[Paraview] Calculation of Lambda2 using ParaView...

[Andy Bauer]

Hi,

The best way to do this would be in a programmable filter. I'm guessing
numpy has some method to compute eigenvalues for non-symmetric matrices.
Beyond that adding the capability to the Python calculator or some other
filter would be needed.



On Tue, Jun 13, 2017 at 3:35 AM, Stefan Melber <Stefan.Melber at dlr.de> wrote:

> Hi Andy,
>
> i think i found the reason: i need the computation of (unsymmetrical)
> eigenvalues for lambda2 - however it seems that the function "eingenvalue"
> calculates only symmetric ones (see https://www.paraview.org/
> ParaView/Doc/Nightly/www/py-doc/paraview.vtk.numpy_
> interface.algorithms.html). How can i calculate the unsymmetric ones with
> ParaView?
>
> Best regards,
>
>      Stefan
>
>
> Hi,
>
> Maybe compute each portion manually and check that against the correct
> values. Also, verifying the gradient calculation is correct is another
> thing to look at. If the grid isn't specified properly then the gradient
> operation will likely be wrong.
>
> On Fri, Jun 9, 2017 at 8:46 AM, Stefan Melber <Stefan.Melber at dlr.de>
> wrote:
>
>> Hi,
>>
>>
>> for vortex-detection i (tried) to calculate the lambda2-criterion (see
>> e.g. https://en.wikipedia.org/wiki/Lambda2_method) with ParaView. The
>> way of calculation is based on this link http://www.iesensor.com/blog/2
>> 016/01/24/using-paraview-to-visualise-vorticity-lambda2-vort
>> icity-q-criterion/.
>>
>> However - the results are a kind of "wrong" - see attached pictures:
>> l2_solver.png shows what the flow solver writes out for lambda2,
>> l2_paraview.png shows what calculated with the algorithm below.
>>
>> Calculation of lambda2:
>>
>> D = Gradients = [d_ij]
>> S = strain(Vel)
>>
>> S     = (D + D^T)/2
>> Omega = (D - D^T)/2
>>
>> lambda = EigenValue_of(S^2 + Omega^2)
>>        = EigenValue_of(S^2 + (D - S)^2)
>>        = EigenValue_of(S^2 + (Gradients - strain(vel))^2)
>>
>> lambda2 = lambda_Y
>>
>> and in ParaView this mean
>>
>> o Vector of velocity with "Calculator".................: Vel =
>> iHat*x_velocity+jHat*y_velocity+kHat*z_velocity
>> o Gradients of Vel with "GradientOfUnstructuredDataSet": switch on
>> "compute gradient" and name it "Gradients"
>> o Calculate lambda with "PythonCalculator".............: lambda =
>> eigenvalue(strain(Vel)**2+(Gradients - strain(Vel))**2)
>> o Caluclate lambda2 with Calculator....................: lambda2 =
>> lambda_Y
>>
>>
>>
>> Does anyone see the error why the calculation of lambda2 with ParaView
>> does not work?
>>
>> Remark: i contacted the author of the blog above already - seems he never
>> tested his calculation and has no idea what is "wrong".
>>
>>     Stefan
>>
>>
>>
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