### 30.1.4. Defining User Functions from Interpolation (Data Input) Functions

A user function can be used in CEL expressions in any place that a standard CEL function (for example, sin, cos, step, and so on) can be used. For a user function with the `Interpolation (Data Input)` option selected, you can enter an expression using the following notation:

- 1D interpolation

      <Function Name>(arg1[units])

- 3D interpolation

      <Function Name>(arg1[units], arg2[units], arg3[units])

When using a system variable, an expression, or a value, you do not need to specify units. For example, a pipe inlet velocity profile might be set by entering:

    inletvelocity( r)

or

    inletvelocity( x, y, z)

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