Y+ range for k-omega SST model. Scaling issues ?

arapha · January 22, 2011, 22:53

Hi,

I am using a k-omega SST model on a flow going through a duct. I should get my y+ between 1 and 5 I believe from what I've seen. I found out about a scaling issue in Fluent thanks to previous posts regarding wheel meshing (Marli Nov 28th). However, I am still having difficulties obtaining the right y+ values from my mesh. When I try to adapt in Fluent it halves the y+ value but horribly increases the number of cells. My mesh y+ is about 60 so that I can't obtain the correct y+ value with a decent sized mesh in Fluent. I'm wondering if this is another scaling issue in Pointwise (importing from CATIA to Pointwise, or in Pointwise, do I have to set the units ? If I go to Properties the ratio of Grid/Database is about 600 ??). Or am I using the adapt function in Fluent wrong ?

Can very small y+ (<<1 ) values lead to erroneous results in Fluent with this k-omega SST model ?

Any thoughts or suggestions greatly appreciated.

Thank you !!

RossFS · January 26, 2011, 23:22

It might be an idea to locate the user guide pdf file that is provided with Fluent as it ought to discuss what the software does in regards to wall functions. Reading up on the wall functions used in Fluent is likely to be able to answer your question.
For example, in ANSYS CFX, the software will run its own wall function to solve the boundary layer when the mesh is coarse. This is done so as to avoid requiring a very large mesh.

Also, can you run inflation layers in your boundary layer zone so that you get a sufficiently small y+ value but do not have an overly large mesh ?
eg 10% increase in height between each cell in the boundary layer.

January 22, 2011, 22:53	Y+ range for k-omega SST model. Scaling issues ?	#1
arapha New Member Join Date: Oct 2010 Posts: 19 Rep Power: 16	Hi, I am using a k-omega SST model on a flow going through a duct. I should get my y+ between 1 and 5 I believe from what I've seen. I found out about a scaling issue in Fluent thanks to previous posts regarding wheel meshing (Marli Nov 28th). However, I am still having difficulties obtaining the right y+ values from my mesh. When I try to adapt in Fluent it halves the y+ value but horribly increases the number of cells. My mesh y+ is about 60 so that I can't obtain the correct y+ value with a decent sized mesh in Fluent. I'm wondering if this is another scaling issue in Pointwise (importing from CATIA to Pointwise, or in Pointwise, do I have to set the units ? If I go to Properties the ratio of Grid/Database is about 600 ??). Or am I using the adapt function in Fluent wrong ? Can very small y+ (<<1 ) values lead to erroneous results in Fluent with this k-omega SST model ? Any thoughts or suggestions greatly appreciated. Thank you !!

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January 26, 2011, 23:22		#2
RossFS Member Join Date: Feb 2010 Location: Australia Posts: 65 Rep Power: 16	It might be an idea to locate the user guide pdf file that is provided with Fluent as it ought to discuss what the software does in regards to wall functions. Reading up on the wall functions used in Fluent is likely to be able to answer your question. For example, in ANSYS CFX, the software will run its own wall function to solve the boundary layer when the mesh is coarse. This is done so as to avoid requiring a very large mesh. Also, can you run inflation layers in your boundary layer zone so that you get a sufficiently small y+ value but do not have an overly large mesh ? eg 10% increase in height between each cell in the boundary layer.