There is a good thread here, take a look: FlowAlongSrf problem
You have to get a reference surface first. You do it by extracting the outer ring surface with _ExtractSrf, then unrolling it with _UnrollSrf. Scale the mesh to fit this unrolled surface exactly, then run _FlowAlongSrf, picking a corner of reference surface, and the corresponding corner of ring surface.It might take a few tries to get it right.