Vortex-induced transient stall

This study analyzes the induced flow by a coaxial vortex ring inside a circular tube applying vortex theory and potential flow theory. The vortex ring itself is generated by bound vortices rotating with the angular frequency \({\varOmega }\). Two results emerge out of the analytic research: first it is shown that induction causes the rotation of the vortex ring. It rotates at the sub-synchronous frequency \({\varOmega }_\mathrm {ind}<0.5\,{\varOmega }\). Second, the ring vortex itself induces an axial velocity at the tube wall. Superimposed with the axial main flow, this results in a stagnation point. Since the vortex strength increases in time, the stagnation point moves upstream. This kinematic effect may falsely be interpreted as a dynamic boundary layer separation. Hence, the results may give new insights into transient stall phenomena in axial turbomachinery.