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Computational evaluation of wind pressures on tall buildings

Authors:

Abstract

At present, the wind engineering toolbox consists of wind-tunnel testing of scaled models, limited full-scale testing, field measurements, and mechanical load/pressure testing. The evolution of computational wind engineering (CWE) based on computational fluid dynamics (CFD) principles are making the numerical evaluation of wind loads a potentially attractive proposition. This is particularly true in light of the positive development trends in hardware and software technology, as well as numerical modeling. The present study focuses on numerical evaluation of wind pressures on tall buildings by using the Commonwealth Advisory Aeronautical Council (CAARC) building model (Melbourne, 1980). The CARRC model has been used extensively to study wind loading on tall buildings in wind tunnel studies and is usually adopted for calibration of experimental techniques. Numerically obtained pressure coefficients on the surface of CAARC building under different configurations of adjacent building are compared with wind tunnel data collected at the RWDI USA LLC laboratory for the present study and from literature. The present numerical simulation uses mostly Reynolds Averaged Navier-Stokes equations (RANS) and Large Eddy Simulation (LES) for selected few cases.
1
Computational evaluation of wind pressures on tall buildings
Agerneh K. Dagnew 1, Girma T. Bitsuamalk2, Ryan Merrick3
1Research Assistant, Civil and Environmental Engineering department (CEE), International
Hurricane Research Center (IHRC), Florida International University (FIU), Miami, Florida,
USA, adagn001@fiu.edu
2Assistant Professor of Wind/Structural Engineering, CEE, IHRC, FIU, Miami, Florida, USA
bitsuamg@fiu.edu
3Senior Technical Coordinator, RWDI USA LLC, Miramar, Florida, USA,
Ryan.Merrick@rwdi.com
ABSTRACT
At present, the wind engineering toolbox consists of wind-tunnel testing of scaled models, limited
full-scale testing, field measurements, and mechanical load/pressure testing. The evolution of
computational wind engineering (CWE) based on computational fluid dynamics (CFD)
principles are making the numerical evaluation of wind loads a potentially attractive
proposition. This is particularly true in light of the positive development trends in hardware and
software technology, as well as numerical modeling. The present study focuses on numerical
evaluation of wind pressures on tall buildings by using the Commonwealth Advisory
Aeronautical Council (CAARC) building model (Melbourne, 1980). The CARRC model has been
used extensively to study wind loading on tall buildings in wind tunnel studies and is usually
adopted for calibration of experimental techniques. Numerically obtained pressure coefficients
on the surface of CAARC building under different configurations of adjacent building are
compared with wind tunnel data collected at the RWDI USA LLC laboratory for the present
study and from literature. The present numerical simulation uses mostly Reynolds Averaged
Navier- Stokes equations (RANS) and Large Eddy Simulation (LES) for selected few cases.
KEY WORDS: Computational fluid dynamics, boundary layer wind tunnel, wind pressure, tall
building, RANS, LES
INTRODUCTION
Buildings, bridges, large span roof structures and other civil structures must be able to withstand
the external loads imposed by nature, at least to the extent that the disastrous damage of natural
force reduced to the acceptable limit (Irwin, 2007). Wind is one of the major forces responsible
for the catastrophic failure and loss of life. Therefore, accurate evaluation and prediction of wind
loads and proper mitigations are very important in reducing the adverse effects of wind in the
built environment. In addition to the mean and the background (due to turbulent fluctuations)
loads on rigid buildings, flexible structures will be subjected to resonant wind loads as well. The
primary source of the along-wind motion is the pressure fluctuations in the windward and
leeward faces due to the fluctuation in the approach flow and its interaction with the building.
Building codes and Standards usually provide loads along the wind direction for common shapes
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in open and suburban terrain category perhaps with the exception of the AS-NZ 2002 code which
attempts to provide provisions in for the cross-wind direction as well. The cross-wind motion is
mainly caused by fluctuations in the separating shear layers. Torsional motion can be caused due
to imbalance in the instantaneous pressure distribution on each face of the building either due to
oblique wind directions, unsteadiness in the approaching flow, partial sheltering and interference
from surrounding buildings or due to own shape and dynamic structural properties. Studies
shows that for many high-rise buildings, the crosswind and torsional response may exceeds the
along wind response in terms of both limit state and serviceability requirements (Kareem, 1985).
Nevertheless, most existing standards, including ASCE07-05, only provides procedure for
evaluation of along-wind effects. For complex cases, the standards refer to physical model
testing in boundary layer wind tunnel (BLWT) facility.
