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Preparing Simulations to Generate CFD Boundary Conditions

You can use DesignBuilder simulations to generate boundary conditions for the CFD calculations. EnergyPlus simulations calculate the overall thermal behaviour of the building in terms of inside surface temperatures and natural ventilation flow rates then CFD allows you to 'zoom in' on the details of the temperature and flow distribution within the space for a particular snapshot in the simulation.

The simulation data that can be used as boundary conditions in CFD calculations are:

• Surface inside temperatures
• Natural ventilation airflow rates through external windows, doors, vents and holes.

To generate the above data clearly the simulation must generate inside surface temperatures and any natural ventilation airflow rates to be included in the CFD calculations. Before starting you should decide on the extent of the CFD domain and the time period you wish to analyse. Then only generate simulation data for these areas and time periods.Otherwise, for large models you might generate an impractical amount of data and overload the software.

The steps required to generate boundary condition data from simulation are:

1. If the CFD simulation is to use natural ventilation airflows through windows calculated by EnergyPlus then you must use the Calculated natural ventilation model option. Scheduled natural ventilation does not calculate airflows through individual windows.
2. Because CFD calculations do not use infiltration flows you may want to switch off Infiltration treatment on the Constructions model data tab. This will help ensure that the flow balance with little or no need for extra correction flows to be introduced on the CFD Boundary Conditions dialog. It also speeds up simulations by reducing the complexity of the Airflow network in EnergyPlus.
3. Switch off the LumRelated Topicsp similar windows on surface model option to ensure that all windows are modelled individually.
4. Prepare the simulation model data and carry out test simulations to check that the model is behaving as expected during the period of interest (e.g. summer design conditions). Look at hourly results.
5. Navigate to the level in the model covering the CFD domain. If you are modelling a single zone in the CFD simulation, then navigate to that zone, if you are modelling a block then go that block or if you are modelling the whole building then go to building level. Then on the Options model data tab select the following output options:
   
   
• Store opening output - ensure that data for windows, vents, doors, holes and sub-surfaces is included in the output.
• Inside Surface Temperature - to store inside surface temperatures for all surfaces and openings in the domain.
• Airflow in - to record the flow in through windows, vents, doors and hole (natural ventilation calculations only).
• Airflow out - to record the flow out through windows, vents, doors and hole (natural ventilation calculations only).
  
  
6. Once the model is prepared, click on the Simulation tab and request hourly or sub-hourly results and run the simulation.
7. Check the simulation results by examining hourly results before feeding them into the CFD calculations. Check that airflow through windows is in the expected direction based on wind direction and expected wind pressures etc.
8. Start the CFD Boundary Conditions Editor and follow instructions for importing the simulation hourly or sub-hourly results
9. Add any assemblies or component blocks with any further boundary conditions needed to represent heat, temperature and flow boundaries imposed by HVAC, lighting and other equipment and occupants.
10. Make sure you have the same total convective heat from your internal gains in the CFD model as you had in the EnergyPlus simulation.
11. Check that CFD results correspond with the boundary conditions provided by the simulation.
    
    

Note 1: It is not currently possible to set flow boundary conditions on internal openings. So although EnergyPlus calculates flow rates through interior openings, DesignBuilder CFD will ignore these flows and re-calculate them using a more detailed method taking into account distributed air velocities and pressures.This means that the flows through internal openings calculated by the CFD won't match exactly with the flows calculated by the EnergyPlus Airflow network. EnergyPlus flow rates through exterior openings are always respected by DesignBuilder CFD when used as boundary conditions.

Note 2: Infiltration airflow is not modelled in CFD and if you are looking at summer time natural ventilation you may wish to switch Infiltration off in simulations. Winter time simulation may need infiltration included if it is significant.