30120130405023

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME

200

CFD ANALYSIS OF WIND DRIVEN NATURAL CROSS VENTILATION FOR

A GENERIC ISOLATED BUILDING

N.S. Venkatesh Kumar1, Prof. K. Hema Chandra Reddy

2

1(Research Scholar, Department of Mechanical Engg., JNTUA, Anantapuram, A.P., India)

2(Registrar, JNTUA, Anantapuram, Andhra Pradesh, India)

ABSTRACT

Natural ventilation, which provides occupants with good indoor air quality and a high level of

thermal comfort with reduced energy costs, has been drawing importance in sustainable strategy in

building designs. This investigation used computational fluid dynamics (CFD) models to study the

ventilation properties for a room with different opening configurations. The 3D steady RANS

equations are solved in combination with the shear-stress transport (SST) k- ω model. The inlet wind

velocity profile is defined according to the logarithmic law in accordance with urban climate.

Ground surface roughness is considered for the analysis. The flow around a building captured the

circulating bubbles upstream and downstream of the building, and the steady flow pressure

coefficient has the same trend as the experimental ones. Air momentum transport seems to be carried

mainly by convection and hardly by turbulent diffusion for cross ventilation.

Keywords: computational fluid dynamics, ventilation, velocity profile, flow pattern, cross

ventilation

1. INTRODUCTION

Natural ventilation, which provides occupants with good indoor air quality and a high level of

thermal comfort with reduced energy costs, has been drawing importance in sustainable strategy in

building designs and is thus attracting considerable interests from designers [1] & [2].

Wind and buoyancy are the driving forces for natural ventilation. Wind pressure differences

along the façade and differences between indoor and outdoor temperatures create a natural air

exchange between indoor and outdoor air. The ventilation rate depends on the strength and direction

of these forces and the resistance of the flow path. Prediction of ventilations rates is difficult as these

physical processes are complex. Hence, it is challenging to control natural ventilation in order to

obtain the required indoor environment conditions.

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING

AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)

ISSN 0976 – 6359 (Online)

Volume 4, Issue 5, September - October (2013), pp. 200-207

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The configurations of building rooms and especially the location of inlet and outlet openings

in relation to dominant wind direction at the site have major effects on the ventilation rates in

buildings. Studies identified that locating Inlet openings near high-pressure surfaces of a building

and exit openings at low-pressure ones produces higher flow rates through windows. Accordingly, an

understanding of air flow around the building is necessary to design well-ventilated residences.

External flows around buildings are very complicated involving severe pressure gradients, streamline

curvature, swirl, separation and reattachment together with the resulting effects of enhancing and

suppression of turbulence.

Empirical models, experimental measurements and computational fluid dynamics (CFD)

simulations are the three approaches available to study natural ventilation. The empirical models, as

reviewed by [3], are often developed from analytical solutions and experimental data. Although the

models are very useful for natural ventilation design, they could not provide sufficient information

on natural ventilation and may not be so accurate. It has been proven that the experimental

measurements are effective as a tool to obtain realistic information about natural ventilation [4]. The

measurements are not only very expensive but also time consuming. Added to this, the data may not

be in great enough detail for understanding the mechanism of natural ventilation.

CFD has a number of clear advantages compared with the other approaches: (1) as opposed

to most experimental techniques including Particle Image Velocimetry (PIV) , CFD provides field

data as regards whole-flow, i.e. data on the relevant parameters in every point of the computational

domain; (2) CFD avoids the sometimes incompatible similarity requirements in reduced-scale testing

because simulations can be performed at full scale; and (3) CFD allows full control over the

boundary conditions and easily and efficiently allows parametric studies to be performed[5]. CFD

models are currently most popular and particularly suited for studying indoor air quality and natural

ventilation, as these are difficult to be predicted using other models. For the above reasons, many

studies on evaluating and optimizing the natural ventilation potential of buildings have employed

CFD.

