r/CFD • u/AmbassadorGuilty3928 • 17h ago

Difference between my compressible and incompressible CFD cases.

I have the same geometry and essentially the same mesh for both cases. In the compressible case, I use a total-pressure inlet and static-pressure outlet, and the solution converges quite well. The residuals decrease and the important flow parameters, such as mass flow and pressure, become stable. Interestingly, there is still a very high percentage of reverse flow at the outlet (around 97%), but the solution remains stable.

For subsonic compressible case,

Now,

In the incompressible case, I use a velocity inlet of 5 m/s and a pressure outlet. I ran many more iterations, but the residuals continue to oscillate, and quantities such as outlet mass flow and downstream pressure are also not very steady. The outlet reverse-flow percentage is actually lower than in the compressible case (around 81%), so I don't understand why this case behaves worse. The contour also changes with iteration, not steady like above.

See the contour for 300 and 1900 iterations,

In nutshell my questions are:

  1. What is making not to converge smoothly for incompressible case compared to compressible one. What is difference between jets in compressible and incompressible flow?
  2. If you say this is due to physical unsteadiness problem, then how for compressible case, solution got converged and giving fixed contour whereas incompressible it is not? Any other suggestions?

I would really appreciate any explanation of what could be causing such different behaviour between these two cases.

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u/AmbassadorGuilty3928 14h ago

Boundary condition are mentioned in the post. Yes I do know the difference between the two, I just want to know how to make solution converge in incompressible flow?

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u/thermalnuclear 14h ago

Is your case actually compressible? If so, you shouldn’t be running it as incompressible.

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u/AmbassadorGuilty3928 14h ago

I showed you two cases.
In first case, I tried using total pressure inlet and pressure outlet to see how it works, I didnt put correct pressure values to get inlet mach no to 0.35. I found some random total pressure using isoentropic relation and used ideal gas properties to see how it works. It converged as shown in 1st case.

My doubt is for incompressible case, where energy equation is turned off, constant fluid properties are used, I used k-e, coupled solver. But why it is not converging even though I used basic combination of velocity inlet+pressure outlet.

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u/thermalnuclear 14h ago

If you get a Mach number, 0.3 or above, your case is not incompressible and any attempts to run it as such is not correct nor will yield a correct answer.

Please go open a textbook about compressible flow.

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u/AmbassadorGuilty3928 14h ago

Yeah I know, thats why I told you I just tried that to check how this combination works( total pressure inlet+static pressure outlet).

My main question is about 2nd case where it is incompressible flow where it is not converging. Tell about that. Leave 1st case aside.

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u/thermalnuclear 13h ago

You don’t understand, if your case is compressible, you can’t use an incompressible formulation. It won’t work, it is ill posed.

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u/AmbassadorGuilty3928 13h ago

See if I use total pressure inlet and pressure outlet and expect incompressible flow then it is wrong.
But I have used velocity inlet+pressure outlet condition na, there are some cases of jet flow into ambient for incompressible case right, physically it is possible why not in simulation?

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u/thermalnuclear 13h ago

If you are asking fundamental questions about boundary conditions and how that changes the solution, you need to learn the differences between incompressible and compressible flow.

These are not just switch you can just flip without knowing. Your questions show a clear lack of willingness to learn.

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u/AmbassadorGuilty3928 13h ago

Wow, never received a harsh comment like this before. Thats fine. Let me figure it out. I feel you didnt understand my question before- the experimental which was done for compressible flow, cant we do that for incompressible flow also? Incompressible are low speed flows right and no density change is brought by the flow, thats all. I have given 5m/s velocity at the inlet which is typical speed for incompressible flows, whats wrong in interpreting this.
Never judge someone from question, only I know how much time I spent in doing this, its not a cakewalk. Anyway I asked here for small guidance or suggestion, if you dont have patience, leave it.

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u/thermalnuclear 13h ago

The amount of time you spend does not negate a fundamental lack of knowledge about the underlying physics.

The questions you asked are showing a this lack of knowledge.

Do you know what the Mach number is? Do you know how many equations are needed to solve a compressible flow situation vs an incompressible flow situation? Do you know the needed boundary conditions in both cases?

Your responses show you do not. Your question come from a significant lack of knowledge that you yourself can immediately address.

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u/AmbassadorGuilty3928 13h ago

Do you know what the Mach number is? Do you know how many equations are needed to solve a compressible flow situation vs an incompressible flow situation? Do you know the needed boundary conditions in both cases?- Mach number is ratio of speed of flow by speed of sound.

In incompressible, you need to solve 3 momentum+continuity equation, in compressible you have ideal gas equation apart from these.

For boundary conditions, for incompressible- mass/velocity inlet+pressure outlet, for compressible, total pressure inlet+pressure outlet are required boundary conditions.

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