CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Conflict between y+, cell size and particle deposition

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   March 6, 2012, 19:03
Default Conflict between y+, cell size and particle deposition
  #1
Senior Member
 
Join Date: Jul 2009
Posts: 211
Rep Power: 9
kingjewel1 is on a distinguished road
Hi there,

I'm hoping you can put me straight here:

I'm investigating particle deposition onto indoor surfaces. Therefore Reynolds numbers are generally less than 10,000. So I'm using a wall function which requires y+ in the first cell to be about 30. Mine are, however, often about 1.

Therefore this gives me incorrect results. However I need sufficient grid resolution for accurate particle deposition.

What are your recommendations?

Regards,
kingjewel1 is offline   Reply With Quote

Old   March 6, 2012, 21:51
Default
  #2
Senior Member
 
calim_cfd's Avatar
 
mauricio
Join Date: Jun 2011
Posts: 136
Rep Power: 8
calim_cfd is on a distinguished road
Quote:
Originally Posted by kingjewel1 View Post
Hi there,

I'm hoping you can put me straight here:

I'm investigating particle deposition onto indoor surfaces. Therefore Reynolds numbers are generally less than 10,000. So I'm using a wall function which requires y+ in the first cell to be about 30. Mine are, however, often about 1.

Therefore this gives me incorrect results. However I need sufficient grid resolution for accurate particle deposition.

What are your recommendations?

Regards,
i'd say turn off your wall functions. y+ = 1 puts you in the in the viscous sub-layer. I guess basically you'll be solving the boundary layers instead of modeling. Then i guess you'll need at least ~15-20 elements along the BL's height with a growth rate of 1.1-1.3.

i haven't rly done any turbulent simulations in openfoam w/o wall functions. Some say that you may need y+ as small as 0.1 to resolve the whole thing properly but i guess that depends on the turbulence modeling/intensity

edit:
guess that you'll have to use low-re models to resolve the BL... try LaunderSharmaKE ras incomp/comp model or the new transition incompressible one kklOmega

set zero gradient on walls and respective values at inlet and outlet!? can any1 with xp in this topic back me up? or maybe correct me?

sry kingj, but i cant help you any better

hope it helps somehow
__________________
Best Regards
/calim

"Elune will grant us the strength"

Last edited by calim_cfd; March 6, 2012 at 22:38.
calim_cfd is offline   Reply With Quote

Old   March 7, 2012, 07:18
Default
  #3
Senior Member
 
Join Date: Jul 2009
Posts: 211
Rep Power: 9
kingjewel1 is on a distinguished road
Quote:
Originally Posted by calim_cfd View Post
i'd say turn off your wall functions. y+ = 1 puts you in the in the viscous sub-layer. I guess basically you'll be solving the boundary layers instead of modeling. Then i guess you'll need at least ~15-20 elements along the BL's height with a growth rate of 1.1-1.3.

i haven't rly done any turbulent simulations in openfoam w/o wall functions. Some say that you may need y+ as small as 0.1 to resolve the whole thing properly but i guess that depends on the turbulence modeling/intensity

edit:
guess that you'll have to use low-re models to resolve the BL... try LaunderSharmaKE ras incomp/comp model or the new transition incompressible one kklOmega

set zero gradient on walls and respective values at inlet and outlet!? can any1 with xp in this topic back me up? or maybe correct me?

sry kingj, but i cant help you any better

hope it helps somehow
This is what I am coming to believe too. But what is odd though, that there are papers out there that seem to get good experimental/numerical comparison without using low-re models. I just don't see how they're doing it with wall functions. Clarification would be greatly appreciated.
Cheers,
kingjewel1 is offline   Reply With Quote

Old   March 7, 2012, 08:11
Default
  #4
Senior Member
 
calim_cfd's Avatar
 
mauricio
Join Date: Jun 2011
Posts: 136
Rep Power: 8
calim_cfd is on a distinguished road
Quote:
Originally Posted by kingjewel1 View Post
.. But what is odd though, that there are papers out there that seem to get good experimental/numerical comparison without using low-re models.
i guess that depends on the flow?

Quote:
Originally Posted by kingjewel1 View Post
I just don't see how they're doing it with wall functions.
try setting usual turbulent setting on patches and zeroGrad on walls and c what you get!

if i have more time i'll try checking some references cuz i'm interested in that 2!!!!!

sry to be of no help
__________________
Best Regards
/calim

"Elune will grant us the strength"
calim_cfd is offline   Reply With Quote

Old   March 7, 2012, 09:57
Lightbulb
  #5
Senior Member
 
calim_cfd's Avatar
 
mauricio
Join Date: Jun 2011
Posts: 136
Rep Power: 8
calim_cfd is on a distinguished road
ok i've looked a bit more into the topic and i confess this is one of the most puzzling topics in cfd for me...i'm still reading Pope's book of turbulence for better modeling theory but atm this is what comes to me from my current knowledge

let's rate:

1:we all know the scale issues related to HiRe lowRe numbers and bounded flows

2:you either have turbulence or you don't!

3:if you do, you have to choose between low or high Re turbulence modeling (lets stick with RAS)

4[edited]: removed

5[edited]: wallfunctions... kinda routine settings for hiRe.. BUT, to resolve a bl you need a lowRe Ras model: have a refined mesh with low y+=1, use calculated type for nut/mut (few say zeroGrad wont change much), zeroGrad for k/epis or very small values like 1e-10 on walls

==>>zeroGrad for k/epis or very small values like 1e-10 on walls: i guess that in the limit they should yield similar results? no? == any comments/corrections to this are most welcome!!! ==


6[edited]: removed

check this pdf,
http://www.google.com.br/url?sa=t&rc...Py7KIzWAuhcT1w

maybe it will point sth you may have missed

guess that's all i can try to help you with!


EDIT: sry about the editing but i corrected some assertions made previously
__________________
Best Regards
/calim

"Elune will grant us the strength"

Last edited by calim_cfd; March 8, 2012 at 15:02.
calim_cfd is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Particle size show on DEM solver chandi STAR-CCM+ 9 May 15, 2013 06:41
Pls help UDF about particle size (Rosin-Rammler) ch3coohminh Fluent UDF and Scheme Programming 7 December 28, 2011 11:09
SPH particle vs. real fragment size Katarina FLUENT 0 April 8, 2009 09:48
Particle Size Distribution with R-R fails? HP FLUENT 6 May 19, 2005 06:35
Particle size distribution-concentration contours Umesh Shah FLUENT 3 July 22, 2003 14:46


All times are GMT -4. The time now is 22:21.