Flapping propulsion near water-air interface

• Applicant: Xiaojue Zhu, Ph.D.
• Subject Area: Fluid Mechanics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 540422505

Numerous flying and swimming creatures use the ground effect to boost their propulsive performance, with the "ground" referring to either a solid boundary or a free surface. While our knowledge of how a solid boundary affects bio-locomotion is relatively comprehensive, the ground effect of a free surface and ist biological implications are not fully understood. In nature, for example, gliding pelicans, flying fish, and swordfish prefer to swim/fly near the (free) water surface and many other examples can be found. Understanding the unsteady ground effect of free surfaces would not only unravel the performance of swimmers in nature but also help improve the design of unsteady underwater propulsors that can efficiently swim near the water surface, taking into account the possible ground effect. This proposal will address several open questions regarding the impact of free surfaces on the dynamics and performance of flapping swimmers. We will first summarize existing literature to gather statistics on fish Froude numbers and then answer the following questions: (a) Does the unsteady ground effect occur when swimming in close proximity to a free surface? (b) How does the ground effect induced by a free surface differ from that of a solid boundary? © Is there an optimal Froude number that maximizes the ground effect? (d) Can the flexibility of a foil enhance the ground effect? We will perform direct numerical simulations in two-dimension and three-dimension to answer these questions.

• DFG Programme: Research Grants