Abstract: Our quest is for the thumb and finger positions that maximize drag in front crawl swimming and thus maximize propulsion efficiency. We focus on drag in a stationary flow. Swimming is in water, but using Reynolds similarity the drag experiments are done in a wind tunnel. We measure the forces on real-life models of a forearm with hands, flexing the thumb and fingers in various positions. We study the influence on drag of cupping the hand and flexing the thumb. We find that cupping the hand is detrimental for drag. Swimming is most efficient with a flat hand. Flexing the thumb has a small effect on the drag, such that the drag is largest for the opened (abducted) thumb. Flow structures around the hand are visualized using robotic volumetric particle image velocimetry. From the time-averaged velocity fields we reconstruct the pressure distribution on the hand. These pressures are compared to the result of a direct measurement. The reached accuracy of ≈ 10% does not yet suffice to reproduce the small drag differences between the hand postures. Graphical Abstract: [Figure not available: see fulltext.].

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The flow field around a full-scale swimmer’s hand model with varying thumb positions is investigated by robotic volumetric PIV. The experiment is conducted in the Open Jet Facility wind tunnel at TU Delft at 15m/s. Quantitative flow field information is constructed with 3D-PTV in a 120 liter volume, encompassing the full hand and arm. The effect of spatial resolution on the time-averaged flow field is investigated. A large-scale recirculating wake behind the hand is accurately identified at a linear bin size of 20mm whereas the accelerated flow between individual fingers can only be resolved at bin sizes below 10mm where 5mm results in a statistically unconverged velocity field. The influence of the thumb is limited to one side of the hand where its presence results in a larger stagnated region in front and larger wake behind the hand, depending on the thumb position. Closing the thumb strengthens the recirculation but results in a smaller velocity deficit downstream, suggesting a smaller propulsive force generation which is considered disadvantageous in competitive swimming. @en