On the formation of vortices under transient and pulsatile inflow conditions in a curved pipe*

Formation of vortical structures  in a 180° curved pipe under transient flows was investigated using particle image velocimetry (PIV) and computational fluid dynamics (CFD) simulations.  Several vortex identification methods were employed to identify various vortices in the flow.  Vorticity transport equation analysis revealed that the origin of the Lyne-Type vortex was around the 30° location from the inlet and close to the curved pipe inner wall. Our study showed that the axis of the Lyne-type vortex (unlike that of the Dean vortices) is primarily in the direction normal to the pipe’s central plane in the vicinity of its origin until around the 60° cross-section, where it starts to gain a weak swirling component normal to the pipe's cross-section. In order to identify the effect of inner wall vorticity on the Lyne-type vortex, additional CFD simulations were performed with a slip wall boundary condition, which revealed the Lyne-type vortex originates from vorticity introduced by the incoming velocity profile close to the inner wall.  During the formation of the Lyne-type vortex, a Dean vortex pair distorts into the two deformed-Dean vortices to provide space to accommodate the Lyne-type vortex.