Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Tianzeng Li, Wenhao Cai, Jiemin Zhan, Tiago M. Barbosa.
The purpose of this study is to analyze the motion status of swimmers during their gliding stage using a numerical simulation method. This simulation strategy is conducted by solving the 3D incompressible Navier-Stokes equations using the Realizable k-ε turbulence closure equations in combination with the Six Degrees of Freedom (6-DOF) method. The uneven mass distribution of a swimmer and the roughness of the surface of the body are taken into consideration. The hydrodynamic characteristics and movement characteristics of the swimmers at different launch speeds were analyzed. The calculated results suggest that an optimal instant for starting propulsive movement is when the velocity of the swimmer decreases by 1.75 m/s to 2.0 m/s from an initial horizontal velocity of 3.1 m/s to 3.5 m/s.
Gliding underwater, which is a major part of the overall performance in competitive swimming, occurs during the starts, between strokes, and after turns [1,2]. A proper gliding posture can offer relatively higher initial velocity and can maintain the momentum of the swimmer for the next stroke, and the proper gliding posture leads to an increase in the overall swimming rhythm.
In the 6-DOF simulation strategy, the meshes of the computational domain should be defined as different functional parts, namely, an active zone that corresponds to the boundary on which the loads are calculated and that delimits a body with defined dynamic characteristics, a passive zone that surrounds and moves with the active zone with non-deformation, a deformable zone where meshes are deforming with time, and constituted by tetrahedral types . Additionally, a stationary zone that is outside the deformable mesh zone is set with structured hexahedral cells to save computing resources, as shown in Fig 3. To investigate the grid sensitivity, the computational mesh is refined using an exponent-based refinement method near the model’s surface with different mesh resolution, i.e., the smallest grid size near the surface is generated as 0.004 m, 0.006 m, 0.008 m, 0.0 1 m and 0.012 m. Cases with different grids are conducted with an initial velocity of 3.5 m/s. Swimmers’ heave and pitch values within the first 0.5 s are compared, as they are sensitive to the grid resolution, as shown in Fig 4. The results were insensitive to further reductions in resolution as the minimum wall normal grid spacing was reduced beyond 8 mm. As a result, the grid resolution of 0.006 m with approximately 7,500,000 cells is conservatively applied in the following studies.
Numerical simulations have been conducted for the different initial horizontal velocities, which are 3.1 m/s, 3.3 m/s and 3.5 m/s. The effective research scope of this simulation corresponds to the underwater glide phase of swimmers holding a traditional streamlined posture.
The aim of this study was to investigate the glide stage of swimmers using a numerical simulation method, the 6-DOF motion. The swimmers’ displacements, orientations and hydrodynamic forces during the glide are presented and are helpful for optimizing and improving the performance of the swimmers’ glide phase.
This paper has presented a numerical study of the gliding motion of swimmers with different initial velocities based on a 6-DOF full-scale body model. The numerical strategy, where the swimmer’s mass distribution and surface roughness are considered, is proven to be feasible for simulating the underwater gliding motion of swimmer. Numerical results show that swimmers will gradually deviate from the horizontal longitudinal position by gliding forward. The maximum lift force appears at the initial stage of the glide when swimmers glide with a relatively high velocity. Calculated results suggest that an optimal instant for starting propulsive movement is when swimmer’s velocity decreases from 1.75 m/s to 2.0 m/s, according to the initial horizontal velocity of 3.1 m/s to 3.5 m/s. In addition, this study, which first attempts to investigate the glide motions of swimmers using numerical methods, can be a reference that is helpful for people to further understand the gliding stage of swimmers. Some results and discussions are helpful for the swimmers and the coaches, such as when to start propulsive movement, how to reduce the drag force, and so on.