Fluid
Any substance that can flow
Density
mass per unit volume of a substance
Upthrust
The upward buoyant force exerted on a body immersed in a fluid
Archimedes’ Principle
An object submerged in a fluid at rest has an upwards buoyancy force, upthrust, equal to the weight of the fluid displaced by the object.
Laminar Flow
-Smooth parallel layers in same direction
-No mixing layers
Occurs:
- Lower speeds
- Higher viscosity fluids
- Smaller flow area
What characterises turbulent flow?
Irregular flow with random variations in pressure, containing eddies (swirls)
What happens to layers in turbulent flow?
Layers mix
How does velocity behave in turbulent flow?
There is a constant change in velocity.
What effect does turbulent flow have on drag?
It increases drag.
Under what conditions does turbulent flow occur?
- Higher speeds
- Lower viscosity fluids
- Larger flow areas.
Viscosity
A liquid’s resistance to flow
-Layers of fluid flowing at different rates
-Shear stress is transferred between each layer; acts as frictional force
-Caused by intermolecular forces between molecules
Higher/ Lower temperature effects on viscosity of liquids…
Higher temperature:
* Lower intermolecular forces, means less shear stress
* Lower viscosity
Lower temperature:
* Greater intermolecular forces means more shear stress
* Higher viscosity
Higher temperature effects on viscosity of gases…
Higher temperature:
* More intermolecular collisions
* Greater shear stress
* Higher viscosity
Lower temperature:
* Less intermolecular collisions
* Lower viscosity
Coefficient of Viscosity formula:
Coefficient of Viscosity (Eta)
= Shear stress/ Velocity gradient
η = (F/A) / (dv/dy)
Terminal Velocity
the velocity of a falling object when its weight is balanced by the sum of the drag and upthrust.
Terminal Velocity Formula:
Weight = Upthrust + Viscous Drag
Stokes’ Law
The equation for viscous drag on small spheres falling through fluids at low speeds.
Stokes’ Law Equation
F = 6πηrv
