Computer Simulation of Air Flow in and Around Building

AN ASSIGNMENT

COMPUTER SIMULATION OF AIR FLOW IN AND AROUND BUILDING

Submitted by:

ADELEKE ABIODUN ADEYEMI

ARC/04/4315

Course:

APPLIED CLIMATOLOGY [ARC 810]

The Department Of Architecture, Federal University of Technology, Akure

In Partial Fulfillment of M. Tech Degree in Architecture

Lecturer in Charge: Prof. Ogunsote

September, 2011

TABLE OF CONTENTS

1.0 Introduction

2.0 CFD in Building Design

3.0 Dynamic Thermal Simulation

4.0Lighting, Day Lighting and Shading

5.0The Use of CFD

6.0Investigation of Wind Load on Buildings and Sculptures

7.0Investigation of Fires and Smoke Eradication

8.0Ventilation and Air Conditioning Inside Building

9.0 Investigation of Comfort Parameters and Air Conditioning in An Office Block Or In Industrial Hall

10.0Method

11.0Results

12.0Physical and Computational Modelingaround Buildings

13.0Assessment of Computer Models Developed To Predict Internal Building Vulnerabilities to External CB Attacks

14.0References

1.0Introduction

Computational modelingtechniques are increasingly being used to successfully predict the internal and external conditions within and around buildings. Building services engineers, architects, developers and clients use the results to evaluate HVAC strategies to ensure specific project requirements are achieved on-site first time. Substantial savings can be made through low cost design and the avoidance of on-site re-design and re-commissioning.
Through a powerful combination of dynamic thermal simulation and CFD, both the energy performance and details of internal comfort of your design may be assessed. An analysis of the feasibility of low cost HVAC and natural ventilation strategies using real data may be performed. Once set up, design modifications may be assessed quickly and cheaply.

Similarly, the availability of daylight, artificial lighting strategies and shading/overshadowing may be rapidly assessed and optimized.

Keywords

Facade, bed, direction, tunnel, smoke gas, spreading fundament forces, loading fluctuation, optimisation.

However, to predict the internal climate in detail, CFD is the only choice. CFD alone can predict hot spots, cold spots, and draught risk and analyze flow paths vital for the assessment of HVAC schemes and natural ventilation strategies.

2.0CFD in Building Design

With ever improving computer technology, the use of CFD in the design of HVAC strategies is becoming more widespread. CFD is the only choice to obtain the important details of the internal climate which result from a particular HVAC design. Flow Analysis Ltd has the expertise necessary to fully utilize the latest CFD software for your design issues, be they internal air flows or external (wind-driven) air flows.

Once the geometry and boundary conditions have been set up, CFD may be used to obtain a 'snap-shot' of the internal climate at a particular representative time-instant. Or, if preferred, a worst case scenario may be examined, such as conditions of zero wind for a natural ventilation scheme. Once the model is set-up, design modifications can be easily incorporated and the simulation re-run at a low cost.
With ever-increasing computing power, it is now feasible to simulate the external (wind-driven) air flows around buildings (see picture above, for example). These simulations may be used to determine pedestrian comfort (increasingly required for planning approval), and pollutant or stack-exhaust dispersal.
CFD can also be used for transient calculations. This may be utilized for HVAC assessment over a small time-period, or more commonly for smoke and pollutant modeling (see the animation at the top of the page).

3.0Dynamic Thermal Simulation

The external environment of a building changes month-by-month, week-by-week, day-by-day and hour-by-hour. Due to its thermal capacity, the fabric of a building responds slowly to these changes. This in turn leads to a more gradual dynamic response of the interior climate as felt by the inhabitants. The prediction of these interior conditions is a vital component in the design of heating and cooling strategies.
Dynamic Thermal simulation is a technique which can predict these changing internal conditions over a time period of up to 1 year. The technique predicts zonal (or room) values for parameters such as air temperature, radiant temperature and air-change rates, using real weather data.

A great strength of CFD is the clear and accessible output formats: colour contours, vectors and streamlines, which are easily understood by the non-specialist. For complex 3-dimensional spaces, animations are a great asset to gain both further insight and bring the simulation to life.
Once the geometry of the building, room or zone has been set-up, it is the known state of the air at, for example inlets and heat sources, which determines the flow and temperature elsewhere. An important source of heat is of course solar radiation. The amount of solar radiation (and reflected long-wave radiation) incident on the surfaces of the building may be obtained from the dynamic thermal simulation described earlier. Other important determining factors, such as flow rate through apertures may also be obtained from a dynamic thermal simulation. Thus dynamic thermal simulation may be used to provide the boundary conditions for CFD.

4.0Lighting, Day lightingand Shading

The availability of adequate natural daylight is of vital importance to the well-being of occupants. State-of-the-art computer simulation software as used by Flow Analysis Ltd is capable of providing a full picture of the daylight factors, lux levels and daylight autonomy throughout your building with rapid turnaround times.

Of course, artificial lighting will almost certainly be required and this can be easily added to the model and optimized in terms of light levels, cost and energy usage.

Shading is also an important issue which may be fully addressed in the software used by Flow Analysis Ltd. For example, by adding surrounding buildings, the shading patterns cast by a new development on existing buildings may be determined.

5.0The Use of CFD Allows Us To:

Evaluate wind forces acting on a building to.
Reduce costs associated with construction materials.
Ensure the viability of the building’s architectural design.
Investigate effective ventilation systems.
Reduce energy costs by determining optimal configuration.
Evaluate configurations in a simulated environment reducing investigation time and costs.
Examine pollution distribution.
Evaluate environmental effect of new constructions.
Ensure that the new building will meet current and proposed environmental regulations
Evaluate smoke evacuation systems.
Ensure the safety of high capacity buildings
Investigation of ventilation in streets or in a city district.

