Biomechanics of propulsion and drag in front crawl swimming Huub Toussaint Institute for Fundamental...
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Transcript of Biomechanics of propulsion and drag in front crawl swimming Huub Toussaint Institute for Fundamental...
Biomechanics of propulsion and drag in front crawl swimming
Huub ToussaintInstitute for Fundamental and
Clinical Human Movement Sciences
Vrije Universiteit, Amsterdam, Holland
www.ifkb.nl/B4/[email protected]
Buoyancy
Weight
Drag Propulsion
How is propulsion generated?
Pushing water backwards
Viewpoints:
Front crawl kinematics
Pushing water backwards?
Hand functions as hydrofoil
Hydrofoil subjected to flow
Hand has hydrofoil properties
Lift and drag force
Adapt to direct Fp forward
Quasi-steady analysis
Quasi-steady analysis:Combining flow channel data with hand velocity data
MAD-system
Propulsion: ResultsQuasi- steady analysis vs MAD-system
Does the quasi-steady assumption fail?
How to proceed?A brief digression
The aerodynamics of insect flight
‘The bumblebee that cannot fly’
Quasi-steady analysis cannot account for
required lift forces
Hence, there must be unsteady,
lift-enhancing mechanisms
Delayed Stall
Unsteady lift-enhancing mechanism
Add rotation…. and visualize flow
Hovering robomoth
3D leading-edge vortex
Delayed stall: the 3D version Leading-edge vortex stabilized by axial flow Can account for ~ 50% of required lift force Key features:
– Stalling: high angle of attack (~ 45º)– Axial flow: wing rotation leads to an axial
velocity / pressure gradient– Rotational acceleration (?)
So what’s the connection?
...back to front crawl swimming
Short strokes & rotations: unsteady effects
probably play an important role
Explore by flow visualization
Our first attempt:– Attach tufts to lower arm and hand to record
instantaneous flow directions
Outsweep
Accelerated flow
The pumping effect arm rotation pressure gradient axial flow
Toussaint et al, 2002
Buoyancy
Weight
Drag Propulsion
Drag:
friction pressure drag wave drag
shipv
Divergent waves
Transverse waves
ship
Effect of speed on wave length
Wave drag 70% of total drag
(of ship)
Length of surface wave
2v2
g
Hull speed for given length (L) of ship:
v
Lg
2
Height of swimmer 2 m:
L 2
v 2 9.812 3.14
1.767 m / s
Hull speed for a swimmer
“Pieter” swims > 2 m/s…..
E T B R C S M J Mean0
10
20
30
40
% o
f to
tal d
rag
subjects
MAD-method
Wave drag as % of total drag
12%
Summary
humans swim faster than ‘hull’ speed wave drag matters at competitive swimming
speeds but is with 12% far less than that for ships where it is 70% of total drag
Interaction length of ship (L) with wave length (l)
hull speed
reinforcement
cancellation
reinforcement
hull speed
Could non-stationary effects reduce wave drag?
Takamoto M., Ohmichi H. & Miyashita M. (1985)
‘Technique’ reducing bow wave formation?
Glide phase: arm functions as “bulbous bow” reducing height of the bow wave
Non-stationarity of rostral pressure point prohibits full build-up of the bow wave
ship
With whole stroke swimming speed increases about 5% without a concomitant increase in stern-wave height.
The leg action might disrupt the pressure pattern at the stern prohibiting a full build up of the stern wave
THANK YOU FOR YOUR
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