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How can values of profile(k, epsilon) not change?

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Old   January 21, 2020, 12:15
Unhappy How can values of profile(k, epsilon) not change?
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Kim Min Kyu
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Hi, I am a beginner in CFD.

I am now trying to calculate wind-induced pressure & force in high rise building.

With experiment data, I could calculate k and espilon at a height of 0 to 600 meters.

Then I made a profile which consists of wind speed, k , and epsilon.

After that, I entered this profile at the inlet as a velocity inlet (also I set outlet as a pressure outlet).

I expected values of wind speed, k, and epsilon would not change as the flow goes on (of course, these values depends on the altitude individually).

However, values of k and epsilon rapidly changed during calculation and converged to some values that I did not expect.

For wind speed case, values are maintained and similar to wind profile in some degree.

I would like to ask how values of k and epsilon would be maintained as originally entered regardless of the progress of flow.

Best regards,
Kim Min Kyu
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Old   January 21, 2020, 14:26
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Lucky Tran
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Values will change according to the solved transport equations. If they change and you did not expect them to, then either the model is wrong or the settings are wrong.

If you want to fix the values of k and epsilon everywhere, then specify k & epsilon on the boundaries as you've already done. Next you'll want to specify k & epsilon on the entire field as an initial condition. Then freeze the turbulence equations in the solver so that it skips over the k and epsilon equations and updates only the remaining equations.

But now I am confused....

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Originally Posted by minque View Post
I expected values of wind speed, k, and epsilon would not change as the flow goes on (of course, these values depends on the altitude individually).
If none of these are changing, why are you even doing CFD? You already know the solution.
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Old   January 21, 2020, 19:46
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Kim Min Kyu
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First of all, I would like to appreciate your reply.

Actually, I didnít want values of k and epsilon to be constant everywhere.

Because Obstacle(building) is located at 330 meters away from inlet (x=330m), I wanted values of k and epsilon to be constant just in front of the obstacle.

I think I might confuse of basic theory of fluid mechanics & CFD, but I think my velocity is already fully developed. Thatís why I wanted values of k and epsilon to be constant before attacking obstacle.
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Old   January 21, 2020, 19:48
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Quote:
Originally Posted by LuckyTran View Post
Values will change according to the solved transport equations. If they change and you did not expect them to, then either the model is wrong or the settings are wrong.

If you want to fix the values of k and epsilon everywhere, then specify k & epsilon on the boundaries as you've already done. Next you'll want to specify k & epsilon on the entire field as an initial condition. Then freeze the turbulence equations in the solver so that it skips over the k and epsilon equations and updates only the remaining equations.

But now I am confused....



If none of these are changing, why are you even doing CFD? You already know the solution.
Also, I would like to ask how to freeze turbulence equations to keep values of k and epsilon.
Sorry for bothering.
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Old   January 22, 2020, 14:14
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In the tree on the left go to solution=> controls.
In the solution controls task pane click equations.

From the ribbon on the top of the screen go to solving=>controls=>equations

You should see flow, turbulence, energy, etc....
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