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/indexsw.htmlH_M_Toussaint@fbw.vu.nl

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

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