Phase-difference on seal whisker surface induces hairpin vortices in the wake to suppress force oscillation.

Seals are able to use their uniquely shaped whiskers to track hydrodynamic trails generated 30s ago and detect hydrodynamic velocities as low as 245 mu ms(-1) The high sensibility has long thought to be related to the wavy shape of the whiskers. This work revisited the hydrodynamics of a seal whisker model in a uniform flow, and discovered a new mechanism of seal whiskers in reducing self-induced noises, which is different from the long thought-of effect of the wavy shape. It was reported that the major and minor axes of the elliptical cross-sections of seal whisker are out of phase by approximately 180 degrees. Three-dimensional numerical simulations of laminar flow (Reynolds number range: 150-500) around seal-whisker-like cylinders were performed to examine the effect of the phase-difference on hydrodynamic forces and wake structures. It was found that the phase-difference induced hairpin vortices in the wake over a wide range of geometric and flow parameters (wavelength, wavy amplitude and Reynolds number), therefore substantially reducing lift-oscillations and self-induced noises. The formation mechanism of the hairpin vortices was analyzed and is discussed in details. The results provide valuable insights into an innovative vibration reduction and hydrodynamic sensing mechanism.