[Paraview] Calculation of Lambda2 using ParaView...

[Stefan Melber]

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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                      _/ *Dr. Stefan Melber-Wilkending*
                    _/_/
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