[javarocket]

Good morning, it is my first day on the forum, I hope to post my question correctly.

I am doing a transient setup of a rotor belonging to an axial compressor and I need to get the same results as a transient blade row analysis, done on the same rotor, in terms of aerodynamic damping.

I am doing this because I will then have to use a whole-stage setup (stator+rotor) to do a flutter analysis, and with the transient blade row analysis it does not work (or at least I have not been able to get it to work).

My problem is that in the transient blade row analysis there is the possibility of entering either the nodal diameter or the inter blade phase angle (IBPA), which cannot be done in the transient setup, where only a certain "phase angle" appears but which seems to be defined differently by IBPA. In fact, if I change the value of the phase angle in the transient analysis, I always get the same aerodynamic damping value, so I don't know how to do it

Does anyone know how to help me?

Thank you

[Opaque]

May I ask why you are trying to model a stage setup?

Do you understand the limitation of using the aerodynamic damping approach to determine if a blade flutters?

As far as I understand, with the use of the aerodynamic damping approach (energy method), the flow conditions in front of the blade under study MUST be uniform, not a wake/distortion created by a passage upstream.

[javarocket]

Dear Opaque, thanks for the reply.

I am trying to model a stage because is the topic of my master thesis.

No, unfortunately I don't know... What are the limitations? I am using this approach because my supervisor told me to get the job done using aerodynamic damping.

Regarding the second sentence, I already have uniform condition at inlet because the stator is downstream (we specifically decided to put it there for this reason).

[Opaque]

Originally Posted by javarocket

No, unfortunately I don't know... What are the limitations? I am using this approach because my supervisor told me to get the job done using aerodynamic damping.
.

Have you considered the blade count for each row and the nodal diameters you will be computing?

Unless you are solving for an equal number of blades on each row and/or a full mode (all blades in both rows), and a Nodal Diameter equal 0, there is no way you will obtain the correct solution for the aerodynamic damping. Why? read the documentation, and carefully find the answer.

Separately, keep in mind you are no longer an undergraduate student, you are a professional who must use technical knowledge to make informed decisions.

[Gert-Jan]

Sometimes students are asked to do things that won't work in order to underline new method that will work.......

[javarocket]

Originally Posted by Opaque

Have you considered the blade count for each row and the nodal diameters you will be computing?

Unless you are solving for an equal number of blades on each row and/or a full mode (all blades in both rows), and a Nodal Diameter equal 0, there is no way you will obtain the correct solution for the aerodynamic damping. Why? read the documentation, and carefully find the answer.

Separately, keep in mind you are no longer an undergraduate student, you are a professional who must use technical knowledge to make informed decisions.

Thank you for your advice, I will search for my answer in the documentation.

Of course, I know, but even professionals sometimes need to exchange views with more skilled people.

P.S. I am not solving with ND=0, but with ND=2, so I don't think this is the problem.

[javarocket]

Originally Posted by Gert-Jan

Sometimes students are asked to do things that won't work in order to underline new method that will work.......

Yes I also thought about this possibility... the problem is there is no much documentation about my problem, that's why I asked in this community

[Gert-Jan]

If it is research, then any question is allowed in my opinion. As long as your supervisor can explain the purpose and you are willing to struggle...

[Opaque]

Originally Posted by javarocket

Thank you for your advice, I will search for my answer in the documentation.

Of course, I know, but even professionals sometimes need to exchange views with more skilled people.

P.S. I am not solving with ND=0, but with ND=2, so I don't think this is the problem.

Hint: unless you use a full/reference model, there is no chance you can get the correct answer for ND = 2 using Analysis Type = Transient, regardless of what incantations of expressions, or user Fortran you use.

You need to use Analysis Type = Transient Blade Row. The documentation should explain why.

[Opaque]

Originally Posted by Gert-Jan

Sometimes students are asked to do things that won't work in order to underline new method that will work.......

Agree; however, my point is to feel empowered to counter-propose to an advisor.

Doing the initial literature research, someone should be able to collect an answer w/o even attempting to run the software.

As a professor ( I used to be ), I would suggest ideas to see if the student pursues them blindly, or analyze them and realize that are not feasible based on some technical reasoning.

[javarocket]

Originally Posted by Opaque

Hint: unless you use a full/reference model, there is no chance you can get the correct answer for ND = 2 using Analysis Type = Transient, regardless of what incantations of expressions, or user Fortran you use.

You need to use Analysis Type = Transient Blade Row. The documentation should explain why.

I already tried with a transient blade row model, but it was not working and my supervisor told me to use the transient one... but if you told me these things I think I should give another try.

I found an error, but I think it could be correlated with the pitch angle imposed in the mixing plane:

+--------------------------------------------------------------------+
| ERROR #555000005 has occurred in subroutine THETA_CONT_FIN. |
| Message: |
| |
| A transition between +/-180 degrees could not be found on side 1 |
| of domain interface: |
| |
| Mixing Plane |
| |
| The algorithm which calculates this value attempts to search for |
| the first element face at this transition. Sometimes this will |
| fail if the pitch angle is incorrect. The pitch angle for this |
| side of the interface is: 21.176 degrees. If this does not |
| seem correct then please carefully examine your interface for any |
| of the following: |
| |
| 1) side 1 has more than 360 degrees of revolution |
| 2) side 1 intersects zero radius |
| 3) side 1 has element faces normal AND parallel to the axis |
| 4) side 1 has element faces at the low radial or axial position |
| which are very thin in the axial or radial direction, or the |
| edges which make up the inner radius/axial position do not form |
| an arc of revolution so that the flow solver can accurately |
| determine the pitch angle. |
| |
| If any of situations 1-3 apply you can try changing Transformation |
| Type to "None" instead of "Automatic". If the 4th situation is |
| the problem then you must explicitly specify the pitch angles |
| of side 1 and 2 of the interface. You may have to change both |
| settings to get the flow solver running.





P.S. Which option do you suggest for the transient blade model row model? Fourier transformation? Because when I use the mixing plane between stator and rotor, I only have the option of setting a Fourier or profile transformation (I don't know why).

[javarocket]

Originally Posted by Opaque

Agree; however, my point is to feel empowered to counter-propose to an advisor.

Doing the initial literature research, someone should be able to collect an answer w/o even attempting to run the software.

As a professor ( I used to be ), I would suggest ideas to see if the student pursues them blindly, or analyze them and realize that are not feasible based on some technical reasoning.

This is true, but I'm also doing an internship, it's my first time using CFX so obviously I don't have the judgment skills of an expert user. I've done a lot of research in the literature, but still my topic is very specific (otherwise it wouldn't be a master's thesis).

[Opaque]

Originally Posted by javarocket

P.S. Which option do you suggest for the transient blade model row model? Fourier transformation? Because when I use the mixing plane between stator and rotor, I only have the option of setting a Fourier or profile transformation (I don't know why).

You really need to read the transient blade row modeling guidelines in the documentation.

You need to understand why there are a few transformation options available; otherwise, you will never be able to set up the problem correctly, and/or it is even possible to model what you want.

Hopefully understanding what "unequal pitch, or pitch change" modeling means may ring a bell.