Wind pressure coefficient determination for greenhouses built in a reclaimed land using CFD technique

Transcription

1 Ref: 1064 Wind pressure coefficient determination for greenhouses built in a reclaimed land using CFD technique Hyun-seob Hwang and In-bok Lee, Department of Rural Systems Engineering, Research Institute for Agriculture and Life Science, College of Agricultural and Life Sciences, Seoul National University, 599, Gwankno, Gwanku, Seoul, , Republic of Korea Abstract Wind load is one of the significant factor for design of greenhouse structure. Among related factors to wind load, wind pressure coefficient on target structure has been studied for successful strucural design of the greenhouse considering safe factor. Korean government announced the plan of constructing new greenhouse complex in reclaimed land. However, a reclaimed land has a different characteristics of surface roughness in comparison to those of rural area and these difference can cause the specific wind environment to the greenhouse facility. Therefore, the effects of different wind environment to the greenhouse structure in an aspect of structural safety should be investigated. CFD technique which has strong advantages of computing qualitative and quantitative detailed information and saving time and cost can be adopted to study the wind pressure coefficient of the greenhouse structure built in a reclaimed land. ESDU program was used for design of vertical wind profiles to consider the characteristics of reclaimed land and the desiged wind profile was applied to the CFD simulation and wind tunnel test as a boundary condition. To validate an accuracy of the CFD model, the wind pressure coefficients of 1-2W arch type greenhouse were measured in the wind tunnel test. Measured wind profile in the wind tunnel showed a good agreement to the desiged wind profile by ESDU and the error showed only 2.7% differences. Based on the validation results, grid size of computational domain was determined as 0.2m, horizontal distance of windward domain was designed as 4H;H was the greenhouse height and standard k-ω turbulence model showed good agreements even though the results of CFD simulation had slightly overestimated values to the results of wind tunnel test. The IOA value was evaluated as which imply that CFD simulation was reliably designed. Based on the validated CFD simulation model, characteristics of wind pressure coefficients according to the various type of greenhouse and wind environment were studied. Keywords: CFD, Greenhouse, Reclaimed land, Wind pressure coefficient, Wind tunnel 1. Introduction Consumption of vegetable and flowering plants has been steadily increasing in Korea, therefore Korean government announced the plan of constructing of new greenhouse complex at Saemanguem reclaimed area to cope with this growth of the consumption. In case of the reclaimed land, new guidelines for structural design of greenhouse has been demanded because there has totally different wind envrionemnt in comparison to the that of inland area. Therefore structural stability according to the wind environment on the greeenhouse structure considering gust frequency and return period of weather condition should be investigated. Wind load which is the one of the significant factor for greenhouse construction guideline, is defined as the force acting on the wall of the greenhouse when wind is blowing aroung the Proceedings International Conference of Agricultural Engineering, Zurich, /8

2 greenhouse. Since wind speed and turbulence are vertically stratified within an atmospheric boundary layer, the characteristics of the vertical wind profile is one of the key factors to investigate the wind load on the greenhouse. As a first step of this purpose, wind pressure coefficients of various greenhouse structure should be evaluated. Wind tunnel test has been widely used to investigate the wind pressure coefficients of the target structure. However, this kind of experimental research is very laborious, expensive and time consuming. Computational fluid dynamics (CFD) which is a powerful tool for analyzing physical phenomena can be an alternative solution for mentioned problem. However simulation study must be initially verified and validated an accuracy of the designed model. The objective of this study is to investigate the wind pressure coefficients of the greenhouse built in the reclaimed land using CFD technology. ESDU program was adopted to design vertical wind profile considering the characteristics of the reclaimed land as a boundary condition of the simulation model as well as the wind tunnel experiment and wind tunnel test was conducted to validate the accuracy of the designed simulation model. Then wind pressure coefficients of the greenhouse were analyzed and evaluated according to various wind condition and stuctural type based on the validated simulation model. 2. Materials and methods The study focused on design and validation of the CFD simulation model using wind tunnel test. To realize proper boundary layer considering the chracteristics of the wind environment in recalimed land, ESDU program was adopted. Using the designed wind profile from ESDU, wind tunnel test was conducted to validate the accuracy of the CFD simulation model. 2.1 Wind pressure coefficient The wind pressure coefficient(cc pp ) can be defined as ratio of mean static pressure (PP ) to mean dynamic pressure at height H (qq HH ) (Equation (1) and (2)). CC pp = PP (1) qq HH qq h = ρρ 2 aaaaaauu HH (2) 2 Where UU HH is the velocity magnitude at height H and ρρ aaaaaa is density of air. 2.2 ESDU (Engineering Sciences Data Units, UK) ESDU (01008, HIS Inc., UK) program was adopted to design the vertical wind profile considering surgace roughness of terrain of target area, hourly-mean wind speed, maximum gust speed and u, v and w components of turbulence. Computed vertical wind profile and turbulence intensity profile were applied to the wind tunnel test and CFD simulation model as boundary conditions. 2.3 Wind tunnel test Wind tunnel experiment was conducted to validate the accuracy of designed CFD simulation model at An-sung city in Korea (TE Solution, Co., Korea). To ensure the accuracy of the experiment results, dimensional differences between full-scale and small scale models, air flow characteristics, data collection and so on must be carefully managed (Lee et al., 2004). Computed vertical wind profile was applied to the wind tunnel test using the number of roughness blocks, barriers and spires. Experimental target greenhouse was 1-2W type 3 spans greenhouse, which is one of typical greenhouse in Korea. The value of 1/40 was applied as the law of similarity law for the scaled greenhouse model. Designed velocity magnitude was 5 ms-1 and five wind directions were tested to investigate the effect of wind direction to the wind pressure coefficients (Table 1). Proceedings International Conference of Agricultural Engineering, Zurich, /8

