Evidence and physical mechanism for vortex-induced vibration of a bluff body without an afterbody

Afterbody-the portion of the body downstream of shear layer separation points-was believed to be essential for the vortex-induced vibration (VIV) of a bluff body. A recent study by Zhao et al. ["Flow-induced vibration of D-section cylinders: An afterbody is not essential for vortex-induced vibration," J. Fluid Mech. 851, 317-343 (2018)] made an important forward to demonstrate "an afterbody is not required for VIV" through water tunnel experiments of a reversed D-section prism. However, our direct numerical visualization showed that the shear layer separation appears at the curved front surface of the reversed D-section prism, leaving a part of an afterbody, which makes their evidence questionable. The present study aims to provide solid numerical and experimental evidence for the statement "an afterbody is not required for VIV" using an elastically mounted triangular prism with one vertex pointing upstream. By conducting two- and three-dimensional direct numerical simulations and water tunnel experiments, we verified that even without an afterbody, the triangular prism can freely vibrate. Furthermore, the physical mechanisms for the excitation and sustenance of the VIV of a bluff body without afterbody are investigated. By decomposing the lift force into the pressure and viscous parts, we discover that the vibration of a bluff body without an afterbody is excited and sustained by the viscous lift component acting on the forebody, in contrast to the VIV of a circular cylinder with an afterbody in which the viscous component always results in energy dissipation and damps the vibration. Some recent experiments showed that the VIV does not occur for the triangular prism. The reasons are also explained: the absence of VIV is due to either high-Re or large mass ratio (structural damping) but not owing to the lack of an afterbody.