a b s t r a c tThe flapping motions of an inverted flag in a uniform flow were simulated using the immersed boundary method. The strain energy of the inverted flag was used as an indicator of the energy harvesting system efficiency. The flapping dynamics of and vortical structures around the inverted flag were examined in terms of the bending rigidity (γ) and the Reynolds number (Re). Three flapping motion modes were observed: a deflected mode, a flapping mode, and a straight mode. A mode intermediate between the flapping mode and the straight mode was identified, the biased mode. The vortical structures in the wake were characterized by three modes: a vortex pair; a vortex pair with a single vortex, and two vortex pairs, during half of the flapping period. The maximum mean strain energy ðE S Þ was obtained when the vortical structures behind the inverted flag formed a vortex pair during the flapping mode.

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Diverse bioinspired antifouling strategies have demonstrated effective fouling-resistant properties with good biocompatibility, sustainability, and long-term activity. However, previous studies on bioinspired antifouling materials have mainly focused on material aspects or static architectures of nature without serious consideration of kinetic topographies or dynamic motion. Here, we propose a magnetically responsive multilayered composite that can generate coordinated, undulatory topographical waves with controlled length and time scales as a new class of dynamic antifouling materials. The undulatory surface waves of the dynamic composite induce local and global vortices near the material surface and thereby sweep away foulants from the surface, fundamentally inhibiting their initial attachment. As a result, the dynamic composite material with undulating topographical waves provides an effective means for efficient suppression of biofilm formation without surface modification with chemical moieties or nanoscale architectures.

The flapping dynamics of vertically clamped three-dimensional flags in a Poiseuille flow was studied numerically by using the immersed boundary method. First, the flapping dynamics of a single flag was explored for comparison. Two distinct flow modes were observed: a flapping mode and a deflected mode. In the flapping mode, periodic vortices shed from the flag are formed, leading to alternating upstroke and downstroke flapping motions induced by the hydrodynamic and restoring forces. In the deflected mode, the flag is initially deflected by the hydrodynamic force and reaches a stationary state; the hydrodynamic force is balanced by the restoring force. For tandem flags, when the gap distance is small, the flags behave as one single flag with a higher bending rigidity. When the gap distance is intermediate, the front flag deflects the oncoming flow away from the rear flag. The flapping motion of the front flag is significantly confined by the presence of the rear flag, which results in an attenuation of more than 50% in its flapping amplitude. When the distance is large, the impact of the rear flag on the upstream flow field is negligible, so the front flag exhibits a flapping amplitude and frequency that are similar to those of a single flag. The vortices shed from the front flag induce the formation downstream of a low pressure region, which results in active flapping in the rear flag with a strong amplitude. There are two vortices shed from the tandem flags in each flapping period. When they are far apart, the phase difference is linearly dependent on the gap distance.