3 2.3 CFD simulation To properly construct the CFD simulation model, boundary layer was designed based on the computed results by ESDU through user-defined function in ANSYS FLUENT solver (ver 14., ANSYS Inc.). GAMBIT (ver 2.3., ANSYS Inc.) which is one of pre-processor tool was used to design the computational domain and meshes. In this study various size of meshes were tested to get the more accurate values and this can be evaluated using the results of wind tunnel test. Various horizontal length (0; no laminar zone, 4 and 8H; H is height of geenhouse) of laminar zone in the computational domain were especially tested to technically maintain the boundary layer from inlet to the windwared of the greenhouse. Four turbulence closure models (Realizable k-ε, RNG k-ε, Standard k-ε and Standard k-ω) were also evluated to find proper turbulence model. After evaluating the accuracy of the designed simulation model, the wind pressure coefficients were computed according to the various type of greenhouse such as 1-2W, wide-span and venlo and various wind environment conditions (wind direction 0, 22.5, 45, 67.5, 90 ). Table 2 shows the initial conditions of CFD simulation model. 2.4 Validation of CFD simulation model To evaluate the accuracy of the CFD simulation model in comparison to the results of wind tunnel test, concept of IOA (Index of agreement) was used. IOA is similar to the concept of R square and the values lie between 0 (no correlation) and 1 (perfectly fitted) (Equation (3)). IIIIII = 1 nn ii=1(pp ii OO ii ) 2 nn ii=1( pp ii PP + OO ii OO ) 2 Where O is observed value in the wind tunnel and P is Predicted value in the CFD simulation 3. Results and Discussions 3.1 Design of wind conditions by ESDU Theoretical wind profile and turbulence intensity profile was computed by ESDU considering the chracteristics of reclaimed land. Based on the computed profile, wind and turbulence profile was designed using roughness blocks, spires and barriers in the wind tunnel test. Measured these profiles were compared with computed theoretical results by ESDU (Figure 3). The average difference of both wind profile was 0.13m/s, maximum difference was 0.35m/s and the average error was 2.7%. In case of the turbulence intensity profile, average error was 0.44%, maximum error was 1.3% and minimum error was 0.05%. There results implied that designed wind envrionment in the wind tunnel reflected the chracteristics of the reclaimed land very well. 3.2 Results of wind tunnel test Figure 4 shows the measured wind pressure coefficients of the scaled 1-2W greenhouse model in the wind tunnel according to the various wind direction. The values of wind pressure coefficients were measured and recorded along the pre-selected and punched measurement holes of wind pressure (C1~C6). When the wind was blowing perpendicular direction to the side wall (wind direction: 90 ), maximum value was found at C5 measument line with the value of 0.48 and minimum value was found at C1 measurement line with the value of (3) 3.3 Validation of CFD simulation model Total six number of mesh size were considered to examine the effect of mesh size on computational results. Figure 5 elucidates the results of grid independent test at selected point. When the mesh size were 0.1 and 0.2m, there was no big difference however, if the size of Proceedings International Conference of Agricultural Engineering, Zurich, /8

