Hydrodynamic performance and wake characteristics of three-dimensional fish in oblique swimming

The remarkable swimming performance of fish has inspired widespread biomimetic applications in underwater robotics. In this study, the oblique swimming of a three-dimensional fish is simulated using STAR-CCM＋, based on the finite volume method and the overset mesh technique. The investigation covers a range of oblique attack angles ( α ) and focuses on the effects of undulation frequency ( f ) and swimming speed ( U BL ) on the hydrodynamic characteristics and wake vortex structures. The results show that for different frequencies and speeds, the trends of the force coefficients with α are consistent; however, swimming speed exerts a stronger influence on the coefficients than does undulation frequency. A critical swimming speed of U BL = 2 BL/s is identified, beyond which the variation in force coefficients becomes relatively mild, whereas no analogous critical frequency is observed. Under high undulation frequency and low swimming speed, the spectrum of the tangential force coefficient displays a multi-peak pattern, indicating greater energy generation but accompanied by higher instability. At large attack angles, the force spectra broaden into a continuous band, revealing energy dispersion and a drag-dominated regime. Notably, at the critical attack angle α = 45 ° , a mixed-vortex mode emerges from the separation and reorganization of the reverse von Kármán (RVK) street, leading-edge vortices (LEVs), and trailing-edge vortices (TEVs), which contributes to unnecessary energy dissipation.