CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

What is Zero Gradient for Opening bound condition?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 13, 2006, 06:53
Default What is Zero Gradient for Opening bound condition?
  #1
Pete
Guest
 
Posts: n/a
Hi... I spent hours trying to get the definition of "Zero Gradient" when specifying "Turbulence" for "Opening" boundary condition.

Can anyone please help if you know what the term means?

The new design of the user manual is very louisy....I find it a lot harder to get what I'm looking for these days.

Thanks for help.
  Reply With Quote

Old   January 14, 2006, 05:29
Default Zero Gradient boundary condition is better?
  #2
Pete
Guest
 
Posts: n/a
Hi.... Really hope that someone can clarify this term for me. What is the effect if zero gradient for turbulence is set at outlet opening boundary condition?

I run a few cases with K-e model using 5%, 10% & zero gradient for turbulence at inlet & outlet. It doesn't seem to change velocity & static pressure distribution in my problem. However, I get a message in Pre saying that zero gradient is better. Has anyone found that this advice is true?

If zero gradient means all variables are set to a constant value at outlet boundary, then it can only be true when you extend the outlet location very far away from flow domain.
  Reply With Quote

Old   January 17, 2006, 16:04
Default Re: What is Zero Gradient for Opening bound condit
  #3
Phil
Guest
 
Posts: n/a
'Zero Gradient' means that the variable is 'fully developed' on the inflow portion of the opening; ie, dphi/dn=0.
  Reply With Quote

Old   January 18, 2006, 03:36
Default Re: What is Zero Gradient for Opening bound condit
  #4
Pete
Guest
 
Posts: n/a
Thanks, Phil. In my case, it means turbulent profile will be the same after the outlet. Right? Would you mind to help explaining the reason or giving some references where this condition works better?

I still can't figure out the difference between specifying a constant turbulence intensity and using Zero gradient condition.
  Reply With Quote

Old   January 19, 2006, 13:47
Default Re: What is Zero Gradient for Opening bound condit
  #5
Phil
Guest
 
Posts: n/a
Sometimes the solution is insensitive to turbulence inflow BCs because the turbulence is often source-dominated (production/dissipation).

Other times the inlet levels do matter. If the flow is fully developed, the zero gradient condition makes sense; it will give a fully-developed turbulence profile. Otherwise it is probably best to specify the inlet level.
  Reply With Quote

Old   February 19, 2010, 11:23
Default zero gradient for inlet BC
  #6
New Member
 
Sudharshani
Join Date: Jun 2009
Location: Melbourne,Australia
Posts: 16
Rep Power: 8
Sudharshani is on a distinguished road
hai Phil,
i would like to know what do u mean by fully developed turbulent flow....?
i am using ANSYS CFX to model supersonic CD nozzle and i would like to know can i use zero gradient under the turbulence section ?
bcos when i use two different options (zero gradient and mediem intensity and eddy viscosity) i got 0.01 diffrence in the mach number values . and also the shock formation inside the nozzle is also diffrent using zero gradient option.

can u expalain when and why we should use "zero gradient".option?
thanking you
sudhar
Quote:
Originally Posted by Phil
;74607
Sometimes the solution is insensitive to turbulence inflow BCs because the turbulence is often source-dominated (production/dissipation).

Other times the inlet levels do matter. If the flow is fully developed, the zero gradient condition makes sense; it will give a fully-developed turbulence profile. Otherwise it is probably best to specify the inlet level.
Sudharshani is offline   Reply With Quote

Old   February 20, 2010, 06:26
Default
  #7
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,809
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
I think you will find Phil has long since past away, this post is over 4 years old. But no matter.

Fully developed flow is flow where the gradient of the variable in the flow direction is zero. That is the variable does not change as the flow progresses as it is "fully developed".

I don't think a zero gradient boundary is appropriate. Some level of upstream turbulence will be present and it looks like in your case it is affecting results so you need to make sure you get it right. What incoming turbulence level are you modelling? You should use this as your upstream turbulence condition.

A second possibility is if minor changes like this change your results then I recommend you move your upstream boundary further away from the region of interest.
ghorrocks is offline   Reply With Quote

Old   November 2, 2012, 21:31
Default zero gradient vs normal to boundary
  #8
New Member
 
Join Date: Apr 2011
Posts: 5
Rep Power: 6
elisun is on a distinguished road
Hi.

I'm doing both steady state and transient analysis for a given case. For the steady state I give constant mass flow at the inlet and pressure at the outlet, and I run it. Then I use the resulted inlet pressure as an inlet boundary for the transient case, and I run the transient case. Then I check the mass flow at the inlet. The mass flow would be oscillating.
I read in cfx manual that we choose the direction of flow to be normal to boundary when we have constant mass flux at inlet.
So my question is should I use zero gradient or normal to boundary for each steady state run and transient run, since I have constant mass flux for steady state but oscillating mass flux for transient. Would it be ok if I use normal to boundary for steady and zero gradient for transient?

Your help is highly appreciated.
elisun is offline   Reply With Quote

Old   November 3, 2012, 05:36
Default
  #9
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,809
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
You choose boundary conditions to match the flow you are trying to simulate. That is the first consideration when choosing boundary conditions.
ghorrocks is offline   Reply With Quote

Old   November 3, 2012, 11:17
Default
  #10
New Member
 
Join Date: Apr 2011
Posts: 5
Rep Power: 6
elisun is on a distinguished road
Right. Which in my case for steady state, normal to boundary and for transient, zero gradient match the flow. I just didn't want there to be any inconsistency between steady state and transient boundary conditions.
elisun is offline   Reply With Quote

Old   June 9, 2015, 05:42
Default turbulence
  #11
New Member
 
goku
Join Date: Jun 2015
Posts: 5
Rep Power: 2
naruto5255 is on a distinguished road
How to specify zero turbulence gradient in pressure outlet boundary condition
naruto5255 is offline   Reply With Quote

Old   June 9, 2015, 06:32
Default
  #12
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,809
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The documentation states this clearly - the exit boundary condition for all convected scalars (including turbulence) is convected from inside the domain. This is a more accurate boundary condition than zero normal gradient.
ghorrocks 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
Periodic boundary condition Arif FLUENT 2 January 17, 2014 07:48
zero-diffusion (constant gradient) boundary condition thomas99 OpenFOAM 4 July 29, 2011 04:59
Calculation of pressure gradient in periodic boundary condition ksaat FLUENT 7 May 16, 2011 03:59
CFX Solver : Sudden crash Hervé CFX 2 June 16, 2008 06:40
How to compute gradient for non-orthogonal grids? Paul Hsieh Main CFD Forum 3 November 11, 2003 05:52


All times are GMT -4. The time now is 11:08.