Fluid Dynamics Flashcards
What is a fluid ?
Something that flows; a substance that constantly deforms under shear stress, no matter how small
Examples of fluids
Liquids / gasses / plasmas
Density equation
Volume (kgm^-3) = mass / unit volume
p = m / V
What is the density of pure water ?
1000 kgm^-3 at 4 °C
What is relative density ?
The ratio of the density of a substance to the density of a given reference material.
Relative density equation for solids
RD = density of substance at 20°C / density of water at 4°C (1000kgm^-3)
Relative density equation for gases
RD = density of substance at 20°C / density of dry air (1.00)
Pressure definition
When a force acts perpendicular to a surface, the pressure exerted is the ratio between the magnitude of the force and the area of the surface
Pressure equation
Pressure = Force / Area
P = F / A
1 Pa = 1 N/m^2
Pressure unit conversions
1 bar = 10^5 Pa
1 atm = 1013.25 millibars , 14.69 psi = 260 mmHg
Absolute pressure equation
Absolute Pressure = gauge pressure + atmospheric pressure
What is a barometer ?
A pressure gauge that measures the pressure of the atmosphere itself
What do pressure gauges measure?
The difference between an unknown pressure and atmospheric pressure.
How does Pressure act in a fluid container? (2)
The forces that a fluid exerts into the walls of a container, and those that the wall exerts on the fluid, always act perpendicular to the walls.
And
The force exerted by the fluid is the same at all points at a given depth.
What is Archimedes principle ?
An object immersed in fluid is acted upon by an upward force that arises because pressures in a fluid increase with depth.
(Upward force on the bottom is larger than the downward force on the top).
What is buoyant force?
The difference between the upward force from the bottom and the downward force on the top of an object immersed in a fluid.
What is the buoyant force equal to?
The buoyant force is equal to the volume of the fluid displaced.
What does upthrust (upward force) equal?
The weight of the water displaced.
What is Streamline flow?
The direction of motion of individual particles is the same as that of the fluid as a whole.
Each particle of the fluid that passes a point follows the same path as those particles which passed the point before.
What is laminar flow?
Streamlined flow in which the fluid moves in layers without fluctuations or turbulence.
What is turbulent flow?
Characterised by the presence of irregular whirls and Eddies; it occurs at high velocities and when the fluids path changes direction sharply (an obstruction).
Viscosity definition
An internal friction that prevents adjacent layers of the fluid from sliding freely past each other when subjected to shear stress.
As the temp of a liquid increases, Viscosity …..
Decreases.
As the temp of a gas increases, viscosity….
Increases.
What is required to keep a viscous liquid moving though a pipe?
Differential pressure across the pipe inlet and the pipe outlet. (Greater pressure at the inlet than the outlet)
As a viscous fluid flows though a pipe, where is the velocity at its maximum?
At the centre of the fluid/pipe. Velocity is ZERO at the walls.
What is skin friction?
The air is slowed and brought to a stand-still when very close to a surface.
The viscosity of the air causes the layers near the skin to retard the surrounding layers along with it.
What is the boundary layer of air?
The layer(s) of air in which the shearing / slowing action takes place between the surface and the full velocity of airflow.
It can be laminar or turbulent.
What are the two types of aerodynamic flow?
Compressible and incompressible.
What is compressible flow?
Flow where the density of the fluid elements can change from point to point (p2 ≠ p1).
The variability of density in aerodynamic flow is important at high speeds.
What is incompressible flow?
Flow in which the density of the fluid elements is always constant.
This does not happen in reality. It is only used to simplify analysis of real physical systems.
Flow rate equation
Flow rate = velocity x cross-sectional area.
FR = v x A
(Litres per second or cubic meters per second)
When is the rate of flow of a fluid constant, even if the pipe or channel varies in size?
When the fluid is incompressible. (Approximately true for most liquids)
Continuity equation
v1 x A1 = v2 x A2
How does a Venturi work?
The fluid velocity in the throat is increased and the pressure is therefore reduced.
The pressure drop can be measured and the flow rate within the throat can be calculated.
As velocity increases, pressure…
Decreases
What two components make up the total pressure exerted by moving liquids ?
Static component (at rest) and dynamic component (pressure equivalent of its velocity).
What is the make up of a pitot-static tube?
Measure total pressure by consisting of two tubes, one with an opening facing the moving fluid and the other with an opening 90° to the direction of flow.
What is the stagnation region of a pitot-static tube?
Where the fluid at the open end of the tube is at rest. The total pressure is also called stagnation pressure.
