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Old Dec 27th 2018, 10:40 AM   #1
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Compressible vs. Incompressible flow

Hello dears,

I have seen many explanations in various references about the definition of compressible flow.
In fact, changing the density of the flow as a result of pressure variations leads to a compressible flow.
Now, my question is that what if the density of the fluid flow changes as a result of temperature changes which come from heat transfer on boundaries.
Imagine a situation in which the fluid flow density is changing by temperature but the Mach number is low. Is it a compressible or incompressible flow?

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Old Dec 28th 2018, 11:44 AM   #2
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Incompressible flow is always an approximation,
However it is far easier to model incompressible flow than compressible.

The question becomes, are the flow conditions close enough to incompressible for the simpler model to be used as an acceptable approximation.

If the fluid has a thermocline, or similar boundary, you may be able to assume incompressible conditions on either side of the boundary,
but you would have to make special provision in any model for the boundary.
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Old Dec 29th 2018, 08:28 AM   #3
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Some points...

Incompressible fluid is a material property

Incompressible flow describes a flow property

Pont 1 - In an incompressible flow, the density of a fluid element doesn't change along its pathline. So you can have an incompressible flow of compressible fluid in the case of low Mach numbers or in stratified flows.

Point 2 - An incompressible fluid always results in an incompressible flow
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Old Mar 6th 2019, 01:59 AM   #4
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When a fluid flow is compressible, the fluid density varies with its pressure. Compressible flows are usually high speed flows with Mach numbers greater than about 0.3. Examples include aerodynamic applications such as flow over a wing or aircraft nacelle as well as industrial applications such as flow through high-performance valves.

Incompressible flows do not have such a variation of density. The key differentiation between compressible and incompressible is the velocity of the flow. A fluid such as air that is moving slower than Mach 0.3 is considered incompressible, even though it is a gas. A gas that is run through a compressor is not truly considered compressible (in the thermodynamic sense) unless its velocity exceeds Mach 0.3. This is important to note because analyses run as compressible can be harder to run, and require more longer analysis times than incompressible flows.
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