Nagi Elabbasi
Certified Consultant
Posted:
5 years ago
Dec 10, 2012, 9:54 AM EST

Hi Line,

Are you sure you are using a 2D axisymmetric model not a 2D planar model? If so you should only model a rectangle with width equal to the radius (not diameter) of the pipe, and COMSOL will automatically add the symmetry boundary condition on the left side of the rectangle.

Nagi Elabbasi

Veryst Engineering

Hi Line,
Are you sure you are using a 2D axisymmetric model not a 2D planar model? If so you should only model a rectangle with width equal to the radius (not diameter) of the pipe, and COMSOL will automatically add the symmetry boundary condition on the left side of the rectangle.
Nagi Elabbasi
Veryst Engineering

Posted:
5 years ago
Dec 10, 2012, 10:26 AM EST

Hi

i was wondering if there is not a factor 2 (/8 instead of /4 ?) missing in there.

I did a quick run in 2D axi SPF aand this gives rather coherent values but is still mesh dependent, and the pressure inlet needs a bout a meter to stabilise fully.

I'm not that astonished that the flow flattens in the middle with high resolution mesh. Also the temperature, you are not using 20°C are you ?, the eta/mu value doest really fit for me.

--

Good luck

Ivar

Hi
i was wondering if there is not a factor 2 (/8 instead of /4 ?) missing in there.
I did a quick run in 2D axi SPF aand this gives rather coherent values but is still mesh dependent, and the pressure inlet needs a bout a meter to stabilise fully.
I'm not that astonished that the flow flattens in the middle with high resolution mesh. Also the temperature, you are not using 20°C are you ?, the eta/mu value doest really fit for me.
--
Good luck
Ivar

Posted:
5 years ago
Dec 12, 2012, 7:16 AM EST

Thank you so much. I still can not get it right but you guys sure did help me on the way.

I changed the model and now I have a 2D axissymmetric model with a radius of 0.05m and a length of 13 m.

you are right I use 323.15K as temperature.

I have an inlet pressure of 0.126024724

and outlet still 0

My maximum velocity calculated by the hagen poiseuille flow is 1.107E-2 m/s

whereas COMSOL have it at 9.98E-3 m/s

This is offcourse closer than before but still I would love it to be a little better.

I have added the COMSOL file if anyone is interested

Thank you so much. I still can not get it right but you guys sure did help me on the way.
I changed the model and now I have a 2D axissymmetric model with a radius of 0.05m and a length of 13 m.
you are right I use 323.15K as temperature.
I have an inlet pressure of 0.126024724
and outlet still 0
My maximum velocity calculated by the hagen poiseuille flow is 1.107E-2 m/s
whereas COMSOL have it at 9.98E-3 m/s
This is offcourse closer than before but still I would love it to be a little better.
I have added the COMSOL file if anyone is interested

Nagi Elabbasi
Certified Consultant
Posted:
5 years ago
Dec 12, 2012, 1:26 PM EST

Close to the theoretical solution, but you should be able to get even closer. Make sure you are using the correct value of the pressure gradient dp/dz. There is a relatively significant pressure loss at the entry which leads to an actual pressure gradient (throughout most of the pipe) that is smaller than what is estimated with P_inlet/ total length.

Nagi Elabbasi

Veryst Engineering

Close to the theoretical solution, but you should be able to get even closer. Make sure you are using the correct value of the pressure gradient dp/dz. There is a relatively significant pressure loss at the entry which leads to an actual pressure gradient (throughout most of the pipe) that is smaller than what is estimated with P_inlet/ total length.
Nagi Elabbasi
Veryst Engineering

Posted:
5 years ago
Dec 12, 2012, 2:40 PM EST

Hi

and have you checked the laminar inflow condition ?, note that the pressure inlet is counting also the extra length, I believe (check your results, as the plots will not show the fictuous inlet region, while the pressure difference should give you te drop in the laminar inflow addition length (at least is was so for some versions ago, this might have changed ;)

--

Good luck

Ivar

Hi
and have you checked the laminar inflow condition ?, note that the pressure inlet is counting also the extra length, I believe (check your results, as the plots will not show the fictuous inlet region, while the pressure difference should give you te drop in the laminar inflow addition length (at least is was so for some versions ago, this might have changed ;)
--
Good luck
Ivar