Drag reduction by flapping a pair of flexible filaments behind a cylinder

The hydrodynamic mechanism of drag reduction by flapping a pair of flexible filaments behind a cylinder was explored using the penalty immersed boundary method. The effects of the phase difference between two filaments, the attachment height, and the flapping amplitude on drag reduction were examined. The flapping filaments weaken the vortex shedding via the destructive interaction between the vortices with the opposite signal. The clapping (out-of-phase) flexible filaments experience a lower friction drag and reduce a form drag of the cylinder, showing a better drag reduction than the snaking (in-phase) flexible filaments and the clapping rigid filaments. A minimum drag is obtained at an appropriate attachment height and flapping amplitude that avoid collision of the filaments and weaken the shear-layer-filaments interaction. The effectiveness ratio of the clapping filaments is higher than that of the snaking filaments. Energy saving can be achieved by avoiding the shear layer-filament interaction at a low flapping amplitude, whereas the filaments can further reduce the drag with greater energy consumption at an appropriate flapping amplitude. In addition, the total drag decreases with increasing Reynolds number, accompanied by a transition of the wake pattern from the 2S mode to the P + S mode.

Automated summary

The hydrodynamic mechanism of drag reduction by flapping flexible filaments behind a cylinder was investigated using the penalty immersed boundary method. The effects of phase difference, attachment height, and flapping amplitude on drag reduction were studied. Flapping filaments weaken vortex shedding through destructive interaction, with out-of-phase filaments showing better drag reduction than in-phase filaments. Minimum drag is achieved at appropriate attachment height and flapping amplitude to avoid filament collision and weaken shear-layer-filament interaction. The clapping filaments are more effective than snaking filaments. The total drag decreases with increasing Reynolds number, transitioning from 2S mode to P + S mode wake pattern.