1. April 2016 Flow Structure Interactions & Pendulum Dynamics in Fluid Jack O'Neill
Abstract
The speed achieved during cycling races continues to increase each year and a
cyclist’s capability of achieving a higher speed is dependent on both physical ability
and technology. Mechanical components have evolved, as has the cyclist’s clothing.
More recently the potential for gains from modifications to the aerodynamic drag of a
cyclist’s legs, by the addition of boundary layer elements, has been realised.
This work was conducted to determine whether a commercially available boundary
layer management system, Aero Trip Strips (ATS), could save energy. The work
considered the influence of wind speed and positioning on a cyclist’s leg.
Laboratory experiments using a pendulum, where the pendulum’s mass represented a
cyclist’s leg, within a wind tunnel to simulate the motion of a cyclist’s leg showed
that energy was saved by the application of ATS. These strips showed a significant
decrease in drag at wind speeds above 15mph. The force exerted by the wind on the
pendulum mass (with no external force applied to oscillate the pendulum) was
measured at various wind speeds and showed that the addition of ATS yielded the
greatest improvement (greatest lowering of force) at a wind speed of 20mph. This
was found to be the onset of the critical flow regime where flow becomes turbulent
and the drag force is significantly reduced.
Laboratory testing also showed that the maximum energy saving (0.541J) produced
by the use of ATS occurred around 30mph. At approximately 35mph the energy
difference between the tests (with and without ATS) began to decrease and the ATS
became less advantageous. The effect of positioning the ATS around the pendulum
mass was explored and showed the optimum position to be 45º.
In addition to exploring the use of ATS, a dimpled surface was attached to the whole
of the pendulum mass. This showed a dimpled surface could reduce drag to an even
lower magnitude than the ATS technology. As this would appear to be more
attractive to a competitive cyclist, it is suggested that sports clothing manufacturers
should consider incorporating dimpled surfaces in their leg wear for athletes.
One resonant frequency of the pendulum oscillation at varying wind speeds was
found during the start of the oscillation at 0.586Hz through Fourier analysis but there
was no evidence of other natural frequencies resulting from vortex shedding or the
wind tunnel vibration. This eliminated the possibility of other factors contributing to
the oscillatory behaviour of the pendulum and influencing the results.
This work concluded that ATS technology delivers significant benefits for
competitive cyclists, and in a sport where winning margins are narrow it could
represent the difference between winning and losing.