Study on wind flow around a pentagon plan shape tall building using CFD

Multi-storey buildings are emerging because of the lack in land availability within urban areas. Mostly, tall buildings are irregular in plan and more complex to wind action. In the present scenario, it is important to study the wind structure interaction for irregular plan shape tall buildings. This paper is focused on understanding the behaviour of pentagon plan shape building under wind load at different wind incident angles 0°, 45°, 90°, 135° and 180°. The wind behaviour is studied using Computational Fluid Dynamics (CFD) and turbulence model k- is used for external flow simulation around the tall buildings. The building models are scaled in the ratio 1:300 and wind velocity at inlet is scaled in the ratio 1:5 respectively. From the results, external pressure coefficient on various faces of the buildings, drag coefficient and lift coefficients are determined for pentagon shape building. The maximum positive external pressure coefficient (Cp) is 0.446 had occurred in Face A at 180° wind incident angle and the maximum value of the negative external pressure coefficient (Cp) is 0.388 had occurred in Face D at 135° wind incident angle.


Introduction
Tall buildings are laterally affected due to seismic and wind load. While considering the tall building wind force is predominant than earthquake loading. Because of lateral force, a very large moment is generated at the base. The magnitude of base moment for a tall building mainly depends upon the slenderness ratio. In addition to this, if the plan of the buildings is of irregular shape, the wind analysis is the critical task which may create more flow situations when wind interacts with structures. Analysis of irregular plan tall buildings are more critical to wind load than regular shape buildings. However, current design standards provide guidelines only for regular and symmetric shapes. There are no standard guidelines and analytical formulas available to calculate the wind effect on irregular plan shape tall buildings. Therefore, it is a necessity to conduct research work on an irregular plan of tall buildings. Building performance has been increased towards windward direction by optimizing its corner size. In this Artificial Neural Networks (ANN) is used to estimate the aerodynamic response of IOP Publishing doi: 10.1088/1742-6596/1850/1/012045 2 building by 3D Large Eddy Simulation (LES) [1]. Wind tunnel experiment is agreed on eight number of L-shaped building models by varying its geometric dimensions. Based on results an empirical relation is expressed for determining wind-induced torque on L shaped high-rise buildings [2]. The comparison exposed that there is some huge deviation between the experimental and numerical results, because of mismatching in inflow boundary condition [3]. An inverse method is developed to estimate the wind response and wind load on high rise buildings by limited response measurements [4]. The size of the recirculation region is high in the infinite cylinder because of the increase in momentum inflex [5]. The wind flow pattern around a tall building under urban wind condition has studied using CFD. All RANS turbulence model has compared to predict the wind pressure on a circular plan high rise building [6]. The wind response on a tall chimney in along and crosswind direction is studied. In that, a semi-empirical relation is derived using structural damping for computing the crosswind effect on chimney [7]. The galloping stability of rectangle and H shape tall building has investigated using CFD. By comparing the mean external pressure coefficient, the windward coefficient good agreement experiment data and the leeward or suction coefficient have some deviation from experiment data. The deviation is because of limitations in the RANS turbulence model [8]. Aerodynamic effect on the various shape of a tall structure vortex formation is studied. By changing some part of the basic crosssection, the aerodynamic damping can be achieved [9]. From the results of the previous investigation, the present paper focuses on investigating the wind effect on the pentagon plan shaped tall buildings using CFD. The numerical analysis is carried for different wind angles from 0° to 180° at an equal interval of 45° and mean external pressure coefficient (C p ) on the different faces of the pentagon model is the core area in this study

2.Governing Equation for CFD
The ANSYS Fluent software has the algorithm to solve the computational fluid dynamic equation.

Continuity equation Non-conservation form
(1) By applying the model of a finite control volume approach on fixed in space model conservation form of continuity equitation is obtained.

The Momentum Equation
The momentum equation is also called as Navier stokes equations. The fundamental principle for momentum equation is Newton's second law of motion.

Non-conservation form for viscous flow
x-direction: y-direction: and z-direction:

Non-conservation form non-viscous flow
x-direction: y-direction: and z-direction:

Conservation form for non-
x-direction y-direction and z-direction

Domain geometry
The building is modelled using 1:300 scale and the buildings are assumed domain was chosen [6]. The distance of 3H as windward and leeward sides domain. The distance between the boundary walls and and isometric view of pentagon building Reynolds Averages Navier-Stokes (RANS) this, the pressure contours of different faces of the pentagon plan shaped building are observed using 1:300 scale and the buildings are assumed as The distance of 3H in upstream and 10H in the downstream side s respectively. Figure 2 shows the geometric representation of The distance between the boundary walls and the building model is provide and isometric view of pentagon building is shown in figure 1. The numerical study is performed using Stokes (RANS) and K -ԑ model by ANSYS Fluent software. Along with this, the pressure contours of different faces of the pentagon plan shaped building are observed Plan view and isometric view of the pentagon building , . (11) as rigid. A rectangular downstream side are provided representation of the building model is provided as 3H. The plan shown in figure 1. The numerical study is performed using ԑ model by ANSYS Fluent software. Along with this, the pressure contours of different faces of the pentagon plan shaped building are observed.

