1. Chapter 15: Human Movement in a Fluid Medium
Basic Biomechanics, 4th edition
Susan J. Hall
Presentation Created by
TK Koesterer, Ph.D., ATC
Humboldt State University

2. 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

3. The Nature of Fluids Fluid:
Air and water are fluids that exert forces on the human body.

4. 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.

5. Laminar versus Turbulent Flow
Laminar flow:
Low velocity relative to fluid medium
Turbulent flow:
High velocity relative to fluid medium

6. 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

7. 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:

8. Flotation Depends on body’s buoyancy and its weight
Weight = buoyant; body floats
Weight > buoyant; body sinks

9. 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

10. 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

11. 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

12. 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

13. Wave Drag Wave drag: Major affect on human swimmers
Factors the affect wave drag
Greater up-and-down motion
Increased swimming speed

14. Lift Force
Lift:
FL = ½CLApv2

15. Foil Shape Foil: Bernoulli principle: Factors that Affect;
Velocity, pressure, and lift force
Coefficient of lift:

16. Foil Shape
Semi-foil shape
Angle of Attack:
Lift/drag ratio:

17. Magnus Effect
Magnus force:
Magnus effect:

18. 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:

19. 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).

20. 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.