some help in my simulation, vent silencer

MuRdOcGT · May 31, 2014, 01:40

hi! first of all i am new in this forum i have been reading some threads for a long time before posting this thread. i hipe you guys can help me out with my simulation in CFX...

I am student and i am doing my thesis and i need to use cfx to predict if my silencer works or not.

i designed a silencer, i already meshed my geometry got several elements around 8 millions. i finally got convergence i want to shere with you if my results are ok because its my first simulation in cfx, i have already done some workshops, and my convergence graph is a litlle different to the workshops.

i dont know how to set up steam or superheated steam , i run this with water at 280c.

prior to all i am verifying my geometric convergence so my fluid for the moment doesnt matter ( that was said by my tutor).. please i hope you can helpme thanks!!!

i attached my txt file and the picture of my graph.

[IMG]

[/IMG]

sorry for my bad english

ghorrocks · May 31, 2014, 07:24

Have you read the FAQ on this? http://www.cfd-online.com/Wiki/Ansys...gence_criteria

MuRdOcGT · May 31, 2014, 16:23

thanks, for your fast reply...

did my run converge? what do you think?

i think my mesh is ok i have 8M elements, should i need to refine it more?

i dont know what to look if is convergin.

sorry i have learned ansys through internet and some workshops, as you all probably know workshops always works.

Pd. thanks

ghorrocks · June 1, 2014, 03:50

Did it converge? Almost certainly not.

Is your mesk OK? New people to CFD make major mistakes in meshing about 95% of the time. So it almost certainly needs work as well.

Read the documentation on obtaining convergence for a start on convergence.

MuRdOcGT · June 8, 2014, 22:04

hi i modified my mesh an create in this link a document with all the things i am doing, i hope you guys check it and see what i am doing wrong, and thanks for answering back.

https://www.dropbox.com/s/zefwb41y0p...0all%20CFD.pdf

ghorrocks · June 9, 2014, 20:14

First of all, the CCL file (or output file) contains everything necessary about the solver in a text format. No need to copy images of CFX-Pre into a document. Attach that in future.

Secondly, I would run this with a coarse mesh to start with, and only after it is running reliably would I consider refining the mesh.

Third - This model appears to have symmetry. If so you might as well make use of it and halve your simulation size.

MuRdOcGT · June 10, 2014, 12:12

Thanks for taking the time to answer back, next time I'll just upload de output file sorry for that, I though it might be easier for you to look and see where my problem is.

I'll try a coarser mesh, but first is water ideal gas the right fluid material for my simulation?

What else do you think I should modify besides a coarser mesh? Thanks and sorry.

ghorrocks · June 10, 2014, 19:56

The coarser mesh is for development only. You will need a fine mesh as soon as it works reliably to get accurate answers.

What material properties - what Mach number is the flow in this device? What pressure or temperature differences? And phase change?

MuRdOcGT · July 10, 2014, 00:47

hi! it finally converged.... my problems were on my boundary conditions. i just put in the inlet and static pressure at the outlet...

I am now facing another problem my run converged using upwind advection sheme. A friedn told me that if i want to be sure about it i can run again using my upwind results and do a high res..... when i do this, it doesnt onverged, what do you suggest me!!!?

thanks

ghorrocks · July 10, 2014, 01:48

Have you read the FAQ I linked to on post #2? It covers all that - see points 3-5.

MuRdOcGT · July 15, 2014, 20:47

hi, again.. i solved my problem it seem that i cant use high res because my model is to complex and my machine isnt enough so it would take ages to run and in my country we have electricity issues so i just made last run with 0.75 in blend Factor..

Now i am facing a problem i hope you can help me, i need to do a accoustic analisis, i just want to know how many Db are lost with my silencer, i was trying Harmonic response but i found it TOO much complicated because all the commands, so do you think another way to do what i want?

Thanks.

Bless

ghorrocks · July 15, 2014, 20:51

In CFX the only way is to run it and monitor the pressure at a point far away and the calculate the sound intensity from the pressure wave. The put your silencer in it and re-run it to get another sound level.

MuRdOcGT · July 16, 2014, 01:00

thanks glenn, how exactly? i dont get it..sorry.

Do i need to monitor a point far away fron the outlet of my silencer? and then put my silencer in that point?

Thanks once again

ghorrocks · July 16, 2014, 07:34

First of all - if you do not know about acoustics and CFD then you are never going to complete this simulation. Make sure you understand how to do an accurate CFD study and you understand how acoustics work.

