[A.L.Verminburger]

I have encountered a distinction being made between compressible and incompressible flows quite often. I don't really understand why it is so important?

I do understand that the problem I am working on has an incompressible flow, i.e. 0.3 > M.

But where do I enable the incompressible setting in Fluent? I do not seem to find the answer neither in the user guide nor the tutorials?

[farzadpourfattah]

if you select in material type "ideal gas" your gas will consider compressible gas.

[sbaffini]

What is hidden from the user, in Fluent, is that selecting different density specifications will activate different solvers, ss said by farzadpourfattah. However, the distinction between compressible or incompressible flows is quite imortant, physically as well as numerically and you should investigate it more deeply.

[A.L.Verminburger]

OK, so Fluent does this automatically, depending on the material specification - got it. It seems that the most significant difference between incompressible and compressible flows is the presence of shock waves and chocked flow in the latter (I would be inclined to associate these phenomena with Aerospace applications).

[sbaffini]

You don't actually need to reach M=1 to see differences. Also, from the numerical point of view, the algorithms are completely different due to the nature of the flow itself. It is more a matter of "will pressure changes cause density changes?" You can't do (direct) aeroacoustics with an incompressible solver, as the pressure is just treated as a lagrange multiplier and not like a thermodynamic variable (elliptic versus hyperbolic nature).

Check this discussion:

http://www.cfd-online.com/Forums/mai...-enigma-2.html

[Tanjina]

Hello,

I have similar problem with defining the compressible/ incompressible fluid. I chose "constant" when I selected the material. But after completing the calculation, from fluent I found two different value for area weighted average velocity and mass weighted average velocity. But it should be the same. Is there any way so that I can assign "incompressible" fluid?

Any suggestion will be highly appreciated.

Regards,
Tanjina.

[vindiggity]

Tanjina were you able to solve this? I'm running into the same problem - I have one velocity inlet and one outlflow and the mass weighted average velocity at the outflow fluctuates wildly and is at some points 100's of times larger than at the inlet. I'm simply using air as defined by fluent and really have no idea why this is happening

[mahbood]

what the activation of "compressibility effect" does (under KW- SST option) ? while we set to the ideal gas model and activate the energy equation?
Also if I activate it in the middle of my calculation, does it have some effects on my results? I mean should I run enough to pass the errors?

[LuckyTran]

Originally Posted by mahbood

what the activation of "compressibility effect" does (under KW- SST option) ? while we set to the ideal gas model and activate the energy equation?
Also if I activate it in the middle of my calculation, does it have some effects on my results? I mean should I run enough to pass the errors?

This isn't a full answer, but that's an additional correction for the turbulence model to include extra compressibility effects. It's not related to compressible effects of the mean flow (the laminar flow for example).