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Insect-proof screenhouses are commonly used to grow plants in warm climates. However, there is relatively little literature on their microclimate compared to greenhouses. This study presents computational fluid dynamics (CFD) results of airflow, temperature, and humidity ratio patterns in a screenhouse with a roof consisting of a large flat insect-proof screen and impermeable walls. First, vertical profiles of velocity, temperature, and humidity at the center of the screenhouse were obtained by 2D steady-state CFD simulations and validated by experimental results. Root mean square error (RMSE) values were used to measure the differences between the two. The lowest RMSE values among simulations with different turbulence models were 0.49 K, 1.26 g kg−1, and 0.05 m s−1 (with the RNG turbulence model) for temperature, humidity ratio, and air velocity, respectively. The main deviation of the CFD results from the experimental results was observed with the air velocity in the upper region of the screenhouse. Inflow and outflow in the leeward and windward parts of the flat roof were observed, respectively. This resulted in large-scale airflow within the screenhouse opposite the outside wind direction at the canopy level. The results suggested that the leeward section of the screenhouse is warmer than the windward one and has a lower humidity ratio. Large-scale rotating airflow formed in the center of the screenhouse, close to the roof, a large area with a humidity ratio similar to ambient conditions. • CFD simulations are used to determine airflow, temperature and humidity patterns. • CFD and experimental results were in agreement except of velocity near screenhouse top. • Airflow within the canopy of an insect-proof screenhouse was opposite to wind direction. • From windward to leeward ends the temperature increased and humidity decreased. [ABSTRACT FROM AUTHOR]

Until about a decade ago, greenhouse microclimates were analysed using a simplified approach assuming a perfectly stirred enclosure. Demands for efficient and sustainable greenhouse operations, stimulated intensive research that focused into the distributed microclimate within greenhouses. Since the distributed microclimate and the air-exchange rate are interconnected the latter is important. Three methods for determining the air exchange in a small naturally ventilated monospan greenhouse were compared: experiments, computational fluid dynamics (CFD) and a model which relates flow rate through openings to their pressure drops. A rose-growing greenhouse was ventilated via two longitudinal side openings, one on the windward and the other on the leeward walls. In the experiments ventilation rates were estimated by means of a tracer gas; in the CFD simulations the decay rate of a virtual tracer gas and calculated airflow rates through the openings were used and in the model, ventilation rates were calculated with an equation relating flow rate through the openings to their pressure drop, using local wind-pressure coefficients. All the methods agreed well up to a wind speed of about 4ms−1; at higher wind speeds the ventilation rate values deduced from the decay of tracer gas, both in the experiments and CFD simulations, were much lower than those obtained with the other techniques. The concept of age of air was used to show that the lower ventilation rates at the high wind speeds were associated with imperfect mixing of the supply air with the greenhouse air, with consequently diminished air-exchange efficiency. [Copyright &y& Elsevier]

An insect-proof screen acts as a mechanical barrier, preventing insects from reaching the plants in a greenhouse and thus reducing pesticide application requirements. However, the exclusion of very small insects requires fine-mesh screens which impede ventilation and thus increase temperature and humidity within the greenhouse. To minimise the effect on ventilation the screen resistance to airflow should be as low as possible. Experiments and computational fluid dynamics (CFD) simulations were used to compare the airflow resistance of flat vs. concertina-shape screens. The CFD results were validated against experimental data and then further simulations were done to explore additional concertina-screen configurations. Folding a fine-mesh screen into a concertina shape reduced airflow resistance: for upstream air velocities of 0.42–0.85 m s−1 a screen with a typical 60° fold allowed, on average, about 23–32% higher airflow than a flat screen with similar through-screen pressure drops. The simulation results allowed a detailed examination of the through-screen and downstream flow patterns, and showed that the downstream velocity profile close to a concertina screen was different to that of a near flat screen. Velocities downstream from the concave parts of the concertina screen were lower than those downstream from the convex parts, resulting in a form of ‘sinusoidal’ velocity distribution rather than the uniform velocity distribution downstream from a flat screen. The CFD results further suggest that for a given pressure drop the airflow may increase with decreasing fold angle; this depends on the pre-folding screen resistance to airflow. [ABSTRACT FROM AUTHOR]

Flow through woven screens is of continuing interest in many fields, including protected horticulture, in which such screens are used mainly to protect crops against insects. Screens may significantly affect crop microclimate, therefore computational fluid dynamics (CFD) simulations have been used by researchers in recent years to predict their effects on the distribution of microclimate parameters within the structures where the crop is grown. In such simulations screens are usually represented by porous media, through which the pressure drop is estimated by means of the Forchheimer equation. The applicability of the Forchheimer equation to estimate pressure drops through woven screens was investigated. Firstly, the results of CFD simulations of flow through realistic models of woven screens were validated by means of wind tunnel tests and by means of a published correlation for calculating pressure drops through such screens. The CFD tool was then used in conjunction with the wind tunnel tests and published correlation results to examine whether the coefficients in the Forchheimer equation are constant over a wide range of Reynolds numbers, and to explore the flow through the screens. It was found that the values of the Forchheimer coefficients varied with Reynolds number, and that the changes in their values can be assumed to be relatively small and not very significant but only up to a Reynolds number, Rei  ≈ 130–160. Moreover, presenting the Forchheimer coefficients as functions of only porosity may lead to erroneous predictions of pressure drops. [ABSTRACT FROM AUTHOR]

In computational fluid dynamics (CFD) analyses, insect-proof screens are usually simulated as porous media. In applying the porous medium approach, the Forchheimer equation with published correlations for the permeability K and inertial factor Y are usually used. In the present study, a comparison between simulations of flow through a realistic woven screen and a porous slab that represented the realistic woven screen indicated that using the published relations for K and Y may lead to erroneous results for pressure drops through screens. An alternative way, basically similar to that used in previous studies, for calculating K and Y is presented; it led to good estimates of pressure drops through various porous media slabs that were used to simulate realistic screen geometries. [Copyright &y& Elsevier]