Abstract: In grass conservation systems, the field drying process of cut grass is an important function since it determines subsequent losses and possible hazardous effects of during silage. The drying process of harvested grass was evaluated using two different numerical approaches. Firstly, an existing experimentally-verified analytical model was used. Several parameters were improved from previous studies such as the evaluation of stomata conductance from outside climate variables. Secondly, a CFD modelling approach was applied to open field drying process of the biomass. The cut biomass in the field was simulated using the equivalent macro-porous medium approach. Experimental values were used to obtain realistic and accurate boundary conditions. The developed CFD model was validated using the existing analytical model that is based on evaporations as estimated from the Penman equation. An acceptable agreement between simulated and measured values of water content was obtained. The mean difference in the estimated outputs from the two models was 8%. Condensation occurred during the night and was correctly simulated by both types of models. In general, a good correspondence was found between the two approaches. The use of the CFD model reveals the climate heterogeneity in the grass area and also, creates the possibility of applying the model as a decision support model for an enhanced treatment of the grass after cutting.
Abstract: A methodology approach to simulate, by means of computational fluid dynamics (CFD) tools, a greenhouse equipped with a fan and pad evaporative cooling system is presented. Using the main aspects of evaporative cooling systems, in terms of heat and mass transfer, the flow and boundary conditions of the simulation model are identified taking into account both the external and internal climatic conditions. The crop (tomato) was simulated using the equivalent porous medium approach by the addition of a momentum source term. The temperature and humidity of incoming air and the operational characteristics of fans were specified to set up the CFD model. Numerical analysis was based on the Reynolds-averaged Navier-Stokes equations in conjunction with the realizable k-∈ turbulence model. The finite-volume method (FVM) was used to solve the governing equations. The 3D full scale model was solved in several differencing schemes of various orders in order to examine its accuracy. This simulation approach was used to identify the critical parameters of microclimate of a greenhouse and the regions where these have to be measured during the experimental processes. The simulation model was validated against experimental data based on the air temperature inside the greenhouse at twenty three points for three ventilation rates. These results showed good qualitative agreement in which the influence of the different airflow rates on greenhouse microclimate, indicated that the proper choice of ventilation rate is a crucial factor in order to improve the efficiency of evaporative cooling systems in greenhouses.
Abstract: Full-scale experimental data and computational fluid dynamics (CFD) methods are used to determine the accuracy of four different turbulence models [standard k-ε, k-ε renormalisation group (RNG), k-ε realisable, Reynolds stress model (RSM)], which are used to describe the turbulent part of air in problems concerning the natural ventilation of buildings. Ventilation rates were measured in a livestock building using the decay tracer gas (CO2) technique. Airflow and temperature patterns were mapped out in a greenhouse with a tomato crop using a three-dimensional sonic anemometer and a fast-response temperature sensor. A commercially available CFD code was used to evaluate the different turbulence models. Average values from experiments were used for boundary conditions. The numerical results are compared with the experimental data, and they showed a good agreement, especially when the k-ε RNG turbulence model was used. The computations of the flow field using the different turbulence models showed noticeable differences for computed ventilation rate, air velocity and air temperature confirming the importance of the choice of the closure model for turbulence modelling.
Abstract: An experimental greenhouse equipped with fan and pad evaporative cooling is analysed using two different models. The first one consists of a numerical simulation approach applying a commercial CFD code. The main aspects of evaporative cooling systems, in terms of heat and mass transfer and both the external and internal climatic conditions were integrated to set up the numerical model. The crop (tomato) was simulated using the equivalent porous medium approach by the addition of a momentum and energy source term. The temperature and humidity of incoming air, the operational characteristics of exhaust fans and the pressure drop occuring in the pad, were specified to set up the CFD model. The second model considers the greenhouse as a heat exchanger. Based on greenhouse structural characteristics, external climatic conditions, pad efficiency and ventilation rate, the air temperature distribution is predicted. The results, concerning the air temperature, provided both by numerical and analytical model, were validated by experimental measurements obtained at a height level of 1.2 m above the ground in the middle of the crop canopy. The correlation coefficient (R2) between computational results and experimental data was at the order of 0.96 for the numerical model and 0.77 for the analytical one, with average percentage error of 3.5% and 7.6%, respectively. The analytical model proved to be a useful simple evaluation tool, but the numerical one provides a more accurate overview of the air flow in the greenhouse showing that fan and pad evaporative cooling system could be effectively parameterized in numerical terms, in order to improve system’s efficiency.
Abstract: Concerning the greenhouse environment, the ultimate goal of an investigation would be to determine the climatic parameters for all locations in the study area. Objective of the present study is to analyse the distribution of air temperature and air velocity of an experimental greenhouse with tomato crop, equipped with fan and pad evaporative cooling system, using geostatistical methods. The main aspects of geostatistics in terms of theoretical background for understanding spatial correlation models and kriging applications are presented. The most common variogram models were fitted to the experimental data sets obtained during summer period from an experimental greenhouse equipped with fan and pad evaporative cooling system. The Kriging approach was applied using the semivariograms corresponded to these data sets. Finally, the prediction maps of air temperature and air velocity were produced in different height levels inside the tomato crop canopy showing a great variability. Geostatistic analysis may be applied to determine not just optimal spatial predictions but also probabilities associated with risk-based analysis in order to improve the suitability and efficiency of climatic controls systems in greenhouses.
