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J.agric.Engng Res. (1998) 70,165-176Article Number. Ag970262Physical Modelling of Natural Ventilation Screens and Windows in GreenhouseWe use a combination of practice and theory research on greenhouse ventilation equipment, is to study greenhouse shutter and Windows service life of the device under low pressure and under different wind and pressure fluctuations. These fluctuations associated with the mean wind speed, average wind speeds are obtained by energy spectrum analysis. It follows that a pressure associated with the mean wind speed, and estimation of gas turbulence on mean wind speed has a big impact. Start the potential relationships between greenhouse ventilation equipment and is based on the groundbreaking research of fluid machinery. On air flow and air pressure and temperature correspond to change this prediction corresponds with data from experimental, by and large, the difference between them is less than 20%.NotationA area,m2 Z height, mCc coefficient accounting for convective effect Z0 surface roughness legth,mCkl Kolmogorov constant （0.5） Cwf friction coefficient Greek symbolCu turbulent kinetic energy constant window angle betwwen the flap of windowand the frame,degF(n) power spectral density ，m2/s confficient of thermal expansion g gravitational acceleration，m/s*s porosity h height ，m k von Karmans constant（0.4）H characteristic depth，m dynamic viscosity,Ns/ m2 L characteristic length ，m wind pressure coefficient n frequency ，HzP pressure of air ，PaP0 mean absolute pressure of air in enclosure ，PaPst stack pressure ，Pa SubscriptsPw wind pressure ，Pa fr flow field Q airflow ，m3/s I inside ru turbulent kinetic energy dissipation rate,m2/s l larger length T absolute temperaturw ，K rmsw root-mean-spure wind velocityu flaid velocity, s smaller length u* friction velocity，m/s st stack V volume w wind Y inertial factor1.Introduction Natural ventilation of greenhouse is an extremely complex process, all parameters of this process depends on the greenhouse (rated value, location and geometry of the window as well as leak areas, and so on) and the external environmental conditions. This phenomenon continued interest can cause natural ventilation in mass and energy imbalances and further serious effects of indoor environments. Since 1954, Morris and Neale1 first on natural ventilation in greenhouse ventilation equipment for experimental studies, it is increasingly taking into account the potential physical effects on ventilation equipment. A few decades later, Baiely2 and Cotton3, and Miguel, along with 4 other experiments demonstrated the plant and greenhouse roof with sunshade NET potential benefits. In recent times, in the fenestration equipment 5 curtain is in order to prevent the entry of insects, thereby reducing use of chemical pesticides. So, research on surface air curtain is also an important issue. Existing on the subject of analysis methods can be broadly classified into the following two ways: (1): the empiricism and semi-empirical methods of research studies on airflow through the 6.13 and curtain 2.5.14. (2): guided by the fluid mechanical formulas of digital solutions. 15.17 belongs to the first type of all models based on the theory that the movement of air driven air circulation between the potential of there is a simple linear relationship. This analysis finds that the greenhouses of indoor air is not compressed, plain pieces of this analysis or based on experience derived from the Bernoulli equation. Despite the simple assumption that in conjunction with specific experiments under the corrective coefficient, is also greatly limits the range of the study results. In the second model, using a digital simulation program to solve the equation of inertia, momentum and energy equations, decision speed, temperature and pressure, and eventually air circulation patterns within the greenhouse. Described in this study are two-fold:(1) Understanding of airflow through the shutter with holes (insulation, shading, pest control) and physical properties of skylights in the Exchange. More important deals with shutter and skylight with holes of factors affecting the service life of the parameter and the poor start.