# (HELP )Compressible vs incompressible flow (QUESTION)

## Main Question or Discussion Point

(HELP PLZ)Compressible vs incompressible flow!!(QUESTION)

Ok! I think I know the difference between the two, which is basically the density. My question is actually when is the equation of properties like (Static temperature, Total temperature, Total pressure, Static enthalpy, dynamic viscosity and so on) different between compressible and incompressible flow?

Thanks for the help in advance

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The simplest definition of compressible flow is that the density is not constant. However, the true definition is that the divergence of the velocity field is not zero. A flow is typically considered compressible when M>0.3.

Either way, nearly every equation and formula changes if the flow is said to be compressible. All of the equations get more complicated because you can no longer treat density as a constant and so its derivatives are no longer zero.

Hopefully that answered your question. If you still have more questions, I suppose just ask them or rephrase the original question if I didn't provide the information you were looking for.

The static temperature is just the thermodynamic temperature, which is just a function of 2 other intensive variables. Same with the static enthalpy and the dynamic viscosity, so nothing changes here.

For incompressible flow, we can say Total Pressure = Static Pressure + Dynamic Pressure
For compressible flow, we can say Total Pressure = Static Pressure + Impact Pressure
We can say that the total pressure equals the stagnation pressure as long as the flow stagnates isentropically (a good assumption unless we have shocks present).

We can always say Total Enthalpy = Static Enthalpy + 0.5*Velocity^2. If we can say dh=c*dT and c is constant (good assumptions for ideal gases), we like to use Total Temperature = Static Temperature + Dynamic Temperature instead.

thank you so much for the replay. it was so helpful