[Paraview] Calculation of Lambda2 using ParaView...

[Andy Bauer]

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/
> 2016/01/24/using-paraview-to-visualise-vorticity-lambda2-
> vorticity-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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