Lecture 07 - FluidMechanics

# Lecture 07 - FluidMechanics - Fluid Fluid Mechanics Review...

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Fluid Mechanics Review

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Properties Involving Mass or Weight of Water Mass Density (often called density) ρ mass per unit volume kg/m 3 ρ = 1000 kg/m 3 at 4 o C Specific Weight γ Gravitational force (weight) per unit volume of water γ = 9810 N/m 3 ( or 62.4 lb/ft 3 ) at 4 o C Relationship between density and specific weight : g 
Physical Properties of Water

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Viscosity Viscosity- measure of a fluid’s resistance to shear or angular deformation. Water is a Newtonian fluid , and there is a linear relationship between shear stress ( τ ) and velocity gradient (du/dy): where the constant or proportionality μ is the dynamic viscosity, and u is the velocity. Viscosity has units N . s / m 2 and is highly dependent on temperature. Water at 20 oC has a viscosity of 1.00 x 10 -3 N.s/m 2 (2.09 x 10 -5 lb-s/ft 2 ) Dynamic viscosity: the ratio of shear stress to the velocity gradient: dy du Kinematic viscosity: the ratio of the dynamic viscosity to the density in which the gradient force dimension canceled out in μ / ρ ν has been defined because many equations include μ / ρ Kinematic viscosity of water at 20 o C is 1.00x10 -6 m2/s (1.08x10 -5 ft 2 /s)
Viscosity Example Water has a dynamic viscosity of 10 poises Water has a dynamic viscosity of 10 poises (dyne-s/cm2). What is the kinematic viscosity in ft2/s? Since 1 lb = 444800 dynes and 1 ft = 30.48 cm, then 1 lb.s/ft 2 = 444800 dyne.s/(30.48 cm 2 ) = 478.8 poises -- the conversion of poise to lb.s/ft2 (for dynamic viscosity) is then: 1 lb.s/ft 2 = 478.8 poises μ = (10 poises) / ( 478.8 poises/lb.s/ft2) = 0.0209 lb.s/ft2 -- The kinematic viscosity is then computed: f f lb 2 32 ) 0209 0 2 2 s ft ft lb s ft ft s / 0108 . 0 / 4 . 62 ) / . )( / . ( 3

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Elasticity Elasticity (or compressibility) is related to the Elasticity (or compressibility) is related to the amount of deformation (expansion or contraction) induced by a pressure change Compressibility is characterized by the bulk modulus of elasticity Ev, defined by: V dV dp E v / or : / d dp E v In most hydraulic engineering applications, water is considered incompressible. Elasticity is sometimes important in Gd t Groundwater unsteady flow in pipelines – e.g. water hammer
Surface Tension Molecular of water below the surface act on Molecular of water below the surface act on each other by forces that are equal in all directions Molecular near the surface have a greater attraction for each other Molecular on the surface re not able to bond in all directions and consequently form stronger bonds with adjacent molecules.

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## This note was uploaded on 03/17/2011 for the course CIVE 310 taught by Professor Zhang during the Spring '11 term at UNL.

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Lecture 07 - FluidMechanics - Fluid Fluid Mechanics Review...

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