influence of surrounding buildings on canopy roof of

International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018

INFLUENCE OF SURROUNDING BUILDINGS ON CANOPY ROOF OF LOW RISE BUILDINGS IN ABL BY CFD SIMULATION A.K. Roy1, Aslam Aziz2, S.K. Verma3, S.K. Sharma4 1

Assistant Professor, Civil Engineering Department, NIT Hamirpur, [email protected] M.Tech. Student, Civil Engineering Department, NIT Hamirpur, [email protected] 3 Associate, Professor, Deptt. of Civil Engineering, PEC University of Technology, Chandigarh, [email protected] 4 Assistant Professor, Department of Civil Engineering, NIT Jalandhar, [email protected] 2

ABSTRACT Columns alone are required to support the canopy roofs and they are most external exposed parts of the low-rise buildings, wind action is directly exerted on both upper and lower sides of the surfaces. These types of roofs are more vulnerable to wind actions and experiences damage during the windstorms. These types of roofs are mainly provided in the agriculture, railway stations, airports structures. The shape of roof, pitch of roof, obstruction under the roofs and direction of wind, effects the intensity of wind forces on the canopy roof. In codes mainly pressure coefficients(CP) are specified for two types of canopy roof buildings, one is troughed free canopy roof and other is pitched free canopy roof buildings. Also, these values are for few incidence angles of canopy roof. Today wind tunnel studies are the one of most important topic, to know experimentally the wind load effect on the different types of models of roofs like canopy roof. Keywords: canopy roof, boundary layer flow. CFD simulation, wind pressure coefficients, wind flow angle.

INTRODUCTION A canopy is an over-head roof or a structure, which are made up of fabrics or metal covering, attached to structure which is able to provide shade or shelter from weather conditions such as sun, hail, snow and rain. A canopy roof having shape also like a tent which is most of time without floor. Architectural canopies provide protection from weather and also provide decoration to the structure (Adanur, 1995). These types of canopies are supported by the building to which they are attached and also, they are supported by posts or frames which are mounted on ground such that to keep it upright, the post should not be less than two. Canopies having fabric cover like gazebo or cabana can also stand alone. Fabric canopies can encounter various design needs. Numerous modern fabrics are bright, long-lasting, easily cleaned, strong and flame-resistant. Vinyl, acrylic, polyester or canvas are used as a material for the fabric canopy roofs. High strength-to-weight ratios materials are provided in the modern frames, also they are corrosion resistant. The proper use of all these properties can provide a safe, strong, attractive and economical products (wikipedia). Letchford and Killen (Killen and Letchford, 2001; Letchford and Killen, 2002) made a parametric study of wind loads on cantilevered grandstand roofs and they derived equivalent


static trapezoidal load distributions for the design of such structures. For simple cantilevered grandstand roofs and canopy roofs they also used some wind load codes and design gidelines (Cook, 1990; 2011). Typically, a roof offers more solid protection while as canopy provides only shade; while as canopies filter and roofs protect. You often hear 'canopy' used to describe the sky-ward protection which provides shade and a little from rain protection which you experience in forest while as a cave is like a roof. This will offer roof-like protection shade, protection from rain, and protection from big, hard, nasty things that fall from the sky (Oosterwal, 2011). Wind loads on free-standing canopy roofs have been studied experimentally(Ahuja and Roy, 2013; Roy et al., 2007a, 2007b, 2007c, 2007d, 2017). There are three types of roof geometries, gable, troughed and mono-sloped roofs. For the various wind directions wind pressure is measured simultaneously, at many points on both top and bottom surfaces of the roof. AS/NZ Standard (Uematsu et al., 2007) specification compare the wind design loads on the windward side and leeward side halves of the roof which are expressed as equivalent static loads. A gable type canopy roof having dimensions 150mm x 130mm (Fig. 1) was studied to determine the wind pressure distribution by wind tunnel effect, in a simulated terrain category 2 as per IS 875 (Part 3), 2015 boundary layer flow at a length of scale 1/40. Perspex sheets are used to make gable type canopy roof model having dimensions (cross section) 1.3 m (width) x .85 m (height) and tested in the closed-circuit wind tunnel. Mean, R.M.S, Maximum and Minimum pressures have been measured on the surfaces of the model for 7 wind directions namely 00, 150, 300, 450, 600, 750 1.3 m (width) x 0.85 m (height).The region close to the leading edge and ridge line having rectangular strips experiences large pressure than the rest of the roof. A wind orientation between 300 and 750 , Major pressures are found both upper and lower surface of the roof ,close to the leading edges line and the ridge (Roy et al., 2007e).

