During this study three experimental setups are explored, all of them aiming to obtain better insights in the physics of human swimming. The drag force of towed objects is examined by performing towing experiments on human swimmers in the swimming facility and by performing tethered sphere experiments in a laboratory facility. The flow of the surrounding water during swimming is visualized and quantified. Footage obtained in experiments at the swimming facility is analyzed by a PIV routine in order to create the velocity and vorticity field of the water, induced during active swimming.The average drag on the human swimmer is validated with previous research (observed drag force F is linear with the squared velocity v2: F=29.3v2). However more insight is gained in the fluctuations of these forces. Analysis based on a dimensionless quantity called the Strouhal number (St), implies that vorticity is shed from the frontal area (probably the head or hands) of a swimmer, which enhances fluctuations in the drag force. Furthermore towing experiments on human swimmers are recommended to be performed at large towing velocities, since the relative error in the force is the smallest for these velocities.Drag force fluctuations are investigated in more detail during tethered sphere experiments. The drag coefficient (CD=0.69) and Strouhal numbers (St=0.14 and St=0.28) for the tethered sphere are observed to be similar to previous research. Uniquely, this study combines synchronized information of the force on the tethered sphere and the position of this sphere in the flow. The fluctuating force is observed to be closely related to the stream wise position of the sphere as well as the radial distance from the original position to the actual position of the sphere.By performing PIV measurements, some interesting flow fields created during human swimming are observed. Moments after the downkick with the legs during butterfly swimming, two opposite signed patches of vorticity are observed. These patches translate through the water because of their induced velocity in one another. These patches are created as a result of the forceful downkick; Newton's third law implies a reaction force of the water exerted on the swimmer. This force is calculated to be 62N, in semi upwards and forward direction.Just before the downkick with the legs (again during butterfly swimming), a large vortex is created underneath the swimmer. Again Newton's third law predicts a force of the water exerted on the swimmer. In this case a force of 63N in the upward direction is obtained.Some new insights in the interpretation of measurements on the drag force of human swimmers are obtained during this study. Fluctuations in this drag force are most probable created due to vortex shedding, as also observed during tethered sphere experiments in the laboratory.Furthermore some large scale flow phenomena (vortices) are created in the flow during active swimming. These vortices exert a force on the swimmer, which eventually provides in the propulsion of human beings during swimming.
Experiments on human swimming : passive drag experiments and visualization of water flow
Custers, G. (Auteur). 31 aug. 2013