I am trying to extract a vortex core in tecplot using the eigen value method( second eigen value <0). My domain is a human arterial tree and I am simulating blood flow across it in fluent. When I extract vortex core using the second eigen value method and compare it with plotting streatraces, I notice that the core exists at some extra regions where the streamtraces do not recirculate at all. I was wondering if someone could explain why is that happening? Also, I wish to understand, CFD analyzer's eigen mode method of extracting vortex core? I would deeply appreciate if someone can explain that to me.