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Old   November 9, 2007, 12:01
Default Turbulence Options / Yplus
  #1
fat bug
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Learning the ropes with CFX.

I'm running a flow simulation with air "compressible". For Turbulence option, I chose "High (intensity=10%)". How do I determine if I made the right choice?

I'm using "SST" as turbulence model. The results showed that Y+ is over 40 at certain places. I zoomed in and noticed that it's at some insignificant corners. Do I need to either fill in the corners or refine the mesh at these corners to get Y+ down?

I'll appreciate all your help!
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Old   November 11, 2007, 06:40
Default Re: Turbulence Options / Yplus
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Glenn Horrocks
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Hi,

High intensity correct choice? If the resulting turbulence level matches your experiment then yes. For some simulations setting the background or inlet turbulence level is critical, for others it is not. Depends on the simulation.

Need further mesh refinement? Again, it depends. If these regions are meaning that proper boundary layer growth or separation cannot be modelled then yes it might affect the results. Otherwise no, it may not be critical.

Glenn Horrocks
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Old   November 12, 2007, 15:14
Default Re: Turbulence Options / Yplus
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fat bug
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Would I be able to read the Reynold's number in the results? Or is there another way to tell the actual percentage of the turbulence to make sure it's under 10%, or maybe in another category?

As far as yplus, I tried to inflate the area that had extremely high numbers (up to 1000). I was able to reduce yplus done to about single digit in 99.9 of the area, the results did not change much (less than 5%). I was surprised to see that.
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Old   November 12, 2007, 17:30
Default Re: Turbulence Options / Yplus
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Glenn Horrocks
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Hi,

Turbulence intensity comes from the experimental results you are trying to match. They should report the inlet or background turbulence level present.

Not all flows are significantly affected by the boundary level. Some flows do not need accurate boundary layer resolution.

Glenn Horrocks
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Old   November 17, 2007, 20:45
Default Re: Turbulence Options / Yplus
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jocko
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maybe it just not the region where the primary flow goes, that is why you dont have any significant change on the result. I think the sst automatic wall fuction can deal with the y plus only up to 200 and swicth to low reynold's number method when the grids become much too fine. Thus if the flow really goes through this region, it should give u asignidicant change after refining the region with y value of 1000.
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