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SportBiomk Swimming MM10
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Transcript of SportBiomk Swimming MM10
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Uwe Kersting, 20111
Biomechanics of Swimming
Center for Sensory-Motor Interaction
Sports Biomechanics
Uwe Kersting MiniModule 10 - 2011
Uwe Kersting, 20112
Objectives
Apply sports biomechanics approach toswimming
Be able to differentiate characteristics ofdifferent swimming styles
Review fundamental concepts on fluiddynamics buoyancy, lift vs. drag
Learn about dophin skin and swim suits
Create a comprehension how all workstogether
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Uwe Kersting, 20113
Contents
1. Introduction: four main swim styles
2. What we can see: characterisation of thefour styles
3. Fluidynamics principles- drag- lift- about surfaces and adjacent materials
4. Simple calculations
5. Finetuning of swim style understanding (?)
6. Summary
Uwe Kersting, 2011
Videos
4 swimming styles & ....
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Uwe Kersting, 2011
Factorial Model ofSwimming Performance
partial times model
TIMEtotal
Startingtime
TurningtimeStrokingtime
Block time Flight time Glide time
Glidedistance
Averageglide speed
Horiz
speed atentry
Changes in
horiz speedin glide
Horizontalimpulses in
glide
Mass ofswimmer
Horizontalimpulse intakeoff
Movementtime
Reactiontime
Vert Vel
at TO
Height
above water
Verticalimpulse intakeoff
Uwe Kersting, 2011
Starting timestart depends on horizontal and vertical impulses
produced on the blockspeed in air greater than speed in water: optimise time in
the air.however, too much height in the start produces
greater downward speed which must be stopped inthe water appears to slow the swimmer down
grab vs sprint starts: grab is faster off the blocks, butsprint start greater impulse (what is the objectiveof the start?)
- beware of first out of the blocks syndrome- when to start stroking? When your glide speed drops to
your swimming speed
maximum impulse in minimum time
I = m * v
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Uwe Kersting, 2011
Factorial Model of
Swimming, contd
TIMEtotal
Startingtime TurningtimeStrokingtime
Strokingdistance
Averagestroking speed
Average strokelength
Average strokefrequency
Turningdistance
Startingdistance
Racedistance
Propulsiveforces
Resistiveforces
Pulltime
Recoverytime
Wavedrag
Surfacedrag
Formdrag
Propulsivedrag forces
Propulsivelift forces
Propulsiveforces (legs)
Propulsiveforces (arms)
Uwe Kersting, 2011
Stroke length
Propulsive forces:lift forces from sculling actionsdrag forces from pull actionlegs contribute to propulsion in whip and dolphin
kicks, but less so in flutter kick
Resistive forces:form drag X-C area (viewed from the front)surface drag typically small, reduced by flutter
kick. Also by speed suitswave drag caused by lifting water above surface
level(minimise rolling and vertical motion ofthe body)
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Uwe Kersting, 2011
Basic propulsion instructionsnew water
Hand to move into still water and accelerate it(generate a force against it)
hand must move in a 3 dimensional curve
if the hand moves in a straight line backwards,it cannot accelerate as much water!
lift
additional force can be gained by pitching thehand so that it acts as a wing (producing liftas well as drag)
this is called sculling
Uwe Kersting, 2011
Factorial Model ofSwimming, contd
TIMEtotal
Startingtime TurningtimeStrokingtime
Averageturn time
Numberof turns
Glidetime in
Turntime
Glidetime out
Turntechnique
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Turnsturns take between 20 35% of race time!the longer the race the more important turns
become
Observational research has shown that a pikedturn is faster than a tucked turn
Distance (yd)
Stroking
Turning
Starting
Percentage
oftotalrace
time
Uwe Kersting, 2011
Physics principles needed forswimming
Need to stay at the surface
Need to produce propulsive forces
Need to minimise resisitive forces
Definitions and concepts
Application to swimming
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Uwe Kersting, 2011
A fluid is any substance that tends to flow orcontinuously deform when acted upon
Gases and liquids fluids with similarmechanical behavior
-- but compressible vs. incompressible
Definition of a Fluid
Uwe Kersting, 2011
Relative Velocity
Velocity of a body with respect to thevelocity of something else such as thesurrounding fluid
The velocity of a body relative to a fluidinfluences the magnitude of the forcesexerted by the fluid on the body
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Uwe Kersting, 2011
Other important FluidProperties
Fluid Density - mass per unit volume(i.e., 1 g/ccm), = 1 g/cm3
Fluid Viscosity - internal resistance of afluid to flow (oil vs water)
In addition:
Temperature & atmospheric pressure affectboth above
Uwe Kersting, 2011
Forces exerted by fluids: Buoyancy
in water, the buoyant force equals theweight of the volume of water displaced.
the centre of buoyancy is at the centreof mass of the volume of water displaced
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Will a body float or not???
