[lapac1]

Hello everyone,

I am performing a CFX analysis for a 3-stage fan section of a turbofan engine. In the Expressions section, I am calculating the pressure difference between the inlet and outlet. When I perform the analysis with "air at 25C," the pressure difference is 150 kPa. However, when I perform the analysis with "ideal air gas," the pressure difference goes up to 10^18 and seems to go to infinity. How can I fix this issue? I want to perform my analysis with "ideal air gas."

[Gert-Jan]

When using "Air at 25C", density will be constant.
When using "Air Ideal Gas", density will be the outcome of local Pressure and Temperature.
So, possibly the values on your Boundaries are unrealistic? Or maybe you have set both the Reference Pressure and a Pressure boundary equal to 0 [Pa]? Or check the units of temperature? Maybe you gave 0 [K] where you mentioned 0 [C]?
If this low hanging fruit does not help, you need to share the output file so we can take a closer look to your settings.

[lapac1]

Thank you for your reply. You can see the related files below.


idealgassettings.png

idealgas.txt

pdiffidealgas.png

[Gert-Jan]

That is a quite complicated system. There could be many things wrong.
Your 1e18 pressure shows something is completely wrong. Make sure you know what you are doing.
However, I would start with 1 [atm] as reference pressure instead of 0 [Pa]. Then at least you have a basis for a realistic density.
Also, did you use the first calculation as a start for the second?

[lapac1]

Solutions were carried out using air as an ideal gas and air at 25°C in different files. I am getting the desired results using air at 25°C. For ideal air gas, I started the reference pressure with 1 atm and inlet pressure with 0 kPa. It still gives the same results. My boundary conditions and other things are looking quite true. I have watched many videos on this, but they are doing the same things I am and getting good results. I couldnt find any solution for this problem for quite long time. If anyone can help me I would be very grateful for that.

[Opaque]

Originally Posted by lapac1

Solutions were carried out using air as an ideal gas and air at 25°C in different files. I am getting the desired results using air at 25°C. For ideal air gas, I started the reference pressure with 1 atm and inlet pressure with 0 kPa. It still gives the same results. My boundary conditions and other things are looking quite true. I have watched many videos on this, but they are doing the same things I am and getting good results. I couldnt find any solution for this problem for quite long time. If anyone can help me I would be very grateful for that.

Since you have already done both simulations, take both output files and compare them using a "difference text file" tool, and compare what is different between both setups.

The only acceptable difference is the content for Air at 25C and Air Ideal Gas, and wherever they are referenced. Any other difference, and you are not comparing the same models.

[lapac1]

I used the "difference text file" tool, but I couldn't see any specific difference between the models. Can you also check it, if it is possible?
https://www.diffchecker.com/LTrVkpvm/

[Opaque]

Originally Posted by lapac1

I used the "difference text file" tool, but I couldn't see any specific difference between the models. Can you also check it, if it is possible?
https://www.diffchecker.com/LTrVkpvm/

The cases seem to be the same, but you are not running it the same way. When debugging model issues, you must compare identical runs including the number of cores/partitions being used.

On the model itself, you have not set initialization values for the simulation; therefore, you are relying on the Automatic behavior which may be different between incompressible and compressible flows.

Also, from the initial scales, your flow is becoming transonic from inlet to outlet in the compressible model --> more difficult to converge than a plain incompressible flow.

Time to reconsider your timescale selection, and allow the "initial guess" to settle into something meaningful.