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Old   November 28, 2019, 01:54
Default Archimedes Screw Generator
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Dylan Sheneth Edirisinghe
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Hello!
(Reducing time step make solver fail, why?)

I am doing simulation on ASG in Ansys CFX. I created quality Hexa mesh using ICEM and set up in CFX considering the gravity flow. Since the rotational axis is not parallel to the gravity axis unable to do steady-state simulation. Therefore I did a transient simulation with 0.1s time step.

Since the screw rotational speed is 53rpm I tried to reduce the time step further to 0.05s after it converges under 0.1s time step. the first few iterations are going well and at some point, solver failed showing fatal overflow error.
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Old   November 28, 2019, 01:57
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First work through the issues discussed on the FAQ: https://www.cfd-online.com/Wiki/Ansy...do_about_it.3F
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Old   November 28, 2019, 02:31
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Dear Mr.Horrocks,
Thank you for the link, and I have read it again and again.
I think;
1) the physics of the simulation set up correctly
2) the mesh is high quality
since it solver complete run for 0.1s time step.
3) the initial condition is already 0.1s time step calculation
4) double precision is already on
5) I tried with 0.05s time step from the beginning the solver fails even without converging. Therefore I use 0.1s for beginning and reduce time step 0.05s after converging. Error happens!!!

Do you know how to find a good time step for buoyancy-driven rotational flow?
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Old   November 28, 2019, 06:44
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The overflow error means your model is very numerically unstable. When you reduce the time step size you can resolve finer transient flow features which can lead to a reduction in numerical stability. This is a possible mechanism by which reducing the time step causes divergence.

In this case you need to improve the numerical stability. If you are sure you are modelling it correctly the next thing to look at is the mesh quality. A higher quality mesh will improve numerical stability, so even though your model converges with the existing mesh at the larger time step improving mesh quality is an important factor in resolving this problem.
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Old   November 29, 2019, 01:54
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Dylan Sheneth Edirisinghe
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Dear Sir,

Here I attached my basic mesh details. Under the 0.1s time step, the RMS Courant number is about 26. According to the courant definition if I may reduce the size of elements further it results in high courant number. But reducing the time step also reduces the courant #. I was read for better solution courant # is about 1.

Since I use the K-epsilon turbulence model I feel my mesh quality is enough.

Thank you!
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File Type: docx Mesh details.docx (100.2 KB, 5 views)
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Old   November 29, 2019, 04:53
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Quote:
Since I use the K-epsilon turbulence model I feel my mesh quality is enough.
I would not have mentioned it unless it was an issue. Improving mesh quality always helps, even when the simulation is already converging as it will then converge better. But yours is not even converging so you definitely would be helped by improving mesh quality.

But after seeing your geometry I think I see other problems. have you got an opening boundary on the top of this device? It is very close to the screw top, and I presume you have a GGI linking the screw to the inlet tube. Having an opening so close to a GGI and perpendicular to the flow direction is going to be numerically unstable. I would recommend you move this boundary further away from the important bit of your geometry, it will greatly improve your numerical stability.
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Old   November 29, 2019, 06:45
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Dear sir,

Thank you for your reply and now I am working to make my opening bit away from the screw. I may let you know as soon as I finish that. Thank you very much, for the important detail!
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Old   November 29, 2019, 07:11
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Like Glenn said...... having the outlet in a vertical position like this is troublessome. I always make the outlet for water in horizontal position, and an opening for air at the top. Then the pressure definition for the water part is clear, making the simulation much more stable.
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Old   December 3, 2019, 07:48
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Dear Gert-Jan,

I have considered your opinion and now I model a down channel with a horizontal opening. As well as I made my top opening away from the rotating domain. Now I am working on the problem. as soon as it completes I let you know.

Thank you very much for your advice.
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Old   December 3, 2019, 09:27
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Dear Glenn and Gert;

Thank you for the advice! but the problem seems to be same!!!

I have made my top opening away from the rotating domain (Screw) as figure 1. And I modify the simulation using the outflow channel and make the horizontal opening too. (the Previous outlet is an opening from rotating domain directly)

Solver manager monitoring graph is illustrated in figure 2.

Firstly, I tried to run having 0.05s time step at the beginning. But without converging the overflow error occur. Then I tried with 0.1s time step at the beginning and after converging I reduce the time step 0.07s. Having increased the power it converges again. Then I further reduce the time step to 0.05s. On the way to convergence the overflow error occurs.

**My monitor variables are torque on 2blade sides, hence the net torque and the power

Please advise; what shall I do?
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File Type: png Figure1.png (103.1 KB, 7 views)
File Type: jpg Figure2.jpg (91.2 KB, 6 views)
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Old   December 3, 2019, 09:36
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The source of your error can be anything.
It could be that it has to do with the outlet. But we are not sure.
Therefore we advised to create a top outlet for air (orientation is less relevant) and a horizontal bottom outlet for water.
I still see a vertical outlet for water at the right hand side, not? Or am I mistaken?

Please make it horizontal at the bottom side.
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Old   December 3, 2019, 09:49
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Alternatively create a backup file at the timestep before it crashes. Then open it in post to find out where the problem is. Look for unrealistic velocities and pressures.......
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Old   December 3, 2019, 18:00
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I have marked up an image with my suggestions.

Modifications.png

In addition to Gert-Jan's suggestions I recommend you include the residuals in the results file and view the residuals in CFD-Post. This will show you the areas of high residuals which are the areas it is having convergence problems. This will tell you what you need to focus on.
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Old   December 3, 2019, 21:20
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Dear Glenn and Gert;

Thank you for your advice.

Since I could not define my boundary condition, here I attached detail picture.

As you said, now I am going to run the simulation having the back-up and view it in CFD post. Further, I will try a simulation having a top channel opening away as you show in the previous attachment.
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