Week 3 Flashcards
Flow state, boundary layers and Reynolds number
What is Reynolds Number?
A ‘dimensionless quantity’ that describes the ‘scale’ of the flow.
Re=(PuH)/(u) or Re= (uH)/v
What does Reynolds Number tell us?
How much the flow is affected by viscosity and whether it is laminar or turbulent.
What do the inertial forces do?
‘Pull the fluid apart’
What do the viscous fluids do?
‘Hold the fluid together’
What does Re<1 mean?
Viscous forces are dominant
What does Re>1 mean?
Inertial forces are dominant
Describe Osborne Reynold’s Experiments.
Dye was fed into a pipe through which water flowed. As velocity increased, patterns in the dye changed. At low speeds, the dye was smooth and uniform, however at higher speeds it underwent apparently chaotic motion. This demonstrated the link between fluid velocity and turbulent flow.
What is laminar flow?
The state where fluid ‘layers’ move over one another.
What is the momentum between ‘layers’ transferred by?
Shear
What is Turbulent flow?
The apparently random and chaotic motion in a fluid motion in a fluid.
What does Re<2300 mean?
Flow is laminar
What does Re>2300 mean?
Flow is turbulent
What are boundary layers?
Are regions of viscous fluid that have lower than free stream velocity close to walls.
What are boundary layers a result of?
Viscosity causing a frictional force between the fluid and the surface.
What do fluid molecules closest to the wall do? and why?
‘stick’ due to electrostatic forces.
