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January 13, 2006, 06:53 
What is Zero Gradient for Opening bound condition?

#1 
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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. 

January 14, 2006, 05:29 
Zero Gradient boundary condition is better?

#2 
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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 Ke 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. 

January 17, 2006, 16:04 
Re: What is Zero Gradient for Opening bound condit

#3 
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'Zero Gradient' means that the variable is 'fully developed' on the inflow portion of the opening; ie, dphi/dn=0.


January 18, 2006, 03:36 
Re: What is Zero Gradient for Opening bound condit

#4 
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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. 

January 19, 2006, 13:47 
Re: What is Zero Gradient for Opening bound condit

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Sometimes the solution is insensitive to turbulence inflow BCs because the turbulence is often sourcedominated (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 fullydeveloped turbulence profile. Otherwise it is probably best to specify the inlet level. 

February 19, 2010, 11:23 
zero gradient for inlet BC

#6  
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Sudharshani
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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:


February 20, 2010, 06:26 

#7 
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Glenn Horrocks
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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. 

November 2, 2012, 21:31 
zero gradient vs normal to boundary

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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. 

November 3, 2012, 05:36 

#9 
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Glenn Horrocks
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You choose boundary conditions to match the flow you are trying to simulate. That is the first consideration when choosing boundary conditions.


November 3, 2012, 11:17 

#10 
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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.


June 9, 2015, 05:42 
turbulence

#11 
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goku
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How to specify zero turbulence gradient in pressure outlet boundary condition


June 9, 2015, 06:32 

#12 
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Glenn Horrocks
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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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