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Compute drag force on a mesh (complete beginner!)

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Old   October 10, 2014, 21:07
Question Compute drag force on a mesh (complete beginner!)
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Matteo Monti
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Hello everyone,
I hope this is not such a dumb question but I spent several days trying to look through the wiki and making trial&error (mostly errors!) to understand how to do this, but I seem not to be able to understand it on my own.

Here is my problem (i think this is quite a basic problem) that I would like to solve with SU2.

I have a 3D mesh of an object. As far as I understand (I have a basic background in 3D graphics) a 3D mesh should be just a set of coordinates of points in a 3D space, and a set of triplets (A, B, C) of points representing triangles. These triangles usually join together to form a discretization of the surface (mesh) of the object represented.

So, I have a mesh of this object, and I would like to use SU2 to just determine, given a velocity v, what the drag force on that object would be if air was to flow against that object at that velocity, with v parallel to the positive x axis.

I read how the configuration file is formed, and I was able to use SU2 to solve the flow for the example meshes. I also think I understood how to interpret the surface_flow file to get the force (correct me if I am wrong!): it is just a set of points representing the pressure as a scalar field over all the vertices specified in the mesh. Now, since pressure has a meaning only when applied to a surface, I would loop over all the triangles in the mesh, compute the average pressure over them (just by computing pressure on (A + pressure on B + pressure on C)/3), and then get the force as a sum weighted by the area of each triangle of the force vector given by the pressure multiplied by the normal versor to the triangle. Does that sound right?

Now, I wanted to understand how I could generate the mesh. I read the wiki (input and output files section) but I couldn't understand. As far as I understood from the wiki, an .su2 file contains somehow the description of some internal structure of the 3D objects, so that if I wanted to describe an object I wouldn't be able to just describe its surface (which sounds to me like the only thing that should be relevant to a CFD simulation!) but I have to come out with some (irrelevant?) internal structure made of tetrahedrons or similar 3D elements. Why that? How can I do it? I have also noticed that one has to write the description of marker elements that actually sound like the mesh I was looking for! But that feels quite redundant: if I have to describe the whole internal structure (again, it sounds irrelevant to a CFD simulation!) the border of that object should automatically be evident! Why repeat that?

So I made this very naive test and tried to just ignore the 3D elements part and just write down one marker for a tetrahedron, on which I tried to compute the flow.

Here is the .su2 file:

0.0 0.0 0.0 0
1.0 0.0 0.0 1
0.5 0.0 0.8660254 2
0.5 0.75 0.4330127 3
MARKER_TAG= tetrahedron
5 0 1 2 0
5 0 1 3 1
5 1 2 3 2
5 0 3 2 3

I setup some basic .cfg file that looks to long to paste here, and tried to run the simulation. I got a SEGMENTATION FAULT! That scared me a lot and I gave up with trying to understand this on my own.

Can someone explain to me, just from scratch, how to do what I need to do?

- I have a set of triangles (mesh) on which I would like to run a simulation
- The simulation consists in air flowing along the positive x direction at some speed v
- I need to know the drag force D that acts on that object

Thank you very much,
and sorry for the long post!

Matteo Monti
matteo.monti is offline   Reply With Quote


configuration file, mesh 3d, mesh file, segmentation fault, su2

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