December 8, 2022 at 10:49 amShuttleSubscriber
Hello there, I am currently simulating a thin gap b/w rotor and stator (Fig. attached) with less than 1 mm in dimension (mesh size is quite large). Initially, the rotor was rotating at around ~10 krpm and I was getting reverse flow and convergence issues, which I solved by extending my domain. Now for my further investigation, I increase the speed of rotor to ~100 krpm and the same issue occurs again, no convergence at all. Is extending the domain the only way to have convergence? Mesh quality looks ok: Skewness (max: 0.78; avg: 0.062), Min. Orthogonal: 0.22, Max. Aspect ratio: 93.87. I also tried to simulate with smaller time steps & relaxation factors, but nothing works.
Boundary conditions: Pressure inlet and outlet
Sol methods: Coupled, Green-gauss, all second ord
Solution controls: all default except energy: 0.6
December 8, 2022 at 12:17 pmRobAnsys Employee
How much mesh have you got in the gaps (cell count) and are the back flow conditions sensible relative to the domain size, scale and conditions? At 100k rpm what is the tip speed?
December 8, 2022 at 12:32 pmShuttleSubscriber
Mesh size is approx. 35 million cells and tip speed based on TS=pi∗D∗RPM/60 = ~665 m/s
I have not much idea how reverse flow behaves, but it looks like the higher the speed, the more reverse flow I am getting at boundaries inlet & outlet, which is also physical. Worth to mention: I am using an exp function for viscosity dependent on temperature
December 8, 2022 at 1:49 pmRobAnsys Employee
As the rotation speed increases so does the axial effect on the generated vortex: look up toroidal vorticies. Is the tip speed really Mach 2? Or do the operating conditions keep it subsonic?
Cell count is just that, how many cells between the rotor tip and the casing? I can model something that's well refined with 10k cells, or excessively coarse with 60M: it's all a question of scale and how I place my sizing & elements.
Reverse flow is common, but you need to be careful if it alters the result. Ideally, the domain is extended to prevent this happening but otherwise the domain is extended far enough that the reverse flow can't alter the flow in area we're interested in.
December 8, 2022 at 2:09 pmShuttleSubscriber
ya, the opt conditions are maintaining the subsonic conditions. I just have the fluid gap as a mesh where down part is rotating and the above is stationary, so size is 35M. But it didn’t answer why after some iterations my residuals especially energy and continuity start diverging or jumping so high?
December 8, 2022 at 2:26 pmRobAnsys Employee
It depends on what the evolving flow field is doing. If you have a rotor-stator is it sliding mesh or moving reference frame.
December 8, 2022 at 2:37 pmShuttleSubscriber
moving reference frame
December 8, 2022 at 2:46 pmRobAnsys Employee
Look at the flow field a bit before it fails. You're looking for odd velocity spikes but also the velocity field in the gaps, plot with node values off to see how well resolved the gradients are.
- You must be logged in to reply to this topic.
Earth Rescue – An Ansys Online Series
The climate crisis is here. But so is the human ingenuity to fight it. Earth Rescue reveals what visionary companies are doing today to engineer radical new ideas in the fight against climate change. Click here to watch the first episode.
Subscribe to the Ansys Blog to get great new content about the power of simulation delivered right to your email on a weekly basis. With content from Ansys experts, partners and customers you will learn about product development advances, thought leadership and trends and tips to better use Ansys tools. Sign up here.
- Suppress Fluent to open with GUI while performing in journal file
- Floating point exception in Fluent
- What are the differences between CFX and Fluent?
- Heat transfer coefficient
- Getting graph and tabular data from result in workbench mechanical
- The solver failed with a non-zero exit code of : 2
- Difference between K-epsilon and K-omega Turbulence Model
- Time Step Size and Courant Number
- Mesh Interfaces in ANSYS FLUENT
- error in cfd post
© 2023 Copyright ANSYS, Inc. All rights reserved.