Turbulent Viscosity Ratio Limited not Vanished yet, but My Case Was Converged

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 May 22, 2016, 01:41 Turbulent Viscosity Ratio Limited not Vanished yet, but My Case Was Converged #1 New Member   Andi Join Date: Jun 2015 Location: Indonesia Posts: 17 Rep Power: 8 Hello there, I have questions I ran simulation about flow across inlet duct HRSG with Fluent 6.3.26, with the parameters were : 1. Turbulence model : Realizable k-epsilon 2. Standard wall function 3. k-epsilon model : Turbulence Intensity (10%) and hydraulic diameter 4. Residual monitor : 10-4 5. Inlet : Velocity inlet 6. Outlet : Outflow 7. Using Energy Equation and i built the superheater tube geometri in my simulation. while I was running the simulation, there was warning about "turbulent viscosity ratio limited to viscosity ratio of 100000 in ..... cells." But in 1000 iterations my case was converged and the warning was not vanished yet. Can somebody explain what's wrong in my simulation? Is my result valid ? I'd appreciate for your help Thanks Regards

 May 22, 2016, 10:42 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,794 Rep Power: 56 In general, the result is invalid and you need to fix the limited viscosity ratio. Make a plot and look for regions where the turbulent viscosity is extremely high. That will tell you where you need to improve. It could be your uniform velocity inlet.

May 24, 2016, 16:40
#3
New Member

Andi
Join Date: Jun 2015
Location: Indonesia
Posts: 17
Rep Power: 8
Quote:
 Originally Posted by LuckyTran In general, the result is invalid and you need to fix the limited viscosity ratio. Make a plot and look for regions where the turbulent viscosity is extremely high. That will tell you where you need to improve. It could be your uniform velocity inlet.
Thanks LuckyTran,

So, do i just refine the grid in that area?

The statement about "limited viscosity ratio" appear, is it only because of my mesh, or something else like wrong boundary condition at the inlet?

 May 24, 2016, 19:39 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,794 Rep Power: 56 It can and usually is caused by wrong inlet boundary conditions. i.e. a high uniform turbulence intensity applied to a uniform velocity inlet is non-physical. It can be caused by the mesh if there are significantly skewed cells.

May 25, 2016, 21:51
#5
New Member

Andi
Join Date: Jun 2015
Location: Indonesia
Posts: 17
Rep Power: 8
Quote:
 Originally Posted by LuckyTran It can and usually is caused by wrong inlet boundary conditions. i.e. a high uniform turbulence intensity applied to a uniform velocity inlet is non-physical. It can be caused by the mesh if there are significantly skewed cells.
i have checked my worst skewness, and it is 0.554523 , is that bad ?

If it was my wrong boundary conditions at the inlet, can you suggest me the right one ?
I have my turbulence intensity set by 10 % (from other paper) and i used hydraulic diameter which is the hydraulic diameter of the inlet, that is 3.1409 m.

Thanks for your help LuckyTran.

 May 25, 2016, 22:29 #6 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,794 Rep Power: 56 The skewness seems okay so it's probably not the cause. Check where the turbulent viscosity ratio is too high and determine where the problem is. Is it near the inlet? Is it near your walls or is it in the core? Or maybe it is not near the inlet but clustered around your tubes.

May 28, 2016, 02:20
#7
New Member

Andi
Join Date: Jun 2015
Location: Indonesia
Posts: 17
Rep Power: 8
Quote:
 Originally Posted by LuckyTran The skewness seems okay so it's probably not the cause. Check where the turbulent viscosity ratio is too high and determine where the problem is. Is it near the inlet? Is it near your walls or is it in the core? Or maybe it is not near the inlet but clustered around your tubes.
The turbulence viscosity is highest at sudden expansion (exit of the duct). Is my mesh too rough at that area ?

And just curious, is there any case that turbulent viscosity ratio could really exceed 10^5 ?

Last edited by Andi_Didi; May 29, 2016 at 03:50.

 May 29, 2016, 02:42 #8 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,794 Rep Power: 56 There are certainly skewed cells in that area. Anyway, you know what locations need improvement. Turbulent viscosity ratio > 10^5 is extremely rare and will certainly never occur for practical scenarios. Maybe for superfluids, critical point transitions, or inside a black hole or something... Since you doing a HRSG, I'll say that it's nearly impossible to have that high a viscosity ratio.

May 29, 2016, 03:41
#9
New Member

Andi
Join Date: Jun 2015
Location: Indonesia
Posts: 17
Rep Power: 8
Quote:
 Originally Posted by LuckyTran There are certainly skewed cells in that area. Anyway, you know what locations need improvement. Turbulent viscosity ratio > 10^5 is extremely rare and will certainly never occur for practical scenarios. Maybe for superfluids, critical point transitions, or inside a black hole or something... Since you doing a HRSG, I'll say that it's nearly impossible to have that high a viscosity ratio.
Black hole.. ? yeah right..

Ok then, thanks for your help. I really appreciate it.

June 10, 2016, 09:09
#10
New Member

Andi
Join Date: Jun 2015
Location: Indonesia
Posts: 17
Rep Power: 8
Quote:
 Originally Posted by LuckyTran There are certainly skewed cells in that area. Anyway, you know what locations need improvement. Turbulent viscosity ratio > 10^5 is extremely rare and will certainly never occur for practical scenarios. Maybe for superfluids, critical point transitions, or inside a black hole or something... Since you doing a HRSG, I'll say that it's nearly impossible to have that high a viscosity ratio.
Lucky Tran, sorry i forgot. In my simulation, i'm using standard wall function for my wall treatment, but i have tube (wall ) with heat transfer condition (conjugate HT). Can i just use standart wall function or i have to use the other else ?

 Thread Tools Search this Thread Search this Thread: Advanced Search Display Modes Linear Mode

 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 OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post olivier FLUENT 11 October 10, 2015 05:49 Balaji FLUENT 5 May 27, 2014 03:47 mechfloeng FLUENT 4 February 6, 2014 12:45 Adrian FLUENT 12 September 21, 2011 04:22 Fan Main CFD Forum 10 September 9, 2006 12:24

All times are GMT -4. The time now is 15:02.

 Contact Us - CFD Online - Privacy Statement - Top