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Recent Researches in Automatic Control Computer fluid dynamics application for establish the wind loading on the surfaces of tall buildings IOAN SORIN LEOVEANU(a), DANIEL TAUS(a), KAMILA KOTRASOVA(b),EVA KORMANIKOVA(b) (a) Civil Engineering Faculty,(b) Civil Engineering Faculty, Institute of Structural Engineering, Structural Mechanics Department. (a) University Transilvania of Brasov, (b) Technical University of Košice (a) Str. Turnului Nr. 5. Brasov. Romania, (b) KSM UIS SvF TU v Kosiciach leoveanui@yahoo.co.uk, danieltaus@yahoo.com, kamila.kotrasova@tuke.sk ,eva.kormanikova@tuke.sk Abstract: - The scope of this paper consists in the Computer Fluid Dynamics apply to establish the wind effects on the surfaces of the tall buildings systems. The tall buildings are in present more and more frequently in the modern cities. The topology of the buildings and the wind directions are more and more important in the case of new tall buildings design. The new materials used in structural engineering with their high mechanical properties, the construction machinery the technologies advances and the economical scope are the principal reasons in the design and execution of taller and more slender structures. In practice the quasi-static computing method used for current structure is exceed by inherit structural flexibility. The wind forces systems that load the buildings surfaces are in practices extremely difficult and expensive. Even the use of wind tunnel is extremely difficult in principal by the problems in the dumping values for the aero-elastic model assuming and Reynolds number scaling. The butt of this article consist in development of a CFD method very useful as a complimentary tool for easy establish the wind tunnel parameters work conditions. In the present paper the CFD modeling is based on Volume Finite Method (VFM) with Total Validation Diminishing Method (TVD) for modeling the flow around complexes buildings surfaces and establishes the pressure variation around them. The new CFD VFM algorithm is developed for minimal errors and based on Riemann and Godunov solvers. The results of this method are compared with the quasi-static models and with the most used numerical method. Key words: Volume finite method, CFD, surface pressure, TVD method, tall buildings, steel structure. 1. Introduction The wind loading is considered by Devenport(1998) The Eurocode and most used standards are in three categories: highlighted by Allsop (2009) as the most flexible a) Extraneously-induced loading based on naturally and inclusive code for normal buildings. The quasi- turbulent oncoming wind. The weak of upstream static methods offered by these codes are only obstructions enhance this categories buffeting. applicable for buildings with structural properties b) Unstable flow phenomenon such as separations, such that they are not susceptible to dynamic reattachments and vortex shedding generate a excitation (Metha, 1998). Thus, the tall buildings, secondary type of forces. those with high slenderness ratios and/or c) The movement-induced excitation of the body asymmetric planes, exceed limitations and are generate by the deflection of the structure create advised to be tested in the wind tunnel. fluid flow too. This phenomenon with a strong This pattern induces inherent structural flexibility unsteady states character gives the complexity of the and heightens concerns regarding the aero-elastic fluid flow around the flexible tall structures. The fluid-structure interaction between the wind and the modern design of flexible tall structures must tall building. The codes of practice have been request to earth quakes events and wind loads, cases formulated with a view to providing an acceptable that represent a state of the art of the civil balance between the overly complex reality and engineering. oversimplified approach. ISBN: 978-1-61804-004-6 433 Recent Researches in Automatic Control Scaled-model wind tunnel testing is an established literature. It proceeds to highlight the user-defined tool among industry design practices. Boundary criteria that must also be satisfied. Finally, the scope layer wind tunnels are capable of quantifying time- for future research on simplifying CFD analyses for dependent surface pressures, including the complex tall buildings is discussed, with a view to producing types of loadings (torsion and acrosswind). The a more efficient and practical solution. model can be used to determine the best orientation The modern simulation is based on turbulence of the proposed building, the case of Burj Dubai models, based on different models of turbulence analysis (Irwin and Baker, 2006). The simulation, used for solving the conservation of mass, nevertheless, it has its own limitations that include momentum and energy equations. Sun et al., 2009 the difficulty to maintain proportionality