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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?