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