The success of application of computational fluid dynamics (CFD) in aeronautical engineering is
very encouraging. Acknowledging the difference between streamlined and bluff body flows, the
use of computational fluid dynamics for predicting wind effects in the atmospheric boundary
layer appears very promising. This is particularly so considering the recent advances in hardware
and software technology, development of reliable sub-grid turbulence models and numerical
reproduction of inflow turbulence (Tamura 2008). At this stage of CWE application, however, a
systematic validation of CWE models through comparison with wind tunnel experiments shall
continue to enhance the confidence and warrant its use for practical applications.
Significant progress has been made in the application of CWE to evaluate wind loads on
buildings (e.g. Murakami and Mochida 1988; Selvam 1997; Stathopoulos 1997; Wright et al.
2003; Camarri et al. 2005; Tamura 2006; Tutar and Celik 2007; El-Okda et al. 2008; Tominaga
et al. 2008a; Cóstola et al. 2009 and others). Significant progress has also been made on the
evaluation of wind load modifications due to topographic elements (Bitsuamlak et al. 2004,
2006; Poggi and Katul 2007). Some countries have already established working groups to
investigate the practical applicability of CWE and develop recommendations for their use for
wind resistant design of actual buildings and for assessing pedestrian level winds, within the
framework of the Architectural Institute of Japan (AIJ) (Tamura et al. 2008, Tominaga et al.
2008b`) and the European cooperation in the field of scientific and technical (COST) research
(Franke et al. 2004). Further, AIJ provides methods for predicting wind loading on buildings by
the Reynolds Averaged Navier Stokes equations (RANS) and LES. Practical applications of
CWE are widespread in areas such as pedestrian level wind evaluation Chang (2006), Lam and
To (2006), Blocken and Carmeliet (2008), Yoshie et al. (2007), and Tablada et al. (2009), where
only the mean wind speeds are required for evaluating pedestrian comfort (Stathopoulos and Hu
2004). CWE applications for wind driven rain are reported by researchers such as Choi (2000),
and Blocken and Cameliet (2004). Some CFD wind flow studies for urban neighborhood
include Zhang et al. (2006), Huang et al. (2006), and Jiang et al. (2008). While most of studies
mentioned above focus on straight winds, studies by Lin and Savory (2006), and Hangan and
Kim (2008) focused on simulation of downburst. Other common uses of CWE, include
augmentation of experimental wind engineering research: Sengupta and Sarkar (2008)
augmented their microburst and tornado wind simulator facility with numerical simulation;
Merrick and Bitsuamlak (2008) used numerical simulation to facilitate selection of an artificial
surface roughness length to be applied on curved surfaced buildings during wind tunnel testing
as a means to compensate for High Reynolds number effects that is usually missing from low
wind speed tunnels.
3
The majority of numerical prediction of wind pressure loads on buildings devoted on the basic
cube shape, because of its geometrical simplicity yet representing the complex features of
building aerodynamics and availability of experimental data (Stathopoulos 2002; Nozawa and
Tamura 2002; Lim et al. 2009). These include full-scale low-rise building structures such as
Silsoe Cube (Wright and Easom 1999, 2003), Texas Tech University (Senthooran et al. 2004)
and Wall of Wind test building (Bitsuamlak et al. 2008). Some of the computational studies for
Tall Buildings include studies by Nozawa and Tamura (2002), Huang et al., (2007), Tominaga et
al., (2008), Tamura (2008) and Braun et al. (2009). Huang et al. (2007) and Braun et al. (2009)
focused on the aerodynamics of Commonwealth Advisory Aeronautical Council (CAARC)
building model and investigate the flow pattern and mean and rms pressure coefficient. CAARC
tall building is considered as one of the most extensively studied building model and popular in
wind tunnel researcher community as well (Wardlaw and Moss 1970 as referenced by Braun et
al. 2009; Melbourne 1980; Obasaju 1992). As a result, the present study also uses the CAARC
building model both for the numerical as well as boundary layer wind tunnel investigation thus
allowing a comparative study among different researcher results which is required perhaps at this
stage of CWE application for building aerodynamics. Different from previous CAARC studies in
literature which focuses on isolated building, the present study include experimental and
numerical aerodynamic analysis of CAAR building model under different orientation of adjacent
building conditions. While the numerical simulation uses Fluent 6.3 commercial software and
employs mostly RANS and LES (for limited case) turbulence models, the experimental part
include boundary layer wind tunnel experiments conducted at RWDI USA LLC, Miramar FL for
the present study.
Unlike flow around streamed line object, analysis of flow around sharp edged bluff-body
involves many difficulties as pointed out by Murakami (1998) and Stathopoulos (1997).
Intensive studies have been done on the suitability of various turbulence models (Murakami and
Mochida 1989; Murakami 1998; Castro and Graham 1999; Saha and Ferziger 1996, 1997).
