mixing flow

kikimon · May 11, 2009, 03:44

The model is basically a large diameter duct with a thin injection tube on the side (T-shape with 2 inlets and 1 outlet). After running the sym, the air speed at the interior goes to the order of 10^16 m/s! During the first iterations, the speed starts going crazy at the inlet of the small tube. Afterwards the whole system has this unrealistic velocity gradients. The inlets speeds are supposed to be around 15 m/s and I`ve tried for boundery conditions the "velocity inlet" and "pressure inlet". I`ve also tried refining the mesh and using various turbulance models. I was hoping to get something close to laminar flow (15m/s) at the center of the tubes. I`d appreciate it if you have any idea of what`s causing this problem

.

zhaopeng · May 11, 2009, 10:40

How did you set the outlet boundary?

kikimon · May 11, 2009, 18:37

zhao peng, thank you for your reply.
I set the outlet boundary as a pressure outlet, with stagnation pressure of 1atm

zhaopeng · May 12, 2009, 01:10

what is the pressure differential between major inlet and outlet?and is the air compressible?

kikimon · May 12, 2009, 01:59

Between the inlet and outlet of the large pipe is 135 Pa and the gauge total pressure of the injection inlet is 540 Pa. This way I got a initialization speed of 15 m/s in the large duct and 30m/s in injection tube.
Thank you again.

kikimon · May 12, 2009, 19:29

I forgot to mention I`m treating it as incompressible beacuse of the low speed of the airflow.

martingariepy · May 12, 2009, 21:55

Dear kikimon

There is something wrong with your boundary condition and/or operating pressure. The veloity you obtain let me think about a pressure differential between the inlet and oulet of around 100000Pa. What might happen is this:if you set your operatiing pressure at, let say 101325, then you should set all the pressure as gauge pressure, then your outlet pressure will be zero and the inlet pressure will be 0+delta_P where delta_p his suppose to be the pressure differential that you want beetween the inlet and outlet.

Hope it's help
Martin

kikimon · May 13, 2009, 02:31

Thank you Martin for your response. I did set the outlet pressure as 0Pa and took into consideration the gauge term. Anyway, the problem is solved, it was the solver equations I was using. I ran 40 iterations with simple-standard and 100 more with coupled-second order and finally got it to converge with reasonalbe quatities. It might be something I should`ve known, but this was the first all-by-myself fluelnt sym, thank you both for your answers.

May 11, 2009, 03:44	mixing flow	#1
kikimon New Member Join Date: May 2009 Posts: 6 Rep Power: 16	The model is basically a large diameter duct with a thin injection tube on the side (T-shape with 2 inlets and 1 outlet). After running the sym, the air speed at the interior goes to the order of 10^16 m/s! During the first iterations, the speed starts going crazy at the inlet of the small tube. Afterwards the whole system has this unrealistic velocity gradients. The inlets speeds are supposed to be around 15 m/s and I`ve tried for boundery conditions the "velocity inlet" and "pressure inlet". I`ve also tried refining the mesh and using various turbulance models. I was hoping to get something close to laminar flow (15m/s) at the center of the tubes. I`d appreciate it if you have any idea of what`s causing this problem .

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May 11, 2009, 10:40		#2
zhaopeng Member zhao peng Join Date: Mar 2009 Posts: 41 Rep Power: 17	How did you set the outlet boundary?

May 11, 2009, 18:37		#3
kikimon New Member Join Date: May 2009 Posts: 6 Rep Power: 16	zhao peng, thank you for your reply. I set the outlet boundary as a pressure outlet, with stagnation pressure of 1atm

May 12, 2009, 01:10		#4
zhaopeng Member zhao peng Join Date: Mar 2009 Posts: 41 Rep Power: 17	what is the pressure differential between major inlet and outlet?and is the air compressible?

May 12, 2009, 01:59		#5
kikimon New Member Join Date: May 2009 Posts: 6 Rep Power: 16	Between the inlet and outlet of the large pipe is 135 Pa and the gauge total pressure of the injection inlet is 540 Pa. This way I got a initialization speed of 15 m/s in the large duct and 30m/s in injection tube. Thank you again.

May 12, 2009, 19:29		#6
kikimon New Member Join Date: May 2009 Posts: 6 Rep Power: 16	I forgot to mention I`m treating it as incompressible beacuse of the low speed of the airflow.

May 12, 2009, 21:55		#7
martingariepy New Member Martin Gariepy Join Date: Mar 2009 Location: Laval Posts: 13 Rep Power: 17	Dear kikimon There is something wrong with your boundary condition and/or operating pressure. The veloity you obtain let me think about a pressure differential between the inlet and oulet of around 100000Pa. What might happen is this:if you set your operatiing pressure at, let say 101325, then you should set all the pressure as gauge pressure, then your outlet pressure will be zero and the inlet pressure will be 0+delta_P where delta_p his suppose to be the pressure differential that you want beetween the inlet and outlet. Hope it's help Martin

May 13, 2009, 02:31		#8
kikimon New Member Join Date: May 2009 Posts: 6 Rep Power: 16	Thank you Martin for your response. I did set the outlet pressure as 0Pa and took into consideration the gauge term. Anyway, the problem is solved, it was the solver equations I was using. I ran 40 iterations with simple-standard and 100 more with coupled-second order and finally got it to converge with reasonalbe quatities. It might be something I should`ve known, but this was the first all-by-myself fluelnt sym, thank you both for your answers.