Why does the minimum velocity become negative only for the 40% porosity model?

[Shunsuke Takai]

Hello,

I am performing a FlowDict simulation on fibrous catalyst layer models and would like to ask about an unexpected simulation result.

To compare the effect of porosity on transport properties, I created five fibrous models with porosities of 40%, 50%, 60%, 70%, and 80%.

The dimensions of each model are:

Voxel size: 0.1 μm
Domain size: 200 × 200 × 200 voxels
(20 μm × 20 μm × 20 μm)

The purpose of this study is to investigate how structural properties such as flow velocity and pressure drop change with porosity.

However, after performing the FlowDict simulation, I found that only the 40% porosity model exhibits negative minimum velocity values, whereas the other porosity models do not.

I attached the graph of the minimum velocity through the thickness direction.

I would like to know:

Is it physically reasonable for the minimum velocity to become negative in this case?
Could this result be caused by the simulation settings or boundary conditions?
Are there any FlowDict settings that I should check first?

My simulation information is:

GeoDict version: 2025.5
Module: FlowDict
Models: Fibrous catalyst layer
Porosity: 40%, 50%, 60%, 70%, 80%
Voxel size: 0.1 μm
Domain size: 200 × 200 × 200 voxels

If additional information such as the FlowDict settings or result files would be helpful, I would be happy to provide them.

Thank you very much for your help.

Best regards,

Shunsuke Takai

[Jonas Schabernack]

Hello Shunsuke,

Thank you for your detailed inquiry. Here are answers to your questions regarding the negative minimum velocities in your 40% porosity model:

1. Physical Reasonableness
Yes, this is physically reasonable. Negative velocity indicates that fluid is locally moving toward the inlet rather than the outlet. Low porosity structures often feature labyrinth-like geometries with high tortuosity. These complex shapes can redirect flow backward into narrow channels or dead ends before it resumes toward the outlet.

2. Comparison with Higher Porosities
As porosity increases, the structure becomes more open and flow paths become more direct. This reduces the likelihood of backflow. You can also see small negative values in your 50% porosity plot. For higher porosities backflow does not occur anymore.

3. Simulation Settings
Since this behavior is physical and expected, you do not need to adjust your solver settings.

The result aligns with expectations for dense fibrous media. The negative values represent local backflow caused by complex geometry rather than a simulation error.

Best regards,
Jonas