In view of above factors, the present work is aimed to employ computational fluid dynamics

to determine the distribution of steady, three-dimensional and turbulent in-room and external flow

field of an isolated generic building. Although the effects of neighboring buildings can be significant,

only an isolated room is considered for the present work to test the basic natural ventilation rules as

affected by relative opening location and different sizes. The CFD simulations are carried out using

ANSYS-CFX, CFD software. The two-equation SST turbulence model is used.

2. CFD SIMULATIONS

Computational settings and parameters for the reference case are outlined and accordingly the

results for this case are presented. Later on, these settings and parameters will be systematically

modified for the parametric study.

2.1. Computational domain and grid The dimensions of the generic isolate building and computational domain were chosen from

[5]. The buildings had dimensions W x D x H = 100 x 100 x 80 mm³ and thickness of the walls is 2

mm (reduced scale) corresponding to full-scale dimensions W x D x H = 20 x 20 x 16 m³.

Computational domain dimensions are calculated as width WD = W +10H, height HD = H + 5H and

upstream and downstream length of 3H and 15H .The resulting dimensions of the domain were W x

D x H = 0.9 x 1.54 x 0.48 m³ (reduced scale), which corresponds to 180 x 308 x 96 m³ in full scale.

The computational grid is made of unstructured mesh with Hexa-core, tetrahedral surface mesh and

prismatic elements on building walls. The prismatic elements at wall boundaries of building are able



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to provide fine mesh for capturing boundary layer. The geometry and mesh is generated using ICEM

CFD, meshing software.

Fig.1: Surface mesh for domain (tetrahedral) Fig.2: Volume mesh on central plane

Fig.3: Dense mesh around

2.2. Boundary conditions The inlet boundary conditions used in the simulations were based on the measured incident

vertical profiles of mean wind speed and turbulence intensity. The inlet wind velocity profile is

defined according to the logarithmic law with Y0 = 0.025 mm (Eq. 1), where 0.3627[m s^-1] is the

ABL friction velocity, the von Karman constant (0.42) and y the height coordinate.

Expression incorporated in CFX ;

(0.3627[m s^-1]/0.42)*ln((y+0.000025[m])/0.000025[m]) (1)

For the ground surface, the no-slip wall functions with roughness height of 0.28 mm are

chosen. No-slip wall functions are also used at the building surfaces, but with zero roughness height.

Zero static pressure is applied at the outlet plane. Free-slip wall functions are used at the remaining



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domain surfaces. The computational domain with boundary condition and velocity profile at inlet is

as shown below.

Fig.4: Applied boundary conditions Fig.5: velocity profile at inlet

2.3. Solver settings

The 3D steady RANS equations were solved in combination with the shear-stress transport

(SST) k- ω model. Advection scheme adapted for solving momentum governing equations is High

resolution and for turbulence equations first order scheme. The convergence criteria are taken as 10-6

.

3. RESULTS AND DISCUSSION CFD simulations of wind driven natural cross ventilation for generic isolated building are

carried out using ANSYS CFX, CFD software. The results presented here are for three cases named

as case I, case II and Case III. Case I refers to cross ventilation with equal area of inlet and exit

openings, both are situated at center of wall. Case II is with larger opening and small inlet area and

situated same as in Case I. The third one represents for equal inlet and exit opening areas and inlet is

situated at bottom center and top center of front and rear wall. The pressure and velocity distributions

are shown by contour plots and velocity vectors.

3.1. Case I The velocity vector field and pressure contours in the vertical center plane are presented

below. Main features of the flow simulated are the standing vortex upstream of the building, the

contraction and expansion of the indoor flow, the recirculation zone behind the room and the

separation zone on the roof. External flow features resembles that of square block.

Fig 6: central plane vertical



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Fig 7: pressure distribution in CPV

The velocity vector field shows the inlet velocity profile which varies with the height and

ground surface roughness which accounts the nature of surroundings. The results are thus expected

to be varying in accordance with building height and surrounding ground roughness.

