Compute fluid dynamics application fo establish the wind loading on the sufaces of tall buildings IOAN SORIN LEOVEANU (a), DANIEL TAUS (a), KAMILA KOTRASOVA (b),eva KORMANIKOVA (b) (a) Civil Engineeing Faculty, (b) Civil Engineeing Faculty, Institute of Stuctual Engineeing, Stuctual Mechanics Depatment. (a) Univesity Tansilvania of Basov, (b) Technical Univesity of Košice (a) St. Tunului N. 5. Basov. Romania, (b) KSM UIS SvF TU v Kosiciach leoveanui@yahoo.co.uk, danieltaus@yahoo.com, kamila.kotasova@tuke.sk,eva.komanikova@tuke.sk Abstact: - The scope of this pape consists in the Compute Fluid Dynamics apply to establish the wind effects on the sufaces of the tall buildings systems. The tall buildings ae in pesent moe and moe fequently in the moden cities. The topology of the buildings and the wind diections ae moe and moe impotant in the case of new tall buildings design. The new mateials used in stuctual engineeing with thei high mechanical popeties, the constuction machiney the technologies advances and the economical scope ae the pincipal easons in the design and execution of talle and moe slende stuctues. In pactice the quasi-static computing method used fo cuent stuctue is exceed by inheit stuctual flexibility. The wind foces systems that load the buildings sufaces ae in pactices extemely difficult and expensive. Even the use of wind tunnel is extemely difficult in pincipal by the poblems in the dumping values fo the aeo-elastic model assuming and Reynolds numbe scaling. The butt of this aticle consist in development of a CFD method vey useful as a complimentay tool fo easy establish the wind tunnel paametes wok conditions. In the pesent pape the CFD modeling is based on Volume Finite Method (VFM) with Total Validation Diminishing Method (TVD) fo modeling the flow aound complexes buildings sufaces and establishes the pessue vaiation aound them. The new CFD VFM algoithm is developed fo minimal eos and based on Riemann and Godunov solves. The esults of this method ae compaed with the quasi-static models and with the most used numeical method. Key wods: Volume finite method, CFD, suface pessue, TVD method, tall buildings, steel stuctue. 1. Intoduction The wind loading is consideed by Devenpot(1998) in thee categoies: a) Extaneously-induced loading based on natually tubulent oncoming wind. The weak of upsteam obstuctions enhance this categoies buffeting. b) Unstable flow phenomenon such as sepaations, eattachments and votex shedding geneate a seconday type of foces. c) The movement-induced excitation of the body geneate by the deflection of the stuctue ceate fluid flow too. This phenomenon with a stong unsteady states chaacte gives the complexity of the fluid flow aound the flexible tall stuctues. The moden design of flexible tall stuctues must equest to eath quakes events and wind loads, cases that epesent a state of the at of the civil engineeing. The Euocode and most used standads ae highlighted by Allsop (2009) as the most flexible and inclusive code fo nomal buildings. The quasistatic methods offeed by these codes ae only applicable fo buildings with stuctual popeties such that they ae not susceptible to dynamic excitation (Metha, 1998). Thus, the tall buildings, those with high slendeness atios and/o asymmetic planes, exceed limitations and ae advised to be tested in the wind tunnel. This patten induces inheent stuctual flexibility and heightens concens egading the aeo-elastic fluid-stuctue inteaction between the wind and the tall building. The codes of pactice have been fomulated with a view to poviding an acceptable balance between the ovely complex eality and ovesimplified appoach. ISBN: 978-1-61804-004-6 433
Scaled-model wind tunnel testing is an established tool among industy design pactices. Bounday laye wind tunnels ae capable of quantifying timedependent suface pessues, including the complex types of loadings (tosion and acosswind). The model can be used to detemine the best oientation of the poposed building, the case of Buj Dubai analysis (Iwin and Bake, 2006). The simulation, nevetheless, it has its own limitations that include the difficulty to maintain popotionality between the scaled tubulence chaacteistics and the scaled building model, especially if the topogaphy is significant (Taanath 1998). Futhemoe, it is impotant to ensue Reynolds numbe effects on the pessues ae kept to a minimum. It is noted by Sun et al. (2009) that a computational appoach has the capability of being moe flexible than taditional wind tunnel expeiments. Fo example, a fully coupled solution between computational fluid dynamics (CFD) and finite element modelling (FEM) can be developed to model the fluid-stuctue inteaction (FSI). A