Discussion Forum

Hi

I do not know of any, but on the other hand "my way" is rather to find out how my physics or the model set-up that allows for such negative concentrations, I would rather suspect a BC issue, a mesh density issue or a time stepping issue, that like that, probably would give you wrong results too, even if you found a way to "clamp" your concentration

--
Good luck
Ivar

hello,

Thanks for the quick reply, but please could you elaborate on your good practice

my wall boundaries are no slip, no gas flux
my gas inlet : normal inflow velocity = 0, gasflux=0.006 kg/m2/s
my liquid inlet : normal inflow velocity=u_in, no gas flux
transport inflow concentration 50 mol/m3
my gas outlet:normal outflow velocity=0 gas outlet
my liquid outlet: 2 times normal outflow velocity u_in/3,gas outlet
1 time pressure, no viscous stress 1 [atm] gasoutlet
3 times transport outlfow

Is there any other boundary condition I can set?

stabilisation:
consistent crosswind Ck,ns 0.1 ck,g0.1
inconsistent isentropic difusion 0.1 O(h) 0.1
transport:
streamline diffusion
crosswind diffusion
0.1/chds.helem
isentropic diffusion 0.1

I have not made a model to my satisfaction, so if you could help me I would greatly appreciate it

best regards
Wouter
nb: it is a 2D model ( rectangle it liquid inlet at the top, gas inlet at the bottem liquid outlets in a separator at the top and the rest of the top is gas outlet)

Hi

I have never used that part of COMSOL physics, and I must say I have problems to understand all your BC's. I have noticed that there is one example using BF in the library: "airlift_loop_reactor", have your tried it and done the exercise in detail, normally there are quite some info in the PDF, in addition to the model.

among others you can define a wall for the fluid, and only inlet or outlet for the gas, I believe you should read a bit more the doc to understand the differences between that type of wall and the inlet/outlet type. this might simplify the physics behind your model and make it solve better (just a guess ;)

But there must be other better tooled than me out there, no ?

--
Good luck
Ivar

Dear All,

I need to flow liquid behaind tube or walls. please is it possible help me. and what is BCs for these tube or walls

regards

Mahmood

Have you solved your problem? I am trying to do a 3d model for human eye and I get negative concentrations. I have laminar flow in the choroid and transport of diluted species in the entire model.

Hello Tomi Kainuvaara

I am sorry to inform you that I did not succeed in solving the problem. The helpdesk did a simplified simulation of my model but they changed mostly the geometry (removed the sharp edges) and used an extremely fine mesh. I am still trying to adapt these improvements to my problem, but I essentialy gave up.

All the best
Wouter

There is a knowledge base article on comsol website about 'negative concentrations' (if you have not already seen it). Other than that, there were few suggestions of implementing convection-diffusion equation with log(c) as variable in place of 'c', where 'c' is your species concentration. I have not tried it myself.

Usually, it is due to mesh density issues and one could use adaptive meshing to solve regions of high gradients.

Good luck with your problem.

There is a knowledge base article on comsol website about 'negative concentrations' (if you have not already seen it). Other than that, there were few suggestions of implementing convection-diffusion equation with log(c) as variable in place of 'c', where 'c' is your species concentration. I have not tried it myself.

Usually, it is due to mesh density issues and one could use adaptive meshing to solve regions of high gradients.

Good luck with your problem.