[Tien Dung]

Hello everyone,

I am trying to obtain the compressor map for the NASA Stage 37 - a transonic axial compressor. I carried steady simulations with 1 rotor and 1 stator, using the SST model.

I was able to reach convergence around the design point (RMS less than 10-6, pressure ratio and efficiency also converged). But when I increased the outlet pressure to find the near-stall point, I got an Overflow problem after about 200 steps. I tried to change the physical time step from 0.0001 to 0.05 (include the residence time) but the problem remained. I checked the y+ for converged cases and y+< 2 in almost all areas.

I ran the same simulations for this compressor before with the k-e model and it converged quite easily.

What do you guys think is the problem? Mesh or boundary condition? Any suggestion is appreciated. Thank you for your time.

[ghorrocks]

What do you guys think is the problem? Mesh or boundary condition?

Neither. At stall the 2 equation turbulence models you have been using are not suitable for these flows as you have large scale transient features. You may need a more sophisticated turbulence model, possible SAS or one of the LES based approaches.

[Tien Dung]

Thank you for your reply.
I understand that at stall, the flow is highly transient, and transient simulation is the best option. But in papers I read, they used SST models for similar compressors and obtained reasonable results with steady simulations. As I said, when I used k-e model, the simulation converged and results came quite close to experimental data. Maybe reducing the effect of transient features can help in my case?

[Opaque]

You mentioned you increased the pressure to move towards the stall conditions, correct?

For what I understand, the current practice to obtain the operating curve is to use the Exit Corrected Mass Flow option.

Once you have obtained a converged solution of your liking, you reduce or increase the specified value to move left/right along the curve using the previous solution as the initial guess.

Keep in mind that as you approach the stall point, the reduction in mass flow specification would be smaller and smaller until the simulation no longer converges.

You can try using a similar approach using the outlet pressure specification, but you may encounter convergence issues depending on the details of the machine you are modeling. The smoother/uniform the curve towards the stall point, the more difficult to solve it becomes (older practice)

Hope the above helps

[Tien Dung]

Originally Posted by Opaque

For what I understand, the current practice to obtain the operating curve is to use the Exit Corrected Mass Flow option.

Once you have obtained a converged solution of your liking, you reduce or increase the specified value to move left/right along the curve using the previous solution as the initial guess.

I will try the exit corrected mass flow approach. Thank you.

[Dinh_Son]

Originally Posted by Tien Dung

I will try the exit corrected mass flow approach. Thank you.

Sorry for repeating an old topic. Have you success with exit mass flow boundary condition, Tien Dung?