CFD Post Result Scalling

nga911 · July 9, 2014, 08:00

Hello,

I'm trying to simulate a large geometry (Airflow around and in a 6 storey building) and to simplify things I scaled down the geometry 1 to 10 before running the simulation, I also scaled down the boundary conditions with the same scale.

However, Now that I've got my results, How do i scale back the value? Namly, The value on the Legend. I've tried to copy the solution Variables (Pressure, Velocity) and creating an expression for that but I've failed, Is there any other way? Also, Is my methodology correct for such situations?

Thanks you!

ghorrocks · July 9, 2014, 08:18

You scale physical models because it is difficult to fit a 6 storey building in a wind tunnel. When you do this you choose a non-dimensional parameter you are going to keep constant (probably Reynolds Number in your case), and the other relevant non-dimensional numbers change (maybe Froude or Mach number, maybe some heat transfer numbers). So when you scale you loose accuracy as you cannot scale and keep all non-dimensional numbers fixed.

The advantage of CFD is you can run the thing at full scale just as easily as at part scale. So you might as well model it full scale and not use any non-dimensional number scaling. This means all physical parameters are being correctly modelled with no approximation.

So, in your case I recommend:
1) If you did not keep the Reynolds Number constant then your simulation results are rubbish and you are going to have to do them again anyway. You cannot simply scale it to the correct answer.
2) When you redo it model it at the full scale as part scale is no benefit in CFD.

July 9, 2014, 08:00	CFD Post Result Scalling	#1
nga911 New Member abdul Join Date: Apr 2014 Posts: 3 Rep Power: 12	Hello, I'm trying to simulate a large geometry (Airflow around and in a 6 storey building) and to simplify things I scaled down the geometry 1 to 10 before running the simulation, I also scaled down the boundary conditions with the same scale. However, Now that I've got my results, How do i scale back the value? Namly, The value on the Legend. I've tried to copy the solution Variables (Pressure, Velocity) and creating an expression for that but I've failed, Is there any other way? Also, Is my methodology correct for such situations? Thanks you!

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July 9, 2014, 08:18		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	You scale physical models because it is difficult to fit a 6 storey building in a wind tunnel. When you do this you choose a non-dimensional parameter you are going to keep constant (probably Reynolds Number in your case), and the other relevant non-dimensional numbers change (maybe Froude or Mach number, maybe some heat transfer numbers). So when you scale you loose accuracy as you cannot scale and keep all non-dimensional numbers fixed. The advantage of CFD is you can run the thing at full scale just as easily as at part scale. So you might as well model it full scale and not use any non-dimensional number scaling. This means all physical parameters are being correctly modelled with no approximation. So, in your case I recommend: 1) If you did not keep the Reynolds Number constant then your simulation results are rubbish and you are going to have to do them again anyway. You cannot simply scale it to the correct answer. 2) When you redo it model it at the full scale as part scale is no benefit in CFD.