Properties and their Variation Methods

There are several basic properties that are necessary to define a solid for use with Autodesk® CFD. Most of these properties can vary with temperature, pressure or scalar, in several different variation methods. These properties and methods are listed in the following table:

Property	Variation Methods
Conductivity The same value for thermal conductivity can be used for all three directions, or each component can be different.	Constant, Polynomial, Inverse Polynomial, Piecewise Linear
Density Density is only used for transient simulations.	Constant, Polynomial, Inverse Polynomial, Piecewise Linear
Specific Heat Specific heat is only used for transient simulations.	Constant, Polynomial, Inverse Polynomial, Piecewise Linear
Emissivity -- Emissivity is used for radiation analyses. The emissivity specified on a solid overrides the value assigned to contacting fluid.	Constant, Piecewise Linear variation with temperature (useful for spectral radiation analyses.)
Transmissivity -- Transmissivity is used for radiation analyses. See the note below for additional information	Constant, Piecewise Linear (variation with temperature), Solar Window, Solar Wall
Electrical Resistivity Only used for Joule heating simulations.	Constant, Polynomial, Inverse Polynomial, Piecewise Linear (varies with temperature)
Wall Roughness Useful for simulating variable roughness height for including the effects of friction.	Constant

Transmissivity

Transmissivity is a measure of how much radiative energy can pass through an object. There are four variation methods are available for Transmissivity:

• Constant
• Piecewise Linear (for transimissity that varies with temperature)
• Solar window
• Solar wall

For the Constant and Piecewise Linear variations, the permissible range of is between 0 and 1. Avalue of 1 indicates that the object is completely transparent, and that radiative energy can pass completely through it. A value of 0 means that the object is opaque. Transmissivity is a unitless parameter, with a default value of 0. The sum of Transmissivity and Emissivity must be less than or equal to 1; if the sum exceeds 1, an error message is displayed when the simulation is started.

The Solar Window and Solar Wall variations are used to simulate solar heating within an enclosure or occupied space. Instead of directly specifying a transmissivity value, however, specify values for the SHGC (Solar Heat Gain Coefficient) and the U-factor. These parameters define the rate at which solar energy passes through walls, doors, windows, skylights, and other exterior features. You can find additional detail in the AEC Solar Heating topic.

Note that absorption of radiation energy by transparent solids is not included in the radiation model.

Assigning Transmissivity

Because the Autodesk CFD radiation model considers fluids to be non-participating, transmissivity can only be assigned to solid parts. It is not possible to simulate radiative heat transfer through dark or “muddy” fluids.

To simulate a Transparent object completely immersed within a fluid:

• Model the object as a solid and mesh it (it cannot be a suppressed part).
• Assign a Transmissivity value between 0 and 1 to the solid material to allow radiation to pass through the object.

To simulate radiative heat exchange between a transparent solid and the environment, such as a window:

• Model the window as a solid in the model.
• Assign a Transmissivity value to the material for that solid.
• Assign a transparent boundary condition to that surface. This boundary condition includes a background temperature

To simulate solar radiative exchange through a transparent solid such as a window:

• Model the window as a solid in the model.
• Select the Solar Window variation for Transmissivity, and assign values for the SHGC and U-factor.
• Enable Solar Heating on the Solve dialog. On the Solar Heating dialog, specify the simulation date, time, and location, and specify the ambient temperature or time variation.

Surface parts cannot be used to simulate transparent media. A non-zero value of transmissivity applied to surface parts is ignored. Likewise, non-zero values of transparency assigned to moving solids are ignored--transparency is not supported for moving solids or within rotating regions.

For more about transmissivity in Radiation Studies

Electrical Resistivity

The resistance per area multiplied by the length of the device. A value for resistivity is required for any solid that is heated by the Joule effect.

The relationship between resistivity and resistance is:

• R = resistance (ohms)
• r = resistivity (ohms-length unit)
• L = length of the device
• A = cross sectional area

For more about Joule Heating