# 2D Surface Airfoil NACA0012

**URL:** https://discourse.paraview.org/t/2d-surface-airfoil-naca0012/6462
**Category:** ParaView Support
**Created:** [February 14, 2021, 8:09pm UTC](https://discourse.paraview.org/t/2d-surface-airfoil-naca0012/6462 "2021-02-14T20:09:57Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![disco\_brocc](https://discourse.paraview.org/user_avatar/discourse.paraview.org/disco_brocc/32/5645_2.png) [@disco\_brocc](https://discourse.paraview.org/u/disco_brocc)
#### Post date: [February 14, 2021, 8:09pm UTC](https://discourse.paraview.org/t/2d-surface-airfoil-naca0012/6462/1 "2021-02-14T20:09:57Z")

</div>

Hi everyone,

I am using ParaView for a simple 2D NACA0012 project. I am trying to analyze the coefficient of lift and drag. But for that, I need the surface. I found some useful steps: [How to get normals of a 2D line and to integrate along it](https://discourse.paraview.org/t/how-to-get-normals-of-a-2d-line-and-to-integrate-along-it/4389)

But it did not work for me. Is there something wrong with my mesh?

 ![paraview_onQLeum382](https://discourse.paraview.org/uploads/default/original/2X/5/57e620dea0ee850fab8e8d1ff8a6a9c859302a9d.png)

Also, I you can find the vtu file [here](https://drive.google.com/file/d/1CFO_aXzPS7d3r2P3LElVKuqeFYQLCn30/view?usp=sharing).

Thanks!

---

<div class="post-metadata">

### Author: ![Kenneth\_Moreland](https://discourse.paraview.org/user_avatar/discourse.paraview.org/kenneth_moreland/32/15033_2.png) [@Kenneth\_Moreland](https://discourse.paraview.org/u/Kenneth_Moreland)
#### Post date: [February 15, 2021, 7:18pm UTC](https://discourse.paraview.org/t/2d-surface-airfoil-naca0012/6462/2 "2021-02-15T19:18:54Z")

</div>

The problem is that your mesh has discontinuities that the `Feature Edges` filter is interpreting as boundaries (what you call the surface). In this screenshot I have selected what would be a single point in a manifold mesh (the square in the upper view), but you can see that there are actually 4 points defined there (the spreadsheet in the lower view). I notice that the fields (density, pressure, and velocity) are slightly different. I don’t know if that is intentional or roundoff error.

 ![2021-02-15_11-57-12](https://discourse.paraview.org/uploads/default/original/2X/9/94a434d1fad6cdaf270be0c8fbf6bdfef46bbe8d.jpeg)

Assuming that whatever is writing your data cannot be “fixed” (there might be reasons for writing the data this way), I think the easiest way to solve the problem is to run the `Clean` filter after the `Extract Surface` filter. The `Clean` filter can merge coincident points and remove these discontinuities in the mesh. I note that not all the point coordinates are exactly the same, so you need to set a tolerance. When I set the `Tolerance` to `1e-6`, that seemed to work well for your data.

Also, when you run the `Feature Edges` filter, turn off all the checkboxes except `Boundary Edges`. For some reason, some of the connections seem to set off other feature edges. (I think the winding is reversed in the triangle cells.)

 ![2021-02-15_12-17-43](https://discourse.paraview.org/uploads/default/original/2X/1/156fa3f2ed8e337fbccac878e02bcedd5d19fb3e.png)

---

<div class="post-metadata">

### Author: ![disco\_brocc](https://discourse.paraview.org/user_avatar/discourse.paraview.org/disco_brocc/32/5645_2.png) [@disco\_brocc](https://discourse.paraview.org/u/disco_brocc)
#### Post date: [February 16, 2021, 4:54pm UTC](https://discourse.paraview.org/t/2d-surface-airfoil-naca0012/6462/3 "2021-02-16T16:54:40Z")

</div>

Hi,

Thanks for the feedback. I also found running the CleantoGrid first seems to do the same job.

Thanks again for the help!