Process of Component & Equipment Characterization for CFD

Large domains and components with tiny details are typical of AEC applications. Many complex features can be simplified through the characterization process to significantly reduce the run time of an analysis. Making decisions about what and when to characterize can be challenging; here the process to characterize items will be discussed.

Keeping the Goal in Mind

When developing the strategies that will be used to characterize items in an AEC application of Simulation CFD, a “big picture” mentality is beneficial.  Keep the overall solution goal in mind and consider how the characterization of a component can reduce simulation complexity.

The first question to ask yourself is “how does the item interact with its surrounding space?” Does it change airflow direction or speed?  Does it add or removes heat? Does it add or remove air?  This insight into the physics helps define how the characterization will be approached. 

For example a group of people in a room will impede flow and produce heat, but a tile in a data center only affects the flow.  A resistance may be used to characterize both, but the occupants would need additional heat generation to provide the model with the proper physical effects.

The people on the left can be replaced with a resistance region on the right to represent them with less detail.

The people on the left can be replaced with a resistance region on the right to represent them with less detail.

 

Above shows a detailed data center floor tile (1) next to a simpler characterization (2) using a resistance material.

Above shows a detailed data center floor tile (1) next to a simpler characterization (2) using a resistance material.


Characterization Process

Often the place to start in the characterization process is the manufacturer’s specifications of the component.

Manufacturing specifications can often be directly used to define standard devices and/or boundary conditions.  If specifications aren’t available, or aren’t detailed enough for a simulation, smaller simulations may be needed to identify the operating parameters. 

For example, the specification sheet for an air conditioning unit should give the flow rate and capacity for heat transfer information that can be applied into the properties for a heat exchanger.  A diffuser, on the other hand, may require a simple simulation or hand calculations of a detailed diffuser in an empty room to obtain the proper component velocities.

  1. Identify the operating characteristics that are necessary to properly characterize the object.
  2. Consider the tools available within Simulation CFD to capture their operating characteristics. Tips are available on the pages that cover Materials and DevicesRepresent the Flow Environment, and Representing the Thermal Environment.
  3. Gather the necessary inputs for model characterization.  When specification sheets are available they are a valuable resource to find flow and heat transfer values.  In the event that the data is not available it will be up to the designer to investigate further.
    • Hand calculations are not uncommon.  For example, calculating an effective conductivity to represent a group of materials in close proximity (like a window) may be required to simplify them into one volume.
    • In more difficult cases, a detailed model of the item to be characterized should be run to gather the information necessary. This information can be used to create a set of simplified parameters or boundary conditions in a larger simulation.
  4. Always validate that the characterization is working as intended.   Run a small model with the characterization that can be compared to a detailed model or hand calculations prior to inserting the characterization into the larger model.

Several of the most common AEC components will be discussed in detail in subsequent pages.  Regardless of the specific component, this fundamental characterization approach will still apply.