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Influences of external forces on objects flying in through fluid (air and/or water) and theirs consequences
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Fluid BiomechanicsFluid Biomechanics
• The study of forces that develop
when an object moves through a
fluid medium.
• Two fluids of interest
• Water
• Air
Fluid forcesFluid forces
• Four major fluid forces of interest:
• Weight
• Buoyant Force
• Drag
• Lift
• Other Forces
FloatationFloatation
• Equilibrium:
• weight of object = buoyant force
• Floatation:
• specific gravity < 1
• weight of object < max. buoyant force
• Specific gravity:
S = Wbody
Wwater
• Center of buoyancy:
The point that is CG of volume of water equal to the volume and shape of the submerged body
CB is closer to the chest region
CG is closer to the pelvic region
Natural floater
True floater
True sinker
Buoyant ForceBuoyant Force • Archimedes’ Principle:
• Object in a fluid is acted on a buoyant force.
• buoyant force = weight of the fluid displaced
by the object
“A body that is partially or totally immersed
in a fluid will experience an upward
buoyant force that is equal to the weight
of the volume of fluid displaced by that
body”
Center of buoyancy & Center of buoyancy & swimming performanceswimming performance
Vwater = +10
Vboat –Vwater = 0 – 10 = –
10
Vwater = +10
Vboat –Vwater = 10 – 10 = 0
Vwater = 0
Vboat –Vwater = 10 – 0 = 10
DragDrag ForceForce
• Along the direction of motion
• Opposite of relative flow
• Slows down the speed
• Resistance• Affected by cross-section area
• Affected by surface smoothness
FD = ½ CDAV2
Skin Friction / Surface DragSkin Friction / Surface Drag
• Boundary layer: layer of affected air
• Depends on• relative velocity• surface area• smoothness of surface• fluid (viscosity)
• fabric (swim suit), shaving body in swimming
Profile Drag/Form DragProfile Drag/Form Drag
• Due to separation of the fluid
from the boundary of the object
• Eddy currents
• Pressure gradient
• Streamlining
Profile Drag/Form DragProfile Drag/Form Drag
• depends on
•cross-section
•shape of the body
•smoothness of the surface
• bicyclist in upright v. crouched position
Wave DragWave Drag • Resistance at the interface, due to wave
LiftLift
•Perpendicular to the direction of motion
• Created by different pressures on
opposite sides of an object due to fluid
flow past the object
• Bernoulli’s principle: velocity is
inversely
proportional to pressure
Bernoulli’s LawBernoulli’s Law
“Where the flow velocity is fast, the
pressure is low; where the flow
velocity is slow, the pressure is
high”
FL = ½ CLAV2
Effect of Lift in SportEffect of Lift in Sport
Variables influencing aerodynamic
lift force
• Angle of Projection; angle between
horizontal and CG of projectile
• Line of Flight
• Attitude Angle; angle between horizontal
and long axis of projectile
•Center of Pressure
•Angle of Attack at Release; angle
between projectile’s long axis and
projection angle
Effect of Lift in SportEffect of Lift in Sport
The Magnus EffectThe Magnus Effect
• The Magnus effect
describes the
curved path that is
observed by
spinning projectiles
The Magnus EffectThe Magnus Effect
• Explained by
Bernoulli’s principle
and the pressure
differences caused
by relative
differences in flow
velocities
Top SpinningTop Spinning
• Velocity of superior boundary
layer decrease
• Velocity of inferior boundary
layer increase
• NET Force; Downward
Back SpinningBack Spinning
• Velocity of superior boundary layer
increase
• Velocity of inferior boundary layer
decrease
• NET Force; upward
• Rebound
flow
NET FORCE (up)
Side SpinningSide Spinning• Lt. spin
Rt. side – low pressure zone
Lt. side – high pressure zone
NET Force; to Rt. side
• Rt. SpinLt. side – low pressure zoneRt.side – high pressure zoneNET Force; to Lt. side