Figure 1 isometric view of canopy roof model (Roy et al., 2007e) The results of the net pressure coefficients provide us information that the main surface shows downward pressure very close to the windward zone, suction at the centre of the roof and again downward pressure at the leeward side of the roof. The values that are mentioned in this paper are different from the recommended code values. Which shows that the downward pressure or suction acts on the roof, so that the resultant forces caused by the cn values are 40% higher than the recommended values ASCE 7 (Poitevin et al., 2013).


Computational Fluid Dynamics (CFD) is a very powerful alternative to know the wind related phenomena on buildings as well as other kind of structures. Many researchers have already carried out significant research on canopy roofs and other structures through CFD modeling and simulation (Khan and Roy, 2016; Roy and Bhargava, 2012; Roy et al., 2012, 2014; Verma et al., 2014). A software package, FLUENT-14 or FLUENT-18.1 (ANSYS 14.5 or ANSYS 18.1) is used for CFD analysis in the present study. FLUENT-14 is a general CFD code based on the finite volume method and an algebraic multi-grid coupled solver. Wind load is one of the important loads to be considered while designing high-rise or low-rise buildings. Relevant codes (AS/NZS-1170-2, 2016; ASCE/SEI: 7-16, 2016; IS 875 (Part 3), 2015)] are available for designing low-rise building for wind action. However, for some specific conditions, very limited information is available with respect to wind loads on canopy roof buildings. Canopy roofs are supported on columns and no walls. Canopy roof buildings are constructed for different number of usage, such as agricultural facilities (barns, etc.), bus and railway stations, carports etc. As Canopy Roofs are light weight structures, so they are unable to provide stability against wind loads. The wind flow pattern and pressure values on the roof surface are compared with experimentally obtained wind loads. The canopy roof buildings selected for the present study are same as have been studied by Roy, (2010). The plan dimensions of the roof have been kept same in all the models, covered in the present study. The length, width and eaves height of the buildings are taken as 12m, 6m and 3m respectively. Only roof slope is changed keeping the above dimensions same. Indian Standard IS: 875 (part-3), 2015 prescribes the pressure coefficients (Cp) for pitched free roofs, for few wind incidence angles and for few roof slopes. Further, available information does not include wind pressure coefficients (Cp) on corners where high pressure or suction is expected especially in case of skew angles.

CFD MODELLING OF CANOPY ROOF Details of Models used for the CFD Simulation In the present study of canopy roof the different dimensions of the domain and the building are as length of the domain 230m, breadth of the domain 100m and height of domain 50m. the project is on interference effect, so two buildings are used one is as model building on which we have to calculate the effect and another building is interfering building. In this case the two buildings are of same dimensions. The length of the building 30m, breadth of the building 15m height of the building 8.8m. Domains and Meshes One of the main role in the CFD modelling is choice of domain, the size of domain and positioning of the building in the domain is also one of the factor (Revuz et al., 2012). Recent studies of CFD used by Franke et al. (2004) for determining the domain size. The section of Franke et al. (2004) is as follows “The inlet, the lateral and the top boundary should be 5 H away from the building, where H is the building height. The outflow boundary should place


at 15 H behind the building to form the flow development, this outflow length should also be functional for an urban area building, where H is replaced by Hmax, the height of the tallest building. To prevent artificial acceleration of the flow over the tallest building, the top of the computational domain should be also at least 5 Hmax away from this building. As given in the Fig. 2 the different dimensions of the domain are shown. The height of the domain should 6H, lateral distance from the sides of buildings in the domain should be 5H. From the front side of the building the minimum distance should be 5H and back side of the building the distance should be 15H.

