## Methodology for CFD-modelling of open spaces

It is vital to have information about air velocities and air patterns when assessing comfort conditions on outdoor spaces. Data can be obtained through full scale monitoring on site or through wind tunnel tests, but both solutions can be very costly. Another way to obtain data is to carry out CFD calculations of the concerned space. This however requires a methodology, as CFD is a very calculation heavy type of software.

CFD (Computational Fluid Dynamics) is a computer simulation tool used to calculate flow data for different kinds of fluids. CFD could be used for studies of the water flow in a river stream, the air movement in an urban space or other fluid situations. The governing flow equations are the Navier-Stokes equations for transport of mass, momentum and energy. If the flow is turbulent an additional set of transport equations is needed. Since the equations are highly non-linear and coupled they are too complex to be solved by simple calculation. Therefore, in order to analyse a specific fluid situation, it is often necessary to use either experimental data (measurements on site or in a wind tunnel) or numerical data from CFD.

Through the last decades computer technology has developed rapidly and made it possible to simulate fluid motions in such ways that the results are very much in accordance with existent fluid situations. CFD can therefore be used as an advisory tool, which presents a realistic picture of a certain flow situation.

The most significant features of CFD are the very big model files and the very long calculation time - the bigger model file, the longer time it takes to calculate. Therefore many CFD users try to decrease the size of the model in order to reduce the calculation time. This often leads to a com-promise in the detailing of the model. Therefore it is utterly important to be aware of how to prioritise during modelling, i.e. which sections requires detailed modelling and which sections do not, in order to obtain reliable results.

The basic principles in using CFD are:

- Model building - definition of a control volume (the river or the urban area) and what is in it (e.g. rocks or buildings;

- Boundary conditions - what comes in and what goes out of the control volume;

- Generating a mesh - divides the model in to smaller pieces;

- Validation of the model - a check to assure the reliability of the model;

- Calculations;

- Evaluation of results.

Most times there is an iterative procedure from the model building to the validation of the model; the model is often revised several times in order to find the optimal model.

The methodology describes these principles in detail and demonstrates the principles on two specific example studies.

CFD (Computational Fluid Dynamics) is a computer simulation tool used to calculate flow data for different kinds of fluids. CFD could be used for studies of the water flow in a river stream, the air movement in an urban space or other fluid situations. The governing flow equations are the Navier-Stokes equations for transport of mass, momentum and energy. If the flow is turbulent an additional set of transport equations is needed. Since the equations are highly non-linear and coupled they are too complex to be solved by simple calculation. Therefore, in order to analyse a specific fluid situation, it is often necessary to use either experimental data (measurements on site or in a wind tunnel) or numerical data from CFD.

Through the last decades computer technology has developed rapidly and made it possible to simulate fluid motions in such ways that the results are very much in accordance with existent fluid situations. CFD can therefore be used as an advisory tool, which presents a realistic picture of a certain flow situation.

The most significant features of CFD are the very big model files and the very long calculation time - the bigger model file, the longer time it takes to calculate. Therefore many CFD users try to decrease the size of the model in order to reduce the calculation time. This often leads to a com-promise in the detailing of the model. Therefore it is utterly important to be aware of how to prioritise during modelling, i.e. which sections requires detailed modelling and which sections do not, in order to obtain reliable results.

The basic principles in using CFD are:

- Model building - definition of a control volume (the river or the urban area) and what is in it (e.g. rocks or buildings;

- Boundary conditions - what comes in and what goes out of the control volume;

- Generating a mesh - divides the model in to smaller pieces;

- Validation of the model - a check to assure the reliability of the model;

- Calculations;

- Evaluation of results.

Most times there is an iterative procedure from the model building to the validation of the model; the model is often revised several times in order to find the optimal model.

The methodology describes these principles in detail and demonstrates the principles on two specific example studies.