between and Castro, 2003 use different CFD turbulence the scaled turbulence characteristics and the scaled model for analyze the wind loads on the tall building model, especially if the topography is buildings. The selection of the turbulence model is significant (Taranath 1998). Furthermore, it is made based on accuracy, computational cost, important to ensure Reynolds number effects on the accessibility and available time for simulations. The pressures are kept to a minimum. It is noted by Sun most complete form of CFD is the Direct Numerical et al. (2009) that a computational approach has the Simulation (DNS) method that uses the direct capability of being more flexible than traditional solution of Navier-Stokes equations for each control wind tunnel experiments. For example, a fully volume. The disadvantage of this method consists in coupled solution between computational fluid the mesh size dimension conditions. The cell mesh dynamics (CFD) and finite element modelling must be smallest that the vortex eddy within the (FEM) can be developed to model the fluid-structure flow for capturing the turbulent effects. So, the cost interaction (FSI). A wind tunnel test relies on the of DNS become extremely high and Knapp (2007) simplified assumption that the scaled aeroelastic conclude that the DNS method should be limited by model can satisfactorily replicate the dynamic a small scale simulation and low Reynolds numbers. properties of the full-scale design. It neglects the The other methods are implemented in ANSYS Inc, influence of higher modes. The application of CFD (2005) and is based on Reynolds-averaged Navier- for practical wind engineering problems has Stokes (RANS) and Large Eddy Simulation (LES) received a lot of research attention over the last and are the two most used method for wind load three decades and has made major progress due to simulations. The RANS methods are based on the the advancement of computer technology. Thus far, two popular most used models, κ−ε and κ−ω, where the leading applications for the built environment κ represent the kinetic energy and ε the turbulent have concentrated on mean wind speeds for areas dissipation rate or ω the specific dissipation rate. including: natural ventilation; pollution dispersion; The RANS method is based on additional empirical and human comfort at street and balcony level equations for establish the turbulent viscosity and (Stathopoulos, 1997). It has proven very difficult for are relatively simple to use and robust and can CFD to acceptably model the complex flow describe the full spectra of turbulence scale. Other interference phenomena induced from buildings. methods are based on transient solutions based on Typical features of this unsteady flow regime spatial filtering approach adapted with subgrid-scale include turbulent length scales and separation for smallest cell dimension eddy. The mesh in that regions larger than the body size of the structure. method must be smallest that in the RANS method This is the reason less work has been performed on but the filtering approach can give more information predicting time-dependent surface pressures on about turbulence areas of the model. In the last these man-made bluff bodies. CFD has not years, was developed a new method based on the developed enough to suggest it could replace wind RANS and LES models, named Detached Eddy tunnel testing in this respect. It does, however, offer Simulation (DES), method where the simplest encouraging potential to act as a complimentary RANS algorithms are used for majority flow tool. In this paper, the various turbulence models domains simulate and LES is used only in the area will be discussed with respect to their ability to of separated flow. The use of DES method in the predict surface pressures and resulting wind loads wind load estimation of tall structures consists in the for a tall building. This includes a detailed review of previous validation studies performed within the ISBN: 978-1-61804-004-6 434 Recent Researches in Automatic Control high turbulent area developed around the buildings and in the specificity of wind flow. 2. Numerical analysis ∂ρ ( ) In this paper we establish the pressure distribution r on the façades of the tall buildings using the TVD + ∇ ρ ⋅V = 0 (1) ∂t algorithms for gas dynamics based on Euler PDE ∂ ( ) ( ) r r r r system of equation for a situation of wind loading ρ ⋅V + ∇ ρ ⋅V × V = −∇p + ρ ⋅ g more closely of the reality. The domain of ∂t (2) computing is established in the figure 1 and the ∂ r wind input speed diagram in the figure 2. The ( ρ ⋅ E ) + ∇ ( ρ ⋅ E + p ) ⋅ V = ∇ ( λ ⋅ ∇T ) (3) system of partial differential equations is give on the ∂t forms of mass conservation (1), momentum conservation (2) and energy conservation (3): Index Value Index Value Index Value Ox [m] Oy [m] Oz [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) Figure 1. Geometrical