A body will float only if:
Wt of body Wt of an equal amount fluid
This can also be stated as:
Weight of body
Weight of an equal amount of fluid
termed: Specific Gravity of Body
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Uwe Kersting, 2011
Specific gravity of a body
Effects of:
1. Volume of air inlungs
2. Age (very young orvery old)
3. Females vs Males
4. Body composition
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Forces exerted by fluids: Buoyancy
weight: Alwaysverticallydownward!!!
Archimedesprinciple:magnitude ofbuoyant force ona given body =
weight of thefluid displaced bythe body
Weight
Weight
BuoyantForce
Weight
BuoyantForce
Weight
BuoyantForce
Uwe Kersting, 2011
Pressure approach
Px = * hx
20
h
P
Px
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In a wave ...A circular shape
is a bad bodysurfer...
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A wave is ...
Uwe Kersting, 2011
Wave Drag =energy loss
motion at theinterface of body andfluid causes waves,
takes energy and slowsdown theswimmer/boat/etc.
But you can use waves!In swimming, the lanemarkers are designedto reduce wave motionbetween lanes
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Uwe Kersting, 2011
DragDrag = resistance force - slowing themotion of a body moving through fluid
Drag force:
FD = drag force; CD = coefficient of drag; = fluid density, AP = projected area of body orsurface area of body oriented to fluid flow; v =relative velocity of body with respect to fluid
2
2
1vACF DD =
Uwe Kersting, 2011
Examples of Coefficient of Drag
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Factors affecting Drag
CD = affected by shape & orientation of bodyto relative to fluid flow
= medium density e.g., air densitydecreases with altitude 1968 OlympicGames in Mexico City (2250m) manyworld records set!
v = greatest effect!!! Theoretical squarelaw if e.g., cyclist at double speed; otherfactors remain unchanged: drag forceopposing increases 4 times!!!!
2
2
1vACF
DD
=
Uwe Kersting, 2011
Form Drag
form drag depends on the cross-sectional area presented to theflow
streamlining is an attempt tominimize form drag
sometimes you want to maximize formdrag:
oar blade
sailing downwind
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What is Form Drag???Separation of flow from boundary andsubsequent re-uniting of the divergentpaths causes a pocket to be formedbehind moving body
Pocket has lower pressure versus highpressure resulting from oncoming airflowstriking the front of the body
Whenever a pressure differential exists, aforce is directed from the region of highpressure to the region of low pressure =FORM DRAG ( = CD)
Uwe Kersting, 2011
An example: streamlining(short)
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Laminar vs Turbulent flow
Laminar flow in parallellayers
Turbulent flow withviolent intermixing offluid
Affected by :
1. form of body2. relative velocity3. surface roughnessof body
Uwe Kersting, 2011
Modifying boundary layer turbulence
you can reduce drag by reducing turbulence
a rough patch on the surface will reduce theseparation angle and thus reduce drag
Images of a ball in a wind tunnel. On the right, the ball had sand gluedto the front of it. Notice the separation angle change
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Speed suitsmany sports now use suits which incorporate rough
patches designed to reduce the separation angleand decrease the drag
Uwe Kersting, 2011
What is surface drag?Example: water rushes past an object, layer
of water in contact with object is sloweddown due to forces the objects surfaceexerts on it - that layer of air slows downthe layer of air next to it etc. .
Boundary layer: region within which fluidvelocity is diminished due to shearingresistance caused by boundary of movingbody
Depending on velocity & nature of body, theboundary layer becomes unstable &turbulent i.e., change from laminar toturbulent flow!
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Surface Dragsurface drag
depends on thesmoothness of thesurface and thevelocity of flow
shaving in swimming big effect???
other examples of
sports to decreasesurface drag.???
Uwe Kersting, 2011
A note on the technologicaladvancement of the swimsuits
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The Perfect MaterialIn the past: Hairless skin better than suitHuman skin: Too porous, turbulence too high
Shark skin: Scales spaced very closely together
Hydrophobicity, turbulence control > Dragresistance slice the water.
Fastskin I and II developedby Speedo
Coverage: Eventually fromfeet to hands
Oxygen bubbles alongstitches
Uwe Kersting, 2011
The Perfect Shape
Following three years of research thatincluded input from NASA, tests onmore than 100 different fabrics andsuit designs, and body scans of morethan 400 elite swimmers, Speedo haslaunched its most hydro-dynamicallyadvanced - and fastest - swimsuit todate.