RANS has been used in wind engineering application due to their simplicity in modeling and
reduced computational cost. Despite the computational cost, LES (large eddy simulation) is
believed to have reached the stage where analysis can be carried out within a reasonable
computational time for complicated turbulent flow around actual buildings and should be
considered as a complementary tool for wind loads evaluation (Tamura, 2008).
NUMERICAL MODEL
Based on a preliminary exploratory study on surface mounted cube, different turbulence models
from literature have been examined for their relative suitability for the present bluff body
application. Figure 1 shows comparison between numerically obtained pressure coefficients on
surface mounted cube by several researches with experimental results. As expected LES
provides superior results that agree well the experimental data obtained from wind tunnel and
full-scale tests. From RANS family of models the RNG k- provides relatively better result
compared to the standard k- model. In the present study RNG k- has been therefore opted due
to its relatively good agreement with BLWT compared to Standard k- and relatively lower
computational resource demand compared to LES. Only for comparison purpose LES has been
applied for Case 1A and Case 1B. Due to the chaotic nature of flow around bluff bodies, the
turbulence flows significantly affected by the presence of walls. The near-wall treatment has a
great impact on the result of the numerical simulation. Murakami et al. (1999) discussed the
difficulty in using no-slip boundary condition at solid walls for flows with high Reynolds
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number, such as the present case. For such flow environment, Werner and Wengle (1991) wall
functions in the viscous sub layer (as provided by Fluent 6.3), which will reduce the use of
excessive fine meshes around the solid walls can be used. While for Case 1A, very fine meshes
are used in the near-wall region in attempt to resolve the low Reynolds number flow up to the
wall surface and Werner and Wengle (1991) wall functions are applied for Case 1B. Case 2 and
3 uses non-equilibrium wall functions as described in FLUENT user manual (2006).
Figure 1 Comparison of mean pressure coefficients by several researchers using several turbulence models.
NUMERICAL AND WIND TUNNEL MODEL SETUPS
The geometrical modeling for the numerical simulation of CAARC building model mimics the
1:400 scale rigid model of the wind tunnel testing. The CAARC model has a rectangular
prismatic shape with dimensions 100 ft (x) by 150 ft (y) by 600 ft (z) height. The flow is
described in a Cartesian coordinate system (x, y, z), in which the x-axis is aligned with the
stream-wise direction, the z-axis is in the perpendicular direction and the y-axis is in the vertical
direction. The computational domain dimensions, boundary conditions and the wind tunnel
configurations and are given in Fig. 2. The driver (inlet), lateral and downstream boundaries are
extended laterally to minimize blockage issue on the aerodynamics of the test building. The
Reynolds number based on building height H and inflow velocity UHat
Hz
is
5
108.3 x
. Open
exposure velocity profile with power low exponent of 0.16 is used during the present BLWT test.
-2
-1.5
-1
-0.5
0
0.5
1
1.5
Cp
Bitsuamlak et al., 2008 (K-e)
Wright and Easom, 2003 (K-e)
Wright and Easom, 2003 (RNG)
Wright and Easom, 2003 (NL)
Lim et al., 2009 (LES)
Wind tunnel (Holscher et al., 1998)
Wind tunnel (Richards et al., 2007)
Full Scale (Richards et al., 2007)
A
D
C
5
Table 1: Study cases
Case Configuration Velocity profile Scale
Case 1A Isolated BLWT (Huang 2007) 1:250
Case 1B Isolated BLWT (present)
Case 1C Isolated ASCE (exposure B)
Case 1D Isolated ASCE (exposure C)
Case 2A full height adj. bldg upwind of CAARC ASCE (exposure B)
Case 2B full height adj. bldg upwind of CAARC ASCE (exposure C) 1:400
Case 2C full height adj. bldg downwind of CAARC ASCE (exposure B)
Case 2D full height adj. bldg downwind of CAARC ASCE (exposure C)
Case 3A half height adj. bldg upwind of CAARC ASCE (exposure B)
In the present study, three building configurations under four different flow fields have been
investigated. Table 1 describes all cases considered for the numerical simulation of CAARC
building model. Case 1 simulates wind effects on the isolated building without considering the
interference action of the surroundings. Case 2 simulates CAARC building with an adjacent
surrounding building having similar height with the test building placed upwind and downwind
of the study building. Case 3 simulates similar to Case 2 but the adjacent building has only half
height of the CAARC model.
The computational domain contains non uniform grid points and the grid stretching ratio for LES
simulation is less than 1.1 to avoid cut-off wave number between neighboring grids, see Fig. 3.
High mesh resolution is used in the low Reynolds viscous sub-layer zone. The inlet velocity
boundary condition comprising four different mean profiles as shown in Fig. 4 has been
simulated. For transient flow simulation generation of inflow turbulence for LES analysis (Case
1A), the velocity fluctuation is generated using spectral synthesizer available in Fluent. Free out
flow boundary conditions are imposed as the downstream of the computational domain. Wall
treatments as described in Numerical Model section are used at the building wall and the ground
surface. At the top and side boundaries are assumed to have zero velocity gradient and
symmetric boundary conditions are applied.
