Suppression mechanism of vortex-induced vibration by the attached and discrete secondary vortices of a harbor seal vibrissa

Abstract

This study is inspired by the extraordinary suppression performance of the intrinsic vortex-induced vibration (VIV) of a harbor seal vibrissa, which is renowned for its excellent detection characteristics in sensing upstream flow information. A physical understanding of the suppression mechanism of vibrissal cylinder oscillation is mandatory for the design of a sensor detection system. In this study, the suppression performances are studied for a vibrissal cylinder at a reduced velocity U_r = U/(f_nw D) of 5, with the corresponding Reynolds number of Re = 3,750, according to the swimming velocity and real geometrical scale for a harbor seal vibrissa. In addition, a verified fluid and structure interaction (FSI) solver implemented with an improved dynamic mesh strategy is adopted. Different transverse spring-mounted oscillation responses are obtained by alternating the angle of attack (AOA). It is found that the attachment and secondary separation of vortices in the near wake significantly influence the shedding pattern and can lead to the distinct suppression of oscillating responses. A pair of attached vortices are observed for the vibrissal cylinder at AOA = 0°, where the oscillating response is almost fully suppressed. Forced vibration is applied to address the role of attached vortices in the suppression mechanism. In contrast, when the suppression mechanism diminishes at AOA = 90°, the vortex-shedding pattern is characterized by unstable discrete secondary vortices. These secondary vortices re-separate from the primary vortices, indicating high stability of the primary vortices in the near wake, which represents the primary mechanism deteriorating the suppression of vortex-shedding. Furthermore, the interaction of attaching and secondary discrete vortices results in a partially suppressed oscillating response for the harbor seal vibrissa at AOA = 45°. Of note, these two mechanisms cause the shedding mode to switch between the “2S” and “P + S” modes while partially suppressing the oscillation response.