When viewing the
results below it should be noted that, while experimental drag readings
represent characteristics of the entire wheel, power reading only represent the
rotating characteristics i.e. drag of the spokes (aero and wire) and skin
friction of the disc and rim. This is because the motor is not working against the
drag of the entire wheel. The wheel is being held in place by the balance,
which is in no way connected to the motor.
8.1 Drag at zero
yaw
Fig 8.1 shows the axial drag of
the wheel at zero yaw angle for all the configurations tested. The full table
of results from which the graph was plotted can be found in appendix C.
The results
displayed in fig 8.1 follow a general trend to higher drags at higher speeds,
although the drag of the spoked wheels does not increase at a consistent rate.
Significantly the spoked wheels all have a similar shaped curve while the disc
and tri-spoke have near straight lines. The results are consistent enough to
postulate that spoked wheels have less drag at low speeds – below 25km/hr.
Although it was
assumed that if the 36 spoke wheel has the highest drag at 40km/hr it will have
the highest drag at all speeds, this is not the case in the results obtained
since, contrary to expectation, the spoked wheel curves cross each other. This
apparent inconsistency might be due to:
i) the high drag
of the support jig and motor which makes the measurement similar to weighing a
bag of sugar on the bathroom scales by weighing yourself and then you and the
sugar! For tare values see appendix C.
ii) the spirit level style of the
balance and the need to re-zero it regularly, which gave potential for human
error in taking the readings.
iii) the very small variations in
drag, so changes in the atmospheric conditions in the lab, errors in setting up
the wheel in the test rig, and any inaccuracy of the balance may have
significantly affected the results.
8.2 Power at zero
yaw
Fig 8.2 shows the
power required to drive the wheel at zero yaw angle for all the configurations
tested. The full table of results from which the graph was plotted can be found
in appendix C.
Fig 8.2 displays
a set of results which are believed to be more reliable than those for drag.
This higher degree of accuracy can be attributed to:
i) a more precise method of
measurement - The voltmeter and ammeter used had digital displays, which
involve no human error and gave readings to 2 decimal places.
ii) the high tare values (see appendix C), which were an
advantage in this case, because they took out most of the inaccuracies due to
the efficiency of the motor.
The below zero
results are probably due to the difficulty of taking accurate tare values at
low speeds. The bare hub and spindle have very little inertia and readings were
erratic.
As far as the
actual power results obtained are concerned, some interesting readings were
taken. It was assumed before the testing was carried out that the disc would be
the most efficient wheel, but the results indicate that this is far from true.
It is mid way down the power scale along with 18 spokes. The tri-spoke was the
easiest wheel to turn, being even more aerodynamic than the 6 spoke wheel. As
expected, the power needed to turn the round spoked wheels increases with the
number of spokes.
8.3 Theoretical
results
The differences in drag between
the various configurations are theoretically analysed in section 6. These
results do not compare exactly with the experimental results, but show a strong
correlation especially with the experimental results for power. They also show
why there are differences between the data collected for each configuration. A
more in depth theoretical analysis with fewer assumptions is needed to give
precise results. This could not be done due to both time and software
limitations.
The theoretically obtained
results for all but the disc configuration are displayed graphically in fig 8.3
and 8.4. It should be noted that these are values for the spokes only and do
not include the power to turn the rim or the drag it produced, the rim being
the same for all tests this does not affect comparison of the results.
Fig 8.4 shows the
tri-spoke as having a much higher power requirement than was found in the
experiment. This is probably due to the assumption made to simplify the
analysis that the spokes meet an uninterrupted free stream flow: in fact the
flow is disturbed by the rim. This does not greatly affect the flow over the
round spokes as they are angled out away from the rim towards the hub so most
of the spoke can be seen by the free stream flow. The aero spokes however, go
straight into the centre of the hub and so are completely hidden behind the
rim. This will result in the aero spokes experiencing a lower pressure and
probably more turbulent flow, which will mean less drag and so less power is
needed to turn the wheel.
Fig 8.3 and 8.4
extend the power and drag curves to 60km/hr to highlight how aerodynamics
becomes increasingly important at higher speeds.
8.4 Drag at
different yaw angles
Fig 8.5 shows the
axial drag of the wheel at varying yaw angles for the disc, 18 spoke and
tri-spoke configurations. The full table of results from which the graph was
plotted can be found in appendix C.
The results shown
in fig 8.5 inevitably contain the same inaccuracies discussed earlier in this
section, but these are not considered to be as great due to the greater
differences in the drag measured and that these were the last tests to be
carried out (practice makes perfect!).
All
configurations show an increase in drag as the wheel is yawed. The 18 spoke and
tri-spoke show very similar characteristics, with evenly spaced curves for each
yaw angle, i.e. the rate of increase of drag with yaw angle is constant.
The disc wheel
shows a greater increase in drag than the other configurations with a sudden
increase in drag somewhere between 15 and 20 degrees. This same phenomenon was
seen by Tew and Sayers[2] and is contrary to the negative drag found by
Greenwell et al[1]. Greenwell’s finding seems improbable (despite being a common
belief in time trialling circles) as a disc at an angle to the wind will
generate lift which is associated with induced drag – not induced
"push"! This forward force may be heavily dependent on the shape of
the disc. The disc tested by Greenwell which gave the most negative drag had
convex sides, another with near zero drag had flat parallel sides, Tew’s had
straight sides which angled out to the hub while the researcher’s model has
concave sides.
The increase in
drag is probably due to the disc stalling which could clearly be seen in the
smoke tests (see section 6). From this it can be deduced that the 18 spoke and
tri-spoke do not stall, or at least not to the same extent, which leads to the
conclusion that the flow passes between the spokes and through the wheel,
increasing the flow on the adverse side and eliminating stall.
8.5 Power at different yaw
angles
Because the wheel
is not pulling itself along in the tunnel, the power readings represent only
the drag of the spokes and the skin friction of the rim or sides of the disc.
They will not be affected by changes in the profile drag of the wheel.
Therefore a large variation in power needed to turn the wheel as the yaw angle
is increased is not expected and little is seen in fig 8.6. On close
examination an increase in power is seen when the disc is at 20 degrees yaw
angle. This corresponds to the increase in drag in fig 8.5. It would be interesting
to see a result for the power needed to turn Greenwell’s disc at this yaw angle
– perhaps the decrease in drag would be at the expense of an increase in power?