**process derive

Description

This post calculation computes the derivative of some fields with respect to a variable (time or another variable). It is computed using a first order formula:

(127)\[\begin{split}\begin{aligned} \dot{f}(t_i) &= \frac{f(t_i) - f(t_{i-1})}{t_i - t_{i-1}} \qquad \text{if the field is derived wrt. time}\\ \frac{\partial f}{\partial x}(t_i) &= \frac{f(t_i) - f(t_{i-1})}{x(t_i) - x(t_{i-1})} \qquad \text{if the field is derived wrt. another variable}\end{aligned}\end{split}\]

Syntax

**process derive \(~\,\) *list_var names base

names is the list of variables to derive with respect to base.

Example

The following example computes the velocity in each direction. It will generate the fields D_time_U1, D_time_U2 and D_time_U3.

****post_processing
 ***local_post_processing
  **file node
  **nset ALL_NODE
  **process derive
   *list_var U1 U2 U3 time

 ***local_post_processing
  **file integ
  **elset ALL_ELEMENT
  **process derive
   *list_var eto11 time
****return