what if Y+ value is more than 300

sache · December 25, 2014, 01:32

I need some clarification.

In case of any fluid flow simulation if in most of area y+ value is coming more than 300 for standard wall funtion, will it give wrong pressure drop result or the result will be acceptible?

as per theory it should be low to capture all boundary layers, but sometimes mesh count goes so high that it is not possible to have fine mesh at surface.

then can i consider whatever pressure drop i get, will be acceptible.

thanks in advance for comments.

Hamidzoka · December 25, 2014, 14:26

In case of using standard wall functions, the behavior of flow at near-wall regions will be solved analytically, i.e. conservation equations will not be solved numerically at very first cell by the walls. Although this brings about some errors in computations, decreases the computation costs and there will be no need to generate dense boundary layer meshes at wall boundaries.
Acceptable level of y+ in case of standard wall functions depends on the employed turbulence model to some extent. But it can be as large as 150 or even 200.
I think reducing the current y+ value from 300 to somewhat around 150 will not impose too much computation costs.

regards

lcarasik · December 25, 2014, 18:20

Quote:

Originally Posted by Hamidzoka

In case of using standard wall functions, the behavior of flow at near-wall regions will be solved analytically, i.e. conservation equations will not be solved numerically at very first cell by the walls. Although this brings about some errors in computations, decreases the computation costs and there will be no need to generate dense boundary layer meshes at wall boundaries.
Acceptable level of y+ in case of standard wall functions depends on the employed turbulence model to some extent. But it can be as large as 150 or even 200.
I think reducing the current y+ value from 300 to somewhat around 150 will not impose too much computation costs.

regards

Also, I would recommend you check what your specific implementation (based on which software you are using) of wall function is being used. This should tell you what value range your y+ value your first cell layer should be within.

sache · December 27, 2014, 21:34

Thank you hamidzoka. Your reply is helpful.

sache · December 27, 2014, 21:35

Quote:

Originally Posted by lcarasik

Also, I would recommend you check what your specific implementation (based on which software you are using) of wall function is being used. This should tell you what value range your y+ value your first cell layer should be within.

Thank u dear

amitray · December 31, 2014, 01:17

Valid range of y+ for using the wall functions approach is in between 30 and 300. Depends on your priority.

December 25, 2014, 01:32	what if Y+ value is more than 300	#1
sache New Member Join Date: Dec 2013 Posts: 17 Rep Power: 12	I need some clarification. In case of any fluid flow simulation if in most of area y+ value is coming more than 300 for standard wall funtion, will it give wrong pressure drop result or the result will be acceptible? as per theory it should be low to capture all boundary layers, but sometimes mesh count goes so high that it is not possible to have fine mesh at surface. then can i consider whatever pressure drop i get, will be acceptible. thanks in advance for comments.

December 27, 2014, 21:34	thank u	#4
sache New Member Join Date: Dec 2013 Posts: 17 Rep Power: 12	Thank you hamidzoka. Your reply is helpful. lcarasik likes this.

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December 25, 2014, 14:26		#2
Hamidzoka Senior Member Hamid Zoka Join Date: Nov 2009 Posts: 282 Rep Power: 18	In case of using standard wall functions, the behavior of flow at near-wall regions will be solved analytically, i.e. conservation equations will not be solved numerically at very first cell by the walls. Although this brings about some errors in computations, decreases the computation costs and there will be no need to generate dense boundary layer meshes at wall boundaries. Acceptable level of y+ in case of standard wall functions depends on the employed turbulence model to some extent. But it can be as large as 150 or even 200. I think reducing the current y+ value from 300 to somewhat around 150 will not impose too much computation costs. regards

December 31, 2014, 01:17		#6
amitray New Member Amit Ray Join Date: May 2014 Location: Chennai, India Posts: 5 Rep Power: 11	Valid range of y+ for using the wall functions approach is in between 30 and 300. Depends on your priority.