[Paraview] Velocity field analysis - lambda2

[David Larsson]

Hi Tim,


thanks for your answer. Your method sound straight-forward. The only issue I have now is that I'm simply not sure how to control the Calculator of paraview. Following your steps, I want to set e.g.:


'Result Array Name': S_squared


and then compute using the calculator something like


(gradient(velocity) + transpose(gradient(velocity)))^2/2


but this gives me a calculator error ('Syntax Error: expecting a variable name'). So I assume I'm simply using the calculator the wrong way (giving input in an incorrect manor).


Could you specify how you give input to the calculator following your outlined steps? Also, do you know the difference between the 'ComputeDerivative' and 'Gradient' is?


Thanks again for the help,


/David



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________________________________
Från: Tim Gallagher <tim.gallagher at gatech.edu>
Skickat: den 7 december 2015 23:16
Till: tim gallagher
Kopia: paraview at paraview.org; David Larsson
Ämne: Re: [Paraview] Velocity field analysis - lambda2

For the sake of people years later who come across this thread, there is a typo in my definition and it should be:

lambda_2 = S_ik S_kj + \Omega_ik \Omega_kj

Tim

________________________________
From: "Tim Gallagher" <tim.gallagher at gatech.edu>
To: "David Larsson" <david.larsson at sth.kth.se>
Cc: paraview at paraview.org
Sent: Monday, December 7, 2015 5:11:30 PM
Subject: Re: [Paraview] Velocity field analysis - lambda2

David,

I haven't taken on lambda-2 yet, but I have been able to build a pipeline for Q-Criterion. It looks something like:

Data -> CellDataToPointData (if it isn't already point data) -> ComputeDerivatives (set the gradient option to vorticity and the tensor option to strain) -> CellDataToPointData -> Calculator (compute |S|^2) -> Calculator (compute |\Omega|^2) -> Calculator (compute Q-criterion)

Since lambda_2 = S_ik S_ij + \Omega_ik \Omega_kj, you can use the same procedure and use just the standard calculators if you want to expand the Einstein summation and plug them all.

Once I've done this, I save the set of 3 calculators as a custom filter. When I use pvpython, I load my custom filter in the server manager and then use the CD2PD and ComputeDerivatives filters as needed. The python calculator might be a better way to do it, but I haven't tried that yet.

Tim

________________________________
From: "David Larsson" <david.larsson at sth.kth.se>
To: paraview at paraview.org
Sent: Monday, December 7, 2015 4:11:39 PM
Subject: [Paraview] Velocity field analysis - lambda2


Dear all,


I'm working with velocity vector fields over multiple time steps. I would like to compute a number of entities for the velocity field - most importantly the lambda2-scalar, but being relatively new to paraview I'm not really sure how to do that.


I can do some simple modifications using the calculator but can't really find my way to grad() or curl() etc. I guess the solution is going over to pvpython (which would also be preferred with regards to doing automated analysis).


Does anyone have any experience with performing numerical analysis of vector velocity fields in pvpython that could give some hints on how to get started? Or does anyone even have performed lambda2-analysis on such?


I've seen some similar questions in the mail-list from before but can't seem to find a conclusive answer, so I thought I'd give it a try myself.


Thanks for the help.


/David


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