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wall y+: physic meaning

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Old   November 16, 2009, 04:32
Default wall y+: physic meaning
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Hi,
I would like to know which is the physic meaning of the wall y+.
Thanks!
sara
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Old   November 16, 2009, 05:14
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y+ is a non dimensional distance from the wall
y+= (y*uf)/(/pho)
y:distance from the wall
uf: friction velocity = (du/dy) for y=0 (at the wall)
*/pho: cinematic velocity

u+=u/uf
u: velocity component parallel to the wall

The physical meaning is that if u have a turbulent flow, for a y+<5, wich is very close from the wall, there is a region called viscous sublayer where the flow is laminar. This region only knows viscous stresses. u+=y+

when 5<y+<30: buffer layer , intermediate region

30<y+<500 (the upper limit can vary from a flow to another): log-region: turbulent stresses became dominant. u+=2.44 ln y+ + 5.5
The velocity profile here is not as much universal as in the viscous sublayer. It does not hold for cases with adverse pressure gradients....If the wall is not smooth, u must change the constants....
(the width of the log-layer is about 1/3 from the boundary layer: the rest is called defect region, and there exists correlations for veloity profile there for equilibrium boundary layers)

In CFD simulations, u can resolve the whole bounday layer and in such case, u need a very fine mesh near the walls (gradients are very important there). This approach is expensive. (u must have y+=1 on the first near wall cell)
U can instead use the wall law: u choose ure first cell to be in the log-law layer, and u dont need to resolve the whole boundary layer. This approach is more economic. However, it can lead to better results than the first one sometimes (if u are modeling the boundary layer with a non suitable equation by exemple: case very frequent when u use the k-epsilon model)
Vietazz, annu, ramya k and 1 others like this.
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Old   November 16, 2009, 05:25
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Ok thanks!
You are very kind!
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Old   November 16, 2009, 05:45
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no problem comrade. If u tell where do u need to apply it i can maybe give u more details
regards
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