wind tunnel test elies on the simplified assumption that the scaled aeoelastic model can satisfactoily eplicate the dynamic popeties of the full-scale design. It neglects the influence of highe modes. The application of CFD fo pactical wind engineeing poblems has eceived a lot of eseach attention ove the last thee decades and has made majo pogess due to the advancement of compute technology. Thus fa, the leading applications fo the built envionment have concentated on mean wind speeds fo aeas including: natual ventilation; pollution dispesion; and human comfot at steet and balcony level (Stathopoulos, 1997). It has poven vey difficult fo CFD to acceptably model the complex flow intefeence phenomena induced fom buildings. Typical featues of this unsteady flow egime include tubulent length scales and sepaation egions lage than the body size of the stuctue. This is the eason less wok has been pefomed on pedicting time-dependent suface pessues on these man-made bluff bodies. CFD has not developed enough to suggest it could eplace wind tunnel testing in this espect. It does, howeve, offe encouaging potential to act as a complimentay tool. In this pape, the vaious tubulence models will be discussed with espect to thei ability to pedict suface pessues and esulting wind loads fo a tall building. This includes a detailed eview of pevious validation studies pefomed within the liteatue. It poceeds to highlight the use-defined citeia that must also be satisfied. Finally, the scope fo futue eseach on simplifying CFD analyses fo tall buildings is discussed, with a view to poducing a moe efficient and pactical solution. The moden simulation is based on tubulence models, based on diffeent models of tubulence used fo solving the consevation of mass, momentum and enegy equations. Sun et al., 2009 and Casto, 2003 use diffeent CFD tubulence model fo analyze the wind loads on the tall buildings. The selection of the tubulence model is made based on accuacy, computational cost, accessibility and available time fo simulations. The most complete fom of CFD is the Diect Numeical Simulation (DNS) method that uses the diect solution of Navie-Stokes equations fo each contol volume. The disadvantage of this method consists in the mesh size dimension conditions. The cell mesh must be smallest that the votex eddy within the flow fo captuing the tubulent effects. So, the cost of DNS become extemely high and Knapp (2007) conclude that the DNS method should be limited by a small scale simulation and low Reynolds numbes. The othe methods ae implemented in ANSYS Inc, (2005) and is based on Reynolds-aveaged Navie- Stokes (RANS) and Lage Eddy Simulation (LES) and ae the two most used method fo wind load simulations. The RANS methods ae based on the two popula most used models, κ ε and κ ω, whee κ epesent the kinetic enegy and ε the tubulent dissipation ate o ω the specific dissipation ate. The RANS method is based on additional empiical equations fo establish the tubulent viscosity and ae elatively simple to use and obust and can descibe the full specta of tubulence scale. Othe methods ae based on tansient solutions based on spatial filteing appoach adapted with subgid-scale fo smallest cell dimension eddy. The mesh in that method must be smallest that in the RANS method but the filteing appoach can give moe infomation about tubulence aeas of the model. In the last yeas, was developed a new method based on the RANS and LES models, named Detached Eddy Simulation (DES), method whee the simplest RANS algoithms ae used fo majoity flow domains simulate and LES is used only in the aea of sepaated flow. The use of DES method in the wind load estimation of tall stuctues consists in the ISBN: 978-1-61804-004-6 434
high tubulent aea developed aound the buildings and in the specificity of wind flow. 2. Numeical analysis In this pape we establish the pessue distibution on the façades of the tall buildings using the TVD algoithms fo gas dynamics based on Eule PDE system of equation fo a situation of wind loading moe closely of the eality. The domain of computing is established in the figue 1 and the wind input speed diagam in the figue 2. The system of patial diffeential equations is give on the foms of mass consevation (1), momentum consevation (2) and enegy consevation (3): ρ + t ρ V t t ( ρ V ) = 0 ( ) + ( ρ V V ) = p + ρ g ( ρ E) + ( ρ E + p) V = ( λ T ) (1) (2) (3) Index Ox Value [m] Index Oy Value [m] Index Oz Value [m] L 96 Y 96 Z 96 L1 6 Y/2 48 H1 21 L2 21 Y1 36 H2 69 L3 46 Y2 33 H3 12 L4 3 Y3 24 L5 36 Y4 24 L6 18 a) b) Figue 1. Geometical domain dimensions. The system of equations is wite as: q = f + g + h (4) Whee: ρ ρu 2 ρu ρu + p q = ρv; f = ρuv ; ρw ρuw E u( E + p) ρv ρw ρuv ρuw 2 g = ρv + p; h( q) = ρvw 2 ( ) ( ) ρvw ρw + p v E + p w E + p (5) The total enegy E epesent the sum of intenal and kinetic enegy and is expession is: 2 2 2 ( u + v w ) p 1 E = + ρ + (6) γ 1 2 And the equation of state fo a g-law polytopic gas consideed in the pesent wok has the shape c p γ = (7) cv Whee c p and c v ae the specific heat at constant pessue, espectively constant volume. 2.1. Bounday and Initial Conditions and input paticulaities ISBN: 978-1-61804-004-6 435
