Numerical Simulation Of Submerged Dolphin Kick Stroke Based On Multi-body Motion

Purposes A numerical simulation method based on multi-body motion is used to study the characteristics of the flow field around the athletes,the change of the net flow force,and the effects of hip flexion and extension angle and ankle joint adduction angle on the submerged dolphin kick stroke.MethodThe body shape data of a male swimmer are obtained by 3D scanning,and the data are reverse reconstructed to obtain the CAD model of the swimmer.According to the characteristics of submerged dolphin kick stroke technique,the joints of the athlete model are separated,and the swimmer model is divided into different segments,and the simulation of submerged dolphin kick stroke is realized by controlling the movement of each segment.The governing equations of fluid flow are Navier-Stokes equation and continuity equation of incompressible viscous fluid,and the turbulence model is RNG k-ε model.The computational fluid dynamics software package ANSYS Fluent 19.0 is used as the solver for calculation and solution.By changing the flexion angle of hip joint at the end of the extension kick,three different kicking modes of submerged dolphin kick are obtained.Athlete models with three different ankle adduction angles are obtained by rotating the lower limb segments.ResultIn this study,the variation trend of net streamwise force is basically consistent with the results of previous studies.Water velocity increases as it passes over the swimmer model’s forehead,hips,heels and knees,where flexion is performed.It slows down as it passes over the swimmer model’s neck,waist and hips,where flexion is performed.At the end of the extension kick,a large vortex structures are shed from the swimmer model’s knee and foot,while during the flexion kick,a series of smaller vortex structures are shed from the swimmer model’s foot.The drag is at its maximum at the end of the flexion kick,and the thrust is at its maximum at the start of the extension kick.The drag of the swimmer model decreases with the increase of the hip flexion angle at the end of the extension kick.The swimmer model with different ankle joint adduction angles has little difference in net streamwise force during submerged dolphin kick stroke.ConclusionThe numerical simulation method of submerged dolphin kick stroke based on multibody motion has certain feasibility.In the process of submerged dolphin kick stroke,the vortex structure are shed off from the surface of the swimmer’s body in the area with a large velocity gradient in the flow field.The shedding of vortex structure mainly occurred in the lower limbs with larger motion range,and the shedding of vortex structures are different in the extension kick and the flexion kick.Propulsion is mainly generated during the extension kick phase and mainly comes from the swimmer’s feet.The flexion angle of the hip joint has a certain influence on the net flow force.At the end of the extension kick,when the flexion angle of the hip joint increases from 20° to30°,the drag decreases.Adduction angle of the ankle has little effect on the net streamwise force.