You did fine. The issue is probably that you selected an isosurface that is close to 0 (and negative!). The value you select depends on your problem, dimensions, Re, etc. I generally try 0.5 and then see whether I need a higher or lower value for a correct representation of my flow. I can’t tell you what will work for you, just that you explore some other values for the isosurface.
I recommend to read these:
- G. Haller, “An objective definition of a vortex,” J. Fluid Mech. , vol. 525, pp. 1–26, 2005.
- J. Jeong and F. Hussain, “On the identification of a vortex,” J. Fluid Mech. , vol. 285, pp. 69–94, 1995.