# How to model species transport only due to diffusion?

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May 25, 2021, 17:51
How to model species transport only due to diffusion?
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CASE- I am trying to model the transient transport of SAMPLE in a hydrogel scaffold. I will attach a picture.
Temperature everywhere is 310K at all times.

GEOMETRY- A solid object (hydrogel geometry) , surrounded by a square enclosure (Fluid zone). The enclosure is initially filled with water.

The sample concentration is applied to the top of the geometry at time=0 (think droplet of sample added on top).

I want this to be the only inlet of the sample. There is no water inlet.

MODELS- Transient, Laminar, Species.
MIXTURE- water and sample. the sample has similar properties as water.

BC's:
Side and bottom walls- Default (0 diffusive flux)
TOP is modeled as a wall with diffusive flux of sample with mass fraction 1. THIS IS INCORRECT FOR ME AS I ONLY HAVE AN INITIAL DROP OF THE SAMPLE,NOT A CONSTANT DIFFUSIVE FLUX.

The sample only enters the geometry from the inlet at T=0. After this initial value, no more sample is entering the geometry. Thus convection = 0

I want to model the transport of the sample as a function of time, and collect concentration contours of the sample.

MY QUESTIONS:

How can i define initial conditions or boundary conditions to accomplish this?
To do this I would need to

A) Have an initial concentration condition at t=0 of the sample at the top wall. Then the sample just moves because of diffusion.

ANSYS only gives me an option of having a constant diffusive flux at the walls.

How can i have an initial concentration of the sample at the top surface and simply have it diffuse through the geometry?

BONUS- Any ideas about how i should select my time step size?

Thanks for any help.
Attached Images
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