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Old   February 20, 2012, 02:27
Default K-kl-transition model y+
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Hi,

I am doing a simulation for 16m Wind Turbine blade. I am using Fluent. I am using k-kl-transition model but I dont know the exact limit of y+ for this model. If it recommends 1 then I cannot get it because I would have to make the mesh finer. (The small cell size near the wall will make the far field cell 5 meters long). What if I use the k-epsilon model? As I think it is much relaxing in terms of y+ by using wall functions. and allows the limit to be greater than or equals to 30.

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Old   February 21, 2012, 01:29
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Any comments guys?
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Old   February 21, 2012, 03:33
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HI

If you think the boundary layer effects are not important or separating flow features are irrelevant to you you can use k-epsilon model but remember the torque you get on the blades without resolving these near wall phenomenons will always be with the correction. K-omega SST model is capable of resolving these near wall phenomenons effectively than k-e but that should comes with a rider...Computationally expensive

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Old   February 21, 2012, 09:29
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Thanks Sreenivas,
In k-w SST I dont get any options like use of wall f/n or enhanced wall treatment. In k-kl-transition the same is the case. I believe that use of wall f/n like in k-e reduces the wall y+ effort and you can use a much coarser mesh.
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Old   February 25, 2012, 12:04
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HI

Yes you will not get that option because if you are using k-w SST,means computationally you will have to resolve your boundary layer ie> you have to keep ur Y+<5.In the viscous sub layer you will have to put at least 30 to 40 layers.

You are true that when ur using wall function approach you have the liberty to use coarser mesh.Solver will take care of the region below the first grid point.

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Old   February 25, 2012, 12:55
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Quote:
I am using k-kl-transition model but I dont know the exact limit of y+ for this model. If it recommends 1 then I cannot get it
Then dont use K-Kl-w model. Yplus 1 or less is pre-requisite other wise you not get any meaning full results.

What is the Reynolds number of your turbine?

Quote:
In k-w SST I dont get any options like use of wall f/n or enhanced wall treatment. In k-kl-transition the same is the case. I believe that use of wall f/n like in k-e reduces the wall y+ effort and you can use a much coarser mesh.
Which version of Fluent, you are using.
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Old   February 26, 2012, 05:33
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SST kw is best model recommended for these kind of problems as long as the flow is fully turbulent but i have a confusion regarding the wall y+ of SST kw in Fluent 13. In fluent 6.3 it was written that SST kw uses Enhanced wall treatment as default but it can switch over to wall functions approach when wall y+ are in between 30-300 but no such thing is written in the user manual of Fluent 13 about SST kw, so i am confused. As par my little knowledge Ansys has made some modifications in the turbulence models, so I want to ask that can we also use SST kw with wall y+ in b/w 30-300 for getting good results?? any body who is sure about this please comment.

Shamoon what is your Re.No?? is it high enough to assume the flow as fully turbulent along the length of the blade??
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Old   February 26, 2012, 07:08
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Quote:
I want to ask that can we also use SST kw with wall y+ in b/w 30-300 for getting good results??
SST with wall function (30-300) is no good, as it looses it advantages. Since SST model uses the K-W model in near wall region and therefore is best for separated flows in boundary layer. You know boundary layer has three regions namely viscous sub layer, buffer layer ( Y+ 5-30,so called, doesn't exist physically) and log layer. As yplus goes from 1 to 100, it gradually switches to k-epsilon model. It is also a myth that turbulence models are always correct for the y+ 30-300 (so called wall function regime), but it possible that outer layer ends at Y+ 80 or may go up to 1000.

To apply boundary condition for turbulence model and mean flow equations we need the boundary conditions at wall (you don't calculate or specify it manually, it is done by solver for you) that is very important. Here comes into action the so called near wall treatment and wall functions. Now it depends upon user that he forces the solver the near wall velocity profile (near wall treatment) or just want to specify the velocity profile by some empirical formula and want to keep the computational cost low (wall function comes into play here). Both approaches have problems:

1. Both are not valid in yplus 5-30 range.
2. As separation occurs, yplus drops to zero and wall function ceases to be valid and you get wrong results
3. For near wall treatment you need yplus 2 for SST and for K-epsilon type models you need yplus 0.2
4. For near wall treatment you need at least 40 layers in boundary layer to properly resolve it.

To address this issues, two new models have been developed

1. salable wall function: For k-epsilon type models. It keeps minimum yplus 11.06 (intersection of viscous sub layer and log layer) by some limiter.
2. Automatic or all yplus or hybrid wall functions: They are designed to address the strict limits for boundary layer meshing:
a) They need 10-15 points in boundary layer
b) Yplus may be from 10-100 and they insensitive to this. Although this is true to zero pressure gradient flat plat flows but for complex geometries and flows (stagnation, separation and adverse pressure gradient) it may not be valid for yplus greater than 10. It has been thoroughly concluded on SA and SST model for complex flows (I can send you papers).

In summary if you want to use the SST model in Fluent 13 or 14

1. They have implemented the hybrid wall function by default.
2. Switch automatically to near wall treatment to wall function according to yplus. For example they may be using wall function for wall plus greater than 30 but will switch to near wall treatment when separation occurs and yplus goes to zero.
3. According to research best yplus should be kept below 10 with 10-15 points in boundary layer.
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Old   February 26, 2012, 21:33
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Hi, I'm quite new to FLUENT. Regarding to the previous posts, May i know where can i identify the y+ value in FLUENT 6.3? Thank you.
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Old   February 26, 2012, 23:32
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contour plot >>>> turbulence >>>>>>yplus
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Old   February 27, 2012, 11:47
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Far thanks for such a great help, it will really help me in clarifying my concepts. One more question, what is the meaning of wall y+ becoming zero and can we capture separation with wall y+ greater than 100 or we have to have wall y+=1 to capture separation?
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Old   February 27, 2012, 11:56
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Yplus goes to zero velocity goes to zero. This is the case when separation occurs. For better prediction yplus should be less than 1. Wall function mesh can capture the separation, but its accuracy is questionable.
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Old   February 29, 2012, 11:56
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Thanks a lot. One more question, what is the effect of turbulence parameters on solution, i mean if we are interested in finding coefficient of lift and drag then do the values of turbulence parameters effect the lift and drag coefficients? what are best guess to specify these parameters?
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