The Bounday conditions used in the solving poblems consist in fee output fom the planes P2,P3,P4 and P5, as in figue 2a and in input fom plane P1. The inlet gas in the computation domain is made fom plane P1 with a speed distibution as in figue 2b. The speed distibution is made in accod with the maximum speed at 60 yea measuement on wind in the most affected aea on India, in ageement with the design noms fo wind load. The speed distibution on Ox is consideed linea on all the inlet aea fo veification the tubulence that can appea in the fluid tap between the two buildings with diffeent height. The inputs fom plane P1 have a pulse shape with equal time between the pulse duation and pause between two pulses. a) b) Figue 2. The plane notation of the domain analyzed, a) and the inlet speed distibution on Ox diection. 3. Results The analyze give the maps of gas speeds, u,v,w enegy and pessue inside the computing domain, the pessues on the façade and the othe sufaces of the tall buildings. Figue 3. The steams line of the gas flow and the pessue in the façade section at time 0.915 s of simulation. a) t=0.152484 s b) t=0.299869 s c) t=0.446137 s d) t=0.592342 s ISBN: 978-1-61804-004-6 436
Recent Reseaches in Automatic Contol e) t=0.731194 s f) t=0.871237 s g) t=1.534835 s h) t=5.39784 s Figue 4. Gas speed U [m/s] in the xoz symmety plane of the domain of analyze fo diffeent time moments. a) t=0.299869 s b) t=0.446137 s c) t=0.592342 s d) t=0.731194 s e) t=0.871237 s f) t=0.985921 s g) t=1.107586 s h) t=1.245531 s i) t=1.390198 s j) t=1.534835 s k) t=1.679116 s l) t=5.39784 s Figue 5. The map of pessue on the facade of the tall building fo diffeent moment of the aplication. a) t=0.871237 s b) t=1.390198 s c) t=2.114628 s d) t=5.39784 s Figue 6. The pessue map on the back suface of tall building fo divese moments of the application. ISBN: 978-1-61804-004-6 437
a) t=0.871237 s b) t=1.390198 s c) t=2.114628 s d) t=5.39784 s Figue 7. The pessue map on the ight suface of the tall building fo divese moments of the application a) t=0.871237 s b) t=1.390198 s c) t=2.114628 s d) t=5.39784 s Figue 8. The pessue maps on the left suface of the tall building fo divese moments of application. 4. Conclusion The gas dynamics modelling based on Eule PDE system of equation can solve the poblems of wind loads on tall stuctues without using the Navie- Stokes PDE system of equation with divese tubulence flow models fo accuate flow dynamics. A combination between the two modelles can be used because the gas speeds ae low in the case of wind loads and the gas is pactical incompessible. The tubulence aea of flow, that in the civil engineeing have a huge aea of the domain (60-80%) in the cases of wind loads on tall buildings can be simulate using the Eule system of equations and the complicated tubulences models can be avoided. 5. Refeences 1.Allsop, A. BS EN 1991-1-4 Tall Buildings. In: ICE (Institution of Civil Enginees), New Euocode on Wind Loading. (London, UK, 11 May 2009). 2. ANSYS Inc. FLUENT 12.0 Theoy Guide. (2009) 3. Building Reseach Establishment. Wind aound tall buildings, BRE Digest 390, (BRE: Watfod, UK, 1994). 4. Bitish Standads Institution. BS 6399-2:1997 Loadings fo buildings Pat 2: Code of pactice fo wind loads. (BSI: London, UK, 1997). 5. Bitish Standads Institution. BS EN 1991-1-4:2005 Euocode 1: Actions on stuctues Pat 1-4:Geneal actions Wind actions. (BSI: London, UK, 2005). 6. Casto, I.P. CFD fo Extenal Aeodynamics in the Built Envionment. The QNET-CFD Netwok Newslette. Vol. 2: No. 2 July 2003, pp 4-7 (2003). 7. Davenpot, A.G. What makes a stuctue wind sensitive? in Poc. The Jubileum Confeence on Wind Effects on Buildings and Stuctues. 8. Poto Alege, Bazil 25-29 May 1998, eds. A. A. Balkema, (Rottedam, Nethelands, 1998) pp 1-14. 9. Iwin, P.A. and Bake, W.R. The Buj Dubai Towe Wind Engineeing. Stuctue Magazine, June 2006 pp 28-31 (2006). 10. Knapp, G.A. Impoved methods fo stuctual wind engineeing. Ph. D. Univesity of Nottingham(2007). 11. Nozu, T., Tamua, T., Okuda, Y., and Sanada, S. LES of the flow and building wall pessues in the cente of Tokyo. Jounal of Wind Engineeing and Industial Aeodynamics, Elsevie, 96 pp 1762-1773 (2008). ISBN: 978-1-61804-004-6 438