Fig: 8 Velocity vector in CPV around the building and in room

The below figures show velocity vector field, velocity and pressure contour in the horizontal

plane at height of 0.04 m. It can be seen that only the area spanned by the cylindrical stream tube is

actually ventilated. The ventilated zones are generally referred as active and non-ventilated as calm

or dead zone. The blue color zone represents the dead zone. The red color in pressure contour

represents stagnation flow field which is located at top of front wall and on two sides of opening.




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Fig: 10 Pressure vector in CPV around the Fig: 10 Velocity contour in CPV around the

building and in room building and in room

3.2. Case II The results presented are for cross ventilation with increased area for exit openings. Supply

and exhaust window location are at the center of opposite walls. The below figures shows the air

flow vector field velocity and pressure contours in the vertical center plane. The plan of the

ventilated zone seems to have larger area at exit and looks resembles a trapezoid. Transport of air

momentum seems to be carried mainly by convection and hardly by turbulent diffusion.




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Fig: 11 Pressure contour in CPV around the Fig: 12 Velocity contour in CPV around the

building and in room building and in room

Fig: 13 Velocity vector field in the horizontal Fig: 14 Pressure contour in the horizontal

plane plane at height of 0.04 m at height of 0.04 m

3.3. Case III The results presented are for cross ventilation with equal inlet and exit openings. Inlet

opening located at bottom ad outlet at top. The below figures shows the air flow vector field and

pressure contours in the vertical center plane. There is an increased area of the calm zone at center of

room as compared to Case I and fresh air is more attached towards the wall. The quality of air inside

of room is decreased. The external flow features are same as that of other cases.

Fig: 15 Velocity vector in the vertical center plane



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Fig: 16 Velocity contour in the vertical center plane

4. CONCLUSIONS

The model for flow around a building captures the circulating bubbles upstream and

downstream of the building, and the steady flow pressure coefficient has the same trend as the

experimental ones. The two-equation turbulence model is a useful tool for predicting in-room flows.

Cross ventilation is useful for ventilating the areas passed by the stream tube between inlet

and exit openings, but outside the stream tube recirculation zones with no domination of fresh air.

The low turbulence level is not enough to enhance momentum transfer from the stream tube to the

otherwise stand still air outside the tube.

Based on the locations of humans inside the room, the wind incidence angle can cause an

increase in velocity magnitude and reduction of calm air zones inside the room.

REFERENCES

[1] Etheridge, D., Sandberg, M, Building ventilation: theory and measurement (New York: John

Wiley and Sons, Chichester; 1996).

[2] Allard, F,.Natural ventilation in buildings: a design handbook (UK: James & James, London,

1998).

[3] Allocca, C, Single-sided natural ventilation: design analysis and general guidelines, M.Sc.

Thesis, Department of Mechanical Engineering, Massachusetts Institute of Technology,

Cambridge, MA. 2001.

[4] Katayama T., Tsutsumi J., Ishii A., Full-scale measurements and wind tunnel tests on cross-

ventilation (J. Wind Eng. Ind. Aerodyn. 1992, 41-44) 2553-2562.

[5] Ramponi R, Blocken B., CFD simulation of cross-ventilation for a generic isolated building:

impact of computational parameters, Building and Environment, 2012, 53, 34-48.

[6] Tarun Singh Tanwar, Dharmendra Hariyani and Manish Dadhich, “Flow Simulation (CFD) &

Static Structural Analysis (FEA) of a Radial Turbine”, International Journal of Mechanical

Engineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 252 - 269, ISSN Print:

0976 – 6340, ISSN Online: 0976 – 6359.

[7] P.S. Jeyalaxmi and Dr.G.Kalivarathan, “CFD Analysis of Flow Characteristics in a Gas

Turbine- A Viable Approach to Predict the Turbulence”, International Journal of Mechanical

Engineering & Technology (IJMET), Volume 4, Issue 2, 2013, pp. 39 - 46, ISSN Print:

0976 – 6340, ISSN Online: 0976 – 6359.

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