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(a) Computational domain and boundary conditions: Case 1.
(b) Wind tunnel configurations.
Isolated test
building
7
(c) Computational domain: Case 3. (d) Computational domain: Case 2.
Figure 2 Computational and wind tunnel set up.
Figure 3 Typical grid used in the present study: Case 1A.
Horizontal section
Vertical section
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Figure 4 Inflow boundary condition (velocity profiles) used in the present study.
RESULT AND DISCUSSIONS
For validation and comparison purposes, both standard  and LES simulations have been used
for Case 1A and Case 1B. Measurements of mean pressure coefficients normalized by the
dynamic pressure at the building height are compared with the present BLWT data as well as
those obtained from literature. The mean pressure coefficients, Cp, on the upper windward,
sidewall and leeward faces are extracted for the isolated building case at 2/3H of the building as
shown in Fig. 5 for various inflow boundary conditions. As shown in Fig. 5, there is a good
agreement between the present LES as well as Braun’s (2009) and Huang’s (2007) numerical
simulation results of the current study with the experimental results on the windward face. The
agreement deteriorates slightly at the sidewalls and improves at the lee-ward wall. The standard
 method over-predicts the pressure coefficient at the stagnation point and at flow separation
point.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
8 9 10 11 12 13 14 15 16
U(m/s)
Z(m) - Model scale
BLWT (Huang et aL., 2007)
BLWT (Present study)
ASCE (Exposure - C)
ASCE (Exposure - B)
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Figure 2. Comparison of mean pressure coefficient.
Figure 3. Mean velocity contour at mid-vertical and 2/3 H horizontal section.
Figure 5 Measurements of mean pressure coefficients over the perimeter at Z/H =2/3: Velocity pressure
normalized by UH (12.7m/s)
Windward wall Leeward wall
Figure 6 Contour plot of mean pressure distribution over the building walls (Case 1A)
-2
-1.5
-1
-0.5
0
0.5
1
1.5
0 0.5 1 1.5 2 2.5 3 3.5 4
X'/Dx
Cp
Pre sent study (ke - C a se 1A )
Pre sent study (LES - C ase 1A)
Pre sent study (LES - C ase 1B)
Bra un et a l., (2009)-numerical
Huang et al., (2007)-numerical
Wind tunnel (B LW T- P resent st udy)
Wind tunnel (Tong Ji Unive rsity)
10
-2
-1.5
-1
-0.5
0
0.5
1
1.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
X'/Dx
Mean Cp
Case 1B (RNG )
Case 1C (Expo sure-B)(RNG)
Case 1D (Expo sure-C)(RNG)
Wind Tunnel (Prese nt study)
Figure 6 shows mean-pressure contour pots of the windward and leeward face of configuration
Case 1A. The horseshoe vortex shape contour generated on the front wall agrees well with
Huang et al. (2007) and Braun et al. (2009) flow field investigation. To asses the effects of
inflow boundary conditions, the present work studies four different inflow conditions as outlined
in table 1. Where each of the categories varies in their velocity profile, turbulence intensity and
turbulence integral length scale. Figure 7 exhibits the comparison of mean pressure coefficients
obtained by numerical simulation with the wind tunnel data. Among all, there is reasonable
agreement between Case 1C and the experiment on the windward face. As expected there is
substantial increase in prediction of mean Cp by the numerical method.
Figure 7 Measurements of mean pressure coefficients over the perimeter at Z/H =2/3 (Case 1): Velocity
pressure normalized by UG (14.63 m/s)
Effect of adjacent building plays an important role on the aerodynamic response of buildings.
Each building site is unique and needs to be examined case by case. In the present study a
limited attempt was made to assess the effect of different adjacent building with different height,
Case 2 with same height as with the study building and Case 3 with half height. The numerical
as well as the wind tunnel test results, reveals the sheltering effect due to the presence of tall
structures upwind of the study building as shown in Fig. 8. Approximately a 150% reduction in
mean pressure coefficient has been observed relative to the isolated case. Figure 9 presents the
same case but when the adjacent building is placed in the leeward direction of the flow. In this
case the interference has no significant on the front wall pressure distribution but there is
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Figure 8 Comparison of mean pressure coefficients over the perimeter at Z/H =2/3 (Case 2A & 2B): Velocity
pressure normalized by UG(14.63 m/s).
