Fluid Studies
What is a fluid? 4 An ideal fluid doesn’t compress, like most common liquids. 4 Gas has fluid-like properties, but is compressible unless compression takes place very quickly. 4 Certain solids have fluid-like properties; think about fine sand flowing.
Two Broad Fields of Study 4 Hydrostatics: fluid at rest 4 Hydrodynamics: fluid in motion
Hydrostatics 4 Density: the amount of space (volume) a certain mass occupies. 4 Units like kg/m 3 4 Can range from zero (space) to almost infinite (black holes)
Pressure 4 Force spread over an area. 4 High pressure can be either a heavy weight over a large area or a small area supporting a relatively light weight. 4 For water, pressure is linear; twice the depth is twice the pressure. 4 For air, pressure is exponential in nature
Quantities:air 4 1 atmosphere is 14.7 lbs/in 2, or 101,000N/m 2. 4 Also measured in bars, Torrs, or inches or millimeters of Mercury. 4 Barometers measure pressure by gauging how high a column of Mercury will grow in a vacuum due to external air pressure. 4 Typically 29 inches or 760mm
Water 4 33 feet of water equals one atmosphere. 4 This great pressure makes it impossible to use a long hose for underwater breathing. 4 Therefore, SCUBA (self-contained underwater breathing apparatus) must be used. 4 Breathing highly compressed air can cause the “Bends” when a diver returns to normal pressures.
Blood Pressure 4 Systolic: pressure in arteries while heart pumps. 4 Diastolic: pressure in arteries while heart relaxes 4 Healthy for most people: 120 over 80 4 Measured in mm of Hg.
Buoyancy 4 An upward force due the fact that the underside of an object is deeper (and under greater pressure) than the top side. 4 Archimedes’ Principle: the buoyant force is equal to the weight of the displaced fluid. 4 Therefore, a greater buoyant exists in a more dense fluid.
continued... 4 Neutral buoyancy: neither rising or sinking when immersed (like a sub or fish) 4 Negative buoyancy: floating. When the buoyant force is greater than the weight. 4 People float because they are less dense than the surrounding water and therefore weigh less. 4 People float better in saltier water!
Buoyancy in Air 4 Why balloons and blimps work. 4 Hot air (or helium or hydrogen) is less dense than cold air... 4 But not by much! That’s why balloons and blimps are so big.
Hydrodynamics 4 Two kinds of fluid flow: laminar and turbulent. 4 Turbulent is very difficult to deal with mathematically; deals with chaos. 4 We restrict studies here to laminar (layered) flow.
Laminar Flow 4 Air (or other fluid) flows along the surface of an obstacle smoothly, forming streamlines. 4 Little energy lost to friction. 4 A blunt object forced through even a laminar fluid will cause turbulence, causing frictional losses.
Bernoulli’s Principle 4 The faster a fluid flows, the lower in internal pressure. 4 Many examples, like standing near a speeding train and being “sucked” in. 4 There is no suck! Just a differential pressure between your back and the train’s passage. 4 The speeding train carries air with it quickly, lowering the pressure around it.
Other examples: 4 Wings: the air over the top travels faster than air under the bottom. 4 Curve ball: side of ball turning in direction of motion travels relatively slower than the side turning away from the direction of motion. 4 Tornadoes: the eye is at a greatly lower pressure due to the rapid winds around it. 4 Sails: formed by the wind into a vertical wing.