Then do a compressible flow simulation with your gizmo in the middle of the domain and your monitor point somewhere far away to represent far field results. You will need to put the outer boundary of the domain much further out.

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May 31, 2014, 07:24		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Have you read the FAQ on this? http://www.cfd-online.com/Wiki/Ansys...gence_criteria

May 31, 2014, 16:23		#3
MuRdOcGT New Member Greydil Join Date: May 2014 Posts: 7 Rep Power: 12	thanks, for your fast reply... did my run converge? what do you think? i think my mesh is ok i have 8M elements, should i need to refine it more? i dont know what to look if is convergin. sorry i have learned ansys through internet and some workshops, as you all probably know workshops always works. Pd. thanks

June 1, 2014, 03:50		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Did it converge? Almost certainly not. Is your mesk OK? New people to CFD make major mistakes in meshing about 95% of the time. So it almost certainly needs work as well. Read the documentation on obtaining convergence for a start on convergence.

June 8, 2014, 22:04		#5
MuRdOcGT New Member Greydil Join Date: May 2014 Posts: 7 Rep Power: 12	hi i modified my mesh an create in this link a document with all the things i am doing, i hope you guys check it and see what i am doing wrong, and thanks for answering back. https://www.dropbox.com/s/zefwb41y0p...0all%20CFD.pdf

June 9, 2014, 20:14		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	First of all, the CCL file (or output file) contains everything necessary about the solver in a text format. No need to copy images of CFX-Pre into a document. Attach that in future. Secondly, I would run this with a coarse mesh to start with, and only after it is running reliably would I consider refining the mesh. Third - This model appears to have symmetry. If so you might as well make use of it and halve your simulation size.

June 10, 2014, 12:12		#7
MuRdOcGT New Member Greydil Join Date: May 2014 Posts: 7 Rep Power: 12	Thanks for taking the time to answer back, next time I'll just upload de output file sorry for that, I though it might be easier for you to look and see where my problem is. I'll try a coarser mesh, but first is water ideal gas the right fluid material for my simulation? What else do you think I should modify besides a coarser mesh? Thanks and sorry.

June 10, 2014, 19:56		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	The coarser mesh is for development only. You will need a fine mesh as soon as it works reliably to get accurate answers. What material properties - what Mach number is the flow in this device? What pressure or temperature differences? And phase change?

July 10, 2014, 00:47		#9
MuRdOcGT New Member Greydil Join Date: May 2014 Posts: 7 Rep Power: 12	hi! it finally converged.... my problems were on my boundary conditions. i just put in the inlet and static pressure at the outlet... I am now facing another problem my run converged using upwind advection sheme. A friedn told me that if i want to be sure about it i can run again using my upwind results and do a high res..... when i do this, it doesnt onverged, what do you suggest me!!!? thanks

July 10, 2014, 01:48		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Have you read the FAQ I linked to on post #2? It covers all that - see points 3-5.

July 15, 2014, 20:47		#11
MuRdOcGT New Member Greydil Join Date: May 2014 Posts: 7 Rep Power: 12	hi, again.. i solved my problem it seem that i cant use high res because my model is to complex and my machine isnt enough so it would take ages to run and in my country we have electricity issues so i just made last run with 0.75 in blend Factor.. Now i am facing a problem i hope you can help me, i need to do a accoustic analisis, i just want to know how many Db are lost with my silencer, i was trying Harmonic response but i found it TOO much complicated because all the commands, so do you think another way to do what i want? Thanks. Bless

July 15, 2014, 20:51		#12
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	In CFX the only way is to run it and monitor the pressure at a point far away and the calculate the sound intensity from the pressure wave. The put your silencer in it and re-run it to get another sound level.

July 16, 2014, 01:00		#13
MuRdOcGT New Member Greydil Join Date: May 2014 Posts: 7 Rep Power: 12	thanks glenn, how exactly? i dont get it..sorry. Do i need to monitor a point far away fron the outlet of my silencer? and then put my silencer in that point? Thanks once again

July 16, 2014, 07:34		#14
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	First of all - if you do not know about acoustics and CFD then you are never going to complete this simulation. Make sure you understand how to do an accurate CFD study and you understand how acoustics work. Then do a compressible flow simulation with your gizmo in the middle of the domain and your monitor point somewhere far away to represent far field results. You will need to put the outer boundary of the domain much further out.