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-   -   MRFSimpleFoam and BCs (https://www.cfd-online.com/Forums/openfoam-solving/109489-mrfsimplefoam-bcs.html)

flowing November 19, 2012 05:55
MRFSimpleFoam and BCs
 
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?

zaynah04 November 19, 2012 06:30
hi
have you been able to solve that?

flowing November 19, 2012 08:34
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.

zaynah04 November 19, 2012 09:21
hi can you please explain to me how you had defined the inlet vel?
thnaks
zaynah

flowing November 19, 2012 11:04
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.

zaynah04 November 19, 2012 11:06
the (1 0 0)

1 define 1m/s in x drection right?
but what does the
Quote:
type surfaceNormalFixedValue;
refValue uniform -1;
means?

Heroic November 19, 2012 11:14
Quote:
Originally Posted by zaynah04 (Post 393012)
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.

linnemann November 19, 2012 22:21
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;
    }

flowing November 21, 2012 08:05
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?

Aadhavan November 22, 2012 14:09
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

linnemann November 22, 2012 20:57
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;
    }

Aadhavan November 23, 2012 02:03
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

skykzhang November 26, 2012 09:30
2 Attachment(s)
Quote:
Originally Posted by linnemann (Post 393743)
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?Attachment 17248

Attachment 17249


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