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By Andrew Lyttle and Matt Keys, Western Australian Institute of Sport, Mt. Claremont, Australia
View the pdf of this article
Digitizing the underwater video footage (left) and deriving the kinematic variables (right)
The underwater phases of modern swimming
form a large component of the total event
time, and can often lead to the difference
between earning a medal and finishing with
the rest of the pack. Optimal underwater starting
and turning techniques require maximizing
the distance achieved by minimizing the deceleration
rate caused by hydrodynamic drag forces.
Currently in elite competition there is a wide range
of underwater technique strategies used by swimmers
with very little scientific rationale applied in
their selection. Previous empirical testing conducted
by Lyttle et al.1 examined the net force produced
during underwater kicking due to the propulsive
force and active drag. Results were compared to
prone streamlined gliding in order to prescribe
an approximate velocity at which to initiate underwater
kicking. The study assumed the steady state
(constant velocity) condition at which the testing
was conducted, so it limited the applicability
to real swimming where the body is continually
accelerated and decelerated.
A study now underway at the Western
Australian Institute of Sport (WAIS) seeks to discriminate
between the active drag and propulsion
generated in underwater kicking with the goal
of prescribing an optimal kick profile in swim starts
and turns. The use of FLUENT CFD will allow variables
such as the amplitude and frequency of the
kicking movement, as well as the effects at different
velocities and depths, to be examined from
a fluid dynamics perspective. The objective information
gained can be used in technique analysis.
An elite swimmer from the WAIS with a proficient
underwater kicking ability has been filmed
underwater, performing both maximal high amplitude,
low frequency dolphin kicks, and low amplitude,
high frequency dolphin kicks. A CFD model
is being developed that is based on an accurate
3D mapping of the swimmer using a Cyberware
WBX whole body laser scanner. The 3D mapping
will be combined with 2D joint kinematic data
recorded from digitized underwater video footage
of dolphin kicks from the same swimmer. Once the
detailed model is imported into FLUENT, it will be
used to model the fluid flow around the upper body
as well as the animated lower limb in order to differentiate
flow lines and calculate propulsive and
resistive coefficients. The dynamic mesh model in
FLUENT will be used to simulate the movement of the lower limbs. The base of the model will be
set up by defining the individual components as
solid bodies and creating a user-defined function
(UDF) to match the output from the digitized kinematics
data.
During the next phase of the study, alterations
to the inputs and model constraints will be investigated
to examine the effects of variations in underwater
kicking form and technique. The objective
information gained from this type of analysis will equip
the sports scientists with the tools to more accurately
provide advice on technique modifications in order
to gain the extra edge at the elite level.
The authors would like to acknowledge the financial
support of the Western Australian Institute of Sport
and Australian National Elite Sports Council –
Sports Science/Medicine subcommittee for funding
this project.
Reference:
- A.D. Lyttle, B.A.B. Blanksby, B.C. Elliott, and D.G.
Lloyd, Net forces during tethered simulation of underwater
streamlined gliding and kicking techniques of the
freestyle turn, Journal of Sports Science, 18, p.801-807,
2000.
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