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Can anybody please check my boundary conditions?

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Old   August 3, 2011, 04:47
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Sukanta Rakshit
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Why the outlet pressure is set fixedValue = zero? It should be zeroGradient.
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Old   August 3, 2011, 06:53
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Thank you Sukanta.

I changed the pressure boundary condition as the following:

Code:
boundaryField
{
    inlet
    {
        type            fixedValue;
        value           uniform 0;
    }

    outlet
    {
	type            zeroGradient;
    }
Now the overall flow field is more reasonable but the velocity is too high at some points and I still have some strange feature (see pic attached).

What do you think?


Thank you!
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Old   August 3, 2011, 07:00
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Balakrshnan Ramakrishnan
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Quote:
Originally Posted by srakshit View Post
Why the outlet pressure is set fixedValue = zero? It should be zeroGradient.
If the velocity is specified at the inlet, usually we take the case that outlet is at atmospheric pressure.
Since we use relative scale for pressure, we assume the atmospheric pressure is zero.
The outlet is to be set to zero if it is at atmospheric pressure.
If you get a pressure below zero, its less than atmospheric pressure.

I hope I am correct.
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Old   August 3, 2011, 07:04
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But why inlet is fixed at zero now? Change in to zeroGradient

You have 30 m/s constant flow. Just tell me one thing if U at inlet is 30 m/s and Pressure is zero in that patch...is it consistent?
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Old   August 3, 2011, 07:09
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Quote:
Originally Posted by srakshit View Post
But why inlet is fixed at zero now? Change in to zeroGradient

You have 30 m/s constant flow. Just tell me one thing if U at inlet is 30 m/s and Pressure is zero in that patch...is it consistent?
I am not able to make the mesh given in this case since I use OF-1.7.x.
But from blockMeshDict I can say there is something wrong with naming the patches.
I guess in this case the patches inlet and outlet are swapped. But I am not sure.


If the inlet is set to some velocity, say 30 m/s, the pressure is set to zeroGradient.
Also if the outlet is set to be with a pressure ( fixedValue )of zero , then the inlet
should be zeroGradient. I do this in my cases and I get good results
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Old   August 3, 2011, 07:10
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Quote:
If the velocity is specified at the inlet, usually we take the case that outlet is at atmospheric pressure.
Since we use relative scale for pressure, we assume the atmospheric pressure is zero.
The outlet is to be set to zero if it is at atmospheric pressure.
If you get a pressure below zero, its less than atmospheric pressure.

I hope I am correct.
I do not think the pressure specified in the solver is relative. So u should either keep it floating or keep the absolute pressure.
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Old   August 3, 2011, 07:21
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Quote:
Originally Posted by srakshit View Post
I do not think the pressure specified in the solver is relative. So u should either keep it floating or keep the absolute pressure.
The solver uses Gauge pressure - reference to the atmospheric pressure- where atmospheric pressure is zero. In absolute pressures negative value is not possible(except in some cases like trees). But in our simulation we get negative pressures all the time. So it cannot be absolute pressure. whatever we do in normal CFD(everyday) is Gauge pressure.

Hope this helps
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Old   August 3, 2011, 07:48
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1. I had to change the inlet condition also because otherwise I got an error which basically said that the solver couldn't find any reference cell for pressure.

2. Pressure is relative, so (I think) you can have 30m/s velocity with 0 pressure because that mean that the pressure s the same as ambient static pressure

3. Inlet and outlet are fine, just checked them in paraview.

4. I ran the simulation for another 1000 iterations and the strange behavior underlined in previous pic disappeared but I'm still getting way too high velocity…
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Old   August 3, 2011, 07:52
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@lovecraft22

Why is there a zero velocity line (vertical streak of blue)at the inlet? is it due to your boundary conditions?
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Old   August 3, 2011, 07:56
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its because the velocity = 30 m/s at inlet.

what is the outlet velocity boundary in this case?
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Old   August 3, 2011, 07:56
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May be you can have a look at my simulation -> see attached pic
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Old   August 3, 2011, 07:59
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That's my inlet condition: he velocity is not 0 but 30m/s.

About the outlet, the velocity is around 3000 m/s!!
It's an hypersonic cylinder!
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Old   August 3, 2011, 08:02
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what boundary condition you are using for outlet?

Also what pRefValue you are using?
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Old   August 3, 2011, 08:08
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Quote:
Originally Posted by Balakrshnan Ramakrishnan View Post
May be you can have a look at my simulation -> see attached pic
Could you upload your boundary conditions?

Thank you!
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Old   August 3, 2011, 08:16
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Try these boundary settings

for pressure

inlet
{
type freestreamPressure;
}

outlet
{
type freestreamPressure;
}

for velocity

inlet
{
type freestream;
freestreamValue uniform (30 0 0);
}

outlet
{
type freestream;
freestreamValue uniform (30 0 0);
}
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Old   August 3, 2011, 08:28
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You have to define one pressure value at least.

I set the pressure value to 0 at the inlet and the rest as you suggested.
Now it's running, let's see what comes out.
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Old   August 3, 2011, 08:41
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Quote:
Originally Posted by lovecraft22 View Post
That's my inlet condition: he velocity is not 0 but 30m/s.

About the outlet, the velocity is around 3000 m/s!!
It's an hypersonic cylinder!
Oops i thought you are just doing low velocity simulations.

which solver you use for hypersonic flows?
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Old   August 3, 2011, 08:43
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I was just kidding about the fact that though I'm running a low velocity simulation yet I get an hypersonic velocity at the outlet…
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Old   August 3, 2011, 08:44
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Quote:
Originally Posted by lovecraft22 View Post
I was just kidding about the fact that though I'm running a low velocity simulation yet I get an hypersonic velocity at the outlet…
Lol...

Which version of OF are you using?
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Old   August 3, 2011, 08:46
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OpenFoam 2.0.0
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