Chapter 15: Human Movement in a Fluid Medium

Chapter 15:Human Movementin a Fluid MediumBasic Biomechanics, 4 th editionSusan J. HallPresentation Created byTK Koesterer, Ph.D., ATCHumboldt State University

Objectives• Explain the ways in which the composition and flowcharacteristics of a fluid affect fluid forces• Define buoyancy and explain the variables thatdetermine whether a human body will float• Define drag, identify the components of drag, andidentify the factors that affect the magnitude of eachcomponent• Define lift and explain the ways in which it can begenerated• Discuss the theories regarding propulsion of thehuman body in swimming

The Nature of FluidsFluid:• Air and water are fluids that exertforces on the human body.

Relative MotionRelative velocity:• Of a body with respect to a fluid is thevector subtraction of the velocity of thefluid from the velocity of the body.

Laminar versus Turbulent FlowLaminar flow:– Low velocity relative to fluid mediumTurbulent flow:– High velocity relative to fluid medium

Fluid Properties• Density (ρ) = mass / volume• Specific weight (γ) = ratio of weight tovolume• Viscosity = internal resistance to flowAtmospheric pressure and temperatureinfluence a fluid’s density, specificweight, and viscosity

BuoyancyArchimedes’ principle: the magnitude of thebuoyant force acting on a given body is equalto the weight of the fluid displaced by thebody.• F b = V d γCenter of Volume: point around which a body’svolume is equally distributed and at which thebuoyant force acts.

Flotation• Depends on body’s buoyancy and itsweight• Weight = buoyant; body floats• Weight > buoyant; body sinks

Flotation of the Human Body• Floatability is a function of body density• For flotation, buoyant force must greater thanor equal to body weight• Orientation of the human body• Torque on the floating human body

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Drag• F D = ½C D ρA p v 2Coefficient of drag:• Depends on shape & orientation of a bodyComponents of total drag force:• Skin friction• Form drag• Wave drag

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Skin FrictionSkin friction, surface drag, or viscous drag:• Fluid particles slowed due to shear stressBoundary layer: layer of fluid immediatelyadjacent to the bodyFactors that affect skin friction drag• Velocity of fluid flow, surface area,roughness, and viscosity

Wave DragWave drag:• Major effect on human swimmers• Factors that affect wave drag– Greater up-and-down motion– Increased swimming speed

Bernoulli’s principle• Pressure in a fluid varies inverselywith the velocity

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Lift Force• Lift: force acting on a body in a fluid in adirection perpendicular to the fluid flow• F L = ½C L ρA p v 2

Foil ShapeFoil: shape capable of generating lift in the presence ofa fluid flowBernoulli principle: inverse relationship betweenrelative velocity and relative pressure in a fluid flowFactors that Affect:– Velocity, pressure, and lift forceCoefficient of lift: unitless number that is an index ofa body’s ability to generate lift

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Foil Shape• Semi-foil shape of projectiles such as the discusand javelin generate some lift force when oriented atappropriate angles with respect to the direction of thefluid flow.• Angle of Attack: angle between thelongitudinal axis of a body and the direction ofthe fluid flow• Lift/drag ratio: the magnitude of the lift forcedivided by the magnitude of the total dragforce acting on a body at a given time

Magnus EffectMagnus force: lift force created by spinMagnus effect: deviation in the trajectory of aspinning object toward the direction of spin,resulting form the magnus force

Propulsion in a Fluid Medium• Resistive drag acts on a swimmer• Propulsive drag: force acting in the directionof a body’s motion• Propulsive drag theory: attributes propulsionin swimming to propulsive drag on theswimmer• Propulsive lift theory: theory attributingpropulsion in swimming at least partially to liftacting on the swimmer

Propulsion in a Fluid MediumVortex Generation• may play a role in swimming propulsionStroke Technique• product of stroke length (SL) and stroke rate(SR).

Summary• The relative velocity of a body with respect to a fluidand the density, specific weight, and viscosity of thefluid affect the magnitude of fluid forces.• The fluid force that enables flotation is buoyancy.• Drag is a fluid force that acts in the direction of thefree stream fluid flow.• Lift is a force that can be generated perpendicular tothe freestream fluid flow by foil-shaped objects.

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