﻿ Problem Descriptor Reference > The Sections of a Descriptor > Definitions

# Definitions

The DEFINTIONS section is used to declare  and assign names to special numerical constants, coefficients, and functions used in a problem descriptor.

In assigning names to the definitions, the following rules apply:

 • Must begin with an alphabetic character.   May not begin with a number or symbol.

 • May be a single character other than the single character t, which is reserved for the time variable.

 • May be of any length and any combination of characters, numbers, and symbols other than reserved words, coordinate names or variable names.

 • May not contain any separators.  Compound names can be formed with the '_' symbol (e.g. temperature_celsius).

 • May not contain the '-' which is reserved for the minus sign.

Normally, when a definition is declared it is  assigned a value by following it with the assignment operator '=' and either a value or an expression.  Definitions are dynamic elements and when a value is assigned, it will be the initial value only and will be updated, if necessary, by the problem solution.

Example:

Viscosity = 3.02e-4*exp(-5*Temp)

Definitions are expanded inline in the partial differential equations of the EQUATIONS section.  They are not represented by a finite element approximation over the mesh, but are calculated as needed at various times and locations.

Redefining Regional Parameters

Names defined in the DEFINITIONS section may be given overriding definitions in some or all of the REGIONS of the BOUNDARIES section.  In this case, the quantity may take on different region-specific values.   Quantities which are completely specified in subsequent REGIONS may be stated in the DEFINITIONS section without a value.

Note: See the User Guide section "Setting Material Properties by Region" for examples of redefined regional parameters.

#### Defining Constant Values

Normally, DEFINITIONS are stored as the defining formulas, and are recomputed as needed.  In rare cases (as with RANDOM elements), this is inappropriate.  The qualifier CONST() can be used to force the storage of numeric values instead of defining formulas.  Values will be computed when the script is parsed, and will not be recomputed.

name = CONST (expression )

Note: Scripts with staged geometry will reparse the script file and regenerate any CONST values.