New buildings change the ventilation of streets, often affecting the wind pattern of the whole district. Alterations in wind pattern can cause adverse effects in the local distribution of pollutants. To define these effects, traffic and meteorological data can be used to perform an atmospheric simulation of the area. Investigation of the wind pattern surrounding your project can ensure environmental compliance and provide valuable information with regards to HVAC development.

Investigations were performed among other places on a new section of the planned Budapest ring road (M0) and a new district, Budapest’s Millennium City Centre. The investigations recommended the installation of a ventilation system within a tunnel on the M0 and alterations in the positioning of air intake vents on many of the buildings in the Millennium City Centre.

6.0Investigation of Wind Load on Buildings and Structures

Wind load must be considered in the design of large commercial buildings particularly those with extensive tent membranes. Although wind tunnel experiments provide accurate and valuable wind load information, the process does not allow a great deal of flexibility. Therefore, wind tunnels are primarily used as a means to validate CFD results. Computer simulations make it possible to explore complex physical phenomena such as the natural flows induced by a temperature gradient or wind blasts and fluid structure interactions (FSI). FSI has crucial importance in the design of several engineering systems since wind loads of bridges and other structures may cause deformation and this way alter the flow of the fluid itself. The use of CFD also allows alterations in the system’s design and operation to be investigated using a simulated environment reducing investigation time and costs.

Our company has performed many wind load investigations involving CFD simulations with wind tunnel validation, including those for the Budapest Sportarena and the tent roof of the Ice Stadium in Essen, Germany.

The effect of wind on comfort and building ventilation

When designing ventilation systems and evaluating landscaping options, it is valuable to have an understanding of the surrounding wind patterns. For a ventilation system, the optimal ratio of fresh to re-circulated air is affected by the pollutants in the intake air and the pressure distribution on the surface of the building. This ratio can be set to avoid the danger of frost and reduce energy losses. The local wind conditions also affect the comfort of passers-by; wind investigations can determine the optimal positioning of footpaths, benches and other outdoor facilities.

7.0Investigation of Fires and Smoke Eradication

When designing communal buildings with high seating capacity, it is important to consider effective fire evacuation systems. With numerical simulation the operation of air injection, suction or radial-ventilator smoke systems can be modelled more effectively than with traditional cold smoke field investigations. The CFD method of modeling is capable of taking into consideration the buoyancy force induced by the temperature gradient of the smoke, providing detailed quantitative analysis for design optimization of the system.

A detailed investigation of the smoke ventilation system in the Budapest Sportarena was performed by introducing a model fire into a numerical simulation environment. The location and intensity of this fire were determined according to the requirements for current fire safety laws.

8.0Ventilation and Air-Conditioning Inside Buildings.

When designing the ventilation system of a large capacity hall, it is a challenge to remove the heat produced by the audience and bright lighting elements while maintaining special conditions required for certain performances. For example, in a figure skating competition the air 1.5 m above the ice-rink must not exceed 18 oC (64 oF) , and in a table-tennis competition there are strict regulations about the maximum air speed around the tables. In the case of theatres or concert halls, it is important to maintain low noise and therefore low air velocities within the hall.

Simulation of the ventilation system can determine the optimal number and placement of intake and exhaust nozzles to meet the specific needs of the hall. By optimizing the nozzles, an effective ventilation system can be implemented with reduced capital and operational costs.

9.0Investigation of Comfort-Parameters and Air-Conditioning in an Office-Block or In Industrial Hall

In office buildings or industrial factories, employee comfort can increase productivity and improve employee longevity. Indoor comfort is largely dependent on air conditions which can be defined in terms of flow conditions, air temperature, humidity, and radiating heat.

There are additional concerns for specific cases. In the case of a large atrium or lobby designed with glass walls, there is the additional complexity of “cold radiation”. In industrial factories, there is often heavy machinery requiring the evacuation of large amounts of heat. There can also be harmful gases which must be removed to comply with strict air purity regulations.

To support ventilation system designs, a CFD numerical simulation of the flow can be carried out, allowing different system configurations to be examined in terms of air flow characteristics such as temperature, velocity, and moisture. The system can also be optimized to avoid intake air leaving the room through hydrodynamic shortcuts without executing its desired effect.

The new air-condition-wall is quickly becoming an economic option for effective temperature control in office blocks with large glass surfaces. The air-condition-wall uses double glass walls in which natural or enforced flow is generated. When combining its use with an air-conditioning system, the effect of extreme weather conditions can be reduced, thereby reducing energy consumption.

The spatial distribution of these can be defined by CFD. Besides this, problems can turn up like the investigation of the necessity of an air curtain, plotting the effects of cold radiation, calculation of heat-losses caused by automatic gates and avoiding waste of hydraulic shortcuts.

10.0Methods

With the generation of a numerical model of the building and its environment, the use of a CFD (Computational Fluid Dynamics) analysis allows simulating beside the air flow also special effects such as exhaust propagation and forced vibrations.

11.0Results

In contrast to discrete pressure and force values as obtained from measurements, the results of a numerical analysis offer the distribution of these parameters along the entire building. Vortex shedding, forced vibrations exhaust and particle spreading can be represented three-dimensionally and time-dependent. Thus, the numerical flow simulation is an interesting and cost-saving alternative to experimental methods, especially in the planning phase of a building.

Building contour

Animation: 3D vortex shedding on a quadratic chimney