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

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

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.

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

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

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

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

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

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?

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.

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?

don't know.( But the porosity of the wall is 0.015) we have to find out from simulation


The wall is completely porous , Not a specific part

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

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

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.

Thank you Dr

I got the simulation what I expected. Now the the simulation is running

Thank you for your kind advice and help

Thank you

Dear all

Please find the attached cross sectional view of artery.The artery length is 100mm
The outer part is porous domain. I introduced free slip between fluid and porous interface . I set fluid - porous interface by using GGI.when I run the simulation I saw momentum and mass -2 is not at all executed . please clarify

Thank you

What is momentum and mass -2? Is that fluid flow in the porous domain?

I think it is fluid flow in the porous domain

So it looks like it is not solving the fluid equations in the porous region. Can you post your CCL?

Dear Dr

Please find the attached my CCL .

I don't know what is the problem
I need to solve this as soon as possible . Please help me in this regards.

Thank you

Reagrds

Where do you get the time step size from? Did you actually do something to show that time step was required or did you just guess?

You have your artery wall set to solid morphology. You probably want this porous (I am not sure about that).

You have the mass momentum model as free slip on the interface. You will want this to be no slip.

Why have you set a max coeff loops of 3? And why a minimum of 1? Remove the min loops and make the max loops something like 10.

Do you need the expert parameter? Have you checked you need it?

I would simplify this model to get the components working. I would model the arterey only (fluid flow only, and the fluid is a newtonian fluid) to make sure the time step and boundaries are working. Then add the porous wall. When that works add the non-newtonian fluid model.

I just guess . I want to show how the shear stress and other parameters changes with respect to time

Yes . I want this as a solid morphology having a porous in nature.
Is it wrong to define solid here? if I remove, will my result vary or not?

I have seen many articles they are using free slip boundary at the interface
Ok I will do

This part I am not sure. your advice is highly needed.


Thank you

Time Step: Do not guess, invariably you will get it wrong. Use adaptive time stepping, with 3-5 coeff loops per iteration. Then the solver will find the correct time step size.

Free slip: sure, you can use free slip but is that what you want? Then you will not get any realistic flow profile in the artery.

Do not put expert parameters in unless you know you need them and you know what they are doing. They are not called expert parameters for nothing.

I am facing the same problem . One cycle completed but there is no fluid flow inside the porous domain.

I changed from transient to steady state analysis, still I did't get.

can I assume porous domain initialization with free slip.


Thank you

Slip or no slip should not matter on the porous interface. If you are getting no flow in the porous region you have a more fundamental problem with your simulation.

I note your permeability is 2e-18[m^2]. I am no expert in porous flows but this sounds pretty low. Wouldn't that pretty much stop flow in the porous region?

I am also thinking . I tried with permeability 1[m^2] . I could not get a flow in the porous domain.

Since I am using turbulent model in the main flow. when the flow enters into the porous region it would be laminar flow . How to handle this ?

Thank you

Rather than randomly trying permabilities, how about working out what the permability actually is?

IT is actually 2e-18[m^2]

OK - so with a permability so low, will you get any flow?

In post #27 I recommended you simplify the model to get the components working. Have you done this?

Yes Dr.
It works fine . When I add a porous layer the momentum and mass for porous region not at all showing any response.
I now changed to transient to steady flow . I set 200 iteration. I could not not see any flow in the porous region.
Now , I am thinking to initialize the porous domain with Cartesian velocity components . But Don't know how to implement with out knowing velocity . Any suggestion

I have already said my suggestion two times - so here it is for a third time. Have you simplified your model (ie just use a newtonian fluid, and a simple geometry) to test that the porous material works as expected for a simple case?

I tested with straight pipe . the wall I consider porous . I simulate with Newtonian flow . I see the same result when I use non Newtonian fluid. Inside the porous domain momentum and mass equation is not solving. I applied free slip boundary condition.

I have a question . at the domain interface is it necessary to introduce source terms


Thank you

No, you should not source terms to model what I understand you are trying to model.

As the porous model is not working on a simpler model I would concentrate on getting it working on the simple model before going back to the full model. I do not know why it is not working for you, but try these things:
1) Try the porous region as a subdomain of the fluid domain rather than a separate domain. This should not need a GGI so you will have to remove that.
2) Change the porosity model options, like loss velocity type, the expert parameter and any others which look interesting. No harm in trying everything.