Mesh generation and turbulence setting
Mesh size is responsible for the accurate result and also for the computational requirement. the computational time and also get the accurate resu domain. The building models and domain respectively are shown in Figure 3. factors concern in regular design office problem. So, the most predominantly used turbulent model is RANS turbulent model in which two study, the external pressure coefficient from RANS-SKE turbulent model

Inlet and boundary conditions
The no-slip wall condition is considered for for side faces of the domain. Inlet velocity is given as 10m/s. In outlet,

Validation with square model
For validation of CFD in this study is checked for dimensions of 50mm x 50mm in plan and height 300mm in ANSYS Fluent. The plan and isometric view of a square building is show domain RANS-SKE turbulent model. shown in table 1.

Pressure distribution
The variations of wind pressure on different surfaces of the building model for 0°, 45°, 90°, 135° and 180° are shown in computational method using the ANSYS Fluent is used C pe using the formula Domain Geometry . Inlet and boundary conditions is considered for bottom face and the free slip wall condition is considered side faces of the domain. Inlet velocity is given as 10m/s. In outlet, zero velocity is given.

Validation with square model
For validation of CFD in this study is checked for a square plan tall building was modelled with the n plan and height 300mm in ANSYS Fluent. The plan and isometric shown in Figure 4. The created model is analyzed in the above SKE turbulent model. Velocity 10m/s is provided at the inlet. [6]  The variations of wind pressure on different surfaces of the building model for 0°, 45°, 90°, 135° and 180° are shown in figure 5 to 9 respectively. The wind pressure obtained by computational method using the ANSYS Fluent is used to calculate the external pressure coefficient .

Domain Mesh
the accurate result and also for the computational requirement. To reduce lt different face sizing is used in both model and mm polyhedral face sizing Time efficiency and not compromising the result are the two n in regular design office problem. So, the most predominantly used turbulent model is SKE) is dominated one [6]. In this pentagon model was obtained lip wall condition is considered velocity is given.
a square plan tall building was modelled with the n plan and height 300mm in ANSYS Fluent. The plan and isometric in Figure 4. The created model is analyzed in the above-mentioned [6]. where V z is the design wind speed and P is the wind pressure. The variation in external pressure coefficients C pe concerning the height and the wake region fo using CFD are represented in figure 10 and

Pentagon model at 45° wind incident angle
At 45° wind incident angle, faces C and B are getting positive external C p of about 0.260 and 0.187 respectively and further faces A, D and E are getting negative external pressure coefficients of about 0.299, 0.183 and 0.362 respectively. All the faces are projected in a tapered manner. The maximum positive external pressure coefficient is 0.274 has occurred on face C which is more than that of 215.15% in face A, 28.18% in face B, 170.29% in face D and 239.26% in face E.

Drag and Lift Coefficient
Wind pressure distribution for the pentagon building model at different wind incident angle is exposed in fig 11. The previous researcher calculated drag coefficient and lift coefficients for the triangle plan shape tall building using CFD. While designing the tall buildings with round and tapped corner buildings are getting lesser drag coefficient and lift coefficient [10]. The drag coefficient (C d ) and lift coefficient(C l ) are calculated using the formulas (16) and C l = . . (17) where V z is the design wind speed, F d , F l is the drag and lift force and A p is the projected area. The table 2 shows the drag and lift coefficient for the pentagon plan shape building.

Wake region
In the current analysis of pentagon building, formations of wake regions are shown in figure 11 (a to e) for the wind angles 0˚, 45˚, 90˚, 135˚ and 180˚ degree. Wake region characteristic of the tall building in an urban environment has examined using CFD [11]. For pentagon building, the distribution area of the wake region is maximum when the full face is projected towards the leeward direction.

Conclusion
Maximum positive external pressure coefficient is 0.447 of face A at 180° wind incident angle, which is more than of other wind incident angle by 5.92%, 41.80%, 1.63% and 25.43% at 0˚, 45˚, 90˚ of face C and 135˚ of face A. Minimum negative external pressure coefficient is 0.389 at face D at 135° wind incident angle, which is more than other wind incident angle by 5.92 %, 41.8%, 1.63% and 25.43% at 0˚ at face A, 45˚ at face E, 90˚ face E and 180˚ of face C and D. From the results, obtained from CFD shows that maximum positive external pressure coefficient is obtained when the building face is exactly perpendicular to the wind and the external C p is decreased when the building face is in the inclined position. Maximum and minimum drag and lift coefficient are experienced when the pentagon model is oriented towards the wind direction at 0° and 45° respectively. The maximum drag coefficient is 0.892 at 0˚ wind incident angle followed by 0.822, 0.735, 0.680 and 0.678 for wind incident angles 90˚, 135˚, 180˚ and 45˚. The drag coefficient and lift coefficient values are directly related to the pressure intensity of the overall building.