(2) Offer a simple actuarial calculation of the wind and the greenhouse effect formula frame shutter and skylight.2.Theory Unconstrained and constrained fluid movement through hole equipment and physical phenomena indicate that this phenomenon of skylights and skylights or equipment with holes and drive circular motion potential difference of characteristic parameters characteristic parameters. Formula accurately describe this phenomenon must take account of these factors. With the formula in Appendix a. As shown in the Appendix, a skylight or curtain with holes through any airflow and drive potential difference of about, is as follows: (/) Q/ t+ Mu Kp-1Q+ YA-1Kp-1/2| Q| Q+0.5(AHCc2)-1| Q| Q=-A ( pw/H+ pst/H) (1) and Q=A which, Q is flow, is gas density, a, is work area, is gas power viscosity, is material of hole product rate (units volume holds fluid of volume), Kp is material of penetration degrees (fluid through media body of capacity), Cc said convection effect coefficient, h is penetration parameter, PW is wind pressure poor, pst is and temperature related of pressure poor, y in Appendix a, in the has defines. In order to facilitate the calculation of air flow through multi-level sealed box or blank space movement, computer networking service must be established. Film sealing area of the greenhouse is a two-story (Figure 1), Figure 2 illustrates the flow theory of networked computing. Physical model for natural ventilation In only one window and roller shutter in greenhouses, network control of the circulating movement is made up of three points, the three contact consists of two resistors are connected together (Figure 2-a). When the roller shutter when there are gaps, network control is made up of three points, but the three points by three resistance threaded together.2.1 Skylights and roller shutter device with holes air flow characteristic parametersAirflow characteristics of permeable materials are divided into porosity and permeability. 18. porosity materials hold the volume of a gas and total material to accommodate gas volume ratio of size between 0 to 1 (0 1). A pore, porosity is 1 ( =1), because the pores are filled with gas. Any permeable material can make use of the gas, the weights and measures of the ability to call it penetration and are consistent with principles of gas motion penetration is not only related to the fluid viscosity and particle diffusion and obstructions when the collision frequency. 18.2 in terms of porous materials, gas molecules of the collision frequency is larger than 103Hz, because gas kinetic viscosity values for 10-5, penetration value that is less than 10-7. and for opening device (Windows, doors), the collision frequency is close to zero so the penetration Kp-. Consistent with the above, connect the two ends (the pores with the larger opening) number is the penetration of Kp. For gap materials, gases can be considered incompressible (1/Cc2 0). By the formula (1) available: (/) Q/ t+ Mu Kp-1Q+ YA-1Kp-1/2| Q| Q=-A ( pw/H+ pst/H) (2) opening ( =1) material penetration Kp-, the formula (1), second and third on the left can be neglected, exporting: Q/ t+0.5 (AHCc2) -1| Q| Q=-A ( pw/H+ pst/H) (3) of the formula (2) is the famous Forchheimer equation, applied to porous materials. Weak gases can be rounded down second and concluded that Darcys law. When the air is stationary publicity (3) simplified Bernoulli equation.2.2 Driving PotentialDriving potential difference is because of airflow caused by temperature difference (static) or wind (causes the air pressure changes) or both roles at the same time. Any one cause can produce a gradient potential stability caused by poor circulation. Wind speed fluctuations will give rise to an additional cycle of movement. Through a greenhouse roof air flow fluctuations can be divided into vibration cycle and Rotary Wo penetration movements. Vibrating circular movement is due to the fluctuation of the wind and indoor air can be compressed. Rotary-Wo penetrating movement is due to the thermal air currents cause rotation of the vortex to seal indoor air impacts.2.2.1 Greenhouse effect on air movementWhen the porous films or blank when there is a temperature difference between inside and outside, there will be a static pressure, which can lead to air movement. Envisage different temperatures on both sides, according to the following equation, pressure on both sides is different: St= g h (4) and = t, expressed many times absolute measurement of temperature difference t and g the