Application of FLUENT on fine-scale simulation of wind field over complex terrain

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1 Onlne system, Scences n Cold and Ard Regons 2010, 2(5): DOI: /SP.J Applcaton of FLUENT on fne-scale smulaton of wnd feld over complex terran Le L 1, LJe Zhang 1*, Nng Zhang 2, Fe Hu 3, Yn Jang 1, WeMe Jang 2 1. Shenzhen Natonal Clmate Observatory, Shenzhen Meteorologcal Bureau, Shenzhen, , Chna 2. School of Atmospherc Scences, Nanjng Unversty, Nanjng, Jangsu , Chna 3. The State Key Laboratory of Atmospherc Boundary Layer Physcs and Atmospherc Chemstry, Insttute of Atmospherc Physcs, Chnese Academy of Scences, Bejng, , Chna *Correspondence to: Dr. LJe Zhang, Assocate Research Professor of Shenzhen Natonal Clmate Observatory, Shenzhen Meteorologcal Bureau, Shenzhen, , Chna. Tel: ; Emal: ljell@163.com Receved: 25 Aprl 2010 Accepted: 28 July 2010 ABSTRACT The state-of-art Computatonal Flud Dynamcs (CFD) codes FLUENT s appled n a fne-scale smulaton of the wnd feld over a complex terran. Several numercal tests are performed to valdate the capablty of FLUENT on descrbng the wnd feld detals over a complex terran. The results of the numercal tests show that FLUENT can smulate the wnd feld over extremely complex terran, whch cannot be smulated by mesoscale models. The reason why FLUENT can cope wth extremely complex terran, whch can not be coped wth by mesoscale models, reles on some partcular technques adopted by FLUENT, such as computer-aded desgn (CAD) technque, unstructured grd technque and fnte volume method. Compared wth mesoscale models, FLUENT can descrbe terran n much more accurate detals and can provde wnd smulaton results wth hgher resoluton and more accuracy. Keywords: FLUENT; Computatonal Flud Dynamcs (CFD); complex terran; wnd feld; fne-scale smulaton 1. Introducton Understandng the characterstcs of surface wnd feld s of partcular sgnfcance for many practcal and academc jobs, such as wnd energy, envronmental mpact, and meteorologcal dsaster rsk assessments. However, over complex terran, the dstrbuton of a wnd feld near the ground s generally heterogeneous, and the observed data can only represent the wnd character n a very lmted area. Thus, the numercal smulaton s the most mportant way to descrbe the detaled wnd speed dstrbuton over a complex terran. Actually, the numercal smulaton of the wnd feld over a complex terran has always been an mportant ssue n the feld of atmospherc scence, and numerous efforts have been appled to ths ssue, and the studes are related to both engneerng applcatons and academc research (Lu and Zhu, 1992; Yamada, 1992; Yan et al., 1997; Yu et al., 1997; Sh and Jang, 2000; Gao and Zhang, 2001; Jang and Peng, 2002; Ma et al., 2006; Wang and Jang, 2006; Zheng et al., 2007; Lu et al., 2008). Tradtonally, studes on wnd feld smulatons appled the mesoscale model as the prmary research tool, n whch terran followng coordnate was used to cope wth the complex terran, and the fnte dfference scheme was used n numercal calculatons. For most mesoscale models, horzontal resoluton should be larger than 100 m, and dfferent levels of smoothness for the terran data should be performed n order to obtan a stable ntegraton (Dudha et al., 2005; Walko and Tremback, 2006). In fact, very few mesoscale models were able to obtan a stable smulaton result over a steep terran.

