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July 20, 2008, 13:03 
influence of yplus

#1 
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Hi,
I have some kind of problem understanding yplus in combination with high/lowre models and their influence on the flow. E.g. if I take a look at a pipe flow, how does the calculated bulk flow and the profile change? Does the flow gets faster or slower, when using a small yplus (<11) for highrenumber models? And most important, how can I see the change in the equations? On the other hand, what would happen with a lowre model with large yplus (>1)? Maybe, there is a good article explaining this? Thanks in advance! Flo 

July 22, 2008, 14:03 
Re: influence of yplus

#2 
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Is the question too stupid? Would be nice to get some hints...
Flo 

July 22, 2008, 14:30 
Re: influence of yplus

#3 
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Your question is unclearly stated. Are you asking about the effect on a CFD solution as yplus is varied, or are you asking how physical properties vary with yplus? The yplus value (in CFD) is primarily a parameter that describes the required resolution of a grid to capture a viscous layer adequately. If you use a grid whose yplus value is too large (and assuming your flow solver does not have some means of compensating, such as wall function BCs) then you will not capture the correct gradients in the viscous layer, meaning you will not get the correct stresses, meaning you will not model the correct viscous effects in the momentum equation, meaning you won't get the correct velocity field, meaning ...
Beyond that, I'm not sure what you are asking for. 

July 23, 2008, 02:30 
Re: influence of yplus

#4 
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Thanks, o.k., but what how can I know, what 'not correct' means? For the two cases lowRe with too large y+ and highRe with too small y+, how do the stresses and bulk velocity field react? E.g. is the bulk velocity faster or slower for these two cases compared to calculations with correct y+ values? Thanks! Flo


July 23, 2008, 09:52 
Re: influence of yplus

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OK  I think I am beginning to understand what you are asking. For the lowRe models, a value of y+ that is too large will result in a failure to correctly capture the velocity gradient (i.e. something like the log law). The flow may tend to look more laminar, and the bulk velocity should reflect this behavior, and drop. For the second case, it depends on how the turbulence model handles the y+. Some formulations of wall functions don't have a problem with y+ values that are "small", and all you're doing is using more grid points than necessary  and the solution should be fine. Other formulations may tend to be unstable. The best approach for your code is to run some cases and see how it behaves.


July 24, 2008, 01:31 
Re: influence of yplus

#6 
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Thanks; so the lowRe model expects the first grid point in the viscous layer and if the grid point is to far away from the wall, it still assumes a viscous/laminar profile!? I'll make a test case for highre. Thanks again. Flo


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