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Old   December 14, 2010, 04:55
Default open channel flow-sst model
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Hi all. I am trying to simulate a free surface flow over a hydraulically rough bed using the sst model. In order to have a decent behavior of this turbulence model I would like to ask how can I assure, in a practical point of view, that I have at least 15 nodes in the boundary layer (value presented in the documentation)?

Apart from the fact that y+ should be more or less equal to 1 is there any other recommendation when creating the mesh?

It would be very helpful to hear the feedback of all.Regards
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Old   December 14, 2010, 05:46
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I don't think you can do hydraulically rough flow without wall functions (ie y+>11). You should not integrate to the wall for this type of flow.

So put the first node at about y+=15, and grade out from there with a ratio of between 1.01 and 1.05. Hopefully that will have about the right BL resolution for you.
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Old   December 14, 2010, 07:50
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Sorry Sir, but i did not fully understand your explanation.

According to the documentation it says that sst model is based on the k- w model and has the same automatic wall treatment, which consists in switching between a low-Reynolds number formulation at low y+values and a wall function approach at higher y+values. Moreover it says that one can account for the roughness effects by considering a hs+ parameter.
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Old   December 14, 2010, 16:31
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You are right, you can model rough walls while integrating to the wall. But it does not sound like a good idea, unless you need detailed information about the boundary layer then it would be best to use wall functions. This means start with a mesh which has the first node at y+=15 to 20 or so.
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Old   December 15, 2010, 04:17
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Hi.

That`s precisely my case. Basically I am computing some quantities near the wall whose value at moment is very low. I think that can be caused by a bad separation prediction. Thatīs the reason why I have changed for sst model.
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Old   December 15, 2010, 04:55
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SST can handle the first node at y+>11 just fine. You don't need to integrate to the wall.
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Old   December 15, 2010, 05:27
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Hi,more two questions please:

Can you tell me why do you consider that`s not a good idea to integrate to the wall and why should I use the first node at y+=15 (or something like this)?

Do you think that even if i put the first node at y+=15, sst will "catch" the recirculation zones with more accuracy?

Best regards
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Old   December 15, 2010, 17:44
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Because a mesh using wall functions and a first node will have a smaller mesh by a factor of at least 15, and probably more like 100. That means 100 times less memory requirements and run time.

If wall functions work, then use them.
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