2 412 Le L et al., 2010 / Scences n Cold and Ard Regons, 2(5): In recent years, the Computatonal Flud Dynamcs (CFD) model has been wdely used n meteorologcal studes, especally n studes on mcro-scale flow and dsperson n urban areas (Meroney et al., 1999; Chang and Meroney, 2001; L et al., 2004; Jang et al., 2006). The powerful capablty of CFD on descrbng the doman boundares wth complex geometrcal shape ganed attenton of meteorologsts, and correspondngly, CFD was expermentally appled n the wnd feld smulaton over a complex terran. For example, Cheng and Hu (2006) tred to generate a computng model grd for CFD smulaton of the wnd feld over a complex terran n lght of realstc elevaton data. Compared wth mesoscale models, CFD can provde smulaton results wth a hgher spatal resoluton (horzontal grd spacng up to 10 m) and can descrbe realstc terran n more detal. However, n Chna, there are currently very lmted studes n ths area whch prevents further applcaton of CFD on related jobs. In ths study, n order to foster an understandng of the capablty of CFD n descrbng fne-scale wnd feld over a complex terran, the commercal CFD software FLUENT was expermentally appled n the smulaton of the wnd feld over a realstc complex terran. 2. Smulaton tools and methods FLUENT s a powerful and flexble general-purpose computatonal flud dynamcs software, whch can be used for engneerng smulatons at all levels of complexty. It offers a comprehensve range of physcal models that can be appled to a broad range of ndustres and applcatons. FLUENT s based on two prncple numercal methods Fnte Element Method (FEM) and Fnte Volume Method (FVM). Both methods nvolve subdvdng the flow doman nto a large number of fnte elements/control volumes and then solvng the governng equatons of flud flow. FLUENT employs Gambt to generate the grd or mesh for the CFD solver and s able to deal wth complex geometres wth unstructured grd generatng technques. A user-defned functon (UDF) module for addng physcal models and accessng external data s ntegrated n FLUENT. The characterstcs mentoned above make t possble for FLUENT to smulate a wnd feld over a complex terran. In order to evaluate the capablty of FLUENT on wnd feld smulatons over a complex terran, a seres of numercal experments were performed, and the observed data collected at the Foyedng weather staton was compared wth the smulaton results. Subsequently, the capablty of the wnd feld smulaton was evaluated wth a numercal experment over a steep valley n south-west Chna. 3. Numercal experments of Foyedng Mountanous area 3.1. Schemes of numercal experments The Foyedng weather staton ( N, E) s located on top of Foyedng Mountan, n the northwest mountanous area of Bejng, wth an alttude over 1,200 m. The smulaton method used n ths study employed a combnaton of the mesoscale model RAMS and FLUENT, and the detaled steps nvolved are presented n Fgure 1: (1) As a frst step, based on a terran elevaton dataset wth a resoluton of 100m 100m, the CAD model of the terran wth a scale of 8km 8km s constructed va the preprocessor of FLUENT, GAMBIT, whose center s located at the Foyedng weather staton; (2) Based on the CAD model, the FLUENT soluton doman and unstructured mesh are generated; (3) Usng reanalyss data from the Natonal Centers for Envronmental Predcton (NCEP) as background felds, a RAMS smulaton s performed, whose doman encompasses the whole smulated doman of FLUENT; (4) The wnd speed, ar temperature and ground temperature data from RAMS smulaton at predetermned tme steps are extracted from the RAMS smulaton fles and fed nto FLUENT as boundary condtons wth Boundary Profle (BP) module of FLUENT; (5) Fnally, FLUENT smulatons are performed to provde results whch are compared to observed data. The method mentoned above was orgnally used n a study on a cty-block scale smulaton of meteorologcal envronment n an urban area. Ths shows that a modelng system that combnes both mesoscale and CFD models s a reasonable method, for t can provde realstc boundary condtons nstead of completely deal profles (L et al., 2007). The grd confguraton and the data delvery processes of the smulaton n Foyedng area s llustrated n Fgure 2. Takng NCEP re-analyss data as background felds, RAMS smulaton s performed wth four levels of nested grds. The doman of the grd at the 2nd, 3rd and 4th 1evel s shown n Fgure 2a, whose center s located at Foyedng. The grd ponts are n all grds, wth horzontal grd resolutons of 25,000 m, 5,000 m, 1,000 m and 200 m, respectvely. The vertcal grd spacng ranges from 30 to 1,000 m, where a stretch rato of 1:1.15 s employed to adjust the grd spacng. The range and the topography of the nnermost smulaton doman (herenafter referred to as D4) are shown n Fgure 2b, n whch the square wth a thck black lne llustrates the smulaton doman of FLUENT. The smulated meteorologcal data n the vertcal cross sectons along the thck black lne wthn D4 s extracted from RAMS smulaton fles and fed nto FLUENT calculaton model as the lateral boundary condtons. Meanwhle, the smulated surface temperature wthn the square of D4 s fed nto FLUENT calculaton model as bottom boundary condtons. It s worth notng that the ncomng surface temperatures for the bottom boundares can partally compensate for the defects of FLUENT whch s unable to process the surface radaton balance n the smulaton. Fgure 2c presents the terran model of the FLUENT smulaton doman, wth a top heght of 3,500 m. The unstructured grd of the FLUENT smulaton doman s shown n Fgure 2d, wth a horzontal resoluton of 30 m.

