Week 14-15 Unit 1 Flashcards
Name some typical features of a value of gas
- each molecule moves at high speed
- there are a high number of collisions between molecules
- weak interaction between molecule
- momentum is the predominant factor which governs the gas properties
What does it mean by ‘continuum’
‘Continuous distribution of matter with no empty space’
We assume that fluids in engineering questions are a continuum, we look at ‘fluid particles’ which are assumed to be large enough to be treated as a continuum
What is a fluid and state 2 fundemetal properties
A substance which deforms continuously under the action of shearing (tangential) forces, however small they may be. Fluids cannot resits deformation due to weak intermolecular forces of attraction.
Fundamental properties:
- they deform continuously when they are subjected to a tangential force
- they cannot sustain tangential forces and be at rest in the same time
Definition of shear stress (1D) and definition of shear rate (1d)
-Force per unit of surface area which acts in the plane tangent to the surface, F/A (Pa)
- variation of shear deformation (α) with time. (s-1). γ=🔼 α/ 🔼t
How will the shear strain respond to a given shear stress for a solid?
In a solid, the shear strain ( ∆α) will be a fixed quantity for a given value of shear stress. This is because a solid can resist shear stress permanently.
Define viscosity
Viscosity is a measure of resistance to shear in a fluid and is related to interactions between molecules.
Viscosity gives rise to a shear stress in the fluid which is proportional in some way to the shear rate. τ proportional to (y)c where c is a constant
What are Newtonian fluids
Newtonian fluids are fluids whos shear stress at any instant is directly proportional to the applied shear rate. Newtons law expressed as τ = η γ = η (δu/δ γ)
Explain briefly what happens with liquids/gases in terms of viscosity and temp. Also what is the dynamic viscosity of air in room temp
Liquids: viscosity decreases which increasing temp as the short range intermolecular forces between the molecules decreases
Gases: viscosity increases with increasing temp as the momentum energy of each molecule increases.
η air = 1.7 *10 -5 Pa.s
What is kinematic velocity equation and where are velocity gradients high
v= η / ρ
Velocity gradients high in a region called the Boundary layer
What are non-Newtonian fluids
Fluids whose relationship between shear stress and shear rate is severely complicated by the face that the dynamic viscosity varies with shear rate.
