CFD Online Discussion Forums

CFD Online Discussion Forums (
-   OpenFOAM (
-   -   motorBike tutorial and Scaling - New geometry used, scaled and now large velocity!!! (

scott July 28, 2010 23:20

motorBike tutorial and Scaling - New geometry used, scaled and now large velocity!!!
Hi guys,

I have a vehicle stl file that I would like to use. I am trying to modify the motorBike tutorial so that it works...

So far I have done the following:

put car.stl into trisurface folder.

open blockMesh dict and multiplied each length by 1000, (as the car was created in mm and is HUGE when I import to paraview with non-modified blockMesh result.

modify snappy dict with car.stl and car as required and run snappyHexMesh -overwrite

transformpoints -scale "(0.001 0.001 0.001)" to convert it back to metres from millimetres. (i then checked the domain and it is now the correct metre length and has the car in it as expected.)

I then change each entry in the folder 0 for "motorbike_*" to car, so it uses the same settings and because I have scaled the mesh it should now be the same size as the motorbike domain. I leave the velocity etc at 20m/s.


simpleFoam on 4 processors



Now, in Paraview I create a plane right down the centre of the car, and and look at the velocities (after 500 iterations) and once rescaling it, i have values for the freestream of around 5x10^10 !!!

Can anyone please give me a suggestion as to why or where the scaling has gone wrong, or what the units may be that Paraview is outputting...

Thanks for your help.


scott July 29, 2010 04:09

Ok, still have the same issue above, but just going to refine the mesh and check the difference to see if it will go away.

Are blockMesh and snappyHexMesh designed to work in the following way?

If i specify more cells in block mesh, does that then set the levels for snappyHexMesh, so if I modify the x y z coords to have twice as many cells, then my refinement levels in snappy will be calculated according to the initial cell size in blockMesh which will now be smaller, so i will get a more refined surface mesh and hence a heap more cells?

If this isn't how it works can someone please let me know how it does work and what the levels refer to in terms of measurement, and how do I adjust that initial measurement.

In another package I used a while ago I could set a base level, and then each refinement level was half of the level above it. Hence I could change the base level, and everything else would be updated. SO to refine my snappy mesh, I am wondering if i can just change the blockMesh size so it is more refined and then keep the same levels in snappy?

Thanks again, any feedback would be useful! Wish this was all in some kind of theory guide!



All times are GMT -4. The time now is 07:37.