Building Occupants: Characterization for CFD

The need to have occupants in AEC applications is common; however the most complex geometry in an analysis can be the person in the room. Sometimes this complexity is warranted, but often it is not. Learn about the different methods of characterizing occupants.

Most buildings are designed for the use of people, and it is tempting to include them in simulations to add a sense of scale or realism; depending on the scope of the simulation their representation may be simplified through characterization or even removed. 

In all cases occupants generate heat and impede air flow, these are the characteristics that must be considered.  A reference about the amount of heat a person generates can be found in Building Science Fundamentals.

The above flow chart should provide enough insight to determine a method of characterization required in different AEC applications.

The above flow chart should provide enough insight to determine a method of characterization required in different AEC applications.

Characterization Method for Small Spaces

In an office or space that would only contain one or two people a human can be included in a simulation.  The detail of the person should be limited by avoiding complex and tiny features, such as facial features, fingers, and toes.

Above depicts an occupant at his desk drawn into a small office space.  The advantage of having an occupant is that thermal comfort can be assessed on a solid object and different areas of the person may have different levels of comfort.

Above depicts an occupant at his desk drawn into a small office space.  The advantage of having an occupant is that thermal comfort can be assessed on a solid object and different areas of the person may have different levels of comfort.  

TIP:  When simulating a slightly larger space, further de-featuring the person to a block or cylinder may be useful.

Characterization Method for Large Venues

When considering the people occupying a theater hall, stadium, or church the occupants are usually tightly grouped. This can be used to the designer’s advantage because entire seating rows or sections can be characterized together.

A single block can replace all of the people and chairs and be defined as a resistance material with free area ratios (FAR) specified for each direction.  Calculating the FAR can be done using the section area measurements in most CAD tools.  A heat load should be applied to match the output of the number of occupants represented by the block (for example 9 people in a row at 100 watts each; 900 watts needs to be assigned to the resistance representing that row).

This group of occupants (left) will partially obstruct air flow in that location.  A simple block added in CAD (right) and assigned as a resistance with a heat load in CFD can simulate that same obstruction with less complexity.

This group of occupants (left) will partially obstruct air flow in that location.  A simple block added in CAD (right) and assigned as a resistance with a heat load in CFD can simulate that same obstruction with less complexity.

In the case that the occupants cannot be placed into a grouping, such as the people on the field in a stadium it would be more appropriate to put a heat flux boundary condition onto the field as opposed to adding them as explicit geometry into the simulation.  

(1) Players on the field can be represented with a surface heat flux on the field.  (2) Observers in the stands can be characterized by a resistance block with a volume heat generation

(1) Players on the field can be represented with a surface heat flux on the field.

(2) Observers in the stands can be characterized by a resistance block with a volume heat generation