Discussion Forum

Hi,

It is possible to obtain a solution with your conditions and laminar flow interface by using some artificial diffusion, the ''inconsistent stabilization' tab under 'laminar flow'. Using a 2D axisymmetric geometry with a rectangle of 6mmX6.5m and 0.25 isotropic diffusion and nitrogen gas from comsol library, you would get a max velocity of abot 45 m/s.

However, you need to try to reduce the isotropic diffusion as well as try to understand whether it's laminar or turbulent by using the Reynolds number you may calculate from the velocity resutls.


Cheers

Thank you for your help, I'm going to seek everything you seed and I'll get back to you....

Hey Amir,

So I have now a solution (thanks a lot) but I doubt of its relevancy (I have a max velocity of 296 m/s), and I wonder how does this 'inconsistent stabilization' option affect on calculations. As a matter a fact my model is slightly more difficult than a simple tube (I used this tube model only to understand the phenomenon) because I have 2 exits (see attached file), and even before knowing the behaviour of the flow, it seems obvious to me that results are false.

So if I got you well, you said that the stabilization helps the solver finding a solution, but not really accurate, and in order to be more precise I have to remove progressively this stabilization??

Thanks again

Hi,

I think you first need to have an idea what laminar and turbulent flows are and what Reynolds number is; then you may want to have a look at the stabilization techniques such as artificial diffusion.

The comsol manuals give a succinct summary, they may be a starting point.

Regarding your results, you may have very high velocities at the outlet because what you are doing is pumping a gas (or is it liquid nitrogen?) over a long distance with a high inlet pressure, but are you sure you have atmospheric pressure at the outlet, wouldn't it be that you simply have an outlet discharging into the atmosphere? Then you would require the flow rate from that outlet because it is going to have locally a much higher pressure than the surroundings. Furthermore, you need to understand whether your expectations of a laminar flow are correct.

Once you have a good grasp of the physical aspects (laminar/turbulent), the numerical aspects (stabilization) you can model virtually any shape of conduit.

Cheers

Ok I see your point, and the reynolds number is not really high (<1) so I would say that my flow is laminar. I took a look at stabilization techniques and I understand the necessity of lowering the isotropic diffusion coefficient, and I will keep 0.07. Regarding the outlet pressure, the two extremities of the tube through which gaz is exiting don't have anything attached to, it's just exit holes, so it's normal that pressure is at 1 atm no??

I will investigate more after lunch, thak you very much!!

Hi,

Re<1 may even be a Stokes flow, I think you'd better have an idea about viscous fluid flows first of all.

I can't say what percentage of artificial diffusion you need or you are allowed that is possible one of the hardest tasks of CFD.

Ok, so you have an open boundary discharging into atmosphere, no that does not mean that it has atmospheric pressure; just think of it, is the gas flowing quickly out of it? If yes, how is it possible that the stream has the same pressure of the atmosphere? If it is moving, it must have a higher pressure that drives it through the atmosphere.

I think you ought to find the the flow rate, maybe even using the results at hand and then decide whether it is acceptable or not.

Cheers

Ok, so you have an open boundary discharging into atmosphere, no that does not mean that it has atmospheric pressure; just think of it, is the gas flowing quickly out of it? If yes, how is it possible that the stream has the same pressure of the atmosphere? If it is moving, it must have a higher pressure that drives it through the atmosphere.

I thought the pressure just out of the tube was atmospheric pressure (or maybe I was just sleeping during class) and that was how we could use bernoulli's theorem for the total head of stream thing. I think the fact that the flow is moving is just the result of inertia.

Anyway the tube is rather tight so it may be normal to have this velocity, but i will try to do some math to determine the speed of the nitrogen in a simple cas, shouldn't be too harsh.

One last quick question : do you know if, for BC (inlet/outlet for instance), the pressure to be entered is absolute or relative pressure???

Thx have a good day

Hi,

Just out of curiosity, if you have a gas what density do you think it has? And given that density what inertia do you think it has?

Finally, atmospheric pressure may be all right for an outlet, it is up to you to determine whether it is, but having an open boundary discharging into atmosphere does not qualify automatically for having an atmospheric pressure across the outlet surface.

Now back to the results, if you have a velocity profile at the outlet you can calculate with comsol the flow rate and thus you can determine whether you have an acceptable result.

Cheers

Obvioously I didn't think of it all, but if i have the flow rate how could I know my model is right?? just verifying I have my inlet flow rate = sum of the two outlet ones??? My point is I have now a result, which seems quite high to me, but I don't know how I could verify it :s... If I don't know the outlet pressure I can't rely on my velocity field and vice versa right??

Hi,

If you have the flow rate you also know the amount of mass which is flowing out, thus you should be able to see whether it is acceptable or not, id est if at a high velocity corresponds a mass flux of tonns per minute then you may start having some doubts.

Again, you need to have a better grasp of the physics of the system, i.e. you need to find a way to control your system.


Cheers

Ok I'll try to do what you said, thank you so much.
I have an inlet mass flow rate of 47g/s and 37 + 12 at the 2 outlets, which is not really disturbing for me ^^....
And for the record my colleague just told me the nitrogen can't be put at that pressure in the tube, it needs to be regulate. So I was maybe bugging out for no reason...

I thank you for your help it is very kind of you to take time to answer me, and I wish you a good day/night

sorry one last question : If i'm not sure what is my outlet pressure, couldn't I use an "open boudary" instead?? The help of comsol states that may be useful if we can't predicate the pressure at this point....

Hi,

So you do have a mass inflow, thus you can simply use that option for the inlet, you just choose mass flow as boundary condition on teh inlet and mass flow rate as mass flow type. Then you can set the pressure at the outlet as zero and consider it to be some sort of relative pressure because now you have a velocity at the inlet, thus the software will calculate how much pressure drop is required to get that much of mass across that much of length.

Cheers

Ok I'll do that...
Thank you again