Round jet model in FLUENT

PurdueME · April 8, 2009, 14:10

I'm trying to do a turbulent analysis of a simple round jet in Fluent 6.3 and am struggling to get a correct solution. I have tried everything I can think of with no success. The "best" settings I have found so far are below, but I still don't get a self-similar solution.

My control volume looks like this:
----------------
| |
|------| |
|------| |
| |
|--------------|

The top boundary is set to outflow (I'm using incompressible flow.)
The right boundary is set to outflow.
The bottom boundary is set to an axis (jet centerline).
The left-bottom boundary is set to a velocity inlet (velocity specified such that Re ~ 5000).
The left-top boundary is set to outflow.
The left-middle boundary and all internal boundaries are set to wall (jet wall).

I'm trying to compare the S-A, k-eps, and k-w equations with default constants (none give me a correct solution).
Second-order solvers for pressure and momentum.
SIMPLE solver for pressure-velocity coupling.

My grid is spaced such that I have 20 grid points along the jet radius.
Control volume extends 20 diameters downstream of jet exit and 5 diameters above jet wall.

Any other settings are default.

Any ideas? This is my first real Fluent project, so maybe there's something obvious I'm missing.

NewtonKF · April 8, 2009, 14:59

Have you tried to turn some of the outflows conditions to wall condition??? Some numerical problem may occur if you use outflow condition in the left-top region, due to recirculation in this zone... Are you trying to correlate those results to some specific paper??? If not, i recommend the paper of Hussain: Velocity measurements in a high-Reynolds-number, momentum-conserving, axisymmetric, turbulent jet...
Good luck...

mettler · April 8, 2009, 16:41

are you getting an answer at all? This is basically just a flow problem, correct? If you plot the residuals you can see if it is diverging or flat-lining. What is your overall grid count? I would try to mesh it a little finer and see what happens. I do agree with the above post about treating the right upper bc as a wall first. I believe you can post pics on here, so a gambit screen shot might help out too.

April 8, 2009, 14:10	Round jet model in FLUENT	#1
PurdueME New Member Andrew Join Date: Apr 2009 Posts: 5 Rep Power: 17	I'm trying to do a turbulent analysis of a simple round jet in Fluent 6.3 and am struggling to get a correct solution. I have tried everything I can think of with no success. The "best" settings I have found so far are below, but I still don't get a self-similar solution. My control volume looks like this: ---------------- \| \| \|------\| \| \|------\| \| \| \| \|--------------\| The top boundary is set to outflow (I'm using incompressible flow.) The right boundary is set to outflow. The bottom boundary is set to an axis (jet centerline). The left-bottom boundary is set to a velocity inlet (velocity specified such that Re ~ 5000). The left-top boundary is set to outflow. The left-middle boundary and all internal boundaries are set to wall (jet wall). I'm trying to compare the S-A, k-eps, and k-w equations with default constants (none give me a correct solution). Second-order solvers for pressure and momentum. SIMPLE solver for pressure-velocity coupling. My grid is spaced such that I have 20 grid points along the jet radius. Control volume extends 20 diameters downstream of jet exit and 5 diameters above jet wall. Any other settings are default. Any ideas? This is my first real Fluent project, so maybe there's something obvious I'm missing.

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April 8, 2009, 14:59		#2
NewtonKF Member Newton KF Join Date: Mar 2009 Posts: 36 Rep Power: 17	Have you tried to turn some of the outflows conditions to wall condition??? Some numerical problem may occur if you use outflow condition in the left-top region, due to recirculation in this zone... Are you trying to correlate those results to some specific paper??? If not, i recommend the paper of Hussain: Velocity measurements in a high-Reynolds-number, momentum-conserving, axisymmetric, turbulent jet... Good luck...

April 8, 2009, 16:41		#3
mettler Senior Member Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 209 Rep Power: 18	are you getting an answer at all? This is basically just a flow problem, correct? If you plot the residuals you can see if it is diverging or flat-lining. What is your overall grid count? I would try to mesh it a little finer and see what happens. I do agree with the above post about treating the right upper bc as a wall first. I believe you can post pics on here, so a gambit screen shot might help out too.