Understanding design variables

A design variable is a simulation quantity of your model that the TMG Correlation solver varies within a specific range during the optimization to reduce the gap between the reference data and the simulation results.

You can apply design variables on the following quantities:

  • Boundary condition parameters
  • Expressions used in the original solution
  • Material properties: thermal conductivity and specific heat
Note:
TMG Correlation creates design variables from quantities defined in boundary conditions, through parameters and expressions, of your solution in the Simulation file, or from material properties defined in the FEM. It does not support design variables created from other entities in the FEM or assembly FEM, such as physical properties and thermo-optical properties.

TMG Correlation lets you apply design variables as a:

  • Substitution—replaces the selected quantity with a new one that is the design variable. The design variable has the units of the original quantity.
  • Multiplier—adds in front of the selected quantity, a multiplier that is the design variable. The design variable is unitless. The same multiplier design variable can be applied to multiple quantities of the same design variable type (parameters or expressions). You cannot apply it to multiple material properties.
    Tip:
    Use a multiplier design variable method when you want to preserve the original formula in your optimized solution, for example if you want to preserve a time or temperature dependence present in the formula.

Boundary condition parameters, expressions, and design variables

A boundary condition parameter is usually the magnitude of a load, constraint, or simulation object, such as the heat transfer coefficient in the Thermal Coupling simulation object, or the heat pickup in a Thermal Stream load. It can be defined as a constant value, an expression, a formula field, a table field, or a table of fields.

When a parameter is defined by a field, in the Original Formula box you see fd("<field name>"). All fields are exported to the thermal solver as tables. You can create only multiplier design variables from parameters defined by fields.

There are two types of expressions:

  • System-defined expressions that you create from the context of a specific application, for example the heat transfer coefficient expression in Thermal Stream load: 0.45 * Ra^0.8 * fluidConductivity/L. Simcenter 3D names this expression, for example p261, and selects the appropriate physical quantity type for the expression automatically.
  • User-defined expressions that you create in the Expressions dialog box, for example Ra, fluidConductivity, and L used in the previous system-defined expression. When you create a user-defined expression, you enter the formula for the expression, name the expression, select the physical quantity type for the expression, and select the units for the expression.

A formula you input in a boundary condition is always stored as a system-defined expressions if you did not create it beforehand as a user-defined expression. Thus, you can create a design variable for this formula either from the parameter list or the expression list. The difference between the two types is where the design variable is applied:

  • For a parameter type of design variable, this variable applies only in the boundary condition selected even if the formula is used in multiple places.
  • For an expression type of design variable, this variable applies everywhere the expression is used.
Tip:
On the Design Variables > Definition page, notice the values in the Original Formula and New Formula boxes to understand where the design variable applies.