## Fluids

#### Input partial derivative equations in CFX

• Jimmyhan
Subscriber

Hi, I am Jimmy, I would like to simulate membrane separation. I found many researchers do this job by using CFX from papers review. Salt concentration near the membrane surface should be cleared by mass flow simulation. So I need input derivative equations in CFX-pre. I have read 'CFX reference guide release 15.0', but I still confused about this. Can you give me some suggestion? or tell me how to input equation like figure showed.

• raul.raghav
Subscriber

Jimmy, I'll mention in general on how to define a derivative (spatial or time) in CFX.

If you need to define the gradient of u in x-direction (du/dx), you can create the following expression:

or

If you need to define the time derivative of x-direction velocity (du/dt), you can create the following expression:

dudt = u.Time Derivative

That being said, I don't think you can define (dC/dy)@wall as an expression.

• Jimmyhan
Subscriber

Hi, Rahul, Thank you for your reply, If I understand you correctly, CFX just defines velocity gradient? but how can I calculate concentration gradient near the wall? Can you give me some suggestion? or recommend some book?

Jimmy

• raul.raghav
Subscriber

Jimmy, what I tried to point out was that you can find the gradient of any variable by attaching the ".grad_x" or ".Gradient X" to the variable.Like temperature gradient in the x-direction (dT/dx) would be possible by defining expressions: "dTdx = Temperature.grad_x" or "dTdx = Temperature.Gradient X".

And I was trying to say that defining the gradient in the normal direction at the wall is not something you would be able to do.

Usually diffusion in CFX is a little complex than it actually is. You would have to define an "Additional Variable" and define the transport equation for the additional variable and provide the kinematic diffusivity. I mentioned this in a post earlier which you can find here: Diffusion Additional Variables

Would you be able to share your workbench archive file so we can take a look into it?

• Jimmyhan
Subscriber

Thank you very very much, I will try to realize.