Fluids- Properties of Fluids and Key Concepts Flashcards
What is a free surface?
The surface of a fluid that is subject to zero parallel shear stress, such as the boundary between two homogeneous fluids.
What is the normal stress on a fluid at rest?
Pressure
What happens to a fluid under the influence of a shear stress?
It deforms continuously no matter this small the stress.
In fluids, what is the stress proportional to?
The strain rate
What is specific weight?
The weight of a unit volume of a substance
What is specific gravity?
The ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C)
What is viscosity?
A property that represents the internal resistance of a fluid to motion. It measures its resistance to deformation.
What is absolute viscosity about?
Resistance to motion
What is kinematic viscosity about?
Resistance to motion under gravity
Formula for dynamic viscosity
μ=τ/(du/dy) μ is coefficient of viscosity (Pa•s) τ is shear stress u is local velocity y is y axis when looking at profile of pipe
Formula for kinematic viscosity
ν=μ/ρ
ν is kinematic viscosity (m^2/s)
How do the dynamic and kinematic viscosités of liquids and gases depend on pressure?
For liquids, they are both practically independent of pressure except for extremely high pressures.
For gases, dynamic viscosity is independent of pressure, from low to moderate pressures, but kinematic depends on pressure since density is proportional to pressure.
Explain the relation between viscosity of liquids and temperature
Viscosity of liquids is caused by cohesive forces between molecules. At higher temperatures, the molecules possess more energy and can oppose the large cohesive intermolecular forces more strongly so can move more freely. Therefore viscosity decreases when temperatures increase.
Explain the relation between viscosity of gases and temperature
Viscosity in gases is caused by molecular collisions. At high temperatures, molecules move faster, meaning more collisions and therefore a greater resistance to flow. So viscosity increases with temperature.
Formula for viscometer
T=FR=4π^2R^3n•L/lμ T is torque (a moment) R is radius of viscometer n• is number of revolutions per second L is length of cylinder l is separation between the two cylinders
Describe surface tension of a flui
For a free surface of a fluid at rest under gravity, the horizontal surface is in tension. The pulling force causing this tension acts parallel to the surface and is due to the attractive forces between molecules of the the liquid.
Symbol for surface tension and units
σ subscript s (will refer to it as σ in other cards)
Units N/m
Formula relating force and surface tension
F=2bσ
Where F and σ act in opposite directions
b is length of wire
What is surface energy?
The work done per unit area. It is the same property as surface tension.
Formulae for equilibrium of a droplet
Force due to surface tension = area x pressure difference
2πRσ=πR^2ΔP
Formulae for equilibrium of a bubble
Force due to surface tension = area x pressure difference
2(2πRσ)=πR^2ΔP
What does the behaviour of a liquid at the surface where it meets a solid depend on?
The adhesive force between liquid and solid molecules compared to the internal cohesive force between the liquid molecules
What is a wetting fluid and describe the relation between the forces?
One where the L-S forces are greater than the L-L forces so the liquid rises slightly at the surface in contact with the solid.
What is a non-wetting fluid and describe the relation between the forces?
One where the L-S forces are less than the L-L forces so the liquid falls slightly at the surface in contact with the solid.