Figure 9 Measurements of mean pressure coefficients over the perimeter at Z/H =2/3 (Case 2C & 2D):
Velocity pressure normalized by UG (14.63 m/s)
-2
-1 . 8
-1 . 6
-1 . 4
-1 . 2
-1
-0 . 8
-0 . 6
-0 . 4
-0 . 2
0
012345
X ' /D x
M e an C p
W ind Tu nne l (P r e se nt stud y)
C ase 2 A (E xp o sur e -B )(R NG )
C ase 2 B (E xp o sur e -C)(RN G)
-2
-1. 5
-1
-0. 5
0
0. 5
1
1. 5
0 0. 5 1 1 .5 2 2 .5 3 3 .5 4 4. 5 5
X '/D x
Me an C p
W i nd tunn e l (Pr e se nt s tud y)
C a se 2 C (E x po s ure -B )
C a se 2 D (E x po s ure -C )
12
reduction in suction pressure coefficient. The presence of the half height adjacent building have
less significant impact on the mean pressure load compared to the effect of full height adjacent
building as can be expected.
Figure 10 Comparison of mean pressure coefficients over the perimeter at Z/H =2/3: Surrounding
interference effects
Mean pressure contours on the building surface are shown in Figs 11a and 11b for Cases 2 and 3
respectively. Flow modification due to complex fluid structure interactions including the
adjacent buildings are also shown in Figs. 11c and 11d for Cases 2 and 3 respectively. Flow
separation points, down washes on windward walls, recirculation behind buildings that are
signature of tall buildings are clearly depicted in these figures. These capabilities of CFD
simulations are very useful in explaining aerodynamics phenomena among wind engineers and to
structural engineers and architects.
-2
-1.5
-1
-0.5
0
0.5
1
1.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
X'/Dx
Mean C p
Case 1B (BLWT - Present study)
Case 1B (Exposure - B)(RNG)
Case 2 (BLWT - Present study)
Case 2A (Exposure - B)(RNG)
Case 3 (BLWT - Present study)
Case 3A (Exposure - B)(RNG)
13
(a) Case 2 (b) Case 3
Mean pressure coefficients
(c) Velocity path lines: Case 2
(d) Velocity path lines: Case 3
Figure 11 Mean pressure coefficient and velocity path lines: Case 2& Case 3
14
CONCLUSIONS AND FUTURE WORKS
Several comparisons between CFD and experimental analysis have been discussed in order to
better understand the current state-of-the-art and assess the potential use of numerical wind load
predictions approaches for practical use. In the past, consistent efforts have been made to make
CFD a better tool for evaluation of wind loads. The application of CFD techniques to predict
wind load, on CAARC model and with different surrounding conditions reveals the suitability of
CFD tools for preliminary assessments and detail explanation of complex building aerodynamic
characteristics. Consistent to reports in literature, numerical data comparison with
measurements in boundary wind tunnel carried out in the present study suggests that an accurate
time-dependent analysis, such as LES analysis is very vital. Planned future studies include detail
LES analysis of buildings incorporating various orientation of surrounding effects.
ACKNOWLEDGMENT
The support from National Science Foundation (through NSF Career award to the second author)
is gratefully acknowledged.
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... The k-ε model was used for modeling the airflow viscosity in this study. The successful use of the k-ε model for analyzing the tall building model can be found in Dagnew et al. (2009), Irtaza et al. (2021, and Mou et al. (2017). Table 1 shows the overall boundary conditions used for this study. ...
... The validation exercise was performed to ensure the reliability of the input parameters in ANSYS. In this case, the Commonwealth Advisory Aeronautical Council (CAARC) standard tall building model results tested by Dagnew et al. (2009) using wind tunnel test (WTT) and CFD was used (Figure 12). The validation model's Reynolds number at 3.8 x 10 5 was based on building height H and the inflow velocity U H at z = H, as stated by Dagnew et al. (2009). ...
... In this case, the Commonwealth Advisory Aeronautical Council (CAARC) standard tall building model results tested by Dagnew et al. (2009) using wind tunnel test (WTT) and CFD was used (Figure 12). The validation model's Reynolds number at 3.8 x 10 5 was based on building height H and the inflow velocity U H at z = H, as stated by Dagnew et al. (2009). Other input parameters were generated using data from Parameters such as air density and solid material type were assumed to suit the condition. ...
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... The CFD model used within the current study was compared to apreviously published research paper that provided an experimental andnumerical comparison of mean pressure coefficient Cp within the wind tunnel [13]. This comparison wasperformed to examine the validity and applicability of the current CFDmodel. ...
... This comparison wasperformed to examine the validity and applicability of the current CFDmodel. The standard k-ε turbulence model is employed in thesimulation and the results are compared against the experimental results [13], as presented inFigure 5. This model waspicked from a pool of several different turbulence models because it matchesthe experimental data well. ...
... Standard k-ε Simulation Results Compared toWind Tunnel Measurements[13] ...
