Define Q in Python Calculator of Paraview
 

Dear all,

I intend to use Q-criterion or Lambda-2 to visualize vortex in Paraview, can anyone show me the correct code I should type in the expression box?
For lambda-2, it should be something like "eigenvalue(strain(Velocity)**2 + (Gradients – strain(Velocity))**2), but there always appears syntax error.
Please help, thank you in advance.

Di