Fluid is the higher level designation for liquids (example: water = incompressible) and gases (example: air = compressible).
Why is there such a difference in behaviour?
Fluids can change their shape easily, depending on the available space; solids cannot.
Pour a liquid (heavier than air) in a vessel and it will cover the bottom of that vessel up to a certain height (its surface level) that depends on the amount of liquid poured.
A gas (usually lighter than air) will fill all the available space and will show no surface.
Gases are compressible, liquids are not.
To demonstrate that, fill one cylinder with a gas (air will do for this example) and another one with water. Close the cylinders with a piston that can move downward in the cylinder. When the free space (the volume) inside the cylinder is reduced, its content will get pressurized. Note the difference: The piston in the water cylinder will not move, while the air cylinder can make quite some travel, depending on how much force you apply. So apparently the gas is compressible, water is not.
This different behaviour can be explained by looking at the molecules. Gas molecules move around freely with lots of space between them. That space will shrink when the available vessel volume for the gas is reduced, resulting in an increase of the pressure the gas exerts on the walls of its vessel. More molecules in a given volume also means an increase in the density of the gas.
The water molecules are already in intimate contact with each other, so there is no space that could be reduced when putting the liquid under pressure. Hence it is incompressible.
For these reasons a protective cushion would rather be filled with air than with water.
When you hit the water cushion it would feel rather hard, while the air cushion would feel more resilient.
compressible fluid changes its volume when external pressure is applied and in-compressible fluid does not change its volume due to external pressure
Newtonian fluids have a constant viscosity regardless of the applied shear rate, while non-Newtonian fluids have a variable viscosity that changes with the applied shear rate. Examples of Newtonian fluids include water and most oils, while examples of non-Newtonian fluids include ketchup and toothpaste.
Some examples of compressible gases include air, carbon dioxide, nitrogen, and helium. These gases can be compressed into a smaller volume under pressure.
Butane gas is compressible. It can be compressed into a smaller volume at higher pressures.
The continuity equation for compressible fluids states that the rate of change of density (Ο) in a fluid is equal to -ββ (Οu), where Ο is density, u is velocity, and ββ is the divergence operator. This equation is derived from the conservation of mass principle in fluid dynamics.
compressible fluid changes its volume when external pressure is applied and in-compressible fluid does not change its volume due to external pressure
Fluids include liquids and gasses. Liquids are not compressible. Gasses are compressible. Water is a liquid and it not compressible.
Newtonian fluids have a constant viscosity regardless of the applied shear rate, while non-Newtonian fluids have a variable viscosity that changes with the applied shear rate. Examples of Newtonian fluids include water and most oils, while examples of non-Newtonian fluids include ketchup and toothpaste.
A fluid which is reduced in volume by an increase in pressure.
Some examples of compressible gases include air, carbon dioxide, nitrogen, and helium. These gases can be compressed into a smaller volume under pressure.
Butane gas is compressible. It can be compressed into a smaller volume at higher pressures.
Some examples of non-fluids are solids (such as ice, wood, and metal) and gases (such as air, oxygen, and carbon dioxide). These substances do not flow and have definite shapes or volumes.
The continuity equation for compressible fluids states that the rate of change of density (Ο) in a fluid is equal to -ββ (Οu), where Ο is density, u is velocity, and ββ is the divergence operator. This equation is derived from the conservation of mass principle in fluid dynamics.
Yes, fluids can exist as both gases and liquids. In general, gases have low density, are compressible, and fill the entire volume of their container, while liquids have higher density, are not easily compressible, and have a definite volume but take the shape of their container.
A non-Newtonian fluid is a fluid whose viscosity changes under stress or deformation. Unlike Newtonian fluids, such as water or air, non-Newtonian fluids do not follow Newton's law of viscosity. Examples include ketchup, custard, and quicksand.
K. Stewartson has written: 'The theory of laminar boundary layers in compressible fluids' 'The boundary layer'
They are both FLUIDS, and basically follow the same Laws of Physics. The biggest difference is that Liquids are NOT Compressible.