Hydrodynamic mechanism of planar motion in cycloidal-propelled underwater vehicles: Configuration transition from H to X

As a novel propulsion device, the cycloidal propeller offers superior maneuverability and controllability. However, there is still insufficient understanding of how cycloidal propellers should be arranged on a vehicle so that their advantages can be fully exploited, enabling more rational thrust distribution and improved control of the pressure and vorticity fields, and thereby optimizing hydrodynamic performance. To clarify the fluid-dynamic principles underlying this problem, this study takes a four-cycloidal-propelled underwater vehicle as the research object and systematically investigates the planar motion performance of two propulsor layouts, namely the H-configuration and the X-configuration, using computational fluid dynamics. First, the two vehicle configurations and their corresponding planar motion modes are established. Then, the hydrodynamic parameters, pressure distributions, and three-dimensional vortex structures of the two configurations during planar motion are comparatively analyzed. Finally, the performance differences caused by the configuration change are interpreted from the perspectives of thrust distribution, wake interaction, and flow-field reconstruction. The results show that the H-configuration provides a higher degree of thrust organization along the motion direction, thereby producing more coherent wakes, reducing inter-propulsor interference, and achieving higher steady cruising speed and propulsion efficiency. In contrast, in the X-configuration, part of the propulsive effort generates lateral force components and is canceled during force synthesis; meanwhile, stronger wake interaction and broader vortex spreading further weaken the effective propulsion, resulting in lower planar-motion speed and efficiency. However, by utilizing the lateral force components generated by the propulsors, the X-configuration can achieve omnidirectional planar motion without changing the body orientation. This study explains, from a fluid-dynamics perspective, the intrinsic mechanism by which propulsor layout determines the planar motion performance of cycloidal-propelled underwater vehicles and can provide a reference for future research on propulsion layout design.