gravitational acceleration, h represents a vertical height difference, coefficient of thermal expansion.2.2.2 Effect of wind speed on the air movement (wind)Wind speed is measured does not quantitative. At t time averages and fluctuations of the instantaneous value can be man-made is an integral part of the sum of the values, its w and uw. Within a time interval of average wind speed and wind pressure on (Appendix b): Pw=0.5 ( 2W+uwuw) (5), w and the average wind speed, uw wind speed fluctuations in value. Some anemometer can be read directly out of the square root of the mean wind speed and average wind speed. Say uw have a Gaussian probability distribution, you can simple use the following formulas instead of formula (5): Pw=0.5 2W+ -1urmsw-1 (6), where urmsw is the square root of the mean wind speed. Ordinary cases the anemometer provided only average wind speed readings, therefore, can not only display the average wind speeds, and do not ignore the impact of fluctuations of the wind wind speed meter is very useful. So to average static dynamic wind speed average of the wind speed associated with. Kinetic energy consumption rate ( ) according to the law of Kolmogorov 21:r Mu 2/3=0.75ckl-1F (n) ( w/2 ) 2/3n5/3 (7), f (n) for the energy spectral density, n indicates the frequency, CKL-Kolmogorov constants ( 0.5). Again uses away from surface z distance of kinetic energy turbulence and kinetic energy consumption ratio 21 Zhijian of relationship 0.5 uwuw=c -0.5r k (z+Z0)2/3 (8) wind speed fluctuations of square root value and average of relationship following: uwuw=3 f (n) ( w/2 ) 2/3n5/3 c -0.5r k (z+Z0)2/3 (9) which, k is Mr Frederick FUNG. Kaman constants ( 0.4), c is constants ( 0.99), z for Measure the height of the wind speed, and Z0 is the surface roughness (22 to find value of Z0 in the reference literature). In order to obtain the gamma, make sure f (n) and n is necessary, and the need to implement an energy spectrum analysis method. 23 this energy spectrum analysis method is used to measure the frequency swing variable variances in the process of continuous change. This analysis not only to determine the gamma value, and you can get some extra information to clarify characterization and structure of turbulent fluctuations, and you can determine the frequency of wind field within the main vortex. Wind speed is usually at a reference height measurement. Should export one area coefficient, the coefficient and relative wind wind Domains domain reference high. This coefficient is determined by the atmospheric boundary layer wind speed on the vertical section of the wind decided to follow the wind direction positioning to close .3.Experinmental studyIn order to confirm the potential of fluctuations, while also testing the applicability of the model, we came up with the following experiment. Experiment is to the East in two (E-W) direction construction of greenhouses. Two greenhouse sizes the same (Figure 3): eaves height of 4.5 m, the roof slope angle 22. , 4.1 meters wide, 6.6 meters long. Inside each greenhouse from the ground 2.9 metres with a LS11 insulation panels (Miguel, and the other 18 verify that penetration 7*10-10m2, porosity of 0.99). In the first part of the experiment, each film in greenhouse is qualified. Second part of the experiment, shutter film opened 0.20 m *3.80 m a very small hole in the Middle, which looks like a horizontal crack.Each greenhouse is firmly seal on the wall with insulation Strip, the ground covered with polystyrene foam insulation, in addition to the Windows on the roof, between two Windows each equipped with *0.90 m active strips the size of 2.05 metres. Windows tilt can rise by 30 degrees. Equipped with an aluminum surface of horizontal cylinder-shaped electric heater (two 8-meter, 0.05 m diameter cylinders, arranged in pairs, each of two interval 0.35 meters, each interval of 1.15 m), as shown in Figure 4. In the experiment, heat insulation below the temperature will be different, heaters were used to suppress such temperature changing.Installed in each greenhouse 25 brass nickel-copper alloy thermocouple to measure heat shield and outdoor temperature up or down. They are evenly distributed: 10 thermocouple is distributed below the insulation panels, distribution of 10 at the top, there are 5 exposure outdoors. In order to ensure a fast response 8 (response frequency is approximately 