“…Usually, the free vortices have a great impact on the aerodynamic forces acting on the body (Polhamus 1971; Li, Dong & Zhao 2018). For example, they have been shown to be capable of enhancing lift (Polhamus 1971; Dickinson & Gotz 1993) or generating thrust (Zhang 2017), and they could also reduce lift or increase drag in case of dynamic stall (Akbari & Price 2003) or drag augmentation (Zhang, Wang & Fu 2017).…”