# How to determine the coefficient drag in Paraview

i have determined a drag force of 220 Newtons at a speed of 25m/s, 1.225kg/m^3 and an approx area of 1.84m^2, i got a mathematical value of 0.3125 but the area is very approximate, is there a way of determining it in processing?

Hi @Lemlurker,

You have to manually specify the frontal area. You can do this in Paraview. So far, the best tutorial on how to do it in Paraview comes from http://www.vespalabs.org/projects/vespa-cfd-3d-model/openfoam-vespa-tutorial

But I had to modify some steps in order for this to work. I’ll step you through (This works for Paraview 5.x):

1. Load the STL file and tick off Camera Parallel Projection.
2. Select Points On in the front view of the car.
3. `Extract Selection` based on the points you selected.
4. `Transform` the extracted points in order to project them on a flat surface. For the next step to work properly, we have also to rotate them so they lie on the XY plane.
5. Apply the `Delaunay 2D` filter.
6. `Integrate Variables` to get the result.

The area measured by this method is of 2.37032 m2, which is very close to the 2.367 m2 I measured using proprietary software.

In step 2, if using Select Cells On, the resulting area is of about 1.9x m2 as it misses many cells with this particular geometry:

Hint: the filters `Extract Selection`, `Transform`, `Delaunay 2D`, and `Integrate Variable` are located under Filters. But you may add the quickly to the pipeline with `Ctrl + Space` and typing their name in the search box.

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thanks for the info, though having a few issues doing it myself, the extract selection button is greyed out and does nothing, any ideas?

Make sure you hit the `Apply` button whenever you add a filter. Also, you should see the vertices coloured pink in order to extract them.

ah thats it, the apply button thanks

@pfernandez I just saw this thread and im trying to follow the steps, however after clicking extract selection, I dont see the transform options, could you tell me where I can find them?

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Hi @intishd ,

The Transform is the next filter you have to apply after Extract Selection. You’ll find it under the Filter menu or by Ctrl+Space and then start typing the name of the filter you want to apply.

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