12Hz), thermocouple is made of very fine wires (diameter of about 2.5*10-5 m). Pressure transducers to measure the pressure with a film layer, indoor measurement of upper and lower (each measured three times). Instantaneous measurement of wind pressures on outdoor locations on the skylight 0.20 metres. Measurement of wind speed anemometer with a fast response (response rate of 9.5Hz), skylight 0.20 metres it is placed at a distance, but also measurable. Flow determination using an automated scan of gas equipment. In the experiment, using a constant flow rate and decay rate of 9.25. through the slit air partition amount minus the total fluctuation through the divisions of the fluctuations. In each greenhouse, on the ground with two small fans and barrels full of holes to scan gases (N2O) separated. In the experiment, each within the greenhouse air samples have to be tested in 18 different locations (9 in partition top, 9 screens below), and under the infrared analysis. Two sample test conducted in 32 days between February 1996 and March: (1) flow through the greenhouse roof and curtain is produced by the wind. Trials should be indoor and outdoor temperature difference less than 2 0.5oC, under the wind speed is higher than 1.5 meters per second. (2) flow through the porous rectangular slits on the shutter and the shutter (0.02 m *3.80 m) simply because the shutter of the upper and lower temperature (under steady conditions). In order to reduce the effects of wind pressure, determination to close all the Windows (the Lee side of window opening 2O). First part of the experiment (air is the role of the wind), wind speed and temperature difference between indoor and outdoor gathering in the presence of data 8HZ in the frequency within 10 minutes. Second part of the experiment (air is slowly gathering), shutter upper and lower temperature stability ( t 1 oC) data collection frequency is under 1.66*10-2HZ (per 60 seconds), acquired under stable conditions.4.Results and discussion4.1 wind velocity and wind pressureAcquisition of spectrum analysis is the wind speed in 10 minutes frequencies are under 8HZ conditions. Sampling frequency is available through the determination of the highest frequency, that is half the sampling frequency (the Nyquist frequency) in this experiment, the 4HZ. Due to the characteristics of wind in the Leeward and Windward will be different, spectral analysis to 9 respectively in two places. Findings of the window in the closed greenhouse painted figure, as shown in figures 5 and 6.Figure 5. Three different wind speeds of power spectral density of wind speed: 1.27m/s (*) 3.49m/s (o), 5.50m/s (+) on the windward side measured from the roof of 0.20 metres (-five-thirds angle) as shown in Figure 5 for the three different wind speeds of power spectral density of wind speed: 1.27m/s,3.49m/s,5.50m/s, Windward 0.20 metres measured from roofs.Figure 6. Wind of 0.52m/s (*) 2.24m/s (O), 3.37m/s (+), on the Leeward side away from the roof of 0.20 metres measured power spectrum density (inclination- -5/3) As shown in Figure 6 to Lee the power spectral density of wind speed measured (wind of 0.52m/s,2.24m/s,3.37m/s, measured in the Lee side away from the roof of 0.20 metres). As depicted in Figure 5 and Figure 6, balanced distribution of energy spectrum, frequency/energy-five-thirds range, in line with the law of Kolmogorov. Similar access to windward and Leeward wind energy spectrum. Two major fluctuations in the energy spectrum at frequencies below 0.1,0.2HZ of the highest point, more frequently than 1HZ nothing special, low frequency domain dominated by the wind. In fact, the wind speed in the major energy vortex in the low frequency range. Proving the Kaimal Bot.8 and 26 research theory. Parameter can be Figure 5 figure 6 energy spectral density and frequency values calculated. Calculation of surface roughness: 0.04 m 22, results as shown in table 1.4.2 Through the curtain and window airTo flow just as caused by the temperature difference (that is, for stable temperature conditions Q/ t 0) as depicted in the figure 7 and Figure 8 (p w 0).Figure 7. Air through the shutter corresponding to the formula (2) and (4) budget rolling temperature difference (measurement data (*)Figure 8. Air through the rectangular slit shutter Centre corresponds to the formula (3) and (4) budget rolling temperature difference (measurement data (*) as shown in Figure 7 to air through the shutter corresponding to the formula in the experiment (2) and (4) shutter temperature difference of the budget. Figure 8 shows the experiment in the air through the rectangular slit shutter Centre corresponds to the formula (3) and (4) shutter the temperature difference of the budgets. Caused by airflow just because the wind speed ( 0), as shown in Figure 9-11. In the diagram, air through the skylight or curtain and porous partition by the formula (2) value of the skylight by the formula (3) corresponds to the value.Figure 9. Prediction of air through the Windward (*) and the Lee side (O) with the Windward relative pressure (window-4O)Figure 10. Prediction of air through the Windward (*) and the Lee side (O), as opposed to the Lee side of pressure (open the window-4O)Figure 11. Predict the relative pressure of air through the barricades and Windward (any one window open, window-20o) Windward (*), the Lee side (O)Figure 12. Forecast airflow through clapboard Center narrow sewing and upwind surface of relative pressure (any surface has a fan window open, opened window degrees for 20o) upwind surface (*), Leeward surface (O) Figure 9 by shows for experimental in the forecast airflow through upwind surface (*) and Leeward surface (O) Shi and upwind surface of relative pressure (opened window degrees for 4O) Figure 10 by shows for experimental in the forecast airflow through upwind surface (*) and Leeward surface (O) Shi and Leeward surface of relative pressure (opened window degrees for 4O) Figure 11 shows the experimental forecast relative pressure of air through the barricades and Windward. Greenhouse in the Windward or Leeward side of a window you want to open (window-20o). Figure 12 shows the slit and experimental Center for forecasting air through the clapboard Windward relative pressure. Greenhouse in the Windward or Leeward side of a window you want to open (window-20o). Figure 7 and Figure 11 depicts due to temperature, the wind field driving air through the holes in clapboard, which showed two different kinds of fluctuations: Darcy fluctuations (Figure 7) and Forchheimer fluctuations (Figure 11). These findings due to Bear,Bachmat19 and Bailey2. Reynolds data (Re= uKp1/2/) is less than the overall value, the flow and promote potential proportional to the greenhouse and normal ambient air temperature range (T25K). Reynolds when the data is larger than the overall value, must be added to air second (Forchheimer fluctuations), that is, when volatility is determined by the wind speed is higher than 0.25m/s. Experimental gas value compared with the forecast flow equivalent to the value of the model and the resulting error is always less than the 20%, beyond the figure 12. Diagram shows most of the dispersion value is attributable to air through cracks, the airflow is subtracted from the total amount of volatility fluctuations obtained through the clapboard. Because there is little difference, so error is very large.5. Conclusion1. For ventilation devices, energy plays an important role within the Vortex is less frequently than 0.10.2HZ caused by turbulent wind speeds.2. It is estimated that this turbulent winds 13% per cent of the mean wind speed, which means that occupy an important position in the total air pressure.3. Description the air through holes in clapboard (concentration of holes in clapboard middle). However, for around the greenhouse and air-flow surges that are caused by the temperature difference within the normal range, secondary air flow which may be ignored, So equation abbreviation into the law of Darcy (MU Kp-1Q=-A p/HP).4. Through the theoretical prediction of airflow through the baffle and vent with the experimental values were close, largely the difference between them is less than 20%.Reference1 Morris L G; Neal F E the infra-red carbon dioxide gas analyzer and its use in greenhouse research. National Institute of Agricultural Engineering Report，Silsoe，UnitedKingdom，19542 Bailey B J Glasshouse thermal screens: air flow through permeable materials. Departmental Note no. DN/G/859/04013. National Institute of Agricultural Engineering, Silsoe, United Kingdom, 19783 Bailey B J; Cotton R F Greenhouse thermal screens: influence of single and double screens on heat loss and crop environment. Departmental Note no. DN/G/982/04013. National Institute of Agricultural Engineering, Silsoe, United Kingdom, 19804 Miguel A F; Silva A M; Rosa R Solar irradiation inside a single span greenhouse with shading screens . Journal of Agricultural Engineering Research, 1994, 59, 61-725 Kosmas S R; Riskowski G L; Christianson L Force and static pressure resulting from airflow through screens. Transactions of ASAE, 1993, 36, 1467-14726 Okada M; Takakura T Guide and data for greenhouse air conditioning. 