Standard Material Definitions for CFD Simulation

Materials define a simulation’s domain components, impacting the physics of a simulation and its results. Simulation CFD provides a comprehensive library of materials along with the ability to define custom materials as needed.

Materials define the properties of parts which impact the physics of a simulation.  Simulation CFD provides a comprehensive library of materials along with the ability to define custom materials as needed.  Before proceeding further, reviewing the following online help topics is recommended:

Fluids

Fluids are the gases and liquids which flow throughout the simulation domain.  For AEC applications, air is by far the most common fluid material.  The use of liquids is typically reserved for the detailed simulation of AEC components such as heat exchangers, which have water flowing through pipes.

Since some devices, such as fans, must be immersed in a fluid, the normal practice is to begin with the assignment of fluid materials.  More information on assigning fluid materials can be found here.


This lab space (left) would require the material assignment of 7 total volumes to air:  the primary air domain (center) along with the 4 supplies and 2 hood exhausts (right).

For fluid material properties, there are 2 critical items to consider:

  • The temperature and pressure environment to which the fluid is being subjected.
  • Whether the properties can remain constant or need to vary locally in the domain.

The software assumes a default temperature and pressure of 20C and 101.325 kPa respectively.  However, the density of air at an altitude of 5,000 feet (e.g. Denver, Colorado, USA) is considerably lower than that of air at sea level due to the decreased pressure (assuming the same temperature at each altitude).  To account for this change from the default assumptions, the scenario or material environment references must be changed accordingly.

Also, by default, the material properties are Fixed, which means that they remain constant across the domain and do not vary based on local temperature or pressure changes.  For AEC applications where natural convection will play a role, the density of the air must be able to change as a function of the local temperature (warmer air is less dense and rises, colder air is more dense and falls).  To enable a fluid property which has been defined as a function of temperature to vary, the fluid material environment must be set to Variable.

For more details on setting material environments, consider the following resource: Environment-driven Properties.


For this natural convection scenario, note how the warmer air rises to the ceiling (top image) and has a lower density (bottom image). Natural convection requires the air density property to vary as a function of temperature.

TIP: To validate the correct environment reference settings, display the density in Results mode and compare against the expected value.

Solids

Solids are rigid bodies in the domain which obstruct the flow of fluids (e.g. air cannot pass through a concrete wall) and also conduct or radiate thermal energy.  More information on assigning solid materials can be found here.

TIP:  As fluid and solid volumes are defined, it is a best practice to temporarily hide them from view to more easily see and select the remaining volumes.  This technique also prevents errors in reassigning materials.

Specific to AEC, the default material library contains a special solid labeled Human, to represent occupants.  The density and thermal properties are similar to those of water since the human body is roughly 70% water.


This lab space (left) requires the definition of 3 different solid materials: Wood for the desk; Steel for the 2 fume hoods; and Human for the 2 occupants. Note how hiding the air volumes (right) makes it easier to work with solid volumes.

Custom Materials

The default material database is largely comprehensive and contains the majority of materials (air, wood, steel, concrete, glass, aluminum, copper, etc.) used in AEC applications.  However, there will occasionally be a need, particularly for devices, that the default database does not cover.  Simulation CFD enables the user to define custom materials, which can be saved for later use to a custom material library.

Details on creating and storing your own custom material and materials database can be found at the following online help topics:

TIP:  The easiest way to create a custom material is to find a material similar to it in the default database, copy it, and then update the name and properties as needed.