Materials: Fluids Flashcards
Define Density
Density = mass/volume; rho=m/v
Define Laminar flow, Turbulent flow and Streamlines
Laminar flow occurs when adjacent layers of fluids do not cross into each other.
Turbulent flow occurs when layers of fluid cross into each other, resulting in the formation of vortices or eddy currents.
Streamlines represent the velocity of a fluid at each point within it. So where there is laminar flow they are straight lines. Where there is turbulent flow they swirl around and form vortices or eddy currents.
Define Terminal velocity, Upthrust, Viscosity and Viscous drag
Terminal velocity - when the resultant of all the forces acting upon the body is zero, then the body reaches terminal velocity.
Upthrust - upwards force equal to the weight of displaced fluid.
Viscosity of a fluid relates to its stickiness and thus to its resistance to flow.
Viscous drag - is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid.
What are the examples of uses of flow of fluids
- Uplift on airplane’s wings
- Drag forces on a car
- Rate of flow in tubes (oil)
How does the rate of flow of a liquid vary with its viscosity?
The viscosity of a liquid relates to its stickiness and thus to its resistance to flow. So the more viscous(sticky) the liquid is the slower speed it flows at.
Describe Archimedes’ principle
Archimedes’ principle states that when a body is totally or partially immersed in a fluid, it experiences an upthrust equal to the weight of fluid displaced.
Upthrust = Weight of fluid displaced
Explain Stokes’ law
Stokes’ law states that for a sphere moving through a fluid, the viscous drag acting on it is given by the equation F=6πηrv, provided that the movement of the fluid relative to the sphere is laminar
F=6πηrv; r - radius v - terminal velocity
Coefficient of viscosity, η is a measure of the resistance to flow for a fluid. It has the units Ns(m^-2) which are sometimes given as Pa s
How does the viscosity vary with temperature?
Viscosity increases with a decrease in temperature. This is because when the temperature of a fluid rises, it’s internal energy increases. The adhesive forces between molecules and the cohesive forces at solid surfaces decrease causing a reduction in viscosity.
This is important for motor oil. As the oil gets hotter, its viscosity is reduced and the frictional forces between moving parts get smaller, reducing wear.
When viscosity does not depend on the temperature, but on the rate of flow, it is called thixotropy (toothpaste).
