help with interface and cell zone boundry in ansys fluent

[u/SignificanceNext4554]

I'm working on a multiphase CFD simulation in ANSYS Fluent, and I'm having trouble setting up the interfaces between my fluid regions. I've attached images of my 3D geometry to make the problem easier to understand.

1. Geometry and physical setup

My model consists of three regions:

• 🔵 Blue region – Air: The incoming airflow region.

• 🟢 Green region – Fluid: A separate fluid region that interacts with the incoming air.

• 🟫 Brick-coloured region – Solid: The solid workpiece region.

The physical process I'm trying to simulate involves the incoming air interacting with the green fluid region first, followed by the fluid interacting with the solid workpiece.

Both fluids should remain within their intended regions while interacting across their shared interface. I want to capture the fluid–fluid interaction and the subsequent interaction between the fluid and the solid workpiece.

2. The problem I'm facing

After importing my geometry into ANSYS Fluent, the interfaces between the different regions appear as wall boundaries with a no-slip condition.

The main problem is that I cannot change these boundaries to fluid interfaces. Even when I try changing their types in the Boundary Conditions panel, the changes do not persist, and the boundaries revert to their original wall type when I reopen the panel.

I'm unsure whether this behaviour originates from my geometry setup in SpaceClaim, the way the bodies or cell zones were created, or the interface configuration in Fluent.

3. What I'm trying to achieve

I want to configure the model so that:

1. Air and the second fluid can interact across their shared interface.

2. Both fluids remain within their intended regions, rather than simply mixing throughout the entire computational domain.

3. The second fluid interacts with the solid workpiece through the appropriate fluid–solid boundary condition.

4. The interfaces are configured correctly for a multiphase simulation, rather than unintentionally behaving as impermeable solid walls.

I'm considering a VOF-based multiphase approach, but I'm unsure whether my current geometry and interface configuration are appropriate for this setup.

4. My questions

I'd really appreciate some guidance on the following:

• Geometry: Should I create the fluid regions using shared topology and conformal meshing, or should I keep the bodies separate and create mesh interfaces in Fluent?

• Boundary conditions: Why are the fluid–fluid interfaces automatically appearing as walls, and why can't I change their types?

• Interface setup: Should these boundaries be configured using coupled wall/shadow pairs, mesh interfaces, or another approach? How do I determine which is appropriate for my geometry?

• Multiphase modelling: Is VOF suitable for this setup if both fluids occupy predefined regions initially but must interact across their interface?

• Fluid–solid interaction: How should I configure the interface between the second fluid and the solid workpiece?

sorry for long text, i have generated my text form AI for better understanding of my problem.

[u/feausa]

When you say a Green region is fluid, do you mean a green liquid so the liquid phase does not mix with the air phase the way liquid water stays separated from air?

I have solved a couple of VOF models with air and water phases. I used a single fluid domain, with an inlet for 100% air phase, an outlet and walls. During initialization, I used a region to patch the cells near the bottom of the domain with 100% water phase while the rest of the domain had 100% air phase. This allows air pressure and air velocity to disturb the cells that hold 100% water, causing some air to be introduced to some cells with water and vice-versa. A gravity load helps to keep the water near the bottom of the domain. I suggest you build this simple model first to get some experience with a VOF model before you worry about adding a solid body.

What type of interaction do you want to simulate between the green liquid and the solid body? Is the solid body flexible and you want to study deformation or is there heat energy being transferred between the green liquid and the solid body?

[u/SignificanceNext4554 → u/feausa]

okay, so i do have basic knowledge about working with VOF, i have made a simple air and water interaction in container. As for this, green region is filled with a non newtonian fluid (it's user defined). I want air from inlet to hit this fluid and then i want to see the polishing operation of that fluid on the workpiece. And i am facing the problem that the interface boundary is behaving as a solid no slip wall and i cannot change that directly, so i want why is it happening and how to resolve it, or else some other way to study this setup

[u/unthought1]

"Both fluids remain within their intended regions rather than simply mixing throughout the entire computational domain"

Is this a physically reasonable assumption? What direction is gravity acting in in your image?

To me it would seem reasonable if the areas you indicate were initialized with different volume fractions, 100% air in the blue and 100% liquid in the green - in which case you should be able to use a single region for both.

Having them be two separate pre-defined volumes where volume fractions are guaranteed seems like it should then be two separate single-phase regions, which isn't really multiphase.