3 Le L et al., 2010 / Scences n Cold and Ard Regons, 2(5): Fgure 1 The flowchart of the RAMS/FLUENT smulaton Fgure 2 The settng of the numercal experment (a): The 2nd, 3rd and 4th grd of the RAMS smulaton doman; (b): The terran of the 4th RAMS grd (10,400m 10,400m), the square ndcates the smulaton doman of FLUENT; (c): The terran model of the FLUENT smulaton doman (8,000m 8,000m); (d): The unstructured grd used n FLUENT smulaton

4 414 Le L et al., 2010 / Scences n Cold and Ard Regons, 2(5): Control equatons of FLUENT are shown as below: u t + u j j u 1 p = + ρ 2 μ u ρ ( u' u' ) + f (1) u j = 0 T T + u t 2 T = k 2 where, u s tme averaged velocty, u' velocty perturbaton, ρ densty, f buoyancy nduced by thermal effect, and T tme averaged temperature of gas n thermodynamc equaton. Turbulence closure model adopted n the smulaton s realzable k-ε model, whch proved to be the best turbulence model ntegrated n FLUENT by wnd tunnel experments (L et al., 2006). Lateral boundary condtons are set as fxed velocty boundary condtons, and wnd speed and temperature data were drectly nputted from the D4 of RAMS smulaton. Fxed boundary condton s used on the bottom boundary, whch used the smulated surface temperature wthn D4. The Boussnesq assumpton s appled to descrbe the mpact of surface heatng on the wnd feld near the surface. The Corols force s neglected because the horzontal scale of the FLUENT smulaton doman s less than 10 km. j (2) (3) The RAMS smulaton s performed for the perod of 0800LST 29 August 2005 to 1400LST 30 August Takng the frst 12 hours as spn-up perod and startng at 2000LST on 29 August 2005, the RAMS smulaton results are extracted from output fles every 6 hours and delvered nto FLUENT for fne-scale smulaton va the boundary profle (BP) module Smulaton results Fgure 3 llustrates the smulated wnd speed and wnd vector at a heght of 10 m above ground at 20:00 LST 29 Aug It should be noted that the wnd feld shown n Fgure 3 s nterpolated from the unstructured grd of FLUENT smulaton doman nto a regular grd. From Fgure 3, t can be seen that the smulated wnd feld near the ground wthn the doman s hghly heterogeneous. The smulated wnd speed over Foyedng s generally small, except n the wndward sde of the mountans and the valley, both located n the northern part of the doman (Fgure 3a). The nfluence of the terran on the wnd feld near the ground s very obvous, and wnd drecton changes dramatcally over the mountanous area (Fgure 3b). In the southwest secton of the doman, there s a northward flow that s blocked by Foyedng Mountan, whch then turns eastward along the southern sde of the mountan. A flow comng from the north jons the eastward flow, whch leads to the formaton of a convergence zone n the area south of Foyedng. The wnd feld near the ground over the whole northern regon s dsorganzed, and varous vortex-type and dvergence-type flow felds are formed n ths area. As can be seen n Fgure 3, the smulated wnd feld from FLUENT s qute complcated, but t does reasonably descrbe the complex terran n the smulaton doman. Fgure 3 The smulated wnd feld n the surface layer over Foyedng at 20:00 on Aug. 29, 2005 (a): Wnd speed (m/s); (b): Wnd vectors

5 Le L et al., 2010 / Scences n Cold and Ard Regons, 2(5): Fgure 4 dsplays the modelng wnd vector at the slce of X = 4,000 m n the Foyedng doman, whch llustrates the capablty of FLUENT on descrbng a three-dmensonal wnd feld. It can be seen that the wnd speed s relatvely hgher n the area about 1,000 m above the ground and the wnd drecton s unform. However, near the ground, the flow feld s much more complex, and two obvous vortces exst around the top of Foyedng Mountan. Fgure 4 The smulated wnd vector at the slce of X = 4,000 m n the Foyedng doman (tme: :00, wnd speed: m/s) 3.3. Comparson wth observed data In order to quanttatvely evaluate the performance of FLUENT on fne-scale smulaton of a wnd feld, a comparson between the smulaton wnd data nterpolated from RAMS and FLUENT smulaton results and the observed wnd data from the Foyedng weather staton s presented n Table 1. From Table 1, t can be seen that both FLUENT and RAMS smulaton results show a large bas when compared wth the observed values. The man reason for ths bas s that RAMS smulaton s drven by NCEP reanalyss data wth relatvely coarse resoluton, and no data assmlaton technque was adopted to mprove the accuracy of the smulaton, whch s beyond the scope of ths study. Due to the above two reasons, the accuracy of RAMS smulaton results cannot be accordant wth the observed data. Snce FLUENT smulaton s drven by RAMS output data, the accuracy of FLUENT s dependent upon factors