Biomechanics of propulsion and drag in front crawl swimming Huub Toussaint Institute for Fundamental and Clinical Human Movement Sciences Vrije Universiteit, Amsterdam, Holland

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 F p forward

Quasi-steady analysis

Quasi-steady analysis: Combining flow channel data with hand velocity data

MAD-system

Propulsion: Results Quasi- 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’ l Quasi-steady analysis cannot account for required lift forces l 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 l Leading-edge vortex stabilized by axial flow l Can account for ~ 50% of required lift force l 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 l Short strokes & rotations: unsteady effects probably play an important role l Explore by flow visualization l 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:

ship v

Divergent waves Transverse waves ship

Effect of speed on wave length Wave drag 70% of total drag (of ship)

Length of surface wave Hull speed for given length (L) of ship:

Height of swimmer 2 m: Hull speed for a swimmer “Pieter” swims > 2 m/s…..

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 ( )

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

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