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... A benchmark high-rise building model was developed and validated with the experimental data of [16]. The benchmark model is used by many researchers, analysing the wind flow around tall buildings [17,18]. Various vegetation configurations will be incorporated onto the high-rise building model with a skygarden. ...
... Wind distribution is an essential factor to define comfort [52]. Existing wind profile in previous CFD simulations used by Dagnew [17] was used to create a suitable wind velocity profile (Figure 4). Given as Equation 11, the atmospheric boundary layer velocity profile based on the power-law was used [31]. ...
... The validated high-rise model was then used to incorporate the skygarden model. Table 4 and compared with the study by Huang et al. [18] Validation of Vegetation and Trees The vegetation model was validated against the study by Manickathan et al. [17], where the domain size of the reference case was 35m in length and 11.5m in height ( Figure 8). Within the 2D domain, vegetation was represented by a 1m square placed 8.5m from the inlet and 0.5m above the ground. ...
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... A velocity flow field is generated by setting one side of the enclosure as the velocity inlet and the other side as the pressure outlet. Following the works of Dagnew [52] and Huang et al. [53], the power-law wind profile was created to account for the wind speed modifications due to presence of urban surroundings, shown in Figure 4 and given by: ...
... Following the works of Dagnew [52] and Huang et al. [53], the power-law wind profile was created to account for the wind speed modifications due to presence of urban surroundings, shown in Figure 4 and given by: ...
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... Another study [4] conducted focused on the numerical analysis of tall structures utilising the Commonwealth Advisory Aeronautical Council (CAARC) building model. In the investigation, a numerical data comparison with measurements in a boundary wind tunnel revealed that an accurate time-dependent analysis is critical. ...
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... The reason being, in LES, the time-dependent inflow boundary is responsible for a more accurate representation of oncoming fluid flow properties. Hence Dagnew et al. (2009) and Swaddiwudhipong and Khan (2002) recommended LES as a complementary tool for wind load evaluation. Huang et al. (2007) performed a comprehensive analysis of the CAARC building model employing numerical simulation where the building is considered as rigid and aerodynamic coefficients and flow patterns around the building were predicted through a CFD commercial software. ...
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The study focuses on the wind loads acting on the standard Commonwealth Advisory Aeronautical Council (CAARC) building model. Numerical modeling has been used instead of employing wind tunnel experiments due to the requirement of high physical space. This is mainly due to the superiority of CFD codes included in turbulence models such as RANS and more precise LES. However, the shortcoming arising through basing the result on numerical investigations is compensated by direct comparison to experimental data available in literature. The study extends beyond the typical rigid building structure giving account to the minor deformations resulting from the impact of the wind force on an aeroelastic structure through coupled two-way fluid structure interaction, while designing the building to have its natural fundamental frequency of 0.2 Hz. An improved Smagorinsky-Lily model was used for all LES models and the results were compared with available wind tunnel experimental data. It was found that coupled LES analysis provided better correlation to wind tunnel data compared to rigid fluid domains with respect to pressure distribution and wake formation despite the additional computation time taken. However, all these readings were dependent on the boundary conditions which needs to be estimated concisely fur future use.
... Xie and Castro [13] performed numerical simulations over the CAARC building applying the Large-Eddy Simulation (LES) approach embedded in the open source CFD code OpenFOAM. They referred to the boundary conditions used for the wind tunnel test of Dagnew and Bitsuamlak (2010) [14] and they compared their results also with Huang et al. [11] . The main feature of their study is the use of the more recent inflow generation approach developed by Kim et al. in 2013 [15]. ...
Thesis
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Starting from the consolidated concept of aerodynamic model, a benchmark is proposed to further assess and extend its capability of correctly identify the dynamic response of a tall building, devoting particular attention to the contribution of higher-order modes and the possible presence of aerodynamic damping. Being the extrema ratio in terms of accuracy and reliability, a full-aeroelastic model of a tall building is presented as the subject of comparison. The complexity involved in the dynamic wind-tunnel scaling led to the definition of a novel semi-automatized procedure. Based on the author’s experience, developed during the project, each step comes with practical advice, often challenging to find in the scientific literature, and food for thought on the worthiness of the design-oriented aeroelastic modeling approach. The design, construction, identification, and validation of a 1:360-scale, a four-level lumped-mass aeroelastic model of the well-known Caarc standard tall building, is presented. Differently to the numerous previous research involving the Caarc building, here it is disclosed in a new guise, featuring torsional and second-order modes, enhancing the challenge for the aerodynamic model test. An extensive experimental campaign is performed at the CRIACIV boundary layer wind tunnel. Tests are performed in turbulent flow for a wide range of velocities and varying the structural damping to be able to address the results for different design criteria. Although the aerodynamic model is generally found to provide useful insight in the building response, the presence of aerodynamic damping and second-order modes are found to be relevant both in terms of base moments and acceleration. From a design perspective, even for "not exceptional" tall buildings, such as the Caarc, the aerodynamic model seems a valid option for early design stages, while the adoption of an aeroelastic model might be a valuable solution in the refinement of loads or serviceability criteria.
