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akshaybagi April 10, 2013 01:50

chord lenght reduction for airfoil analysis using openfoam
I am using OpenFOAM for analysis of NACA4412 airfoil. chord length is 1m. Now i want to analyse the same for different Re by varying the chord lenght. I dont know how to vary the chord length.
Please help:(

hakonbar April 10, 2013 04:03

As long as the Reynolds number is the only dimensionless quantity that's relevant for your analysis, then doubling the inlet velocity or halving nu (the kinematic viscosity) will have the same effect.

If you do want to increase the chord length, then there are different ways of doing it, depending on if you're using blockMesh or snappyhexMesh.

For blockMesh, you simply double the parameter "convertToMeters", which you'll find near the top of your blockMeshDict.

For snappyHexMesh, you need to launch paraFoam, and open the .obj or .stl-file that defines the wing geometry (you can find it in the "constant/triSurface" folder of your case directory. Apply the "transform"-filter, and set the parameter "scale" to 2 in all directions. Save as an stl-file.

akshaybagi April 12, 2013 03:09

@hakonbar: I am not using either hexmesh or snappy. I mesh it in gmsh and convert it to openfoam mesh. I have used digitiser for now but it approximates the profile, so it may not be the accurate profile. Any other suggestions?

hakonbar April 12, 2013 05:46

I've never used gmsh myself, so I don't know, sorry. As long as the wing is the only solid object in your analysis, and you don't have to define its size in relation to any other object in the flow field, then i don't think it would necessary to increase its size - you could just change some other parameter like I suggested earlier.

akshaybagi April 12, 2013 05:52

Digitiser is working fine now:)

immortality April 12, 2013 07:22

hi Ashkay
What does digitiser do?
I have a problem like you.can i use digitiser to change geometry?
I've meshed in gambit and want to use in openfoam.could digitiser help me in changing in naca0012?
Where can i download it?is it free?

akshaybagi April 13, 2013 01:09

Hi immortality
Digitiser is used to generate profiles from image files. ex if you have an image file of an airfoil, you can generate its profile from the image using digitiser.
This open source, digitizing software converts an image file showing a graph or map, into numbers.
Link: You'll get it here for free.
I dont think you can use it to change geometry.
You say you have meshed in Gambit. What you can do is you can convert the mesh file into OpenFOAM mesh in OpenFoam using the command 'gambitMeshToFoam' and use it in OpenFOAM.:)

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