define "yplus" in custom field function in fluent

Mutlaq · May 15, 2020, 09:25

when I want to compute y+ in fluent it computed for the first layer only. so how can I define y+ in CFF I Know the law of y+=y*ut/kin.visc

but what the Y means in fluent how can I define

anyone can help?

vinerm · May 15, 2020, 09:35

y is just coordinate normal to the wall. The challenge is not with y but with $u_\tau$ . This depends on wall shear stress and is not easy to define for cells that are not adjacent to the wall. The major challenge is in separating wall shear from other shear layers.

Mutlaq · May 15, 2020, 10:01

Quote:

Originally Posted by vinerm

y is just coordinate normal to the wall. The challenge is not with y but with $u_\tau$ . This depends on wall shear stress and is not easy to define for cells that are not adjacent to the wall. The major challenge is in separating wall shear from other shear layers.

I got the y+=20 but in the spanwise direction as the no. of the layer that I put.

and plot the y+ vs u+ in the same direction but that doesn't mean anything because of that not in the normal wall direction.
I know that define y+ isn't easy but I need the code or formula of it because of all standard plots in books and researches are plots with y+

vinerm · May 15, 2020, 10:07

It depends on your objective. No one plots

vs

from CFD since the graph is actually input to CFD. If you want to do it, you have to write a UDF and it won't be easy. The graphs that you see in books is based on experimental data and there is no benefit of regenerating it using CFD. If you are trying to do it for verification, then you have to either come up with a way to separate wall shear from turbulence shear or do multiple simulation with different velocities so that both,

and

, change for each case.

Mutlaq · May 15, 2020, 10:19

Quote:

Originally Posted by vinerm

It depends on your objective. No one plots

vs

from CFD since the graph is actually input to CFD. If you want to do it, you have to write a UDF and it won't be easy. The graphs that you see in books is based on experimental data and there is no benefit of regenerating it using CFD. If you are trying to do it for verification, then you have to either come up with a way to separate wall shear from turbulence shear or do multiple simulation with different velocities so that both,

and

, change for each case.

thank you very much.

the last point. Would you recommend using the periodic boundary conditionز
I actually used it but I need your opinion

vinerm · May 15, 2020, 10:22

That depends on the simulation. Periodic condition exists because it is useful but the usage is based on whether the physics and geometry, both, are periodic or not.

Mutlaq · May 15, 2020, 10:31

Quote:

Originally Posted by vinerm

That depends on the simulation. Periodic condition exists because it is useful but the usage is based on whether the physics and geometry, both, are periodic or not.

I used the periodic to get fully developed for the straight channel then I used the profile to get the outlet condition of periodic as an inlet condition for another channel that has the same mesh, inlet, and outlet area but has a wavy in the bottom wall. so I used periodically only to get fully developed for a small length

vinerm · May 15, 2020, 10:33

That's an appropriate use of translational periodicity.

Mutlaq · May 15, 2020, 10:36

Quote:

Originally Posted by vinerm

That's an appropriate use of translational periodicity.

so that good choice and not wrong?

vinerm · May 15, 2020, 10:39

Yes, that's correct choice.

Mutlaq · May 15, 2020, 10:40

Quote:

Originally Posted by vinerm

Yes, that's correct choice.

Thank you very much

Mutlaq · May 18, 2020, 13:33

Quote:

Originally Posted by vinerm

Yes, that's correct choice.

Mr. vinerm can I ask you about adaption mesh in fluent if I need to use it or no?

vinerm · May 18, 2020, 15:23

Mesh adaption is process to refine (or coarsen) the mesh in specific regions. If you already have the mesh that can resolve the required fields, then you don't need to worry about mesh adaption. Mesh adaption is one of the least used approach in most of the simulations.

Mutlaq · May 19, 2020, 00:04

Quote:

Originally Posted by vinerm

Mesh adaption is process to refine (or coarsen) the mesh in specific regions. If you already have the mesh that can resolve the required fields, then you don't need to worry about mesh adaption. Mesh adaption is one of the least used approach in most of the simulations.

thank you for reply

Mutlaq · May 27, 2020, 10:10

Quote:

Originally Posted by vinerm

Mesh adaption is process to refine (or coarsen) the mesh in specific regions. If you already have the mesh that can resolve the required fields, then you don't need to worry about mesh adaption. Mesh adaption is one of the least used approach in most of the simulations.

