Chapter 15:Human Movementin a Fluid Medium

Basic Biomechanics, 4th edition

Susan J. Hall

Presentation Created by

TK Koesterer, Ph.D., ATC

Humboldt State University

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

The Nature of Fluids

Fluid:

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

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.

Laminar versus Turbulent Flow

Laminar flow:

– Low velocity relative to fluid medium

Turbulent flow:

– High velocity relative to fluid medium

Fluid Properties

• Density () = mass / volume• Specific weight () = ratio of weight to

volume• Viscosity = internal resistance to flow

Atmospheric pressure and temperature influence a fluid’s density, specific weight, and viscosity

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:

Flotation

• Depends on body’s buoyancy and its weight

• 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 than

or equal to body weight• Orientation of the human body• Torque on the floating human body

Drag

• FD = ½CDApv2

Coefficient of drag:• Depends on shape & orientation of a body

Components of total drag force• Skin friction• Form drag• Wave drag

Skin Friction

Skin friction, surface drag, or viscous drag:• Fluid particles slowed due to shear stress

Boundary layer:

Factors that affect skin friction drag• Velocity of fluid flow, surface area,

roughness, and viscosity

Form Drag

Form drag, profile drag, pressure drag:

Pressure differential:

• Factor that affect form drag;– Relative velocity, pressure gradient, and

surface area• Streamlining helps to minimize form drag• Cyclists drafting helps to minimize form drag

Wave Drag

Wave drag:

• Major affect on human swimmers

• Factors the affect wave drag

– Greater up-and-down motion

– Increased swimming speed

Lift Force

Lift:

• FL = ½CLApv2

Foil Shape

Foil:

Bernoulli principle:

Factors that Affect;

– Velocity, pressure, and lift force

Coefficient of lift:

Foil Shape

• Semi-foil shape

Angle of Attack:

Lift/drag ratio:

Magnus Effect

Magnus force:

Magnus effect:

Propulsion in a Fluid Medium

Propulsive drag:

• Resistive drag acts on a swimmer

Propulsive drag theory:

Propulsive theory:

Propulsive lift theory:

Vortex Generation:

Stroke Technique:

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).

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 than 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.