Research Division Summer Placement 2011

Richard Stocker Professor Kwing-So Choi

An investigation into the effects of finger spacing in swimming using PIV

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Introduction It is common practice in swimming lessons to teach students to swim with their fingers completely closed, making their hands like a paddle, as this is believed to be the most efficient way of swimming. Despite this practice, it has been shown that professionals swim with their fingers slightly separated, allowing a small amount of water to flow between them. The reasons for this are as of yet unknown, but research has been done in this area and its conclusions suggested that there could be a link between finger spacing and the drag on a swimmers hand. As an increase in drag without an increase in effort would be beneficial to a swimmer, it is useful to study this area further to more fully understand this correlation. This research aims to confirm the results of previous research and to further study the flow by using particle image velocimetry which will give more of an insight into the flow around the hand and particularly behind the fingers of a swimmer.

Aims/Objectives Aims · To understand the mechanisms of a swimmers hand and be able to more accurately predict the most efficient stroke. · To be able to optimise the finger spacing of a swimmers stroke as a result of these findings. Objectives · Run tests to analyse the drag pattern and intensity of a swimmers stroke under different hand angles and accelerations. · Attempt to visualise the flow behind a swimmers hand and show qualitatively the difference in the flow due to finger spacing. · Use a PIV laser system to analyse the flow behind a swimmers hand in detail.

Acceleration 2.7 m/s^2, finger spacing angle 15°

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Acceleration 2.7 m/s^2, finger spacing angle 0°

Acceleration 2.7 m/s^2, finger spacing angle 10°

Acceleration 2.7 m/s^2, finger spacing angle 5°

Acceleration 3.3 m/s^2, finger spacing angle 0°

Acceleration 4 m/s^2, finger spacing angle 0°

Drag pattern analysis results

The drag analysis tests were performed to evaluate the correlation between the drag the hand is subjected too and angle of the fingers relative to each other. The variables were acceleration and finger spacing angle, with three constant acceleration settings, 2.7, 3.3 and 4 m/s^2, and 4 finger spacing angles in 5 degree intervals from 0 to 15° The structure of the graph remained constant throughout the results and consisted of two peaks, the first one likely to be caused predominantly by viscous drag forces and the second larger peak caused by pressure drag forces. When analysing the data gathered it was shown that neither variable had a significant effect on the maximum value of the second peak on the drag vs time graph so it was decided that the subsequent PIV testing should focus on the first peak, which showed significant variance with both acceleration magnitude and finger spacing angle. The maximum drag value in the first peak increased with an increase in the constant acceleration value, as shown by the three graphs to the left. This was shown at all finger spacing angles . When the effect of finger spacing angle on the magnitude on the drag force on the early stages of the swimmers stroke was analysed, the data showed that there was an increase in the magnitude of the first drag peak as finger spacing angle increased up until a finger spacing angle of 10° after which a decrease in drag was shown, with the 15° finger spacing showing less drag than with the fingers fully closed. This was shown to be true at all accelerations and would suggest that a 10 degree finger spacing would be ideal while swimming, although for a more exact angle more precise tests with a finger spacing range around 10 degrees would have to be performed. These tests also show that the effects of finger spacing and acceleration on the drag of a swimmers hand seem to act independently of each other, with both correlations appearing to be uniform as the other variable was changed. This research is the first to focus on the viscous drag peak with previous research focusing on the smaller differences in the second peak which reached the conclusion that as the acceleration increased the ideal finger spacing angle also increased. These tests however introduced a maximum velocity of 1.6 m/s as it has been shown that is the terminal velocity of a swimmers stroke, and that has alleviated the drag variance. This suggests the second drag peak is heavily dependant on the velocity of the hand with acceleration and finger spacing angle playing a minor role, if any, in the maximum drag value of the second drag peak.

PIV analysis testing results

PIV testing was done in an attempt to study the nature of the flow behind the hand in more detail and to gain a better understanding of why a swimmer opening their fingers slightly has an effect on the drag their hand is subjected to. The variables and parameters for these tests were identical to those done for the drag testing analysis so that the results can be used together to give a broader evaluation of what is occurring during the constant acceleration of a swimmers hand. The PIV tests showed some significant results as highlighted by the comparison between a flow at an identical acceleration, 2.7 m/s^2, but with the sequence above being done with a finger spacing angle of 0° and the sequence below having a finger spacing angle of 10°, shown to have the optimum drag value. When the results of the experiment were theorized, it was expected that at relatively wider finger spacing angles the flow behind the hand would be more elongated and turbulent than with the fingers closed. This is due to the flow of water between the fingers breaking up and disturbing the flow behind the hand, so that while a closed hand would have one wide vortex behind the fingers, a spaced hand would have more narrower vortices following each finger. This would lead to more 3d effects and therefore a wider wake, which combined with the added turbulence should lead to more drag. The observations from the PIV testing confirm these observations, with the closed hand (above ) showing a very clear vortex trailing the hand (shown on some pictures in green) and a relatively undisturbed flow behind the hand. The wider finger spacing (below) shows many small vortices forming at varying heights and distances behind the hand and a much longer flow trailing the hand. The flow also looks more disturbed which would indicate there are 3d effects prevalent, which not the case in the flow behind the closed hand.

Acknowledgements I would like to thank professor Kwing-So Choi, Dr Faycal Bahri and Dr Richard Whalley for their substantial help and support throughout the project, and to Andy Matthews for his technical support. I would also like to thank EPRSC for giving me the funding to have this opportunity and give me valuable engineering experience.

Diagram key A: Force transducer B: Accelerometer C: Traverse system D: Pivot E: Water level F: Hand

Experimental set up

The experiment was conducted in a roughly 2m by 1m by 1m water tank. Above the tank there is a traverse system supported by a steel structure that is placed in the middle of the width of the tank and operates along the longest side. Attached to the traverse is a spacer block and a pivot. The pivot is connected to a long arm which has a composite model of a human hand at the bottom. This means that the arm can rotate about the pivot and the hand can move linearly along the tank in one direction. The experiment is therefore conducted by accelerating the traverse system to propel the hand forward through the water. To measure the drag and acceleration of the hand an accelerometer and force transducer are both attached to the set up. The force transducer is attached to the spacer block so that it touches the highest point of the arm. By calibration of the force transducer and measurement of the distances of the upper point of the arm and the centre of the hand from the pivot point, the drag on the hand can be calculated. For the PIV experiments the laser was set up behind the tank so it could be lined up with a space between fingers, while the camera was lined up to the side of the tank so that it was perpendicular to the hand, thus allowing the flow behind the hand to be captured.

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The apparatus from the side (left) and the back (right)