[cardioCFD]

Hi,

In CFX built-in functionality for modelling chemical reactions (variable composition or reacting mixture), is there a way to define a component that is not being transported (neither by advection nor by diffusion)?

If I use additional variables + sources to model the reactions, there is a possibility to define a diffusive transport variable with a very small diffusivity, but this option is not available when the working fluid is a variable composition mixture or reacting mixture, the options are only transport equation, algebraic equation, etc.

Any advice is highly appreciated.

[ghorrocks]

What type of substance is it you are trying to model?

[cardioCFD]

Originally Posted by ghorrocks

What type of substance is it you are trying to model?

Thank you so much for your advice. I am actually trying to model blood clotting. This includes modelling solidification which is not available as a CFX built in functionality. Moreover, modeling this process as a multiphase process is my last option due to the associated complexities.

To model clot deposition, I would like to define a stationary component, which is actually the clot. So once the clot is formed it sticks there. But there is no such possibility in CFX, the only available transport models, are transport equation, algebraic equation, ...

As I explained in the previous post, currently I defiend an additional variable as you also suggested, as a diffusive transport variable with a very small diffusivity and some source value.

I was wondering if I can do the same for a chemical component to be able to use CFX built in functinality of modelling reactions. Using the built in functionality gives the opportunity that cfx ensure non negative values and smooth convergence.

[ghorrocks]

ANSYS Support has some examples of solidification I think. It would be worth talking to them and seeing what hints they can give.

One idea could be to model the clot as a normal fluid reactive species but use a momentum source term on that species only to stop motion, or at least slow it down.

[cardioCFD]

Originally Posted by ghorrocks

ANSYS Support has some examples of solidification I think. It would be worth talking to them and seeing what hints they can give.

One idea could be to model the clot as a normal fluid reactive species but use a momentum source term on that species only to stop motion, or at least slow it down.

Thanks a lot Glenn. I will talk to the ansys support and see. Re the momentum source, I have tried this, but as far as I know we can only apply momentum source to the whole mixture (working fluid) in CFX, and not on the transport equation for specific species. Am I right?

One other idea but wanted to explore but could not:
CFX solves scalar transport equations for each of the species, something like this:

dB/dt = diffusion of B + advection of B + rate of generation of B

I was wondering if I could apply some sigmoid function on the advection and diffusion terms:

dB/dt = F * (diffusion of B + advection of B) + rate of generation of B

Where F is a sigmoid function that reduces transport of B by diffusion and advection as concentration of B is greater than some threshold value. But I could not really find a way to do this.

Any further advice is appreciated. Thanks a lot.

[ghorrocks]

Yes, I suspect you are correct. Momentum sources get applied to the whole mixture. Oh well, forget about that idea then.

You cannot change the equations like you describe. This is all hard coded into the software so is not possible to change. The only part of the equation which the user has access to is the source term, so if you want to try that approach you will need to do it through the source term.