3: heat loss due to air infiltration of heated greenhouses. Journal of Agricultural Meteorology (Tokyo), 1973, 28,223-2307 Kozai T; Sass S A simulation natural ventilation for a multi-span greenhouse. Acta Horticulturae, 1978, 87, 39-498 Bot G P A Greenhouse climate: from physical processes to a dynamic model. PhD dissertation, Agricultural University of Wageningen, The Netherlands, 19839 De Jong T Natural ventilation of large multi-span greenhouses. PhD dissertation, Agricultural University of Wageningen, The Netherlands, 199010 Montero J I; Anton A; Biel C Natural ventilation in polyethylene greenhouses with and without shading screens. Proceedings of the International Seminar and British-Israel Workshop on Greenhouse Technology, Bet Dagon, Israel, 1990, 65-7111 Fernandez J E; Bailey B J Measurements and prediction of greenhouse ventilation rates .Agricultural and Forest Meteorology, 1992, 58, 229-24512 vans Ooster A Using natural ventilation theory and dynamic heat balance modeling for real time prediction of ventilation rates in naturally ventilated livestock houses. Ag. Eng. 94 Milano, Report N. 94-C-012, 1994 13 Bullard T; Menses J F; Merrier M; Papadakos G The mechanisms involved in the natural ventilation of greenhouses. Agricultural and Forest Meteorology, 1996, 79, 61-7714 Batemans L Assessment of criteria for energetic electiveness of greenhouse screens. PhD dissertation. University of Ghent, Belgium, 198915 Okushima L; Sass S; Nara M A support system for natural ventilation design of computational aerodynamics. Alta Horticultural, 1989, 248, 129-134 16 Outworker E N J; Voskamp J P; Alaskan Y Climate simulation and validation for an aviary system for laying hens. Ag. Eng. 94 Milano, Report N.94-C-063,199417 Mastitis A; Bot G P A; Pacino P; Scarascia-Mugnozza G Analysis of the decency of greenhouse ventilation using computational fluid dynamics. Agricultural and Forest Meteorology, 1997, 85, 217-22818 Miguel A F; Van de Barak N J; Silva A; Bot G P A Analysis of the airflow characteristics of greenhouse screening materials. Journal of Agriculture Engineering Research, 1997, 67, 105-11219 Bear J; Bathmat Y Theory and Applications of Transport Phenomena in Porous Media. Dordrecht: Kluwer Academic20 LeBron G; Clout A thermo dynamical modeling of fluid flows through porous media: application to natural convection. International Journal of Heat and Mass Transfer, 1986, 29, 381-39021 Tenneco H; Lumley J L A First Course in Turbulence. Cambridge, MA: MIT Press, 197222 Euro code ENV 1991-2-4. Basis of design and action on structures: wind action. NNT Delft, 199523 Turkey J W The sampling theory of power spectrum estimates. Symposium on Application of Autocorrelation Analysis to Physical Problems. Washington, Office Naval Research 1950, 47-6724 Clarke R M Observational studies in the atmospheric boundary layer. Quarterly Journal Royal Meteorology Society, 1970, 96, 91-11425 Sherman M H Tracer-gas techniques for measuring ventilation in a single zone. Building and Environment, 1990, 2526 Kemal J C; Wynyard J C; Izumi Y; Cote O R Spectral Characteristics of surface-layer turbulence. Quarterly Journal Royal Meteorology Society, 1972, 98, 563-58927 Miguel A F; Van de Barak N J; Silva A; Bot G P A Forced fluid motion through openings and pores. Building and Environment 1998(in press)28 Walker I S; Wilson D J Evaluating models for superposition of wind and stack elect in air infiltration. Building and Environment, 1993, 28, 201-21029 Peixoto J P; Fort A Physics of Climate. New York, USA: American Institute of Physics, 1992Appendix A: through the vents, pore velocity equations of motionFor is linear of airflow through a can penetration of material, its campaign equation can description as 18.27 (/)u/t+Kp-1u+YKp-1/2uu+(/2)uu/j（/）（2u/j2）=-pt0/j (A1) with Y=4.3610-2-2.12 total pressure 28 Pt0=Pw+Pst formula in the, u is variable speed, is density, Pw pressure is by wind or mechanical caus