Coupling modes of three filaments in side-by-side arrangement

A viscous flow past three filaments in side-by-side arrangement is studied by a numerical simulation and is accompanied by a previously established linear stability analysis. Other than the coupling modes reported previously, which include the in-phase mode, symmetrical mode, and out-of-phase mode, three additional modes are identified in the nonlinear regime by systematically varying the separation distance between the filaments. These modes are the half-frequency mode, irrational-frequency mode, and erratic flapping state. The dynamic characteristics of each mode at the saturated state is described, including the flapping amplitude, frequency, drag force, and mechanical energy of the filaments. Four typical vortex structures are observed in the wake of the filaments and are described as the coalesced vortices, symmetrical vortices, erratic vortices, and independent vortex streets. The vortex structures are closely related to the coupling modes and the dynamic characteristics of the filaments. As the Reynolds number is increased or as the bending rigidity is reduced, the filaments gain more energy and ordering of the coupling modes may change. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3659892]