How to plot Q-criterion in wind turbine blade case
 

I want to see the tip vortex of the blade with flow visualization
I'm solving steady state for one blade by using periodic conditions
I'm using Moving Reference Frame as the blade rotation zone, see the attached figure, it is my mrf zone.
I know that q criterion needs to set in define>custom field function
The q criterion should be as q=0.25*(W*W - S*S)? It means just find the velocity -> vorticity magnitude and derivatives -> strain rate in the custom field function ? Do not consider the tensor form with vorticity and strain rate ??
I don't know how to choose the iso-surface setting. In iso-surface, I first select the q custom function and then key the iso-values, and should be selected the "From surface and From zones" in Iso-surface interface ??
Can anybody tell me how to set the q-criterion in iso-surface ?

Thanks
Forgive my poor English