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Y+ should be considered even for laminar flow?

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Old   February 14, 2007, 11:46
Default Y+ should be considered even for laminar flow?
  #1
Kentyn
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Hi, everyone,

We have discussed in the Forum a lot on the YPLUS treatment for turbulence modeling. Now I learned (see below) that the Y+ considration shlould taken even for the laminar flow. This is new for me.

It is strange because I knew the log-law for the "law-of-the-wall" was used for identifying the viscous layer,blending layer, and the fully turbulent layer. Therefore, it will be applicable for turbulent case. However, according to the statement below, it seems to come to a conclusion that all viscous models need to take care of the choice of YPLUS. This point is not clear to me.

Could any one clarify WHY? Hope to get your comments.

--------------------------------------------------------- There are certain specific recommendations for first cell height (Yp). Resolution of the boundary layer plays a significant role in the accuracy of the computed wall shear stress and heat transfer coefficient. This is particularly true in laminar flows where the grid adjacent to the wall should obey

Yp* Sqrt (Uinf/Mux) <= 1

where, Yp is the distance to the wall from the adjacent cell centroid; Uinf is the free-stream velocity; Mu is kinematic viscosity of the fluid and x is distance along the wall from the starting point of the boundary layer

Proper resolution of the mesh for turbulent flows is also very important. In the near-wall region, different mesh resolutions are required depending on the near-wall model being used.
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Old   February 16, 2007, 10:47
Default Re: Y+ should be considered even for laminar flow?
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Torra
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Hi, Kentyn,

Thanks for posting this question which is not very clear in FLUENT user guide. However, It is really very important.

In fact, the Boundary Layer Theory is applicable to both "Laminar" and "turbulent" flows, as shown in the Book of Schliting (2000).

Like you,I also wonder how the FLUENT deals with the near-wall treatment. What is the references they used to test the laminar flow? It seems that there is nothing mentioned in its USER GUIDE.

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Old   February 16, 2007, 18:09
Default Re: Y+ should be considered even for laminar flow?
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sam
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Isnt It a direct numrical simulation?
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Old   June 2, 2012, 22:29
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Thiru
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the definition of y+ = y*friction velocity/kinematic viscosity. The friction velocity in turn is defined as sqrt(wall shear stress/density). So everything comes down to computing wall shear stress. The wall shear stress can be computed for both laminar as well as the turbulent flow depending upon the case under consideration. For instance, calculating the wall shear stress for the laminar flow on a flat plate goes with blasius solution. And for turbulent flow the expression for tau (wall shear stress) is different. I suppose the wall units could be used for both laminar as well as the turbulent boundary layer. The other possible explanation could be, the wall units are based only on the viscous sublayer of the turbulent flow which is itself laminar and where almost 60% of turbulent production occurs. No problem in using wall units for laminar and turbulent flow. While studying drag reduction computationally it is very important that the far fields are fixed with respect to non dimensionalised wall units,i.e, 50*y+, for the entrance effect,etc., regardless of laminar or turbulent flow. There are lot of journals explaining laminar flow considering wall units.
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Old   September 26, 2012, 06:22
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Naimah
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Hi all,

I have question regarding the wall shears stress and the velocity gradient

Based on Fluent user guide:
Shear-Stress Calculation in Laminar Flow
In a laminar flow , the wall shear stress is defined by the normal velocity gradient at the wall as




In the straight pipe, this will not have any problem to calculate for the normal velocity gradient.
How about in the slanted pipe? which velocity represent the normal velocity gradient?


I have this problem because my geometry that I have is an irregular geometry. I need to verify why I have the high WSS at 1 region but Im not sure which velocity will represent this.


Hope someone can help me.


Thank you so much.


Regards,
Naimah





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