Fluid Statics Flashcards
Pressure transmission
Pressure is transmitted through a fluid
The transmission is fast but not instantaneous
The transmission speed depends on the speed of sound in the medium and the shape of the container
What is Pascal’s Law?
e.g. hydraulics presses use it for a force amplification factor
Also know as the principle of transmission of fluid pressure, Pascal’s Law states that a pressure change at any point in a confined incompressible fluid is transmitted throughout the fluid such as the same change occurs everywhere (height variations neglected)
What is absolute pressure?
Pressure wrt a vacuum
Patm = 101kPa
What is gauge pressure?
Pressure measured relative to local atmospheric pressure
Pgauge = Pabs - Patm
What is the equation of fluid statics?
dp/dz* = (rho)(g)
where z* is defined as the depth from the free surface
What are the assumptions and conventions used when using p = rho(g)(z*) for hydrostatic conditions?
- Pressure is assumed to increase linearly with depth => density of fluid is constant; ie. fluid is incompressible
- Gauge pressure used since Patm said to be 0
Why is it generally possible to neglect hydrostatic pressure changes in gases?
The density of a gas is generally much smaller than that of a liquid.
1.23kg/m^3 (air) compared with 1000kg/m^3 (water)
What is a manometer and what is it used for?
A manometer is used for measuring pressure difference between two locations in a flow
Manometers are commonly liquid filled U-tubes
Pressure differences cause liquid to sit at different levels on each side of the U-tube, which allows for delta p to be calculated
What is a Free Body Diagram?
An FBD displays the object including all contact and external forces acting on it
In which direction does the pressure force always act relative to a (CV) surface?
A pressure force always acts perpendicularly relative to a surface
What is the centre of pressure?
The centre of pressure is where the resultant needs to act to exert the same moment as the pressure distribution
(usually h/3 from base for triangular pressure distributions)
How is the centre of pressure found?
Resultant moment/Resultant force, then taking care that distance was measured from right point; if not then adjust that distance
How to calculate more easily the pressure force on submerged angled surfaces?
For submerged angled surfaces, one can decompose the pressure forces into a constant and linearly distributed load
How to use the projection method?
- The shape of a surface does not influence the horizontal force acting on it
- The vertical force can be determined by recalling that the pressure is equal to the weight of the fluid per unit area
useful to determine the forces on structures with arbitrary geometries; consists of projecting onto horizontal and vertical planes; does not provide information about the centre of pressure
What is Archimedes’ Principle?
A body totally or partially immersed in a fluid is subject to an upward force equal in magnitude to the weight of the fluid it displaces
