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-   -   help me to set suitable outlet boundary condition (http://www.cfd-online.com/Forums/cfx/120720-help-me-set-suitable-outlet-boundary-condition.html)

shaswat July 12, 2013 21:51

help me to set suitable outlet boundary condition
 
Hi
I am simulating blood flow through multiple stenosis condition.

( currently I set two serial stenosis separated few mm apart)

I know the inlet velocity and time averaged pressure which I have taken from journal.

The inlet velocity u(t)= Uave+Uave*sin(4*pi*f*t-(pi/2))
measured Time average pressure proximal to the stenosis is 89 mm Hg



I have only these data. If I set inlet velocity u(t) at the inlet ,what is the boundary conditions at the outlet . what is the suitable initial condition for the transient flow analysis?.

(I am very concern about pressure drop at each stenosis).

Thank you
Shashwat

ghorrocks July 13, 2013 07:27

If you do not know the conditions at the exit boundary then you cannot perform a simulation. You MUST know the conditions at the exit to perform a simulation. So if you do not know the conditions there you must move the exit boundayr to somewhere you do know them.

shaswat July 13, 2013 11:47

Quote:

Originally Posted by ghorrocks (Post 439512)
If you do not know the conditions at the exit boundary then you cannot perform a simulation. You MUST know the conditions at the exit to perform a simulation. So if you do not know the conditions there you must move the exit boundayr to somewhere you do know them.

Thanks Dr Glenn
my professor told that put "out flow boundary condition" But I have read in one of the articles "stress free boundary condition" applied at the outlet.

[A stress free boundary condition was specified at
the outlet. Adequate distal length was ensured for
accurate determination of pressure drops due to the
lesion and for the convergence of the calculations-Guidewire flow obstruction effect on pressure drop-flow relationship in moderate coronary artery stenosis]

I have confused with the outlet boundary condition. what is outflow and stress free conditions? Please help me to put suitable boundary conditions

ghorrocks July 13, 2013 18:42

If you only want to know the pressure drop (and the flow is incompressible) then pressure is relative. This means you can specify any pressure you like so it might as well be zero.

There are many different types of outflow conditions. You can use specified pressure with convective conditions for other variables (this is what CFX uses), some people use zero normal gradient and some people use other conditions (eg stress free). In the end they tend to be very similar and rarely make any significant difference to results.

shaswat July 14, 2013 22:23

1 Attachment(s)
Thank you for your reply
Please find the attached cross sectional view of artery.The artery length is 100mm

I have considered two cases
1.Rigid artery ( wall has no permeable)
2. wall having permeable ( Porous domain)

I have confused with setting boundary conditions in the case 2 ( porous domain).
I set fluid - porous interface by using GCI.
I set other portions as wall and no slip condition was executed.
is it correct?

I also tried free slip boundary conditions, but solver return with error immediately

your advice is important to proceed further

Thank you

ghorrocks July 14, 2013 22:53

Can you successfully run simulations with no FSI or mesh motion? Simplify your simulation down to what does run, then add the complexity one bit at a time - checking it works every time you add something new.

shaswat July 14, 2013 23:38

Quote:

Originally Posted by ghorrocks (Post 439706)
Can you successfully run simulations with no FSI or mesh motion? Simplify your simulation down to what does run, then add the complexity one bit at a time - checking it works every time you add something new.

Thank you so much

I set now outlet boundary as areaAve(Pressure)@outlet

what does it mean?
Is it relative pressure of the outlet from the inlet pressure?
If it is correct no need to put any value of pressure at the outlet. Am I correct?


Please clarify

next,

"Before applying the transient pressure and flow,
The models were axially stretched by 10% of the initial length and pressurized to a mean physiologic pressure of 89.04 mmHg to
account for residual stresses".The above statement I have taken from one of the journals . I need your help to understand this statement in cfx point of view.

Thank you

Thank you so much

ghorrocks July 15, 2013 20:29

Why have you set the outlet pressure to be the average of the outlet pressure? This sounds wrong to me - you should set it to the known fixed outlet pressure value (probably 0Pa).

shaswat July 16, 2013 05:33

Quote:

Originally Posted by ghorrocks (Post 439895)
Why have you set the outlet pressure to be the average of the outlet pressure? This sounds wrong to me - you should set it to the known fixed outlet pressure value (probably 0Pa).


Thank you for your reply

Dear Dr Ghorrocks

I introdce transient velocity at the inlet and

I introduce the outlet boundary condition ( may be wrong) as shown below



BOUNDARY: out
Boundary Type = WALL
Location = F43.41
BOUNDARY CONDITIONS:
MASS AND MOMENTUM:
Option = Specified Shear
SHEAR STRESS:
Option = Cartesian Components
xValue = 0 [Pa]
yValue = 0 [Pa]
zValue = 0 [Pa]
END

Kindly give your comments on it. what is wrong? or which condition shall I use this?

Thank you

ghorrocks July 16, 2013 18:53

You are using a wall as your outlet boundary !!?? This is obviously a bad idea.

shaswat July 17, 2013 02:54

Quote:

Originally Posted by ghorrocks (Post 440152)
You are using a wall as your outlet boundary !!?? This is obviously a bad idea.