Abstract: In the present study a commercial CFD code was used in order to investigate the dispersion of a pesticide inside an arch type tunnel greenhouse with continuous side vents. The greenhouse was cultivated with a tomato crop planted in double rows, which, during the period of measurements had a height of 1.5 m. In parallel, measurements were carried out in order to experimentally determine the decay rate of pesticide concentration. Air samples were continuously taken at seven points inside the greenhouse (six air pumps and one gas analyser). In the 3D numerical model calculations were done for several wind directions and wind speeds, using the experimental values as boundary conditions. The final solution for every case of wind direction and wind velocity was obtained, firstly by a converge solution under steady – state condition; and secondly by an unsteady one, where at the time which equals to zero the air volume in the experimental greenhouse was considered to contain a mixture consisting of air and the used pesticide. The simulation results that were in the same order of magnitude with the experimental values only during the first 5-6 minutes of unsteady solution and only qualitatively good agreement for the rest time, while the decay process was simulated. Even if different wind characteristics, as boundary conditions, were introduced in the CFD model every minute, the agreement remained in a qualitative level, showing that the concentration of the pesticide inside the greenhouse is a function not only of the greenhouse ventilation rate but also of the pesticide evaporation-volatilisation rate from the greenhouse crop, soil and cover surfaces. As the simulation model almost always underestimates the concentration levels inside the greenhouse, more investigation has to be carried out in order to obtain better agreement between measured and estimated values of pesticide concentration inside the greenhouse.
Abstract: An experimental greenhouse equipped with fan and pad evaporative cooling is simulated numerically using a commercial CFD code. The main aspects of evaporative cooling systems, in terms of heat and mass transfer and both the external and internal climatic conditions were integrated to set up the numerical model. The crop (tomato) was simulated using the equivalent porous medium approach by the addition of a momentum and energy source term. Preliminary calculations were carried out and validated by experimental measurements, in order the pressure drop occurred in crop model due to air flow, to be determined as a function of leaf area, stage of crop growth and cultivation technique. The temperature and humidity of incoming air and the operational characteristics of exhaust fans were specified to set up the CFD model. The numerical analysis was based on the Reynolds-averaged Navier-Stokes equations in conjunction with the RNG k-epsilon turbulence model. The finite-volume method (FVM) was used to solve the governing equations. The 3D full scale model was solved in several differencing schemes of various orders in order to examine its accuracy. The simulation results were validated with experimental measurements obtained at a height level of 1.2 m above the ground in the middle of the crop canopy at 23 and 8 points, concerning air temperature and air humidity respectively. The correlation coefficient between computational results and experimental data was at the order of 0.7419 for air temperature and 0.8082 for air relative humidity. The results showing that the evaporative cooling system for greenhouses could be effectively parameterized in numerical terms, providing a useful tool in order to improve system’s efficiency.
Abstract: In this study numerical and experimental results concerning the dispersion of a tracer gas (N2O) inside an experimental greenhouse with a tomato crop were presented and analyzed. The proposed simulation concerns a greenhouse block located in the University of Thessaly near Volos, the continental area of eastern Greece. The tracer gas was injected into the greenhouse, and after its homogenization, vents were opened and simultaneously wind velocity, wind direction and gas concentration were recorded. Air samples were continuously taken at six points in the greenhouse and in two positions outside the greenhouse using a multiple inlet infrared gas analyser. In the numerical model, calculations were made for several combinations of wind speed and direction. In first set, used to validate the numerical model, experimental values were used for the boundary conditions. The final solution for every case was obtained, firstly by a convergent solution under steady - state conditions and secondly by an unsteady one, where at time zero the volume of the experimental greenhouse was considered to contain a new mixture of 'air' and 'N2O'. Good overall agreement was found between the experimental and simulated data. The simulation results provided useful information about the emission of N 2O around the experimental greenhouse and are a first step towards the determination of the behaviour of pesticides around greenhouses.
Abstract: A methodology of application of a numerical technique in order to simulate the dispersion of pesticides from greenhouses is presented. The proposed simulation approach is based on preliminary calculations that concern the greenhouse-block located in the University of Thessaly near Volos, on the coastal area of Eastern Greece. A 3D simulation model was developed using the commercial CFD code Fluent with the realizable k-ε turbulence model in order to simulate the air flow inside and around the experimental greenhouse. The ventilation process, through which the pesticide pollutants are emitted from the experimental greenhouse's side openings, was analyzed by simulating the dispersion of a tracer gas (Nitrous Oxide, N2O) which was used for the first sequence of experiments. The simulation results provided useful information about the dispersion of N2O around the experimental greenhouse, showing that wind characteristics dominate both the ventilation rate and dispersion of the tracer gas. Through this approach the determination of the critical points, where the measurement instruments have to be located during the experimental process are identified according to wind velocity and direction. In addition, useful conclusions were obtained concerning the set up of simulation model, showing how to improve the accuracy of computational predictions.
Abstract: The distribution of the incident solar energy across different spectrum regions (i.e. UV, PAR, NIR) on the surfaces inside a greenhouse plays a major role in its energy balance and also affects basic plant physiological processes, as well as pathogen, weed and insect development. In this paper a global radiation transfer model is used, along with a solar energy model and various well-established computer graphics techniques, in order to implement an algorithm, which computes the greenhouse global solar transmissivity. The algorithm can compute the transmissivity for greenhouses of arbitrary geometric complexity, based on the greenhouse position and orientation, its CAD model, and the optical properties of its surfaces. Multiple reflections and transmissions, based on surface-to-surface visibilities are computed on a finite-element mesh of the greenhouse surfaces, in order to compute a physically correct and accurate irradiance distribution. A number of simulation tests are presented, which indicate that the proposed method provides physically sound results. As a case study, the daily average of a greenhouse’s (clear sky) global transmissivity was computed on winter solstice, for E-W and N-S orientations. In accordance to published experimental results, the spatially-average transmissivity was found to be higher for the E-W orientation, accompanied though by a higher standard deviation (non-uniformity).