that affect the accuracy of RAMS smulaton. However, the relatve error between FLUENT smulatons and observatons s 45.5%, whch s less than that of RAMS (55.3%). Also, the results (wnd speed and drecton) at dfferent tmes for FLUENT smulaton are closer to observatons than those from RAMS smulaton except for 02:00 LST on 30 August, 2005 (Table 1). Ths ndcates that RAMS smulaton results can be "revsed" by FLUENT to some extent. Table 1 The comparson between the observed data and smulated data. Tme (LST) Elements Observaton RAMS FLUENT Aug.29, 2005 Wnd speed (m/s) :00 Wnd drecton ENE SW NNW Aug.30, 2005 Wnd speed (m/s) :00 Wnd drecton N N NNE Aug.30, 2005 Wnd speed (m/s) :00 Wnd drecton ENE NNE NE Aug.30, 2005 Wnd speed (m/s) :00 Wnd drecton ESE N S

6 416 Le L et al., 2010 / Scences n Cold and Ard Regons, 2(5): The smulaton results over Foyedng ndcate that FLUENT can reasonable smulate near-ground wnd felds over a complex terran, and the method of combnng FLUENT and RAMS as a modelng system can provde wnd smulaton results wth hgher resoluton and accuracy. However, FLUENT has ts own defects, such as not beng able to provde large-scale background felds. Thus, n further studes, assmlaton technques should be appled to observed data to mprove the accuracy of the whole system. 4. The fne-scale smulaton experments of the wnd feld over steep terran 4.1. Confguraton of experments The numercal experments ntroduced n secton 3 show that FLUENT can descrbe the heterogeneous detals of the wnd felds over a complex terran wth hgh spatal resoluton. Another noteworthy aspect of FLUENT s that t can provde wnd feld smulaton results n extremely steep terran due to ts capablty of copng wth the complex geometry, whch could not be realzed by most mesoscale models. To valdate the capablty of FLUENT on descrbng the wnd feld over extremely steep terran, a numercal test s performed n a valley area wth steep terran located n southwestern Chna. Ths valley has extremely steep mountans on both sdes, and the relatve heghts of the mountans are over 1,000 m and the maxmum slope angle can reach 70 degrees. Although prevous numercal tests have proven that mesoscale models, such as RAMS and RBLM, are able to cope wth the boundary layer processes over complex terran (Jang et al., 2007), they could not provde stable ntegraton n ths valley area. The FLUENT smulaton doman over the steep valley s shown n Fgure 5, whch s approxmately 8,000 meters by 5,400 meters n the horzontal drecton. It can be seen that the terran of computatonal doman s very steep. The numercal methods for most mesoscale models are based on a fnte dfference scheme, whch means that the model terran must be smoothed before numercal calculatons, thus ensurng computatonal stablty. A hybrd mesh system was used n the numercal experment. Hexahedral cells were used n most areas of the doman, whle trangular cells were used n areas where topography drastcally changed over a short dstance (Fgure 5b). The applcaton of hexahedral cells can reduce the total number of cells n the doman and consequently can reduce the tme spent on numercal ntegraton. Fgure 5 The smulaton doman over a steep valley (a): The terran; (b): The unstructured grd January was the chosen smulaton perod. The Ljang weather staton les n an area near the smulaton doman, and the observed data shows that the domnant wnd drecton s westerly n January. Thus, t s assumed that nflow wnd drecton s westerly n the smulaton. Meanwhle, t s assumed that the wnd profle s approxmated as a power-law, and the lapse rate of ar temperature s approxmated as the dry adabatc reducton rate. The confguratons mentoned above are mplemented nto FLUENT va the UDF module Smulated results and analyss Fgure 6 dsplays the smulated wnd at the heght of 20 m, from whch t can be seen that even f the topography s steep n the doman, FLUENT stll can reasonably smulate the wnd feld near the ground. The flow speed s relatvely low on the leeward sde of the mountan and wthn the canyon bottom, but s qute hgh whle crossng the rdge wth a hgh elevaton. Fgure 6 shows that the flow s sgnfcantly nfluenced by the steep terran. The ar flow runs across the valley along the rver n the bottom of the valley, and a stable vortex exst on the leeward sde of the mountans. In the smulaton, these partcular characterstcs of the wnd feld are reasonably descrbed by FLUENT. Although the lmtaton of the observatonal wnd data prevents further quanttatve assessment on the accuracy of the smulaton over the valley regon, the qualtatve analyss can stll verfy the capablty of FLUENT provdng stable and convergent numercal solutons over steep terran.

