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increase the mesh and change in results?

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Old   July 30, 2015, 15:40
Question increase the mesh and change in results?
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A.heydari
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Hello:
Why increase the mesh, the results are far from the reality?
Of course, after every picture you can see, the results
Especially in right side of semi-cylinder on the the experimental results distances!
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Old   July 30, 2015, 15:46
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What do you mean by increasing the mesh ?

What are you increasing ? Your images from left to right are increasing the mesh element size, not the mesh element count.
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Old   July 30, 2015, 16:22
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What is the difference between the element size and element count there? And what to do with my question?
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Old   July 30, 2015, 18:29
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I do not understand your question either. Can you write it again more clearly?
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Old   July 31, 2015, 01:09
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hi;
I have some exprimental data. After each of these conditions mesh changed .Data modeling has been compared with the results. In the next section of the half cylinder is not good results. And every time fine mesh results more Different. Why? What should I do?
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Old   July 31, 2015, 01:15
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That means your mesh is not fine enough to achieve adequate mesh insensitivity. You need a finer mesh still. It is normal for the results to jump around when the mesh is way too coarse.

This assumes that the remainder of your simulation is correct, of course.
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Old   July 31, 2015, 01:37
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Dear ghorrocks;
With the increase in the fine mesh model can be meshed together in the third dimension. While I'm a two-dimensional model. (1 mm thick two-dimensional model. The maximum length of 5 meters is my model).
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Old   July 31, 2015, 01:43
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I do not understand your last comment.

Also flow over a cylinder has regimes where it is three dimensional. If you model a three dimensional flow with a two dimensional model you will get unpredictable results. Are you sure a two dimensional model is appropriate for the flow regime you are in?
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Old   July 31, 2015, 01:58
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High time software solution, I decided to do a two-dimensional model. Is this wrong?
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Old   July 31, 2015, 02:32
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If you are saying you chose to do a two dimensional simulation of a three dimensional flow because the simulation time was too long - if you want an accurate answer then yes, that is wrong.
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Old   July 31, 2015, 02:48
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Why wrong? More Info
Your suggestion to solve this problem?
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Old   July 31, 2015, 02:50
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Quote:
Originally Posted by ghorrocks View Post
If you are saying you chose to do a two dimensional simulation of a three dimensional flow because the simulation time was too long - if you want an accurate answer then yes, that is wrong.
Why wrong? More Info
Your suggestion to solve this problem?
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Old   July 31, 2015, 02:51
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Desmond is right. If you change your flow conditions to one where it is a two dimensional flow (possibly by reducing the flow velocity) then you can do a two dimensional model. There are good results to compare against for this flow, so it is a good benchmark simulation to make sure you can do an accurate simulation before taking on the more difficult three dimensional simulation.
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Old   July 31, 2015, 03:01
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Quote:
Why wrong? More Info
If you model a 3D flow with a 2D model you have unphysically constrained the flow. Your simulation will have an error, and it will probably be large.

Quote:
Your suggestion to solve this problem?
That is pretty obvious, isn't it? Model it in 3D.
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Old   July 31, 2015, 03:02
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thanks alot
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