Thank you

Kindly refer the attachment diagram . I am really really struggle to set inlet and outlet of the porous domain. Since the fluid is flowing through the luman ( fluid domain), I am struggling to set the boundary condition at the inlet and outlet of the porous domain ( artery wall)
I assume that the flow will come down when it is flow along the porous wall. I activated buoyant flow in the y direction ( flow is in the Z direction) Is it necessary to activate buoyant flow?
I set inlet velocity =0 m/s and outlet= 0 pa for the porous domain.

Please help me to put suitable boundary conditions on the porous domain

Thank you

ghorrocks July 17, 2013 03:00

If the inlet is 0m/s there is no flow, so of course the pressure is 0Pa.

shaswat July 17, 2013 04:44

Quote:

Originally Posted by shaswat (Post 440211)
Thank you

Kindly refer the attachment diagram . I am really really struggle to set inlet and outlet of the porous domain. Since the fluid is flowing through the luman ( fluid domain), I am struggling to set the boundary condition at the inlet and outlet of the porous domain ( artery wall)
I assume that the flow will come down when it is flow along the porous wall. I activated buoyant flow in the y direction ( flow is in the Z direction) Is it necessary to activate buoyant flow?
I set inlet velocity =0 m/s and outlet= 0 pa for the porous domain.

Please help me to put suitable boundary conditions on the porous domain

Thank you


Thank you Dr
If I introduce symmetry boundary condition at the inlet surface and outlet surface , what will be the effect?.

I know that the flow is parallel to the symmetry boundary condition

If the fluid enters through porous wall and due to Leakey on the surface the flow will comes out but the flow is not perpendicular to the surface . It should be parallel to the surface . DO you think that the symmetry boundary condition is suitable for this instead of putting inlet = 0 m/s and pressure 0 pa at the outlet.
what do you think about if I activate buoyant flow option?

ghorrocks July 17, 2013 06:22

Your last post seems to suggest you have no idea how to model this at all. In this case can you post an image of what you intend to model - all of it, not just one little bit - and show what the flow path is intended to be?

shaswat July 17, 2013 08:34

3 Attachment(s)
Quote:

Originally Posted by ghorrocks (Post 440266)
Your last post seems to suggest you have no idea how to model this at all. In this case can you post an image of what you intend to model - all of it, not just one little bit - and show what the flow path is intended to be?

Dear Dr Ghorrocks
Please find the attached model of our studies.
fig1-fluid domain , fig 2- artery Porous domain, fig 3 sectional view of combined fluid domain and porous domain.
We consider blood flow through multiple stenosed artery.currently we are taking single stenosis case.
We have two cases;
1. Blood flow through rigid artery wall
2. blood flow through artery wall which is porous in nature.
we are examine the pressure drop across the stenosis for the above cases under same physiologic transient velocity and pressure condition.
we found from simulation, pressure drop_in rigid artery< pressure drop in porous artery.
But my lecturer told in other way around .

so, I confused with boundary conditions setting at the porous domain
please help me to set the boundary conditions in overall.

I need your advice and help in this regard please.

ghorrocks July 17, 2013 18:49

The fluid flow path is important. How much flow goes through the walls? And what is the source of the fluid coming through the walls (ie everywhere, or a specific location)? How much through the inlet and outlet? Any other flow sources or sinks?

shaswat July 17, 2013 19:40

Quote:

The fluid flow path is important. How much flow goes through the walls?



And what is the source of the fluid coming through the walls (ie everywhere, or a specific location)?




How much through the inlet and outlet? Any other flow sources or sinks?




Quote:

How much flow goes through the walls?
don't know.( But the porosity of the wall is 0.015) we have to find out from simulation

Quote:

And what is the source of the fluid coming through the walls (ie everywhere, or a specific location)?

The wall is completely porous , Not a specific part

Quote:

How much through the inlet and outlet? Any other flow sources or sinks?
The average inlet velocity is 0.2 m/s

For your information
we use SST model . we are expecting low Re turbulence
Wall thickness 1mm

POROSITY MODELS:
AREA POROSITY:
Option = Isotropic
END
LOSS MODEL:
Loss Velocity Type = Superficial
Option = Isotropic Loss
ISOTROPIC LOSS MODEL:
Option = Permeability and Loss Coefficient
Permeability = 2e-18 [m^2]
Resistance Loss Coefficient = loss coefficient
END
END
VOLUME POROSITY:
Option = Value
Volume Porosity = 0.15
END
END
SOLID DEFINITION: artery
Material = Artery wall
Option = Material Library
MORPHOLOGY:
Option = Continuous Solid
END
END


Thank you

ghorrocks July 18, 2013 08:57

The the inlet should be 0.2m/s (or a time function with an average of 0.2m/s), the outlet a pressure boundary at 0Pa and the other wall of the vessel has some function to control the amount of fluid which goes through the wall.

shaswat July 18, 2013 19:37

Quote:

Originally Posted by ghorrocks (Post 440534)
The the inlet should be 0.2m/s (or a time function with an average of 0.2m/s), the outlet a pressure boundary at 0Pa and the other wall of the vessel has some function to control the amount of fluid which goes through the wall.

Thank you Dr
Can you please explain bit about the "some function to control the amount of fluid which goes through the wall". We interface fluid porous . what else required to define?
Thank you

ghorrocks July 19, 2013 06:13

That has to start with you - you say fluid goes through the porous wall all over. So what controls it? You need some function to drive it. A defined flux? Maybe a concentration gradient? Maybe a constant value? So "some function" is a vague reference to the wide variety of functions you can use to define this flow.


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