Scott Riewald,
Biomechanics Director,
USA Swimming
Barry Bixler,
Principal Engineer,
Honeywell Engines and Systems
(swimcfd@aol.com)

USA Swimming, the national governing body for competitive swimming in the United States, has been using FLUENT to evaluate the flow around the hand and forearm of a swimmer during the propulsion phases of the freestyle and butterfly strokes. The goal is to “design” the optimum stroke to achieve peak propulsive performance for elite swimmers. Phase I of this multi-year project, the determination of steady-state lift and drag coefficients for the hand and arm, has been completed.

Flow pathlines show significant boundary layer separation and turbulence

The FLUENT CFD model for these analyses used the standard k-e turbulence model in conjunction with non-equilibrium wall functions. All analyses were second order, and adaptive meshing was applied to increase the accuracy of the results. The force coefficients, evaluated at angles of attack ranging from -15 to 195 degrees, and for various states of water turbulence, compared very well with coefficients developed experimentally in a wind tunnel, a tow tank, and a flume.

Hand and arm at a 45° angle of attack

Although the hand has often been compared to an airfoil in swimming literature, a polar diagram developed from the CFD analyses shows the aerodynamic efficiency of the hand to be significantly less than that of an airfoil of similar aspect ratio. At velocities achieved by competitive swimmers, FLUENT flow pathlines reveal the flow to be highly three-dimensional with significant boundary layer separation. Large vortices form on the downstream side of the hand, and smaller tip vortices twirl off the fingertips in a manner similar to those flowing from aircraft wings or turbine blades.

Position of the hand/arm at the beginning of the propulsive phase of the freestyle stroke, and oilfilm lines from a FLUENT CFD analysis

The successful comparison of the FLUENT results with experimental data has validated the chosen CFD modeling techniques, and will allow the research to progress into Phase II of the project. Sponsored by the Sport Science and Technology division of the US Olympic Committee, the Phase II FLUENT analyses will assess the effects of arm and hand acceleration and deceleration on the swimmer’s ability to generate propulsive forces.

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