Chapter 15: Human Movement in a Fluid Medium

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Chapter 15Human Movementin a Fluid Medium

• Basic Biomechanics, 4th edition
• Susan J. Hall
• Presentation Created by
• TK Koesterer, Ph.D., ATC
• Humboldt State University

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Objectives

• Explain the ways in which the composition and
  flow characteristics of a fluid affect fluid
  forces
• Define buoyancy and explain the variables that
  determine whether a human body will float
• Define drag, identify the components of drag, and
  identify the factors that affect the magnitude of
  each component
• Define lift and explain the ways in which it can
  be generated
• Discuss the theories regarding propulsion of the
  human body in swimming

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The Nature of Fluids

• Fluid
• Air and water are fluids that exert forces on the
  human body.

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Relative Motion

• Relative velocity
• Of a body with respect to a fluid is the vector
  subtraction of the velocity of the fluid from the
  velocity of the body.

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Laminar versus Turbulent Flow

• Laminar flow
• Low velocity relative to fluid medium
• Turbulent flow
• High velocity relative to fluid medium

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Fluid Properties

• Density (?) mass / volume
• Specific weight (?) ratio of weight to volume
• Viscosity internal resistance to flow
• Atmospheric pressure and temperature influence a
  fluids density, specific weight, and viscosity

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Buoyancy

• Archimedes principle the magnitude of the
  buoyant force acting on a given body is equal to
  the weight of the fluid displaced by the body.
• Fb Vd ?
• Center of Volume point around which a bodys
  volume is equally distributed and at which the
  buoyant force acts.

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Flotation

• Depends on bodys buoyancy and its weight
• Weight buoyant body floats
• Weight gt buoyant body sinks

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Flotation of the Human Body

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

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Drag

• FD ½CD?Apv2
• Coefficient of drag
• Depends on shape orientation of a body
• Components of total drag force
• Skin friction
• Form drag
• Wave drag

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Skin Friction

• Skin friction, surface drag, or viscous drag
• Fluid particles slowed due to shear stress
• Boundary layer layer of fluid immediately
  adjacent to the body
• Factors that affect skin friction drag
• Velocity of fluid flow, surface area, roughness,
  and viscosity

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Wave Drag

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

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Form Drag

• Form drag, profile drag, pressure drag
• Pressure differential between the lead and rear
  sides of a body moving through a fluid
• Factors that affect form drag
• Relative velocity, pressure gradient, and surface
  area
• Streamlining helps to minimize form drag
• Cyclists drafting helps to minimize form drag

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Bernoullis principle

• Pressure in a fluid varies inversely with the
  velocity

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Lift Force

• Lift force acting on a body in a fluid in a
  direction perpendicular to the fluid flow
• FL ½CL?Apv2

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Foil Shape

• Foil shape capable of generating lift in the
  presence of a fluid flow
• Bernoulli principle inverse relationship between
  relative velocity and relative pressure in a
  fluid flow
• Factors that Affect
• Velocity, pressure, and lift force
• Coefficient of lift unitless number that is an
  index of a bodys ability to generate lift

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Foil Shape

• Semi-foil shape of projectiles such as the discus
  and javelin generate some lift force when
  oriented at appropriate angles with respect to
  the direction of the fluid flow.
• Angle of Attack angle between the longitudinal
  axis of a body and the direction of the fluid
  flow
• Lift/drag ratio the magnitude of the lift force
  divided by the magnitude of the total drag force
  acting on a body at a given time

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Magnus Effect

• Magnus force lift force created by spin
• Magnus effect deviation in the trajectory of a
  spinning object toward the direction of spin,
  resulting form the magnus force

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Propulsion in a Fluid Medium

• Resistive drag acts on a swimmer
• Propulsive drag force acting in the direction of
  a bodys motion
• Propulsive drag theory attributes propulsion in
  swimming to propulsive drag on the swimmer
• Propulsive lift theory theory attributing
  propulsion in swimming at least partially to lift
  acting on the swimmer

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Propulsion in a Fluid Medium

• Vortex Generation
• may play a role in swimming propulsion
• Stroke Technique
• product of stroke length (SL) and stroke rate
  (SR).

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Summary

• The relative velocity of a body with respect to a
  fluid and the density, specific weight, and
  viscosity of the fluid 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 the free stream fluid flow.
• Lift is a force that can be generated
  perpendicular to the freestream fluid flow by
  foil-shaped objects.

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The End