Properties of Fluids - I - Lecture Slides | CEE 3500, Study notes of Civil Engineering

Download Properties of Fluids - I - Lecture Slides | CEE 3500 and more Study notes Civil Engineering in PDF only on Docsity! 1 Ch 2 – Properties of Fluids - I Prepared for CEE 3500 – CEE Fluid Mechanics by Gilberto E. Urroz, August 2005 Distinction between a solid and a fluid (1) ● Molecules of solid closer together than those of fluid ● Solid: intermolecular forces larger than in fluid ● Elastic solid – deforms under load – recovers original state when unloaded ● Plastic solid: – deforms under sufficient load – continues deforming as long as load is applied – does not return to original state 3 Distinction between a solid and a fluid (2) ● Intermolecular forces in fluid not large enough to hold elements together ● Fluid flows under slightest stress and continues flowing as long as stress is present 4 Distinction between a gas and a liquid (1) ● Fluids: gases or liquids ● GAS: – Molecules farther apart – Very compressible – Tends to expand indefinitely ● LIQUID: – Relatively incompressible – If external pressure removed, does not expand – May have a free surface (subject to its own vapor pressure) 5 Distinction between a gas and a liquid (2) ● VAPOR: – Gas whose T and P very near the liquid phase – Steam is a vapor, state near that of water ● GAS: – Super-heated vapor, far away from liquid phase – Volume of gas or vapor greatly affected by ΔT and ΔP ● Thermodynamics – Study of heat phenomena – Important if significant ΔT or phase changes involved 6 Distinction between a gas and a liquid (3) 7 Density, Specific Weight ● Density, or mass density, ρ [rho] = mass per unit volume (kg/m3, slug/ft3) ● Specific weight, γ [gamma] = weigth per unit volume (N/m3, lb/ft3 = pcf) ● Related by 8 Specific Volume, Specific Gravity ● Specific volume, v: volume per unit mass (ft3/slug, m3/kg) ● Specific gravity for a liquid is the dimensionless ratio where ρL = density of liquid, ρW = density of water at a standard temperature (either 4oC, or 60oF) ● For gases, the reference density is not standard. It must be specified. ● See Sample problems 2.1, 2.2, pp. 15-16 9 Compressible and incompressible fluids ● Compressible: variable density ● Incompressible: constant density ● No real incompressible fluid exists, assumed so if Δρ is small as P changes, e.g., – Liquids usually – Gases if ΔP small relative to absolute pressure ● Liquid compressibility important in pressure waves ● Air: – Incompressible: ventilating system, fly < 250 mph – Compressible: high-velocity pipe, fly > 760 mph (speed of sound) 10 Compressibility of liquids (1) ● Bulk (volume) modulus of elasticity, Ev (kPa-abs, psia) ● Ev represents the Δp required to produce a unit change in specific volume (Δv/v) ● Ev = f(T,p) for liquids [e.g.,see Table 2.1, p. 17] ● Use absolute pressures, thus, units: psia, kPa abs 11 Compressibility of liquids (2) 12 Compressibility of liquids (3) ● For a fixed mass of liquid at constant temperature, the bulk modulus does not change much on a moderate range of temperature (e.g., see Table 2.1, p. 17, for water). ● In this case (refer to figure in previous slide), we can write: ● See Sample Problem 2.3, p. 18.