# If a fluid is incompressible,is it correct to say that the flow may or may not be incompressible?

From my understanding,when there is no change in volume due to pressure applied on a fluid,the fluid can be called as incompressible.As there can be still be change in volume due to other properties,can I say that flow may be compressible?

• Compressibility can be tested by applying equal but opposite pressures on the fluid, for which no flow would occur.
– r13
Dec 19, 2023 at 20:33

No, the fluid is still incompressible, but the container it is in can change volume...

When you use the incompressible fluid and apply pressure to it then the fluid´s density will not Change i.e.,volume of the incompressible fluid does not changes even with the application of external pressure. May be the outer part within which it is stored might undergo volume Change.

• What is the “outer part”? May 24, 2018 at 12:09
• May be cylinder or container as you have metioned. May 24, 2018 at 12:16

Fluids includes gas, liquids and some solids which flows, as you know gas is compressible, in some case fluid flow is considered as compressible like in the case of a rocket design, aircraft etc...

When you say "incompressible" that is what it is by definition. Other things can change and the density of the fluid can change but it is still incompressible. Nothing in the real world is completely incompressible. For example 40°F water at atmospheric pressure has a density of 62.4 lb/ft³ while at 500 psig, that goes up to 62.5 lb/ft³. That is not much of a difference so for the purposes of calculation, we assume cold water is incompressible.

There are cases like cavitation, where due to high velocity of fluid the pressure drops and vapour forms. This vapour is compressible though the fluid might be in-compressible.

Similarly, in the cases where the pressure drops due to loss in potential energy (the flow is upward) the pressure may reduce and again due to that vapour may form.

In these cases, some portion of the flow path shows that the flow is compressible.

No fluid is perfectly incompressible: they'll all change (specific) volume significantly if the pressure changes associated with the flow pattern are big enough. So being incompressible is strictly speaking not a property of the fluid alone, nor of the flow pattern alone, but of the combination of the two.