Fluid Properties Flashcards
Specific density (ρ)
Mass per unit volume (mV⁻¹)
Specific volume (ν)
Reciprocal of specific density (ρ⁻¹)
Specific weight (γ)
Weight per unit volume (ρg)
Specific gravity
Density of the fluid per unit density of water
What is viscosity for?
Telling us how fluids flow and deform
When does shear stress happen?
When particles of a moving fluid nice relative to each other
Viscosity in gases
Molecules are only weakly kept in position by molecular cohesion. As adjacent layers move there is continuous molecule exchange. The layers influence each others speed.
If temperature of a gas increases, the momentum exchange between layers will increase thus increasing viscosity.
Viscosity will also change with pressure - but under normal conditions this change is negligible in gases
Viscosity in liquids
There is molecular interchange between adjacent layers. The molecules are much closer than in gases hence the cohesive forces hold the molecules in place more rigidly.
Increasing the temperature of a fluid reduces the cohesive forces and increases the molecular interchange.
Reducing reducing cohesive forces reduces shear stress, while increasing molecular interchange increases shear stress.
As pressure increases the relative movement of a molecule requires more energy hence viscosity increases.
Newton’s law of viscosity
Shear stress => τ=FA⁻¹
Shear strain => Φ=dx/dy (change in width and length of liquid indirections perpendicular to force)
Proportionality constant (μ) => τ=μdV/dy
Newtonian fluid
μ is. Fluid obeys Newton’s law
Viscosity (μ)
Property of fluid offering resistance to shear deformation because of cohesion and interaction between molecules.
Fluids with high viscosity deform more [what?] than fluids with low viscosity
slowly
Velocity gradient or rate of shear strain
dV/dy
Coefficient of dynamic viscosity
Shear force per unit area required to drag one layer of fluid with unit velocity past another layer a unit distance away.
Kinematic viscosity (ν)
Ratio of dynamic viscosity to mass density (μ/ρ)
