Now, I will go to what is a vortex line also we have come across what is vortex lines? Suppose, I have a flow and if omega bar is the vorticity vector and let so if omega bar is the vorticity vector above this is the omega bar the viscosity vector. And then I draw a tangent at this flow that is d r bar, what will happen if omega bar is parallel to d r bar; that means, the vorticity vector is in the direction of the tangents.