# open channel flow-sst model

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 December 14, 2010, 05:55 open channel flow-sst model #1 Senior Member   Join Date: Jan 2010 Posts: 110 Rep Power: 8 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

 December 14, 2010, 06:46 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,717 Rep Power: 99 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.

 December 14, 2010, 08:50 #3 Senior Member   Join Date: Jan 2010 Posts: 110 Rep Power: 8 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.

 December 14, 2010, 17:31 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,717 Rep Power: 99 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.

 December 15, 2010, 05:17 #5 Senior Member   Join Date: Jan 2010 Posts: 110 Rep Power: 8 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.

 December 15, 2010, 05:55 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,717 Rep Power: 99 SST can handle the first node at y+>11 just fine. You don't need to integrate to the wall.

 December 15, 2010, 06:27 #7 Senior Member   Join Date: Jan 2010 Posts: 110 Rep Power: 8 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

 December 15, 2010, 18:44 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,717 Rep Power: 99 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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