# Calculate expression

**URL:** https://discourse.paraview.org/t/calculate-expression/4662
**Category:** ParaView Support
**Created:** [June 24, 2020, 4:37am UTC](https://discourse.paraview.org/t/calculate-expression/4662 "2020-06-24T04:37:20Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![nsuri](https://discourse.paraview.org/user_avatar/discourse.paraview.org/nsuri/32/4131_2.png) [@nsuri](https://discourse.paraview.org/u/nsuri)
#### Post date: [June 24, 2020, 4:37am UTC](https://discourse.paraview.org/t/calculate-expression/4662/1 "2020-06-24T04:37:20Z")

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Hello,

I have imported unsteady results in .vtu format into ParaView. I want to calculate the below expression:

![paraview_community](https://discourse.paraview.org/uploads/default/original/2X/e/eb3c2940103b9e1593df6397d0f4e7a6b23a8c38.jpeg)

It is the relative rate of change of the velocity vector. To apply this, I need to access velocity data at subsequent time steps, but I am not sure how to do this. Request guidance over this. Thank you.

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### Author: ![cory.quammen](https://discourse.paraview.org/user_avatar/discourse.paraview.org/cory.quammen/32/11193_2.png) [@cory.quammen](https://discourse.paraview.org/u/cory.quammen)
#### Post date: [June 24, 2020, 1:58pm UTC](https://discourse.paraview.org/t/calculate-expression/4662/2 "2020-06-24T13:58:51Z")

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The **Force Time** filter will let you “freeze” a dataset at a certain timestep.