Fields, expressions and plugins

Discussion

You can define boundary conditions using:

• Fields.
• Expressions.
• Plugin functions.

The following diagram shows how fields, expressions, and plugins interact within the Simcenter 3D thermal workflow.

These methods allow boundary condition magnitudes to vary with:

• Spatial independent domains such as Cartesian, cylindrical, spherical, or parametric coordinates.
• Non-spatial independent domains such as time, temperature, or frequency.
• Combined independent domains including spatial and non-spatial variables such as space and time together.

What is a field?

Fields represent functional relationships. They define how a dependent domain varies with an independent domain. Use a field to define boundary conditions that vary:

• With a variable, for example, temperature.
• Spatially.
• With a combination of an independent variable and spatial location.

Example:

Formula field

50 000 W/m² heat-flux pulse from time = 0 to time = 1.0 s: if (time<=1.0[sec]) then 500.0[W/m²] else 0.0[W/m²]

Table field

You can create fields either from the context of a specific application or directly from the Simulation Navigator. If you create a field from the context of a specific application, the dependent domain is predefined.

What is an expression?

An Expression is a mathematical relationship that defines how boundary conditions, material properties, and physical properties vary with spatial and non-spatial variables.

The formula for an expression can be:

• A real numerical value, where the value of the expression is constant.
• A functional relationship, where the value of the expression can vary.

Example:

You can create expressions either:

• From the context of a specific application.
• In the Expressions dialog box.

Auto-generated expressions

The CAE expression system automatically creates expressions for many commonly used variables. In most cases, the name of the auto-generated expression is identical to the variable name itself.

Example:

The software automatically generates the expression: temperature. To use temperature in another expression, simply enter: temperature. In this case, temperature is treated as an expression name, not as a string constant, so quotation marks are not required.

Lookup functions in expressions

Expressions can access model data using lookup functions. The following table lists the examples of lookup functions that are commonly used in formulas for CAE expressions.

In the table, curve name, field name, point name, and variable name are text string names of curves, fields, points, and variables. Because they are string constants, they must be enclosed in quotation marks.

Example:

Function	Description
CONDVAL("condition parameter name",time)	Returns the value for a locked expression that the software creates for a condition parameter at the specified time in the active condition sequence.
FD(“field name”,argument1,......)	Returns the value for a field.
TEMP(“point name”)	Returns the temperature of the solid at the input point.
UG_VAR(“variable name”)	Returns the current value for a variable.

Thermal-flow functions

Use the thermal-flow functions in expressions for the thermal solver to evaluate thermal-flow properties, at run time, for each element in the boundary condition selection.

Example:

Properties	Examples
Total temperature	ABS2REL(v1,v2,v3,v4,i,v5) returns the relative temperature of a fluid.
Fluid properties	CONDFL(v1,i,v2) returns the thermal conductivity of a fluid.
Void properties	VA(i1,i2) returns the convecting area of a void

Choosing between expressions and formula fields

The following table compares the capabilities of formula fields and expressions.

Functionality	Formula field	Expression
Spatial variables coordinate systems	Absolute or local	Absolute only
Number of non-spatial variables	One	Unlimited
Dependent domain	Scalar or vector	Scalar only
Evaluating the formula/expression	Pre-solve	During solve

You can:

• Use fields for spatial variation.
• Use expressions for parametric and time-dependent behavior.

Using fields in expressions

You can use a Field in an Expression as:

• Basic operations:

  fd("field name",optional_argument1,optional_argument2,.....)

• Embedded mathematical operations:

  fd(“Table field(1)",time + 0.2)

• Composite operations:

  fd(“Table field(1)“, fd(“Table field(2)“),fd(“Table field(3)“))

• Vector-valued functions
• Fields with complex dependent domains

What is a plugin function?

A plugin function is a user-written extension that adds custom behavior to the Simcenter 3D thermal solver. It allows you to implement correlations, material models, or physics not available in built-in expressions.

Plugin functions are typically written as external libraries in C or C++ and loaded by the solver at runtime. Once loaded, they can be referenced directly in the simulation in the same way as standard expressions.

Plugin functions allow you to:

• Implement advanced convection and heat-transfer correlations.
• Model custom material behavior.
• Include proprietary engineering methods without exposing intellectual property.
• Access solver-computed quantities during the analysis.
• Update correlations dynamically at every solver iteration.

How plugin functions work

Plugin functions provide direct access to solver runtime data through a full C++ interface. During the solution process, the solver evaluates the plugin repeatedly and supplies the current simulation state as input. The solver calls the plugin function for each element or boundary location where the associated condition is applied. This allows the plugin to compute localized quantities such as heat-transfer coefficients, heat pickup, or custom source terms using current solution values.

Because the plugin is evaluated during the solve, the returned values can continuously adapt to the evolving simulation conditions.

Predefined API methods

You define plugin functions using API methods that are supported by Simcenter 3D and by the thermal solver. You can find:

• A list of API methods in the Thermal Solver API Manual
  • Simcenter 3D methods
  • Thermal solver methods
• A description of the API methods in the C++ header files located in the [installation_path]\ugstructures\evalplugin\src folder
• Examples of .cpp and .cmd files in the plugin_examples\thermal_solver folder in the tmg patch.

Set UGII_BASE_DIR environment variable to point to Simcenter 3D installation directory.

Hands-on material

To gain experience with the topics discussed here, complete the following:

• Using fields to define boundary conditions
• Learn thermal plugin basics