Analysis of free-surface effects on swimming by the application of the computational fluid dynamics method

Abstract

In this study, free-surface effects on swimming are investigated by the application of the computational fluid dynamics method for the first time. The major goal of this study is to represent a new methodology to numerically investigate free-surface effects on swimming. Utilizing a two-dimensional solver with steady flow conditions, the numerical results for swimming at a fully submerged level are validated with the data presented by previous researchers. The validity of the results is also examined through the study of mesh independency and the control of a critical turbulence feature, entitled as non-dimensional wall distance (y+). Finally, for the analysis of swimming at the free-surface level, a two-dimensional time-dependent solver with a well-known multiphase mathematical model is implemented to simultaneously investigate two-phase flow interactions and wave propagation in the proximity of the swimmer’s body. The development of different flow topology features, including major vortex cores and secondary recirculation zones as well as turbulence mixing and flow updraft and downdraft, are investigated and used to compare free-surface swimming against fully submerged cases. While the drag coefficient continuously decreases for higher advance velocities for a fully submerged case, a gradual augmentation is observed for the drag coefficient at a free-surface swimming level. The obtained results suggest that while the proportion of wave drag is ignorable at low swimming speeds below 1 m/s, in the case of higher speeds, its amount enhances significantly and constitutes about 15% of the total drag. Moreover, the proposed methodology is demonstrated to be efficient in the simulation of flow characteristics and phase interactions in swimming at the free-surface level.