Study on the Influence of Relative Chord Length and Frequency of Flapping Hydrofoil Device on Hydrodynamic Performance and Bank Slope Scour

A flapping hydrofoil device is an innovative device for enhancing the hydrodynamics of small rivers. While increasing the flow velocity of the river, it inevitably causes different degrees of scouring on the bank slope. This study aims to find an optimal combination of flapping hydrofoil parameters to maximize the pushing-water performance while minimizing the impact on bank slope scour, which is of great significance for the device’s application and environmental protection. Based on the finite volume method and overlapping dynamic grid technology, this paper selects the maximum bank slope scouring section as the research plane for numerical simulation. In order to expand the scope of application, the relative chord length c* (the ratio of chord length to river channel width) is introduced as a research parameter, and the influence of different relative chord lengths c* and frequencies f on the pushing-water performance of the device and the degree of bank slope scouring is systematically analyzed. The research results show that the near-shore current mean scouring force increases significantly with the increase in f and c*. The pushing-water efficiency will increase with c*, and will gradually increase with the increase in f and then tend to be stable. When c* = 1/2 and f = 2.5 Hz, the pushing-water efficiency reaches 51.04%, but at this time, the impact on bank slope scour is the most serious. When c* is reduced to 1/8, the bank slopes are not scoured even at the maximum frequency f = 2.5 Hz, and the pushing-water efficiency is 24.59% at this time. As c* decreases, the threshold frequency at which scour does not occur on the riverbank increases gradually. In addition, when c* is constant, decreasing f will significantly reduce the scouring force, but will have little effect on pushing-water efficiency. In order to achieve the purpose of this study, the parameters of flapping hydrofoil are recommended to be larger relative chord length and smaller motion frequency combinations.