Dynamics - Fluid Mechanics Flashcards
Fluid
A substance that flows/changes shape of the volume it occupies.
Examples of fluids
Gases or liquids
Liquid differ from gas…
Intermolecualr forces keeping them together.
Densit
Mass divided by volume occupues, or mass per unit volume
How density differs with material?
Air is 1.20kg/m3 whilst ice 0.92x10^3 kg/m3
Force in a fluid..
Any surface in contact with the fluid due to collisions by the molecules
Example of a surface and a fluid presse.
A surface in a fluid to remain at rest must experience equal magnitude of force on both sides.
Equation for pressure?
p = F/A
How is pressure measured?
In pascals.
Describing pressure in a fluid…
Bottom and top have area A with elevations y and y+ dy respectively, with volume dV = A dy, mass is dm = p dV = pA dy and weight is dw = dm g = pgA dy.
What is dp/dy equal to?
Negative density x gravity, so as y increases, pressure decrease, pressure decreasing as move up in the fluid
Hydrostatic Equilibrium
The balance between the pressure gradient force and gravitational pull.
What is the pressure difference between two points equal to?
The negative density x gravity x by change in heights of the two points.
Pascals Law
Pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel.
Absolute Pressure
This is the pressure of having no matter inside a space, or a perfect vaccum
Gauge Pressure
The excess pressure above atmospheric pressure
What does P0 - p = -pg(y2-y1) -pgh tell you?
That pressure p at depth h is greater than pressure p0 at surface by an ammount of pgh
When is gauge pressure negative?
If pressure is less than the atmosphere
Archimedes Principle
This says that when an object is completelty or partially immersed in a fluid, the fluid exerts an upward force on the object equal to the weight of the fluid displaced by the object.
Description of archimedes principle..
Object buoyancy, each point of an object is acted on by dF, sum of the surface forces equal in magnitude to the weight of fluid inside the sruface
For a body to float in air…
Its total weight must equal the weight of air displaced by the body…
Why does buoynacy occur…
The fluid below an object exerts greater pressureo n the object than the fluid above it due to difference in pressure between top and bottom of the submerged object.
Ideal Fluid
Describes an incompressible fluid meaning density cannot change.
Flow Line
Measures path of a particle
Streamline
A line that is tangenital to the instantaneous velocity direction.
Laminar Flow
This is where two or more streams of fluid flow adjacenet without mixing other than by diffusion across their boundary
Turbulent Flow
These are irregulary motions of the fluid elements from high flow rates of abrupt velocity changes
Continuity Equation
This is that mass input minus the mass outpue equals the change of mass
Description of con equation…
A1 and A2 with velocities V1 and V2 during dt move a distance where volume at A1 is the same at A2
Bernoullis Principle
Says that an increase in speed of fluid occurs simultaneously with decrease in static pressure
Mass Flow Rate
The mass of a substance of which pressure passes per unit time.
