# Issue while extract U\_Z on a surface , after clipping using alpha.water scalar
**URL:** https://discourse.paraview.org/t/issue-while-extract-u-z-on-a-surface-after-clipping-using-alpha-water-scalar/15720
**Category:** ParaView Support
**Created:** [November 13, 2024, 3:46pm UTC](https://discourse.paraview.org/t/issue-while-extract-u-z-on-a-surface-after-clipping-using-alpha-water-scalar/15720 "2024-11-13T15:46:59Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@Surya1005](https://discourse.paraview.org/u/Surya1005)
#### Post date: [November 13, 2024, 3:46pm UTC](https://discourse.paraview.org/t/issue-while-extract-u-z-on-a-surface-after-clipping-using-alpha-water-scalar/15720/1 "2024-11-13T15:46:59Z")
Hello , My main idea is to extract the U\_Z of a surface with aircore in middle and water around, So in order to extract just the water surface I clipped the surface using alpha.water with 0.5 and then integrated varibales and finally I was using calculator to find out the kinetic energy Ke- (0.5\*((U\_Z)^2))/9.81. When i do that it is giving me an error saying : “Vector variable name is already registered as a scalar variable name”. Please suggest me a way to extract the U\_Z of the water surface.
In this pipeline the first clip and slice are used to get the overall out of design , later is the process i followed as mentioned in the first para.
I have also tried multiplying U\_Z with kHat and stuff but didnt work, please help me out
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [November 14, 2024, 9:50am UTC](https://discourse.paraview.org/t/issue-while-extract-u-z-on-a-surface-after-clipping-using-alpha-water-scalar/15720/2 "2024-11-14T09:50:45Z")
Do you somehow have two variables with the same name ?