Fundamentals Flashcards
What is a fluid?
A fluid is a substance that deforms continuously under the application of a shear stress.
What constitutes as a fluid?
Gases and liquids
What are some features of fluids?
- Fluids flow and change shape
- All fluids respond to an applied force or shear stress – they experience a continuing and permanent distortion
What are fluids regarded as?
A continuum
What can be considered when thinking about the actions of forces on fluids?
- The behaviour of every molecule in a given flow
- The average effects of the molecules in a given volume
When does a continuum prevail?
When the number of molecule in a given volume is great that the affects are constant or change smoothly over time
When can the continuum approach be justified?
- Number of molecules in a given volume is large
- The scale of interest is not too small
What is the definition of density?
mass per unit volume
How can specific gravity be calculated?
S = density(fluid)/density(water)
How can specific weight be calculated?
lambda = density*g
When is air treated as incompressible?
At velocities«speed of sound
Is water compressible or incompressible?
Incompressible
Which fluid needs larger pressures to compress them?
Liquids
What does each fluid element have?
Has a unique velocity
What does fluid velocity depend on?
Time
What is stress defined as?
Force per unit area
How can stress act at a surface?
- Can be normal to the surface
- Or can be a tangential force
What can fluid motion cause?
Fluid motion can cause stresses
What can stresses cause?
Stresses can cause fluid motion
How do normal stresses behave?
Tend to compress / expand the fluid particle without changing its shape
How do shear stresses behave?
- Give rise to forces that shear the particle & deform its shape
- Changes shape without changing volume
What type of stresses do stationary fluids feel?
Normal only
What type of stresses do moving fluids feel?
They feel both normal and shear
How is viscosity defined?
Viscosity is a measure of how much resistance a fluid has to shear
How is viscosity quantified?
Quantified by the dynamic viscosity ‘mu’
When is kinematic viscosity used?
When density variations can be neglected (motion)
How is kinematic viscosity related to dynamic viscosity?
v = ‘mu’/density
How is volume flux determined?
dQ= u*dA