7 Le L et al., 2010 / Scences n Cold and Ard Regons, 2(5): The reason why FLUENT can provde a stable and reasonable smulaton of the wnd feld over steep terran les n the partcular technques adopted by FLUENT, whch are scarcely used n mesoscale models. These technques are summarzed as follows: (1) Wth CAD technque and grdded elevaton data, the realstc terran can be reproduced n the calculaton model of FLUENT. The calculaton model s establshed and dscretzed n the Cartesan coordnate system, whch ensures that the model can accurately descrbe the steep terran. (2) The unstructured mesh technque s used n FLUENT, whch ensures that smulaton doman wth arbtrary geometry can be dscretzed wth acceptable mesh system. (3) The fnte volume method ensures that FLUENT can provde stable numercal solutons n an unstructured grd system wthout smoothness of the model terran. (4) The UDF and the BP module of FLUENT make t convenent for users to setup boundary condtons through smple programmng. The two modules also make t possble for users to make a lnkage between FLUENT and mesoscale models or observed data. Actually, wth the UDF module and the BP module, FLUENT becomes an open and flexble calculaton platform based on CFD technque nstead of a sealed commercal software. Fgure 6 The smulated wnd feld over the steep terran (wnd speed: m/s) (a): The whole doman; (b): A part of the doman 5. Conclusons and dscussons In ths study, the CFD software FLUENT s expermentally appled n a fne-scale smulaton of the wnd feld over complex terran. The two numercal tests n ths study show that FLUENT has partcular advantages when appled to the wnd feld smulaton. Due to these technques, such as CAD technque, fnte volume method and unstructured mesh technque, FLUENT can present a stable and reasonable smulaton of the wnd feld over extremely complex terran, whch can not be dealt wth by most mesoscale models. Actually, the technques used n FLUENT, such as unstructured mesh and fnte volume method, can be ntroduced to mesoscale models, whch undoubtedly wll enhance the capablty of mesoscale models n copng wth complex terran. Ths study offers an mportant reference for academc research and practcal jobs on wnd energy, envronmental mpact, and meteorologcal dsaster rsk assessments. FLUENT and other smlar CFD software are expected to serve n the above-mentoned and related research and jobs. Fnally, t should be ponted out that though FLUENT smulaton results presented n ths paper have llustrated that FLUENT can clearly and reasonably descrbe the fne structure of the wnd feld over steep and complex terran, ts major mert, as a general-purpose CFD software, s to cope wth flud dynamcs and thermodynamc problems wth complex geometry ncluded n the smulaton doman. Unlke mesoscale models, FLUENT does not ntegrate models descrbng many physcal processes n a real atmosphere, such as surface energy balance processes and cloud mcrophyscs processes. Thus, t should be kept n mnd that the applcaton of FLUENT n the realstc atmosphere has ts own lmtatons and the valdaton of the model s necessary and mportant before actual smulatons are performed. Acknowledgments: The authors are very thankful to Bejng Regonal Clmate Center, Bejng Meteorologcal Bureau for provdng workng platform for ths study. Ths project s supported by the Natonal Natural Scence Foundaton of Chna ( , and ) and the "Mn-project on detaled survey and evaluaton of wnd energy resources" supported by Natonal Clmate Center of Chnese Meteoro-

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