Figure 2 Domain size used for the CFD simulation (Revuz et al., 2012). Also, the angle provided in the roof is 300 which makes the roof two faces one face called as windward side on which the incoming wind strikes and other side is called leeward side or backward side on which the wind leaves. The building model also consist of columns and its dimensions are (15x15). The model is as shown in the Fig 2.

In this project the model dimensions are taken after analysis of different project models and their results are also studied. The model figures in Figs. 3 is representing the domain with the canopy roof including interfering building. The model figures in Figs. 4 represents the meshing of the canopy roof including interfering building through ICEM CFD tools(ANSYS Inc., 2015). Wind

Figure 3 Canopy building in domain

Figure 4 Canopy roof building in meshing


Turbulence Model Researchers have already examined different turbulent models for their relative suitability for the atmospheric boundary layer airflow (Blocken et al., 2007, 2008; Hooff et al., 2015). It has been observed that for this kind of problem the realizable k-ε model (Shih et al., 1995)is most suitable. The commercial CFD code Ansys Fluent 14.5 (ANSYS Inc., 2015) is used to solve the 3D Reynolds-averaged Navier–Stokes equations and the continuity equation using the control volume method. Closure is obtained using the realizable k-ε model (Shih et al., 1995). Pressure–velocity coupling is taken care of by the SIMPLE algorithm. Pressure interpolation is second order. Second-order discretization schemes are used for both the convection terms and the viscous terms of the governing equations. Boundary Condition

500

500

450

450

400

400

350

350

H eig h t (m m )

Height (mm)

For real physical representation of the fluid flow suitable boundary condition that actually simulate the real flow is required, there is always great difficulty in defining in detail the boundary conditions at the inlet and outlet of the flow domain that is required for accurate solution. In the wind tunnel experimental study, the measured velocity profile at the test section inside the wind tunnel is shown in in Figs. 5(a) and Variation of turbulence intensity with height at the test section is shown in Figs. 5(b).

300 250 200 150

M

l

300 250 200 150 100

100

50 50

0 0 0

5

10

15

Wind velocity (m/s)

20

25

0

0.5

1

1.5

2

2.5

3

Turbulence intensity (%)

Figure 5 (a) Measured velocity profile at the test section inside the wind tunnel and (b) Variation of turbulence intensity with height at the test section (Roy, 2010) At the upwind boundary, a velocity inlet )was used with the following expressions for the along-wind component of velocity, U which is similar to the experimental study. U z 1.8715 log z 16.94 1 Standard representation of the velocity profile in the ABL is as shown below. u∗ z z U z ln 2 κ z u∗ k z or k z 0.5 I U 3 C The inlet turbulence dissipation rate profile ε from [21] is given by ∗

(4)


where, is the height co-ordinate, the von Karman constant (~0:42), the scaled aerodynamic roughness length corresponding to a power-law exponent of 0.15 (here: 0.03/300 0.0001 ) and ∗ the friction velocity related to a horizontally homogeneous (stable) ABL flow. The sides and the top of the computational domain are modelled as slip walls (zero normal velocity and zero normal gradients of all variables). At the outlet, zero static pressure is specified. The standard coefficients of the realizable k-ε model were used. The Reynolds number for the flow is 2.65 × 106, using the building height, , and the velocity at as reference values. At the downwind boundary, a pressure outlet was used, with the relative pressure specified at 0 and backflow conditions for and set to those of the inlet. In the domains, however, backflow was not observed because the downwind boundary was sufficiently far from the building. Numerical Simulation and Validation The velocity profile obtained by fluent was compared with the velocity profile of the wind tunnel experimental study as shown in Fig. 6. It is observed that by incorporating all the consideration and boundary condition the inlet velocity profile are very much similar as it was in the experimental study.

