Hydrodynamic Mechanics of Aquatic Locomotion

The hydrodynamic mechanics of aquatic locomotion examine how organisms and engineered systems move through water by generating and manipulating fluid flows to produce thrust, lift and control. Two principal propulsion modes dominate: undulatory swimming, in which rhythmic waves travel along the body or fins to interact with the fluid, and oscillatory or flapping propulsion, where appendages beat to shed vortical structures. Performance emerges from the interplay between kinematic patterns, muscle or actuator constraints and the surrounding fluid’s response. Scaling laws now link swimming speed to body length and oscillation frequency, revealing distinct regimes for small and large swimmers. Collective behaviours such as schooling exploit flow-mediated interactions to enhance efficiency via coordinated phasing. Advances in experimental techniques—particle image velocimetry, force transducers, high-speed imaging—and in numerical simulation have deepened understanding of thrust generation, wake dynamics and energy transfer. Insights from this research inform the design of bio-inspired underwater vehicles, where optimising efficiency, manoeuvrability and speed is vital for applications from environmental monitoring to autonomous exploration.

Research from Nature Portfolio

Recent studies have established robust scaling relations for undulatory swimmers across diverse species, demonstrating that oscillation frequency scales with body length through a crossover near 0.5–1 m. Below this threshold, frequency tuning is governed by muscle physiology, while above it the fluid–structure interaction dictates an inverse length dependence to avoid cavitation at high speeds. Investigations into collective locomotion show that flapping swimmers within arrays self-organise into distinct modes of constructive vortex interaction, yielding enhanced group speed and reduced power consumption. Complementary work with bio-inspired robots and live fish pairs reveals that followers achieve hydrodynamic benefit by adopting a linear phase shift—vortex phase matching—relative to a leading swimmer, enabling energy savings without reliance on visual or lateral line feedback.

Hydrodynamic Mechanics of Aquatic Locomotion publication trend

The graph below shows the total number of articles in hydrodynamic mechanics of aquatic locomotion across all publications each year (not limited to Nature Index journals).

Technical terms

Undulatory swimming: A mode of propulsion in which travelling waves along the body or fins generate thrust through fluid–structure interaction.

Flapping foil propulsion: Thrust generation by oscillatory movement of fins or hydrofoils, shedding vortices to propel the body.

Vortex phase matching: A strategy wherein phase differences between adjacent swimmers’ strokes align to exploit shed vortices for energy savings.

Strouhal number: A dimensionless parameter (frequency × amplitude / speed) that characterises the efficiency of oscillatory propulsion.

Added mass: The effective inertia of fluid that must be accelerated along with a moving body, increasing its apparent mass.

References

1. Wave devouring propulsion: An overview of flapping foil propulsion technology. Renewable and Sustainable Energy Reviews (2023).
2. Analysis of Rowing Force of the Water Strider Middle Leg by Direct Measurement Using a Bio-Appropriating Probe and by Indirect Measurement Using Image Analysis. Cyborg and Bionic Systems (2023).
3. Scaling the tail beat frequency and swimming speed in underwater undulatory swimming. Nature Communications (2023).
4. Undulatory Propulsion at Milliscale on Water Surface. Advanced Science (2024).
5. Hydrodynamic schooling of flapping swimmers. Nature Communications (2015).
6. Vortex phase matching as a strategy for schooling in robots and in fish. Nature Communications (2020).