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Unformatted text preview: Sec. 1.11.9 1 Chapter 1 Sections 1.11.9 Fluid Mechanics EM 3313 Sec. 1.11.9 2 Whats the Point? Context Fluids have many characteristics that make them different from solids. Thats why we are in this class. Motivation We are unfamiliar with many important characteristics of fluids. Learning objective Demonstrate knowledge of the properties of fluids through the mastery of definitions and discussions of their effects Sec. 1.11.9 3 1.1 What is a fluid? Definition: Stress force per unit area Normal stress acts perpendicular to a face Shear stress acts parallel to a face From our perspective, there are two states of matter A fluid is a substance that deforms continuously (flows) under the application of a shear stress, no matter how small A fluid at rest must have zero shear stress (hydrostatic). Only a normal stress =p A solid does not deform continuously (flow) for an arbitrarily small shear stress Two classes of fluids A liquid has closely packed molecules with strong cohesive forces (retains volume and forms a free surface) A gas has has widely spaced molecules with weak cohesive forces (no definite volume) Sec. 1.11.9 4 1.1 Continuum assumption Fluids are aggregations of molecules The distances between molecules is large in comparison to the molecular diameter Molecules are free to move with respect to one another Space between molecules about about 106 mm for std air which yields about 10 18 molecules in a mm 3 How do you define density? Mass per unit volume We have to be careful as to how we choose the volume If we choose a volume too small, the density will fluctuate based on the number of molecules that are in the volume If we choose a volume too large, the density may vary because of aggregate variations Define V * to be just right About 109 mm 3 for std air, about 10 9 molecules * * lim V m V V = Sec. 1.11.9 5 1.1 Continuum assumption (cont) Most engineering problems are concerned with physical dimensions much larger so that density is essentially a point function Fluid properties can be treated as varying continually in space Such a fluid is called a continuum Differential calculus can be used There are cases where this approach is not appropriate Rarified gas flows (upper atmosphere) Nano flows Aircraft flying through a cloud (depending on the droplet size and density) Sec. 1.11.9Sec....
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This note was uploaded on 08/21/2008 for the course EM 3313 taught by Professor Thompson during the Spring '08 term at Mississippi State.
 Spring '08
 Thompson
 Fluid Mechanics

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