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

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 = ½CDApv2 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 = ½CLApv2

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.