domain dimensions. The system of equations is write as: The total energy E represent the sum of internal and kinetic energy and is expression is: q = f ( q ) + g ( q ) + h( q ) (4) Where: E= p γ −1 2 1 ( + ρ u 2 + v 2 + w 2 (6) ) ρ ρu ρu ρu 2 + p And the equation of state for a g-law polytropic gas considered in the present work has the shape q = ρv ; f ( q ) = ρuv ; ρ w ρuw γ= cp E u (E + p ) cv (7) (5) ρv ρw Where cp and cv are the specific heat at constant ρuv ρuw pressure, respectively constant volume. g ( q ) = ρv 2 + p ; h( q ) = ρvw 2 ρvw ρw + p v(E + p ) w(E + p ) 2.1. Boundary and Initial Conditions and input particularities ISBN: 978-1-61804-004-6 435 Recent Researches in Automatic Control The Boundary conditions used in the solving India, in agreement with the design norms for wind problems consist in free output from the planes load. The speed distribution on Ox is considered P2,P3,P4 and P5, as in figure 2a and in input from linear on all the inlet area for verification the plane P1. The inlet gas in the computation domain turbulence that can appear in the fluid trap between is made from plane P1 with a speed distribution as the two buildings with different height. The inputs in figure 2b. The speed distribution is made in from plane P1 have a pulse shape with equal time accord with the maximum speed at 60 year between the pulse duration and pause between two measurement on wind in the most affected area on pulses. a) b) Figure 2. The plane notation of the domain analyzed, a) and the inlet speed distribution on Ox direction. 3. Results The analyze give the maps of gas speeds, u,v,w the pressures on the façade and the other surfaces energy and pressure inside the computing domain, of the tall buildings. Figure 3. The streams line of the gas flow and the pressure 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 Researches in Automatic Control e) t=0.731194 s f) t=0.871237 s g) t=1.534835 s h) t=5.39784 s Figure 4. Gas speed U [m/s] in the xOz symmetry plane of the domain of analyze for different 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 Figure 5. The map of pressure on the facade of the tall building for different moment of the aplication. a) t=0.871237 s b) t=1.390198 s c) t=2.114628 s d) t=5.39784 s Figure 6. The pressure map on the back surface of tall building for diverse moments of the application. ISBN: 978-1-61804-004-6 437 Recent Researches in Automatic Control a) t=0.871237 s b) t=1.390198 s c) t=2.114628 s d) t=5.39784 s Figure 7. The pressure map on the right surface of the tall building for diverse moments of the application a) t=0.871237 s b) t=1.390198 s c) t=2.114628 s d) t=5.39784 s Figure 8. The pressure maps on the left surface of the tall building for diverse moments of application. 4. Conclusion The gas dynamics modelling based on Euler PDE wind loads and the gas is practical incompressible. system of equation can solve the problems of wind The turbulence area of flow, that in the civil loads on tall structures without using the Navier- engineering have a huge area of the domain (60- Stokes PDE system of equation with diverse 80%) in the cases of wind loads on tall buildings turbulence flow models for accurate flow can be simulate using the Euler system of dynamics. equations and the complicated turbulences models A combination between the two modelles can be can be avoided. used because the gas speeds are low in the case of 5. References 1.Allsop, A. BS EN 1991-1-4 Tall Buildings. In: ICE (Institution of Civil Engineers), New Eurocode on Wind Loading. (London, UK, 11 May 2009). 2. ANSYS Inc. FLUENT 12.0 Theory Guide. (2009) 3. Building Research Establishment. Wind around tall buildings, BRE Digest 390, (BRE: Watford, UK, 1994). 4. British Standards Institution. BS 6399-2:1997 Loadings for buildings – Part 2: Code of practice for wind loads. (BSI: London, UK, 1997). 5. British Standards Institution. BS EN 1991-1-4:2005 Eurocode 1: Actions on structures – Part 1-4:General actions – Wind actions. (BSI: London, UK, 2005). 6. Castro, I.P. CFD for External Aerodynamics in the Built Environment. The QNET-CFD Network Newsletter. Vol. 2: No. 2 – July 2003, pp 4-7 (2003). 7. Davenport, A.G. What makes a structure wind sensitive? in Proc. The Jubileum Conference on Wind Effects on Buildings and Structures. 8. Porto Alegre, Brazil 25-29 May 1998, eds. A. A. Balkema, (Rotterdam, Netherlands, 1998) pp 1-14. 9. Irwin, P.A. and Baker, W.R. The Burj Dubai Tower Wind Engineering. Structure Magazine, June 2006 pp 28-31 (2006). 10. Knapp, G.A. Improved methods for structural wind engineering. Ph. D. University of Nottingham(2007). 11. Nozu, T., Tamura, T., Okuda, Y., and Sanada, S. LES of the flow and building wall pressures in the center of Tokyo. Journal of Wind Engineering and Industrial Aerodynamics, Elsevier, 96 pp 1762-1773 (2008). ISBN: 978-1-61804-004-6 438

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