# Spillway modelling

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• Last Post 21 September 2018
Prasanna posted this 11 September 2018

I am trying to model spillway in 2D. The problem of floating point exception is always troubling me. Can anyone helpme in this regard.

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Kremella posted this 11 September 2018

Hello,

Please elaborate and provide some more details of your model. Please provide some screenshots as well as explain your boundary conditions. You might also want to take a look at the mesh statistics.

Floating point error generally occurs when the solver encounters a really small value in the denominator.

Another questions - does this happen right at the beginning or somewhere in the middle of a simulation?

Are you running a steady or unsteady problem?

In short, you might have to explain your problem better so we are able to help you.

Thank you.

Best Regards,

Karthik

Prasanna posted this 11 September 2018

Boundary conditions: Velocity inlet, pressure outlet, wall

velocity is 0.892m/s

the model is scaled to 1:100.

Yes, the floating point error is in the beginning. min orthogonal quality is 0.245, max. ortho skew is 0.7547, max. aspect ratio is 12.6.

Prasanna posted this 11 September 2018

seeta gunti posted this 11 September 2018

Hello Prasanna,

Thanks for the screen shots.

As I can see the screenshot  and understand that k and epsilon are causing the divergence. I suggest to run with first order upwind scheme with lesser URFs. Can you try reducing the URFs of K & epsilon to 0.6 and run. If you still facing the divergence, you can still reduce to 0.5 or 0.4 and run the case. One more observation is reverse flow at the outlet. May be you can still extend your outlet and continue the run to avoid reverse flow at outlet. Kindly follow these two suggestions and  let me know if you still faces the issue.

Regards,

Seeta

Prasanna posted this 11 September 2018

If you can observe in the mesh, the boundary condition at the top most part of the geometry is also pressure outlet. can I understand that the reverse flow is because of not the left most outlet but it is rather because of the BC on the top. can you please provide clarification regarding this.

Prasanna posted this 11 September 2018

Im sorry to say that,  the problem has not yet resolved with URFs as 0.5 for both k and epsilon and first order upwind scheme

rwoolhou posted this 11 September 2018

Can you remesh and force a pave mesh onto the model? Don't use any edge sizing in GAMBIT and when you mesh the face select Pave (it looks to be defaulting to sub map).  You probably also need to refine the mesh over the top of the weir: read up on VOF model and adaption in the documentation.

As an aside, I'd advise learning either DesignModeler or (ideally) SpaceClaim and ANSYS (Workbench) Meshing as they're the current tools used in industry.

Prasanna posted this 11 September 2018

Thank you so much. I request you to suggest me any tutorial that helps me in creating the geometry in design modeller for spillway shape.

Kremella posted this 11 September 2018

Hello,

This is our Youtube page with loads of learning material on 'How To'. I'd suggest you use these videos to get up to speed with DM and WB meshing tools.

I hope this helps.

Best Regards,

Karthik

Prasanna posted this 11 September 2018

Thank you karthik. Will surely use these.

Prasanna posted this 11 September 2018

Thank you karthik. Will surely use these.

Prasanna posted this 15 September 2018

hello, after following all your recommendation, im facing the above mentioned problem. Can anyone try to help me in this regard

rwoolhou posted this 17 September 2018

I suspect you need to reduce your timestep: the warning (for once) is fairly helpful.  Depending on the flow velocity 0.01s may be too high.

Kremella posted this 18 September 2018

Hello Prasanna,

please estimate your time step such that your Courant number does not exceed 1. This will ensure a stable solution. You might want to use the minimum grid size while estimating this time step size to be conservative.

please let us know what you find,

Best,

Karthik

Prasanna posted this 19 September 2018

Hello,

Thank you very much for the suggestion. It worked out and the problem is solved

Prasanna posted this 21 September 2018

Hello,

I have some doubtful results in fluent as detailed in the image. The B.C are velocity inlet, wall and pressure outlet. Can you please let me know where i am going wrong in my simulation. The operating pressure is set as 101325 pa.

Prasanna posted this 21 September 2018

Hi,

I felt this information could add to clarify my doubt. The velocity provided at the inlet is 0.892 m/s. The inlet in the present case is taken as the height over the crest of the spillway.