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Old   July 29, 2013, 16:41
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Quote:
Originally Posted by sh.d View Post
hi dear mauricio
thanks for your answer.
But I did not answer my question:
What is the difference between value and tangential velocity?Is this value scalar or vector?
hi
i cannot access OF right now but i guess that this value is a scalar . The tangential velocity might be a scalar or a vector depending on what OF is asking. i cant check the files atm. when i reach them ill try to post a better reply.
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Old   September 14, 2013, 10:10
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Quote:
Originally Posted by calim_cfd View Post


hi
pressureinletoutletvelocity (in) bcs are to be used with inletoutlet (out) bcs for the u field

and you use totalpressure(in) and fixedvalue(out) for the p field

these bcs guarantee a well-posed problem in case the user has only pressure measures and backflow expected

try that
l8r and gl
Hey, regarding calim_cfd's post, what if I just have the static pressure and definitely need this pressure on my inlet surface (not calculated backwards from velocity), is there a suitable and clean way to make the inlet patch fixedValue in the pressure field? Using the other bcs above?

I also use simpleFoam, so I want to keep rho out of my calculation until postprocessing.
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Old   October 4, 2013, 17:46
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Hi All,

About the boundary condition presureinletoutletvelocity, I have a question:

for outflow, zero gradient, ok, it is clear.

for inflow, from the description we can see that for inflow the velocity is obtained from the patch-face normal component of the internal-cell value. Actually it is also zero gradient! So what is the difference between this case and outflow?

Any comments?

Quote:
Originally Posted by calim_cfd View Post
the bcs i mentioned they kinda form a consistent pair and regarding the usage,, i guess you can use it for outlets, but then you have to handle the signs..

here's the description, guess you've seen it already.. i haven't tried it thou

pressureInletOutletVelocity
Code:
Description
    Velocity inlet/outlet boundary condition patches for where the pressure is
    specified.  zero-gradient is applied for outflow (as defined by the flux)
    and for inflow the velocity is obtained from the patch-face normal
    component of the internal-cell value.
pls give it a try if u have the time and keep us post!
ty
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Old   October 9, 2013, 00:08
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B T KANNAN
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Velocity is a vector quantity.
Use it as shown below...

tangentialVelocity uniform (0 0 0); // this is an additional condition that you can specify on a patch
--
KANNAN
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Old   December 10, 2013, 17:42
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Quote:
Originally Posted by Aadhavan View Post
Hi Fomers and Calim;

I am simulating a fan using MRFSimpleFoam,
I have given the boundary condition as follows,
0/U

inlet
{
type flowRateInletVelocity;
flowRate 2;
value uniform (0 0 0);
}
outlet
{
type zeroGradient;
}

0/P

inlet
{
type zeroGradient;
}

outlet
{
type fixedValue;
value uniform 101325;
}

the simulation ran upto 10000 Iteration,
I have attached the Residual plot as well, please have a look.
but the result is not matching with experimental result.

actually inlet pressure should be less than outlet pressure but I an getting other way around.

I am trying to fix this issue by giving different BC as follows,

0/U

outlet
{
type zeroGradient;
}
inlet
{
type pressureInletOutletVelocity;
value uniform (0 0 0)

0/p

inlet
{
type totalPressure;
gamma 0;
p0 uniform 101325; // total pressure
value uniform 101325;
}

outlet
{
type fixedValue;
value uniform 101260.5; // static pressure
}

while execute the solver, I am getting the following error,

Create mesh for time = 0

Reading field p

Reading field U



--> FOAM FATAL IO ERROR:
Cannot find patchField entry for cyclic ILR0
Is your field uptodate with split cyclics?
Run foamUpgradeCyclics to convert mesh and fields to split cyclics.

file: /home/cerecam/OpenFOAM/OpenFOAM-2.0.1/tutorials/incompressible/MRFSimpleFoam/exCompDomain/onlyPressure/0/U::boundaryField from line 25 to line 76.

From function GeometricField<Type, PatchField, GeoMesh>::
GeometricBoundaryField::GeometricBoundaryField
(
const BoundaryMesh&,
const DimensionedField<Type, GeoMesh>&,
const dictionary&
)
in file /home/cerecam/OpenFOAM/OpenFOAM-2.0.1/src/OpenFOAM/lnInclude/GeometricBoundaryField.C at line 271.

FOAM exiting


please guide me how to approach this problem,
I am really struggling to solve this problem.
please help me,

thanks,
Aadhavn
Hi Aadhavn,
i simulate the centrifugal fan as well and get higher static pressure at inlet and lower static pressure at outlet. I set the massflow at inlet with zeroGradient pressure and at outlet zeroGradient velocity and fixedValue pressure zero.

have you or anyone now how to fix this ?

thanks
-nash
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Old   January 23, 2014, 07:36
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Hi All,

If the inlet and outlet are very far from the flow of interest like the airfoil here, I mean the computational domain is large compared to the airfoil, Can I use pressureinletoutletvelocity (for U) and totalPressure (for p) for both inlet and outlet?

