Added Mass Of Human Swimmers: A Comparison Of Computational And Experimental Results

Abstract

Added mass is the amount of water a swimmer has to accelerate in addition to his body during changes in velocity. It is an important concept in determining the total body drag of swimmers during unsteady motion, as well as propulsive forces acting form accelerating hands and feet. PURPOSE: The aim of this study was to find added mass of a human body using experimental and computational methods, and to compare the two methods. METHODS: For one male adult subject, added mass was found experimentally using vertical oscillations. The subject were connected to a 2.8m long bar with handles, attached with springs (stiffness k=318N/m) and a force cell. By oscillating this system vertically and registering the time period of oscillations it is possible to find the added mass of the swimmer, given the known masses of the bar and swimmer. Additionally the added mass was found using computational fluid dynamics (CFD) calculations. The body surface and volumes of the subject was determined using a computer tomography scan technique. The 3D body- and water model was created using the Gambit software (Fluent®, Inc. Hannover, USA) producing a hybrid mesh of 900 million cells for use in the CFD calculations. A finite volume model including a standard k-epsilon turbulence model was used to solve the Navier-Stokes equations of fluid flow using the Fluent Software (Fluent®, Inc. Hannover, USA). The acceleration of the water flow was set to a=0.4 m·s-2, and the inlet velocities were 0.1, 0.5, 1.0, 1.5, 2.0, 2.5 m·s-2. RESULTS: For the male swimmer, with a body mass (BM) of 81.6 kg, and a characteristic length of 2.39m (reaching height) the added mass was found experimentally to be 21.6kg, and using CFD to be 26.6±4.3 kg. CONCLUSIONS: The added mass of male humans seems to be in the proximity of 26.5 or 32.5% of BM using oscillation or CFD measurements respectively. There is a relatively close agreement between AM found using experimental and CFD methods. The different AM values found using these two methods is most probably due to the subject's experimental conditions during oscillations involve small body posture movements, whereas with the CFD model the body are completely stiff. The results indicate that CFD can be used to estimate the added mass of human swimmers. For the future, unsteady effects of human swimming hydrodynamics should be investigated.