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Cyberholmes July 27, 2011 11:34

Outlet Problems
 
Hello,

I am experiencing a problem with the outlet in my system. The outlet is a perfect circle in a perfectly spherical chamber, and at the beginning it works exactly as it should, but then it behaves strangely. In the center, it is still an outlet, but a ring around the center becomes an inlet. Eventually, the entire outlet becomes an inlet (although the outer ring is still blowing stuff into the chamber faster than the center). I have posted my T, U, and p boundary and initial conditions below. If anyone could help me out with this, I would really appreciate it.

T:

Code:

internalField  uniform 300;

boundaryField
{
      Hotspot
      {
            type            zeroGradient;
      }
      Outlet
      { 
            type            zeroGradient;
      }
      Nozzle
      {
            type            fixedValue;
            value            uniform 310;
      }
      NozzleTip
      {
            type            zeroGradient;
      }
      ChamberWall
      {
            type            fixedValue;
            value            uniform 310;
  }
}

U:

Code:

internalField  uniform (0 0 0);

boundaryField
{
      Hotspot
      {
            type            fixedValue;
            value            uniform (0 0 0);
      }
      Outlet
  {
              type            inletOutlet;
              inletValue        uniform (0 0 0);
              value            $internalField;
        }
        Nozzle
        {
              type            fixedValue;
              value            uniform (0 0 0);
        }
    NozzleTip
        {
              type            zeroGradient;
        }
    ChamberWall
        {
              type            fixedValue;
              value            uniform (0 0 0);
    }
}

p:

Code:

internalField  uniform 266645;

boundaryField
{
      Hotspot
      {
            type            zeroGradient;
      }
      Outlet
      {
    type            outletInlet;
            outletValue  uniform 266645;
            value          $internalField;
      }
      Nozzle
      {
            type            zeroGradient;
      }
      NozzleTip
      {
            type            zeroGradient;
      }
      ChamberWall
      {
            type            zeroGradient;
      }
}

Thank you.

Cyberholmes July 29, 2011 11:35

Can anyone shed any light on this?

Cyberholmes August 2, 2011 10:59

I hate to keep shamelessly bumping this to the top, but I really have very little time in which I need to figure this out, and I don't have a clue what is wrong. I can tell you more now: that after some time my outlet becomes very cold (55 K) and the pressure and the temperature at the outlet are "striped", in that every other plane of cells parallel with the outlet is very cold or low pressure, while the others are hot or high pressure.

Thank you for your help.

kwardle August 4, 2011 10:41

It helps to let people know what solver you are running...

Depending on your solver, I don't think you want outletInlet for p. You might try totalPressure for p combined with pressureInletOutletVelocity for U. In fact, you might try changing the BC on U first with p as you have it and see how that changes what you observe. It seems to be a mismatch with your U/p BC types.

BTW, do you have an inlet somewhere or just an outlet? It seems strange that you have zeroGradient for U on NozzleTip. Again, for people to give useful help we need a little more info. Otherwise, most will just skip on by...while you bump, bump, bump

Cyberholmes August 4, 2011 11:29

Thank you for your response!

I am using an altered version of rhoReactingFoam, which has been changed to include an explicit heat source (I use a file in the constant directory called energySourceProperties to set a constant power input to a region, which acts as a heat source).

I do have an inlet in the chamber, which is in the form of a small nozzle protruding from one side of the chamber. The outlet is much larger in diameter than the inlet and is at right angles to the direction of the inlet.

I have more information now as well. The heat source that I am using, which is at the center of the chamber, is a very small sphere. What I am actually doing is that I have created a small spherical boundary at the center, because I want to model the interaction of the fluid with this surface, and I am creating a heat source which is a spherical shell around this spherical boundary. I have set the BCs for this physical boundary as follows

U:
fixedValue uniform (0 0 0)

p:
zeroGradient

T:
zeroGradient

and then I originally set all of the turbulence parameters (alphat, epsilon, k, mut) to match the settings for all of the other walls, with the wall functions. The problem is, the velocity around this center sphere quickly reaches a magnitude of 400+ m/s, which is entirely unrealistic. A colleague of mine suggested this may be the cause of the strange behavior at the outlet, which involves "stripes" of high and low pressure and temperature (each row of cells is alternatingly very high p and T or very low), and has the appearance that matter is flowing in through the outlet. It may be the case, though, that matter is being compressed at the outlet because of the strangely high velocity from the center.

Sorry for the originally missing information, and if anyone needs more info to help try to understand the issue, I'd be glad to post it.

Thank you.

Cyberholmes August 4, 2011 11:31

More clarification: the NozzleTip is the actual inlet, on the end of the nozzle. I have zeroGradient set for U and p on the NozzleTip because it is not actually a surface. The nozzle is initially not on, but at 0.2 seconds I switch the velocity BC to a flowRateInletVelocity at 1 g/s.

Thanks again.


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