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The aim or objective of the project is to determine interference among two adjacent tall buildings. Computational fluid dynamics was used after modelling the structures in stiff full-scale forms. Using the interference effects from another square plan tall buildings of the same height at 0° WIA, ANSYS CFX analyses pressure variations on sidewalls of a square plan tall building. It can be said that these buildings are closely located – from 0.4h to h, h being a height of a building. Using the square plan shaped building in its isolated condition as a point of comparison, the analysis also compares it to square shape buildings located in other places. Comparisons are illustrated using the interference factors (IF) and IF contours. At times there is recorded a deviant pressure distribution. Apart from that, it is noticed that the interfering buildings have a shielding and channelling influence over the square plan formed building. When there is interference on square plan-shaped building faces, the pressure distribution becomes unpredictable. We get the Cp for each face of the square plan-shaped building in each interfering situation by simply multiplying IF and Cp separately with the integral factor of two situations in an isolated case.
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The paper describes a study about effects of upstream hills on design wind loads using two mathematical approaches: Computational Fluid Dynamics (CFD) and Artificial Neural Network (NN for short). For this purpose CFD and NN tools have been developed using an object-oriented approach and C++ programming language. The CFD tool consists of solving the Reynolds time-averaged Navier-Stokes equations and turbulence model using body-fitted nearly-orthogonal coordinate system. Subsequently, design wind load parameters such as speed-up ratio values have been generated for a wide spectrum of two-dimensional hill geometries that includes isolated and multiple steep and shallow hills. Ground roughness effect has also been considered. Such CFD solutions, however, normally require among other things ample computational time, background knowledge and high-capacity hardware. To assist the enduser, an easier, faster and more inexpensive NN model trained with the CFD-generated data is proposed in this paper. Prior to using the CFD data for training purposes, extensive validation work has been carried out by comparing with boundary layer wind tunnel (BLWT) data. The CFD trained NN (CFD-NN) has produced speed-up ratio values for cases such as multiple hills that are not covered by wind design standards such as the Commentaries of the National Building Code of Canada (1995). The CFD-NN results compare well with BLWT data available in literature and the proposed approach requires fewer resources compared to running BLWT experiments.
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Previous studies have shown that Computational Wind Engineering (CWE) is still in its infancy and has a long way to go to become truly useful to the design practitioner, The present work focuses on more recent studies to identify progress on outstanding issues and improvements in the numerical simulation of wind effects on buildings. The paper reviews wind loading and environmental effects; it finds that, in spite of some interesting and visually impressive results produced with CWE, the numerical wind tunnel is still virtual rather than real and many more parallel studies - numerical and experimental - will be required to increase the level of confidence in the computational results.
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Three-dimensional engineering simulations of momentum-driven tornado-like vortices are conducted to investigate the flow dynamics dependency on swirl ratio and the possible relation with real tornado Fujita scales. Numerical results are benchmarked against the laboratory experimental results of Baker (1981) for a fixed swirl ratio: S= 0.28. The simulations are then extended for higher swirl ratios up to 5 = 2 and the variation of the velocity and pressure flow fields are observed. The flow evolves from the formation of a laminar vortex at low swirl ratio to turbulent vortex breakdown, followed by the vortex touch down at higher swirls. The high swirl ratios results are further matched with full scale data from the Spencer, South Dakota F4 tornado of May 30, 1998 (Sarkar, et al. 2005) and approximate velocity and length scales are determined.
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This paper reviews the current state of the art in the numerical evaluation of wind flow over different types of topographies. Numerical simulations differing from one another by the type of numerical formulation followed, the turbulence model used, the type of boundary conditions applied, the type of grids adopted, and the type of terrain considered are summarized. A comparative study among numerical and experimental (both wind tunnel and field) existing works establishing the modifications of wind flow over hills, escarp-ments, valleys, and other complex terrain configurations demonstrates generally good predictions on the upstream but problematic predictions on the downstream areas of the complex terrain. Comparisons are also made with provisions of the current wind standards as well as with speed-up values calculated using guidelines derived from theoretical models.
