drag calculation

[ArtyB]

Hi,

Can anyone give me some examples of how to calculate the drag on an object using Elmer? I'm not sure if it's a postprocessor task? I'd also like to simulate with moving wall boundary conditions (road vehicle-want the air-road/underside of vehicle to be modelled too). Would I have to use a moving mesh to achieve that?

Elmer looks good, thank you, but need a bit of help getting started...

A

[raback]

The are two different ways.

1) The more accurate and consistant one uses the residual of the discrete system r=Ax-b where A does not include the effect of BCs. For Navier-Stokes equation the residual includes the nodal force vectors needed to reproduce the flow field. To activate the computation add to the Solver section the following keyword.

Code: Select all

  Calculate Loads = Logical True

Internally the velocity + pressure is called 'Flow Solution' and therefore you should obtain 'Flow Solution Loads' vector with DIM+1 components.

You still need to sum up these contributions. That you can do with the SaveScalars solver (see Models Manual). You should operate on the desired component and sum up the values on a given boundary. The following might give you the forces in x and y direction.

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Solver 2
  Procedure = "SaveData" "SaveScalars"  
  Filename = dragforce.dat
  Operator 1 = boundary sum
  Variable 1 = Flow Solution Loads 1
  Variable 2 = Flow Solution Loads 2 
End

When you also add to the desired boundary the flag.

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  Save Scalars = Logical True

The overall procedure is far from elegant but most of the functionality was not designed for this purpose. This could be made more transparent for the user with some effort...

2) There is also a specific solver that uses integration points on the boundary. This does not give the optimal accuracy but has the advantage that you may separate forces on junctions. See the 'Fluidic Force' solver in Models Manual for more details.

BR, Peter