Biomechanics and Medicine in Swimming XI

The Effect of Angle of Attack and Depth on Passive
Drag
Pease, d.l. 1 , Vennell, r. 2
1 Australian Institute of Sport, Canberra, Australia
2 University of Otago, Dunedin, New Zealand
In order to quantify the effect of angle of attack and depth on drag force
during underwater gliding the current study was undertaken. This aim
was achieved by utilising a flume and an anatomically accurate mannequin
whose orientation could be precisely controlled. Measurements
were obtained with the mannequin oriented with the longitudinal axis
at angles of attack of -4, -2, 0, +2 and +4 degrees relative to water flow.
Measurements were taken at depths ranging from 0.2 – 0.8m and at
velocities ranging from 0-2.55ms -1 . There was generally little effect
from the angle of attack other than near the surface (0.2m) where
there was a tendency for the wave drag to be lower for the negative
angles. This finding indicates that as swimmers approach
the surface it may be beneficial to adopt a slight negative angle of
attack in order to minimise the drag force.
Key words: swimming, passive drag, wave drag, angle of attack, flume
IntroductIon
Previous research into the drag forces (both active and passive) acting
during human swimming has shown differences due to body size, shape,
velocity and depth. However, one factor, which has not previously been
investigated, is that of the angle of attack or pitch angle of the athlete’s
body relative to the water flow. Angle of attack is a factor, which has
been highlighted as an issue in previous study but due to the inability
to systematically control this factor it has been generally described as a
limitation.
Therefore, the present study was undertaken to try and quantify the effect
of angle of attack on the drag forces acting upon a streamlined human
swimmer by utilising an anatomically accurate mannequin whose orientation
relative to the water flow direction could be precisely controlled.
It was hypothesized that, as angle of attack changed, there would
be significant changes in the magnitude of the total drag force as well
as changes in the relative contribution of the component forces, viscous,
form, and wave drag. These changes would be primarily due to the
changes in exposed frontal area with increasing (positive and negative)
angles of attack away from the zero angle.
Based on the findings from previous research (Vennell, Pease, &
Wilson, 2006) there are increases in the total drag force, and more specifically
the wave drag contribution to that total force, as depth of the
swimmer decreases. Therefore, another aspect of the current study was
to examine the interaction between angle of attack and submergence
depth and the measured drag force.
Previous studies have quantified body angles similar to angle of attack
during free surface swimming (Zamparo, 2006; Zamparo et al.,
2008) with the aim of using that angle as an indicator of body position.
In general the angle of the trunk to the horizontal is used to represent
angle of attack. In those studies body angles of approximately 15 degrees
were found. However, during the streamlined portion of a swimming
race when the athlete is fully submerged, and not moving on a
fluid boundary, it was theorised that the angle of attack would be much
less due to the freedom of the athlete’s body to move in the vertical as
well as the horizontal plane. This is unlike surface swimming where the
movement trajectory is fixed and essentially limited to the horizontal
plane. By allowing for movement in the vertical plane as well as the
horizontal, the trajectory of the centre of mass is more in line with the
angle of the body thereby reducing the angle of attack relative to the
surrounding water flow.
chaPter2.Biomechanics
Therefore, the current study examined smaller angles of attack which
may be achieved by fully submerged swimmers in a streamline position
such as that experienced following starts and turns.
Methods
All testing was conducted in the aquatic treadmill or ’flume’ at the University
of Otago, as described by Britton, Rogers, and Reimann (1998).
In order to achieve the desired control over position of the body
relative to water flow it was necessary to utilise an anatomically accurate
mannequin rather than live subjects. The mannequin used was the
same as that described in previous research (Bixler & Pease, 2006; Bixler,
Pease, & Fairhurst, 2007; Vennell et al., 2006) and had a total surface
area of 1.859m 2 which allowed for the determination of an estimate for
viscous frictional force.
The mounting structure used to support the mannequin was similar
to that used in the previous studies. However, in the previous work the
mannequin was mounted via a towing arrangement through the fingertips,
which allowed for free movement of the rest of the mannequin’s
body. Therefore a new mounting was necessary which allowed for the
maintenance of orientation at all times. In the modified structure the
mannequin is fixed directly to a vertical aerofoil spar, which is attached
at the back of the mannequin. This aerofoil spar is then bolted to the
same vertical pole and mounting structure described by Vennell et al.
(2006). The vertical spar is infinitely adjustable in terms of mannequin
depth between 0 and 0.9m. However, for the purposes of the current
study the depths utilised were 0.2 – 0.8m at 0.1m increments. Depth
was measured as the distance between the water free surface and the
central longitudinal axis of the mannequin.
The vertical pole then was clamped to a horizontally oriented triaxial
load cell, AMTI model MC3-6-1000, with the Z axis parallel to
the water flow direction. The load cell was in turn interfaced with an
AMTI MCA6 amplifier and finally to a PowerLab unit which allowed
for continuous data collection. Vertical position of the mannequins was
controlled by raising or lowering the vertical spar and pole. The position
was then fixed by a set screw at the gimbal, and a scaffolding clamp attached
in series with the tri-axial load cell.
In order to obtain the optimal drag-velocity curves for the mannequin,
data was collected for 13 velocities: 0, 0.34, 0.55, 0.75, 0.95, 1.16,
1.36, 1.57, 1.77, 1.94, 2.15, 2.36, and 2.55 ms -1 respectively at each of
the tow depths described previously. These conditions were repeated for
each of the angles of attack; -4, -2, 0, +2, and +4° respectively. Angles of
attack were based on a 0° orientation defined as that position with the
minimum projected frontal cross sectional area. Six tests of five seconds
in duration were undertaken for each test condition.
The projected frontal areas for these tested angles of attack were:
0.1282, 0.1156, 0.1079, 0.1124, and 0.1280m 2 respectively. While attack
angles greater than those tested may be exhibited in real situations,
due to the uncertainty of the magnitude of load changes beyond the
angles measured, limits were set in order to minimise the possibility of
damaging either the mannequin or the testing support structure. Secondly,
if angles greater than those tested had been utilised there would
have been protrusion of the mannequin through the water surface at
the minimum depth, which would have changed the wetted area and
thereby affected the results.
In order to detect any effects of angle of attack on the drag forces,
correlation coefficients were determined between the angles of attack
and the respective drag force components. In order to utilise correlational
analyses a linear relationship between angle of attack and drag was
hypothesized due to the linearly decreasing depth of the leading edge
of the mannequin. During the pilot testing the data were analysed for
normality of distribution and were deemed to be suitable for parametric
analysis. Due to the limitation of only utilising the single mannequin
the magnitude of the correlation coefficient needed to be very high in
order to achieve statistical significance (r≥.858 for significance at the 0.1
level and r≥.959 for significance at the .05 level)
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