4 mesh was bigger than 0.2m, the values showed the decreasing trends. In this study, 0.2m was chosen as the proper mesh size considering the accuracy of the computed values and economical number of meshes. Various horizontal length of the laminar zone were studied to technically maintain the wind and turbulence profile from inlet to the windward of the greenhouse. From the comparison of values of IOA, 4H of the horizontal distance of laminar zone was more proper with the value of (Table 3). In case of the turbulnece model, standard k- ω model showed more higher values of IOA in comparions to those of other turbulence model according to the various given wind conditions and the average IOA value was (Table 4). 3.4 Computation of wind pressure coefficient according to the type of greenhouse and various wind environment After validating the accuracy of CFD simulation, the wind pressure coefficients were computed according to the type of greenhouse (venlo, 1-2W, wide-span) and varios wind envrionment condition. For example, in case of venlo type greenhouse, when the wind was blowing perpendicular direction to the side wall of the target greenhouse, the maximum wind pressure coefficient was found at the pressure-receiving surface with the value of 0.7 and minimum value was found at the first span roof with the value of Computed results of 8-span venlo type greenhouse were compared with the published criteria in Netherlands (NEN3859) and there was no big differences(table 5,6). Wind pressure coeffients of 8 span 1-2w type greenhouse were also computed and the tendencies were simliar to those of venlo type greenhouse. Average value was found at perpendicular direction to the side wall with the maximum value of 0.65 and minimum value was found Conclusion ESDU program was used to compute the wind and turbulence profile to consider the chracteristics of the reclaimed land and these computed results were applied to the CFD simulatiuon model and wind tunnel test as the boundary conditions. Based on the wind tunnel test, the accuracy of the CFD simulation model was validated and the proper mesh size was chosen as the 0.2m, horizontal distance of laminar zone for technically setting the wind profile at the windward area was recommended 4H (H was height of the facility) and the application of the standard k-ω turbulence model showed more accurate computed results than the other turbulence models (average IOA value=0.745). This work is still progressing to compute the values of wind pressure coefficients according to the various type of greenhouse and wind conditions then, wind load will be also evaluated to consider safe factor at the reclaimed land. 5. ACKNOWLEDGEMENTS This study was carried out with the support of "Cooperative Research Program for Agricultural Science & Technology Development (Project No. PJ009492)", Rural Development Administration, Republic of Korea 6. REFERENCES Lee, I., Lee, S., Kim, G., Sung, J., Sung, S., Yoon, Y. (2005). PIV verification of greenhouse ventilation air flows to evaluate CFD accuracy. Transactions of the ASAE, 48(5); Lee, I., Kang, C., Kim, G., Heo, J., Sase, S. (2004). Development of vertical wind and turbulence profiles and wind tunnel boundary layers. Transactions of the ASAE, 47(5): Nederlands Normalidatie-instituut. (2004) NEN 3859 Proceedings International Conference of Agricultural Engineering, Zurich, /8

5 Figure 1 Wind tunnel, roughness block setting to design wind profile and experiment scenes of measuring wind pressure coefficients Figure 2 Schematic diagram of wind tunnel and measurement procedure of wind pressure coefficients Figure 3 Comparison of theoretical computed wind and turbulence profile by ESDU and measured values in the wind tunnel test Proceedings International Conference of Agricultural Engineering, Zurich, /8

6 Figure 4 Measured wind pressure coefficients of scaled 1-2W greenhouse in the wind tunnel according to the location of measurement line and wind directions Figure 5 Grid independence test of CFD simulation model Table 1 Experimental conditions of wind tunnel test Scaled wind speed Wind direction Measurement frequency 5 ms -1 0, 22.5, 45, 67.5 and Hz Table 2 Initial conditions of CFD simulation model Contents Values Density of air kgm3 Viscosity kgm-1s-1 Type of mesh Quad type Wind velocity 30 m/s Reference height of wind velocity 4.7 m Wind velocity profile Computed results from ESDU Turbulent intensity Computed results from ESDU Number of mesh 1,800,000 ~ 4,500,000 (depending on the mesh size) Wind direction 0, 22.5, 45, 67.5 and 90 Proceedings International Conference of Agricultural Engineering, Zurich, /8

7 Turbulence model Length of laminar zone Realizable k-ε model, RNG k-ε model, Standard k-ε model and Standard k-ω model 0 (no laminar zone),4 and 8H (H is the height of the greenhouse) Table 3 Computed IOA values according to the horizontal distance of laminar zone of computational domain Distance of laminar zone IOA value (dimensionless) 0 H H H Table 4 Results of computed IOA values according to the turbulence model and wind direction Wind direction Realizable k-ε RNG k-ε Standard k-ε Standard k-ω Table 5 Comparison of CFD computed results and NEN3859 at the 8-span Venlo type greenhouse WD(0) CFD NEN3859 End wall(f) End wall(r) Roof Side wall(in) Side wall(out) Roof CFD NEN3859 A B C D E F G H I J K L M N O P Proceedings International Conference of Agricultural Engineering, Zurich, /8

8 Table 6 Comparison of CFD computed results and NEN3859 at the 8-span Venlo type greenhouse WD(0) CFD NEN3859 End wall(f) End wall(r) Roof -0.3 Side wall(in) Side wall(out) Roof CFD NEN3859 A(N) B(N) C(N) D(N) E(N) F(N) G(N) H(N) I(N) J(N) K(N) L(N) M(N) N(N) O(N) P(N) Table 7 Comparison of CFD computed results at the 8-span 1-2W type greenhouse 0 90 End wall(f) End wall(r) Roof Side wall(in) Side wall(out) Roof 0 90 A B C D E F G H I J K L M N O P Proceedings International Conference of Agricultural Engineering, Zurich, /8