind force (wind effect) \n and buoyancy force (stack effect) based on the temperature rise (temperature difference \n between internal and external airs). The theory based on Bernouilli's equation indicates \n that the natural ventilation rate varies linearly with external wind velocity and area of vent \n openings. Previous works (for example, Baeza et al., 2005) have shown the features. \n When stack effect exists, the natural ventilation rate varies linearly with the square roots \n of height of openings and temperature rise. \n The air movement in the naturally-ventilated greenhouses is affected by the shape, \n size and arrangement of vent openings as well as external wind velocity and direction. Fig. \n 1 shows the airflows caused by wind effect under isothermal condition in a single-span \n greenhouse equipped with continuous ridge and side vents when wind blows into a side \n wall (Sase et al., 1984). It was obtained by a wind tunnel test using a 1/10 scale model. It \n was indicated that the airflow patterns were influenced primarily by the shape of vent \n opening through which the external air entered. The incoming air through the windward \n side vent tended to move up immediately by the influence of inclined vent flap and \n mainly followed the inner surfaces of the roof. In the space to be occupied with a crop, \n the reverse flow due to secondary circulation resulted in the significant decrease in air \n velocity. With the roll-up vents in side walls, on the other hand, the incoming air moved \n directly toward the leeward side vent causing the air velocity in the crop space to increase. \n The investigations of various arrangements of vents showed that the airflow pattern \n depended primarily on the shape of the windward side vent and similar airflow patterns \n were observed whenever the windward side vent and some of other vents were open. \n Figure 2 shows a transition of airflow patterns affected by the combination of \n wind and stack effects for a same greenhouse with all hinged vents open (Sase et al., \n 1984). The stack effect was simulated using a heated floor and Archimedes number was \n taken into account to ensure the similarity of the model test. Under calm conditions as \n shown in top drawing of Fig. 2, the cool incoming air through both side ventilators \n followed horizontally on the floor, and as it was heated during this movement it went up \n at the center of the greenhouse. This airflow determined the temperature distribution \n characterized by the horizontal increase of temperature from both side walls to the center \n of the greenhouse and caused a high temperature in the center vertical space. \n For the case of combined action of wind and stack effects, the airflow pattern","bbox":{"left":0.133514,"top":0.354885,"width":0.7336509999999999,"height":0.55247}},{"text":"314","bbox":{"left":0.129481,"top":0.921407,"width":0.030252,"height":0.01901200000000003}}],"images_bbox":[],"images_bbox_not_text_overlap":[]}