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-   -   What is Zero Gradient for Opening bound condition? (http://www.cfd-online.com/Forums/cfx/22039-what-zero-gradient-opening-bound-condition.html)

Pete January 13, 2006 06:53

What is Zero Gradient for Opening bound condition?
 
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.

Pete January 14, 2006 05:29

Zero Gradient boundary condition is better?
 
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.

Phil January 17, 2006 16:04

Re: What is Zero Gradient for Opening bound condit
 
'Zero Gradient' means that the variable is 'fully developed' on the inflow portion of the opening; ie, dphi/dn=0.

Pete January 18, 2006 03:36

Re: What is Zero Gradient for Opening bound condit
 
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.

Phil January 19, 2006 13:47

Re: What is Zero Gradient for Opening bound condit
 
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 February 19, 2010 11:23

zero gradient for inlet BC
 
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.


ghorrocks February 20, 2010 06:26

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.

elisun November 2, 2012 21:31

zero gradient vs normal to boundary
 
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.

ghorrocks November 3, 2012 05:36

You choose boundary conditions to match the flow you are trying to simulate. That is the first consideration when choosing boundary conditions.

elisun November 3, 2012 11:17

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.

naruto5255 June 9, 2015 05:42

turbulence
 
How to specify zero turbulence gradient in pressure outlet boundary condition

ghorrocks June 9, 2015 06:32

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.


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