Hi Mr. vinerm I need to ask you how can I calculated CLF number any mesh size I need to use with y or x or z

delta t= (CFL*mesh size/velocity)

vinerm · May 27, 2020, 11:39

For CFL, you need to use a scale and not exact value of x, y, or z size of cell. Usually, cube root of volume is used, provided the mesh is hex. If it is tet, then you can use cube root of $(V_{cell}\sqrt{72})^{(1/3)}$ as length scale.

Mutlaq · May 27, 2020, 20:20

thank you very much

Mutlaq · June 1, 2020, 06:45

Quote:

Originally Posted by vinerm

For CFL, you need to use a scale and not exact value of x, y, or z size of cell. Usually, cube root of volume is used, provided the mesh is hex. If it is tet, then you can use cube root of $(V_{cell}\sqrt{72})^{(1/3)}$ as length scale.

Hi Mr. Vinerm I want to ask you about this type of mesh if the mesh good or no, Because as I mentioned earlier I used a periodic inlet profile take it from a straight channel and another reason the mesh in someplace is very close in the streamwise direction

https://drive.google.com/file/d/1Lhn...ew?usp=sharing

vinerm · June 2, 2020, 09:38

I am afraid I cannot access Google Drive. So, better to attach a few pictures here in the post.

Mutlaq · June 2, 2020, 10:07

Quote:

Originally Posted by vinerm

I am afraid I cannot access Google Drive. So, better to attach a few pictures here in the post.

this is a mesh

May 15, 2020, 09:25	define "yplus" in custom field function in fluent	#1
Mutlaq Member Ali Suleiman Join Date: Nov 2019 Posts: 43 Rep Power: 6	when I want to compute y+ in fluent it computed for the first layer only. so how can I define y+ in CFF I Know the law of y+=y*ut/kin.visc but what the Y means in fluent how can I define anyone can help?

May 15, 2020, 09:35	Yplus	#2
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	y is just coordinate normal to the wall. The challenge is not with y but with $u_\tau$ . This depends on wall shear stress and is not easy to define for cells that are not adjacent to the wall. The major challenge is in separating wall shear from other shear layers. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

May 15, 2020, 10:07	Yplus plot	#4
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	It depends on your objective. No one plots vs from CFD since the graph is actually input to CFD. If you want to do it, you have to write a UDF and it won't be easy. The graphs that you see in books is based on experimental data and there is no benefit of regenerating it using CFD. If you are trying to do it for verification, then you have to either come up with a way to separate wall shear from turbulence shear or do multiple simulation with different velocities so that both, and , change for each case. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

May 15, 2020, 10:22	Periodic BC	#6
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	That depends on the simulation. Periodic condition exists because it is useful but the usage is based on whether the physics and geometry, both, are periodic or not. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

May 15, 2020, 10:33	Good	#8
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	That's an appropriate use of translational periodicity. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

May 15, 2020, 10:39	Correct	#10
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	Yes, that's correct choice. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

May 18, 2020, 15:23	Adaption	#13
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	Mesh adaption is process to refine (or coarsen) the mesh in specific regions. If you already have the mesh that can resolve the required fields, then you don't need to worry about mesh adaption. Mesh adaption is one of the least used approach in most of the simulations. Mutlaq likes this. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

May 27, 2020, 11:39	Cfl	#16
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	For CFL, you need to use a scale and not exact value of x, y, or z size of cell. Usually, cube root of volume is used, provided the mesh is hex. If it is tet, then you can use cube root of $(V_{cell}\sqrt{72})^{(1/3)}$ as length scale. Mutlaq likes this. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

May 27, 2020, 20:20		#17
Mutlaq Member Ali Suleiman Join Date: Nov 2019 Posts: 43 Rep Power: 6	thank you very much

June 2, 2020, 09:38	Mesh	#19
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	I am afraid I cannot access Google Drive. So, better to attach a few pictures here in the post. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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