you must indicate an open boundary or a source of matter  which is equal at the opposite of the flux at the boundary.

If you have the same physics in both domains, flux continuity is ensured by default. Thus you don't need to specify anything. 

thanks Mr OLIVE

but I dont know the flux quantity. I set the open boundary in the two domains, but any diffusion didn't occured.

thanks Louisnard, but when I didn't specify anything, It was set to no flux in the boundary in two regions.

Masoud..by default, boundaries are considered by wall, so no flux.

you can define the two domains as pairs and select flow continuity in the menu eg: non-isothermaflow/pairs/laminar flow/ flow continuity .

to obtain flux quantity, you must find the expression of this value for every physic, for example in transport of dilute species, the normal flux of species on a boundary  is define as chds.ntflux_c, c is the concentration of the specie; the prefix chds correspond at the physic; nitf for nonisothermalflux, etc..

I hope it helps.

Thank you Mr Olive but I don't understand what you mean.

I have two region geometry and 'transport of diluted species' in each of them. how can I pair them and set flux continuity? Can you explain with more details or with a simple example?

really appreciated

Sorry, I was wrong, in transport of dilute species, by default, you have continuity at the boundaries unless you specify no flow or unless the variable have not the same name in the two domains; it's for non isothermal flow that you have wall by default.

For pairs, I have never use it, I had just seen flow continuity for this option. I think it must be specify with geometry.

Good continuation.

I don't specify no flow but I have the variable with not same name in two regions. on default it's on no flux. now how can I set the flux continuity in boundary between the two domains?

You must give them the same name or at the boundary in the region1 you specify c1=c2 and in the region 2, c2=c1; the best would be to consider the two regions in the same model.

I cant do this! because there is a partition coefficient between these two regions and this is the cause that I want to specify flux continuity. on the other hand, the diffusion coefficient in these regions are not the same. so I have to use two models for these regions.

Thanks Mr OLIVE, I finally understand. I set two transport properties (each for one region) in one physic. It's solved. 

thanks

Mr. Masoud Hasany,

I have a similar trouble with you. Could you explain more detail how to set up the flux equilibrium on interface in the middle of two other region?

ok

download the example file in which you can understand that.

https://mega.co.nz/#!LckGjQjZ!A7oKstXpKG3IFNOgd2kSd9sPWdoImHFHLnutsKgLPto

Thanks Masoud Hasany.

I will check it. 

Dear all

Hi,

I have a problem in transport of diluted species and I hope you can help to find a solution.

It's a very simple problem; I have a chamber in which a fluid flows. At the bottom of the chamber one species is produced and then diffused to the fluid. Note that, since fluid flows in the chamber I consider both convection and diffusion in my model. I want to define a boundary condition as -D (dC/dx)=n at the bottom of the chamber.

In this equation, D is diffusion coefficient, C is concentration of species, and n is a mathematical expression for flux.

Do you know how should I add this boundary condition to my model? I used Flux boundary condition and entered the right expression (n) of the equation. Is it right? Does COMSOL consider the flux expression (n) as equal to diffusion (the left expression of the equation)?

Hi

there is two different things that you mentioned. first the convection and diffusion of the matter in your chamber which is not related to your boundary condition. you must solely add convection and diffusion physics to your model. and for your boundary condition, if you set the flux boundary condition and give it a number, it just gives the model measure of the matter per unit of surface which will be entered in your chamber. 

Hello

I think I have a similar problem. Please see the attachment. The upper domain is for air and the lower thin domain is for warer. Water is being evaporated from the surface. I am using Transport of concentrated species physics and applying Convection-diffusion for both domain. Problem is, I can not set the mass flux boundary condition on the interface. In comsol, when I try to assign a flux on this boundary, it shows 'not applicable' (see the figure attached). Do you have any idea how to fix this thing? Thank you.

Hello Mr. Masoud Hasany,

I am experiencing a similar problem as yours. I have added 2 Transport of Diluted Species physics and hence cannot satisfy continuity of flux by default. Is there an easy way to do that?

Alternatively, if I use one TDS physics and add two Transport Properties, I will get continuity of flux automatically but how do I incorporate discontinuity in concentration at interface due to partition coefficient?

Thanks in advance!

Tao Wang Masoud Hasany

dear Mr Tao Wang did you find the way to define the concentration discontinuity and flux continuity in two regions with different diffusion coefs?

In my model this discontinuity is dependent to the flux normal to the surface....

Tao Wang thnx dear Wang for your answering .

but my problem is a little different , i want to specify this kind of boundary condition between two adjacent domains with different diffusivity coefs : witch says that concentration jump is proportional to the total flux accross the domains .please check the attachment photo

Tao Wang yeah exactly it is not constant and its proportional to the total flux (convective + diffiusive) across the boundary