Because I use the above options but near the inlet BC there are always unphysical solutions like very large velocity, sometimes like a vortex (positive and negative appearing at the same time there). I am not sure if this is caused by the not reason BC setting.

Thank you for any comments.

Quote:
Originally Posted by calim_cfd View Post


hi
pressureinletoutletvelocity (in) bcs are to be used with inletoutlet (out) bcs for the u field

and you use totalpressure(in) and fixedvalue(out) for the p field

these bcs guarantee a well-posed problem in case the user has only pressure measures and backflow expected

try that
l8r and gl
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Old   January 23, 2014, 10:22
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It is possible that your boundary condition setting give you unphysical results. In my work I tried this kind of boundary condition setting (pressureInletOutletvelocity for inlet and outlet) and they give me wrong results as you.

what is your physical inlet boundary condition? If is a flux that enter in the domain there isn't reason to set a pressureInletOutletVelocity in the inlet for U.
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Old   January 23, 2014, 10:29
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Actually it is an open atomshperic inlet (or strictly it is not an inlet). Sometimes the flow can enter the domain but sometimes it will flow out. How does your unphysical look like? Mine is always: unphysical pressure (very large) and so unphysical velocity. Actually the flow there should be very 'quiet'. Thank you.
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Old   January 23, 2014, 10:42
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I had an unphysical reverse flow but the value of p and U are ok.

What is your outlet?The same of the inlet?
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Old   January 23, 2014, 10:44
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When I use the same condition for inlet and outlet, i.e. pressureinletOutletVelocity for U and totalPressure for p, the flow near the outlet is correct. The unphysical problem lies near the inlet.

I think reverse flow from this kind of BC is normal......
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Old   January 23, 2014, 11:05
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not in my case, but my simulation had also mesh problem so I'm not sure that the problem is the boundary condition setting.. In any case, for a well posed problem you have to set the pressure in one side and the U in the other.. With your boundary condition setting you set the pressure both in inlet and outlet...Try to change, if possible, the outlet boundary condition
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Old   January 23, 2014, 11:08
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Thank you for your suggestions.

Now I am trying:

for inlet:

U: pressureinletOutletVelocity
p: totalPressure

for outlet:
U: inletOutlet
p: zeroGradient

The case is running and not sure this will improve it. Theoretically speaking this should be fine. Did you try the above options in your simulations?
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Old   January 23, 2014, 11:41
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I didn't try their because I decided to change the boundary and put a wall on one hand (I haven't the necessity tho have two open side in my simulation). I'm not sure that your new setting work properly beacause when there is outflow in both boundaries the soler setted the pressure in both side.

Say me if with this setting the solver work without problem
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Old   January 23, 2014, 12:31
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Hi giack,

If I use the following setting for the totalPressure:

Code:
 //        type            totalPressure;
//        p0              uniform 0;
//        U               U;
//        phi             phi;
//        rho             none;
//        psi             none;
//        gamma           1.4;
//        value           uniform 0;
So this options correspond to the following operation in the source code:

Code:
operator==(p0p - 0.5*(1.0 - pos(phip))*magSqr(Up));
Here what is pos(phip)? I am a little confused about this.
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Old   January 27, 2015, 04:45
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Quote:
Originally Posted by openfoammaofnepo View Post
Code:
operator==(p0p - 0.5*(1.0 - pos(phip))*magSqr(Up));
Here what is pos(phip)? I am a little confused about this.
It is meant to act like a boolean. It checks for the sign of phip - i.e. the flux on the patch. If the flux is positive or zero pos(phip) will become "1" otherwise it'll be "0". So depending on the direction of the flux it's either...

operator = p_{0p} - 0.5 \cdot [(1 - 1) \cdot |U_p|^2 ] = p_{0p}

... or ...

operator = p_{0p} - 0.5 \cdot [(1 - 0) \cdot |U_p|^2 ] = p_{0p} - 0.5 \cdot |U_p|^2

As a positive flux should indicate a flux out of the domain it'll just apply p_{0p} while if there's a flux into the domain it'll subtract the U_p term from p_{0p}. I guess that's why this BC is suitable for being used for a velocity inlet as it changes the pressure according to U for an inward flux...
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Old   January 27, 2015, 05:48
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Thank you. I got the reply one year later!
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Old   July 28, 2016, 10:38
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Quote:
Originally Posted by calim_cfd View Post


hi
pressureinletoutletvelocity (in) bcs are to be used with inletoutlet (out) bcs for the u field

and you use totalpressure(in) and fixedvalue(out) for the p field

these bcs guarantee a well-posed problem in case the user has only pressure measures and backflow expected

try that
l8r and gl
does this have to be used with a multiphase case or with just one?
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