Figure 6 Velocity profile of wind in the domain. In the above Fig. 6 the inlet velocity flow on the domain is shown. As the flow takes place the velocity in the domain is different at different positions, it is because at bottom there is less velocity as compare to top of the domain. In Fig. 6 at left side there is shown wind velocity by color as well as height and in the domain the different color represents different velocities. The velocity in domain increases from bottom to top. Results obtained through CFD simulation are fairly good and compared with experimental results [8] and wind standards available of different countries. Pressure coefficient for all four faces of isolated building (Setup-1) at 0° wind incidence angle (Fig. 6) is presented in tabular form as given in Table 1.

RESULTS AND DISCUSSIONS Different models are used to measure the wind pressure, more than 50 data of 5 seconds interval are collected and analyzed. wind pressure data at each tap location are used to calculate Mean, R.M.S., Maximum and Minimum pressure values. The different values of pressure coefficient(Cp) are measured by using mean wind velocity at the different roof heights. (5)


The convention provided in the given paper is as the upper side of the roof wind flow is positive due to positive pressure and below the roof their negative pressure so suction is created, which makes the roof to get uplifted. The net wind-induced pressure and force on the roof due to the combined effect of the upper and lower roof surfaces are defined as positive in the vertically downward direction. The relationship is like the following expression:

p net  p upper

 p lower .

(6)

The wind incident angle θ are measured as θ = 90º when wind coming directly perpendicular to ridgeline of the canopy roof. The wind velocity vectors of the flow are as shown in Fig. 7.

Figure 7 Velocity vector on canopy roof with interference on canopy roof building. The velocity vectors shows the flow of wind velocity on the roof as well as the columns of the building. As the wind strikes on the upper surface of the roof, the arrows are shifted upwards, the concentration of arrows is increased on top of the roof. This shows the increased pressure on the top of the roof on windward side. Also, the concentration of arrows are decreased on leeward side so on leeward side suction pressure is created. In the Fig. 8 pressure contours are shown on the top of the canopy roof and below the roof.

Figure 8 Pressure coefficient or pressure contours on upper and lower side of roof in canopy roof building. As in Fig. 8 at the top the wind flow increases and pressure and below the roof wind flow is due to the suction pressure. In the canopy roof the roof angle is 300 and the interfering building is at 900 to the main building on which we are calculating the effect.


Figure 9 Streamline flow on canopy roof. In the Fig. 9 the velocity streamlines on the canopy roof are shown in which the front building is the interfering building and other is model building. When the wind is allowed to flow through the domain, eddies are created on the lower side of the canopy roof. These eddies formation are due to negative pressure or suction pressure. This suction pressure leads to uplift the roof and makes destruction of the roof. A comparative study of interference on canopy roof on low rise buildings by Roy, 2010 experimental studies and the given CFD simulation of my study. The different values of pressure coefficient found are on top and below the canopy roof are compared to the codal provisions. The data found in both studies and code are as following in the table-1. Table. 1: Cp mean Values on canopy roof with 900. Cp mean values on upper surface. IS-875 Part 3 -2016 (Isolated)

Face A. Roy et al. 2010 Wind Tunnel Exp. Values (Interference)

CFD values Local Pr. Co-eff. (Interference)

IS-875 Part 3 -2016

Roy et al. 2010 Values (Interference)

CFD values Local Pr. Co-eff. (Interference)

Max -ve Cp values.

-2.0

-0.79

-1.29

-1.9

-0.88

-1.29

Max +ve Cp values.

+1.9

--

+0.01

+1.9

--

+0.08

Pressure values

Face B.

Cp mean values on lower surface.

Pressure values

Face A.

Face B.

Max -ve Cp values.

-2.0

-0.86

-0.15

-1.9

-0.58

-0.24

Max +ve Cp values.

