Evolution of incipient cavitation around two cylinders with different headforms

Incipient cavitating flows around two axisymmetric bodies with blunt and conical headforms respectively are investigated both experimentally and numerically. The Lagrangian coherent structures (LCS) defined by the ridges of the finite-time Lyapunov exponent (FTLE) method are employed to investigate flow structures and the motion of incipient cavities. The partially averaged Navier-Stokes (PANS) method is evaluated by comparison with experimental data, and it is found that the PANS model with parameter f_k = 0.2 (where f_k is the ratio of unresolved to total kinetic energy) gives good predictions of the scale of the separation region and the time evolution of the morphology of incipient cavitation. The incipient cavities exhibit a hairpin-shaped structure, traveling arbitrarily and unattached to the body surface. During the evolution process, the incipient cavities move downstream, with some circumferential motion. The FTLE contours and the trajectories of tracer particles reveal significant circumferential flow in the detached vortex structures around the two cylinders. The greater the distance downstream from the cylinder head, the more pronounced is the circumferential motion. Furthermore, it is found that the motion of the incipient cavities is closely related to local flow behavior. The circumferential flow around the blunt-headed cylinder is stronger than that around the conical-headed cylinder. This provides a reasonable explanation for the more pronounced circumferential motion of incipient cavities around the blunt-headed cylinder.