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The purpose of this paper is to find a more accurate method to evaluate pedestrian wind by computational fluid dynamics approach. Previous computational fluid dynamics studies of wind environmental problems were mostly performed by simplified models, which only use simple geometric shapes, such as cubes and cylinders, to represent buildings and structures. However, to have more accurate and complete evaluation results, various shapes of blocking objects, such as trees, should also be taken into consideration. The aerodynamic effects of these various shapes of objects can decrease wind velocity and increase turbulence intensity. Previous studies simply omitted the errors generated from these various shapes of blocking objects. Adding real geometrical trees to the numerical models makes the calculating domain of CFD very complicated due to geometry generation and grid meshing problems. In this case the function of Porous Media Condition can solve the problem by adding trees into numerical models without increasing the mesh grids. The comparison results between numerical and wind tunnel model are close if the parameters of porous media condition are well adjusted.
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A hybrid RANS/LES approach, based on the Limited Numerical Scales concept, is applied to the numerical simulation of the flow around a square cylinder. The key feature of this approach is a blending between two eddy-viscosities, one given by the RANS model and the other by the Smagorinsky LES closure. A mixed finite-element/finite-volume formulation is used for the numerical discretization on unstructured grids. The results obtained with the hybrid approach are compared with those given by RANS and LES simulations for three different grid resolutions; comparisons with experimental data and numerical results in the literature are also provided. It is shown that, if the grid resolution is adequate for LES, the hybrid model recovers the LES accuracy. For coarser grid resolutions, the blending criterion appears to be effective to improve the accuracy of the results with respect to both LES and RANS simulations.
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This article synthesizes the literature on the meteorology, experimental simulation, and wind engineering ramifications of intense downburst outflows. A novel design of a large-scale test facility and experimental evidence of its validity are presented. A two-dimensional slot jet is used to simulate only the outflow region of a downburst. Profiles of mean velocity and turbulence quantities are acquired using hot-wire anemometry. Comparison with the literature provides empirical evidence that supports the current approach. A geometric analysis considers the validity of applying a two-dimensional approximation for downburst wind loading of structures. This analysis is applicable to power transmission lines in particular. The slot jet concept can be implemented in a large boundary layer wind tunnel to enable large-scale laboratory experiments of thunderstorm wind loads on structures.
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The present study aims to generate turbulent inflow data to more accurately represent the turbulent flow around a square cylinder when the inflow turbulence level is significant. The modified random flow generation (RFG) technique in conjunction with a previously developed LES code is successfully adopted into a finite element based fluid flow solver to generate the required inflow turbulence boundary conditions for the three-dimensional (3-D) LES computations of transitional turbulent flow around a square cylinder at Reynolds number of 22,000. The near wall region is modelled without using wall approximate conditions and a wall damping coefficient is introduced into the calculation of sub-grid length scale in the boundary layer of the cylinder wall. The numerical results obtained from simulations are compared with each other and with the experimental data for different inflow turbulence boundary conditions in order to discuss the issues such as the synthetic inflow turbulence effects on the 3-D transitional flow behaviour in the near wake and the free shear layer, the basic mechanism by which stream turbulence interacts with the mean flow over the cylinder body and the prediction of integral flow parameters. The comparison among the LES results with and without inflow turbulence and the experimental data emphasizes that the turbulent inflow data generated by the present RFG technique for the LES computation can be a viable approach in accurately predicting the effects of inflow turbulence on the near wake turbulent flow characteristics around a bluff body.
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The effect of buildings on flow in urban canopy is one of the most important problems in local/micro-scale meteorology. A large eddy simulation model is used to simulate the flow structure in an urban neighborhood and the bulk effect of the buildings on surrounding flows is analyzed. The results demonstrate that: (a) The inflow conditions affect the detailed flow characteristics much in the building group, including: the distortion or disappearance of the wake vortexes, the change of funneling effect area and the change of location, size of the static-wind area. (b) The bulk effect of the buildings leads to a loss of wind speed in the low layer where height is less than four times of the average building height, and this loss effect changes little when the inflow direction changes. (c) In the bulk effect to environmental fields, the change of inflow direction affects the vertical distribution of turbulence greatly. The peak value of the turbulence energy appears at the height of the average building height. The attribution of fluctuations of different components to turbulence changes greatly at different height levels, in the low levels the horizontal speed fluctuation attribute mostly, while the vertical speed fluctuation does in high levels.
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This paper presents numerical results of pedestrian-level wind environment around the base of a row of tall buildings by CFD. Four configurations of building arrangement are computed including a single square tall building. Computed results of pedestrian-level wind flow patterns and wind speeds are compared to previous wind tunnel measurement data to enable an assessment of CFD predictions. The CFD model uses the finite-volume method with RNG k-ε model for turbulence closure. It is found that the numerical results can reproduce key features of pedestrian-level wind environment such as comer streams around corners of upwind building, sheltered zones behind buildings and channeled high-speed flow through a building gap. However, there are some differences between CFD results and wind tunnel data in the wind speed distribution and locations of highest wind speeds inside the corner streams. In locations of high ground-level wind speeds, CFD values match wind tunnel data within ±10%.