- February 14th, 2008
Extreme tight fit: Streamlinebody shape reduce (bad)vibrations.
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FINA hits the brakes but too lateIntention: Ban all hi-tech suits before WC 2009
Failed to address 136enquiries
-> all suits were allowed
Super materials: 100%Polyurethane, Hydrofoil
A male suit, mind you
Uwe Kersting, 2011
Restrictions by FINA 1st Jan 2010Surface covered: Men swimsuit shall not extend above the
navel nor below the knee and for women shall not cover theneck or extend past the shoulders nor shall extend belowthe knee.
Type of material: The material used for swimsuits can be only"Textile Fabric(s)" defined for the purpose of these rulesas material consisting of, natural and/or synthetic,individual and non consolidated yarns used to constitute a
fabric by weaving, knitting, and/or braiding.Additional rules for: surface treatment, flexibility, variety
of materials, thickness, buoyancy, permeability,construction etc
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World Records Never
To Be Broken?
FINA: Records set at WC 2009 willstand.
Though the changes won't go into effect at the worldchampionships that begin Sunday in Rome, they will hangover the competition, seemingly wagging a finger at everyworld-record setter wearing a suit that will never beallowed again in a major swimming championship.
- The Washington Post, July 2009
~ 130 WRs broken since launch of high-tech suits
Uwe Kersting, 2011
Magnus Effectif a ball spins in flight, it will drag some of the
air close to the surface with it. This createsan area of high pressure and an area of lowpressure on opposite sides of the ball
this pressure imbalance will make the ball curve
in flight
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Uwe Kersting, 2011
An example: streamlining
Uwe Kersting, 2011
Bernoullis principleConsider a foil shape fluid flows over the
curved side & is accelerated while on the flatside it remains virtually unchanged
This difference in velocity of flow creates low
pressure on curved side & high pressure on flatside
Remember that force is directed to foil fromarea of high pressure to area of low pressure..causing lift = more effective!!!
Now! Do the stroke!
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Uwe Kersting, 2011
freestyle stroke (butterfly is verysimilar)
Uwe Kersting, 2011
breaststroke pattern relative to the a) swimmer and b) pool
b)a)
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virtually no pull in breaststrokescull out, then scull in
LIFT forces from L & R limbs add to propulsion insculling motion
DRAG forces cancel in sculling motion
direction of movement
DRAG force
LIFT force
direction of movement
LIFT force
DRAG force
direction of movement
DRAG force
Uwe Kersting, 2011
Lift and Drag Forces on a Discus(Wind v = 25 m/s)
Angle of
Attack
(Degrees)
Lift (N) Drag (N) Lift/Drag
0 0.000 0.036 0.000
10 0.135 0.047 2.890
20 0.331 0.128 2.579
30 0.348 0.254 1.371
40 0.264 0.297 0.890
50 0.268 0.380 0.705
60 0.214 0.466 0.459
70 0.148 0.511 0.290
80 0.079 0.525 0.151
90 0.000 0.551 0.000
Angle of attack angle btwlongitudinal axis of a body &direction of fluid flow
Need to take some drag intoaccount to enable lift!
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Back to swimming reality: Relationships
between stroke length and frequency
SF for freestyle, butterfly and breaststroke aresimilar and greater than for backstroke
as race distance increases, SL increases, SFdecreases and speed decreases
differences in ability are due primarily to strokelength, with better swimmers having greater SLs
to increase speed in the short term (i.e., on the day)increase stroke frequency
to increase speed in the long term (i.e., over theseason) train to increase stroke length video
Uwe Kersting, 2011
Summary
Factor (subjective) model be aware of thecomplexity of mechanical factors in swimming
resistive and propulsive forces what are they, andhow can you maximise propulsive and minimiseresistive forces
starts a case of optimising distance in the air plusdistance in the glide
strokes how do the principles of new water andlift influence stroke shape?
understand the relationships between SL and SF
turns play a much more important part in racesover 100m that most swimmers realise. Turnpractice is essential!
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Uwe Kersting, 2011
Points to rememberForces exerted by fluidsbuoyancy magnitude and location (+ effect on floating position)
Bernoullis Principle increased velocity of flow results indecreased pressure
lift & drag forces
result from objects being in a fluid flow.
The drag force is aligned with the flow and the liftforce is perpendicular to it.
Maximum lift at 45, zero lift at 0 and 90.
Form drag depends on the X-C area presented to the flow andgeometry
important in streamlining and minimizing frontalarea
Surface drag is comparably small but may be decisive
Uwe Kersting, 2011
Acapulco ...
or what I left out...
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