+1.9

--

+0.24

+1.9

--

+0.3

The above table gives different values of the Cp at two different sides of the roof. The maximum negative value of Cp on face A, upper side of roof is -1.29, but according to IS:875-2016 the value is -2.0 that is for the isolated building. Roy et al have been worked on the same model but they have used this model in the wind tunnel to get the pressure coefficient values. Roy at al also got the values of Cp near to CFD values of cp. Roy at al got -0.79 maximum negative values on the face A of upper side of canopy roof and in CFD the value is -1.29. Similarly, For the lower side of the roof and two faces of roof, face A and face B the Cp values are calculated in Roy et al and CFD. These values are also compared and at least there is no big difference between the experimental values and CFD values. But in case of IS:8752016 code there is some differences. it is because in code the isolated building values are given but not the interference values of buildings. Apart from that the Cp values in the CFD are local pressure coefficient values.


CONCLUSION CFD offers a very powerful alternative to predict the wind related phenomena on buildings of different kind of structures. Flow separation and visualization can be achieved accurately by the CFD simulations, which helps in understanding the flow phenomenon. The pressure values, flow streamline, velocity vector and numbers of related parameter variables etc. throughout out the roof surface can be determined by the help of CFD analysis. Different turbulence model is available with their differences in accuracy for the CFD simulation. So, while running CFD simulation the turbulence model should be carefully defined to get the accurate result of the problem. The result accuracy depends upon the exact scale of model, proper meshing of model, defining all the physical properties exactly as the realistic to environment conditions and one of the most important parameter is meshing of the model geometry. Many complex and complicated model can be examined with less effort by the help CFD analysis and the designing criteria of the structure system or any fluid flow related system can be considered approximately. REFRENCES 1. Adanur, S. (1995). Wellington Sears Handbook of Industrial Textiles, Lancaster, U.S.A.: Technomic Publishing Company, New Holland . 2. Ahuja, A.K., and Roy, A.K. (2013). Parametric Study of Wind Loads on Canopy Roofs. In The Seventh International Structural Engineering and Construction Conference on “New Developments in Structural Engineering and Construction,” University of Hawaii at Manoa, Honolulu, United States , pp. 1099–1103. 3. ANSYS Inc. (2015). ANSYS Fluent Theory Guide, Southpointe, 275 Technology Drive, Canonsburg, PA 15317: ANSYS, Inc. Release 14.5 . 4. AS/NZS-1170-2 (2016). Australian/New Zeeland Standard - Structural Design Action, Part 2: Wind Action, SAl Global Limited under licence from Standards Australia Limited, Sydney and by Standards New Zealand, Wellington . 5. ASCE/SEI: 7-16 (2016). Minimum Design Loads for Buildings and Other Structures, Reston, Virginia 20191: Structural Engineering Institute, American Society of Civil Engineering . 6. Blocken, B., Carmeliet, J., and Stathopoulos, T. (2007). CFD evaluation of wind speed conditions in passages between parallel buildings — effect of wall-function roughness modifications for the atmospheric boundary layer flow. J. Wind Eng. Ind. Aerodyn., 95, 941–962. 7. Blocken, B., Moonen, P., Stathopoulos, T., and Carmeliet, J. (2008). Numerical Study on the Existence of the Venturi Effect in Passages between Perpendicular Buildings. J. Eng. Mech., 134, 1021–1028. 8. Cook, N.J. (1990). The designer’s guide to the wind loading of building structures. Part 2: Static structures, London: Butterworths . 9. Hooff, T. Van, Leite, B.C.C., and Blocken, B. (2015). CFD analysis of cross-ventilation of a generic isolated building with asymmetric opening positions : Impact of roof angle and opening location. 85. 10. IS 875 (Part 3) (2015). Indian Standard Design Loads (Other than Earthquake) for Buildings And Structures - Code of Practice, Part 3 Wind Loads, New Delhi: Bureau Of Indian Standards . 11. Khan, M.M., and Roy, A.K. (2016). CFD Simulation of Wind Effects on Industrial RCC Chimney. In Civil Engineering Conference-Innovation for Sustainability (CEC - 2016), Hamirpur (HP) – 177005: Department of Civil Engineering, National Institute of


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