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Old   November 19, 2012, 05:55
Default MRFSimpleFoam and BCs
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Dear all,

I have some questions regarding a simple case for MRFSimpleFoam.
I have a cylindrical domain in which one side is inlet, the other is outlet and in it there is a small fan that is rotating.
Ideally, I want to let the fan determine the input flowfield.
For this problem I used laminar flow which I plan to change it to turbulent when the inlet velocity is determined.

I tried to use the following boundary conditions:

Inlet
U: zeroGradient
p: zeroGradient

Outlet
U: zeroGradient
p: fixedValue

Walls
U: fixedValue 0
p: zeroGradient

With the above settings I cannot get convergence and in the flow field there are backflows happening in various areas of the domain, which seems logical due to U zeroGradient at inlet and outlet.
I also tried to use pressureInletVelocity for U at the inlet but the results were worse.

If I impose U: fixedValue at the inlet, then everything works and I get convergence quite quickly.

Any ideas on this?
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Old   November 19, 2012, 06:30
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zaynah K.
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hi
have you been able to solve that?
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Old   November 19, 2012, 08:34
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I was able to solve the problem only when I had defined an inlet velocity.
I have not been able to solve the case if I want the fan to determine the inflow.
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Old   November 19, 2012, 09:21
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hi can you please explain to me how you had defined the inlet vel?
thnaks
zaynah
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Old   November 19, 2012, 11:04
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for the case that worked you can either define e.g.

inlet
{
type fixedValue;
value uniform (1 0 0);
}

or

inlet
{
type surfaceNormalFixedValue;
refValue uniform -1;
}

both of the above work.
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Old   November 19, 2012, 11:06
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the (1 0 0)

1 define 1m/s in x drection right?
but what does the
Quote:
type surfaceNormalFixedValue;
refValue uniform -1;
means?
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Old   November 19, 2012, 11:14
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Quote:
Originally Posted by zaynah04 View Post
the (1 0 0)

1 define 1m/s in x drection right?
but what does the means?
surfaceNormalFixedValue uses the outward normal vector (the vector normal to your inlet, pointing outside of the domain). If you give it a value of -1, it should give you a velocity of 1 m/s into your cylindrical domain.
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Old   November 19, 2012, 22:21
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Pretty easy to let the fan determine the inflow.

U
Code:
    inlet
    {
       type     pressureInletVelocity;
       phi      phi;
       value    uniform (0 0 0);
    }
    outlet
    {
        type            inletOutlet;
        inletValue      uniform (0 0 0);
        value           uniform (0 0 0);
    }
p
Code:
    inlet
    {
        type            totalPressure;
        p0              uniform 0;
        phi             phi;
        U               U;
        rho             none;
        psi             none;
        gamma           1.0;
    }
    outlet
    {
        type            fixedValue;
        value           $internalField;
    }
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Old   November 21, 2012, 08:05
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Linnemann thank you very much.
totalPressure and pressureInletVelocity worked very well.

Maybe I could have realized it after studying a little more the manual.

From what I understand, totalPressure BC is linked with pressureInletVelocity, pressureInletOutletVelocity, etc conditions or the opposite?
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Old   November 22, 2012, 14:09
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Hi Linnemann,
The explained boundary conditions are fine, but i have small doubt.
how to define are where to define the quantity of the flow.

If i want to define 5 m3/s. how to define this with the above given example BC.


Thanks,
Aadhavan
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Old   November 22, 2012, 20:57
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Hi

With the above BC's you cant define the volume flow.

Then you switch to the below BC's

U
Code:
    inlet
    {
        type              flowRateInletVelocity;
        flowRate  constant 0.5; // m3/s for incompressible and kg/s for compressible
        value             uniform (0 0 0);
    }
    outlet
    {
        type            zeroGradient;
    }
p
Code:
    inlet
    {
        type        zeroGradient;
    }
    outlet
    {
        type            fixedValue;
        value           $internalField;
    }
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Old   November 23, 2012, 02:03
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Hi Linnemann,
Thanks for your reply, This BC already i am using.
I want to use pressureInletVelocity BC, if I use this, how can I understand that, at what flow rate it is the simulation is going on.

in my case the fan is designed for 5m3/s, if I want to see the performance at different flowRate, can I use this BC (pressureInletVelocity), if so how can I implement it.


thanks,
Aadhavan

Last edited by Aadhavan; November 23, 2012 at 14:58.
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Old   November 26, 2012, 09:30
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Quote:
Originally Posted by linnemann View Post
Hi

With the above BC's you cant define the volume flow.

Then you switch to the below BC's

U
Code:
    inlet
    {
        type              flowRateInletVelocity;
        flowRate  constant 0.5; // m3/s for incompressible and kg/s for compressible
        value             uniform (0 0 0);
    }
    outlet
    {
        type            zeroGradient;
    }
p
Code:
    inlet
    {
        type        zeroGradient;
    }
    outlet
    {
        type            fixedValue;
        value           $internalField;
    }


Hi Linnemann,
i have some doubts with the U value of inlet. if write it like :
type fixedValue;
value uniform (0.5 0 0);
OK?

if i have a model like this: inlet U=0.5, outlet contact the atmosphere. the fluid flow from the inlet to the outlet.
Attacted are the Pd and U , run the interFoam, wrong... can you give me some advises?P.jpg

U.jpg
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