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Old   December 2, 2017, 20:00
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  #21
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I used the default model exponent 2 each of the increase near volumes and boundaries.

But now after your recommendation in this new version, i will try to change the model exponent value from 2 to 1 or something like that till to find the converge.
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Old   December 10, 2017, 07:10
Question I m still working in the new 18.2 version of cfx but the problem has not been solved
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Hello Glenn again

I have been trying many things in the new 18.2 version but ı could not handle with convergenge problem of the drag force and mesh folding in the new version of cfx. ı want to show some picture for the trim of the ship in the ex and new versions of the ansys. in the ex version the trim converge in 1500 iterations but in the new version i could not handle.

You said that you can change the smoothing options of the mesh in the set up.

I used both of 2 (increase near boundaries and increase near small volumes) with the model exponent from 1 to 10. bu it didnt work.

can you give me any advice about mesh smoothing options ?

here are my pictures, you can see from the starting of itearion in the 16.2 after 50-60 iterations the trim started to change from -0.02 radian to -0.002 radian. but in the 18.2 version the trim started to change from -0.03 to +0.05. then after 160 itearions the itearion stopped.

Best.

note:sorry ı could not upload the photos
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Old   December 10, 2017, 07:28
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Make sure you read the release notes to see what changes have been made to new versions of CFX. That will hopefully help you find what changed.

But looking back over this thread, I wonder whether your approach is the best one. Rather than model the body as a rigid body which opens up issues of moving mesh and stabilising a rigid body - how about you do a series of fixed mesh simulations at various heave, yaw, pitch and immersion depths. These should be straight forward simulations (but getting them accurate is still challenging), and will give you the forces acting on the vessel versus pitch, yaw and immersion depth. To find the steady state condition you then find where the lift force equals the mass of the object. You can also do transient simulations with a simple ODE solver.

Wouldn't this series of simple fixed mesh simulations be a better method to start with? Or is there some reason this approach is not appropriate?
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Old   December 10, 2017, 16:15
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You are true in your side of view Glenn. But from my naval architect point of view ı did this comparision in cfd.

I had the towing tank test results of dtmb 5415 ship model with trim-sinkage and the resistance.

If I use only the fixed mesh there is no problem in every version. but for example if the experiment test resistance result is 110 Newton, I will find 90 Newton with fixed mesh.

On the other side if i use moving mesh ı will find 112 Newton. Becasue the dynamic sinkage's added resistance is coming. And the result is getting better.

And also if ı checked the trim and sinkage results with the experiment everything was ok.

And also i m using a different type of marine cfd code which is called "numeca fine marine". This program is only for marine cfd. everything is reday to use in the program. the program automaticly sets everythin (domain geometry, air water vof, water line margin line, longitudionally bouyancy point and that points gyration radiuses and mass moments etc). When i compare the results with numeca fine marine and ansys 16.2 everything is same and ok

How can i send this situation to any ansys customer service for fixing this problem in the new versions Glenn ?

Best.





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Originally Posted by ghorrocks View Post
Make sure you read the release notes to see what changes have been made to new versions of CFX. That will hopefully help you find what changed.

But looking back over this thread, I wonder whether your approach is the best one. Rather than model the body as a rigid body which opens up issues of moving mesh and stabilising a rigid body - how about you do a series of fixed mesh simulations at various heave, yaw, pitch and immersion depths. These should be straight forward simulations (but getting them accurate is still challenging), and will give you the forces acting on the vessel versus pitch, yaw and immersion depth. To find the steady state condition you then find where the lift force equals the mass of the object. You can also do transient simulations with a simple ODE solver.

Wouldn't this series of simple fixed mesh simulations be a better method to start with? Or is there some reason this approach is not appropriate?
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Old   December 10, 2017, 17:38
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If you run a fixed mesh with the vessel at final sink and trim that should give you an accurate result, shouldn't it?

If you have a ANSYS TECS agreement which includes support then contact them according to that agreement. If you don't have TECS ANSYS is not going to listen to you much.
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