Environment-Driven Properties

For many simulations, material properties are constant, and do not need to vary. For other simulations, such as natural convection and high-speed compressible simulations, material properties do vary physically. To simulate these models correctly, the material properties must be allowed to vary. The Material Environment dialog, accessible from the Material quick edit dialog as well as the Design Study bar, allows you to indicate that the properties of the material will vary. This ensures a more accurate representation of the physical model.

The Default material database contains a single version of each material. Most are defined with variable properties.

When applying a material, specify if it is Fixed or Variable, and define the operating conditions of the scenario. The material properties at these conditions are computed, and either held constant or used initially and varied.

To specify Fixed or Variable

Many materials in the Default material database are defined with varying properties. To specify if they are constant or vary in a scenario:

1. Click Set... on the Environment line of the quick edit dialog.

   (Alternatively, right click on the material in the Design Study bar, and select Edit material environment reference...)

2. Select either Fixed or Variable.

   • Select Fixed to compute properties at the specified Environment, and keep them constant.

     • To specify a specific value for a property that is defined as varying, copy the material to a custom database, change the property variation to constant, and specify the value.

   • Select Variable to vary properties as defined in the material. Do this for natural convection and compressible analyses.

     • Only properties defined with a variable method vary.
     • This dialog is not available for materials defined only with constant properties.

     The material condition (Fixed or Variable) is shown adjacent to the material name on the Design Study bar. This makes it easy to know if the correct variability is selected for the analysis type. Note that Fixed is the default setting.

     Note: Select Variable when properties must change. Examples include natural convection and compressible analyses.

     For an example of setting varying properties...

To specify the scenario Environment settings

The scenario Environment settings affect all materials that have Use scenario environment enabled on the Material Environment dialog.

To change the Environment settings

• In the Design Study bar, right click on the Material branch, and select Edit scenario environment reference... (Alternatively, click Scenario Environment from the Materials context panel.)
• Specify the Pressure and Temperature on the Scenario Environment dialog.

To change the Environment for an individual material

If materials operate at different conditions (such as two fluids in a cross-flow heat exchanger), the scenario Environment value is not sufficient to define their properties. To set the Environment conditions for an individual material:

• Click Set... on the Environment line of the quick edit dialog.
• Uncheck Use scenario environment.
• Specify the Pressure and Temperature conditions.

Examples

To specify constant property, high temperature air:

• Select Air.
• Specify the Environment temperature and pressure.
• Select Fixed to use constant properties.

Autodesk® CFD computes the properties at the Environment.

To use Air with a specified density:

• Open the Material Editor, and copy Air from the Default database to a custom database.
• Modify the variation method for density to Constant, and specify a value. Save and exit the Material Editor.
• On the quick edit dialog, select the air material from the custom database.
• Open the Material Environment dialog, and select Fixed.
• Apply the material to the model.

Property Initialization for Natural Convection

For natural convection analyses, property values must be allowed to vary (by selecting Variable) from the Material Environment dialog.

By default, initial fluid (air) property values are computed based on the lowest specified temperature boundary condition in the analysis. This is the desired behavior for simulating cooling of a device by natural convection.

For situations in which the device is heated by natural convection, it is better to initialize the fluid (air) properties based on the Environment temperature. To do this, enable the following flag in the Flag Manager:

use_property_ref_temp