Physics 160 Biomechanics
Fluid Mechanics
What is a Fluid?
Fluid Dynamics
Important quantities for fluids
• Pressure – Force per unit area
• Density – Mass per unit volume
• Viscosity – measure of amount a fluid resists
flowing
• When a fluid flows over an object the amount
of friction is determined by the viscosity of the
fluid and the smoothness of the object.
Buoyancy
• Archimedes’ Principle
– Buoyant force equals weight of
fluid displaced
– If an object’s density is greater
than 1.0g/cm3 it will sink in fresh
water, if it is smaller than this it will
float.
– An iceberg is 91% underwater!
Boundary Layer
Boundary Layer
The thickness of the BL depends
on:
1. the shape of the object - thin
short objects have smaller BL long objects give the BL space
over which to grow
2. the roughness of the surface smooth surfaces have less
friction and a smaller BL.
3. the velocity of the fluid - very slow
moving fluids have very orderly
flow patterns termed LAMINAR
Turbulence
As the layers mix and swirl over one another, the BL
becomes turbulent
If the swirling is regular and repeatable, it is termed a
vortex or an eddy
The region in the BL where the orderly laminar layers
start to mix together, but before they really start
swirling, is called the transition region.
The transition region is usually in a fairly small region.
Boundary Layers
Form Drag
Laminar flow
Larger form drag
Turbulent flow,
Smaller form drag
Form Drag
Reducing Drag
Drafting at 25mph
Wave Drag
• Due to swimmer creating a wave in the water.
• Increased by up and down movement.
• Lane lines tend to reduce wave actions.
Angle of Attack = α
Small drag and lift
Larger lift
No lift
Downward lift
Bernoulli’s Principle
High velocity gives low pressure
Magnus Force
Examples
1. A 700 N man has a body volume of 0.08 m3.
If submerged in fresh water, will he float?
2. How much body volume must a 60 kg
person have to float in fresh water?