Messages

Hello Shunsuke,

You can visualize local backflow by using thresholding on the velocity field:

• Load the flow volume field and select the relevant velocity component depending on the defined flow direction (e.g., Z).
• Apply a threshold to display only values smaller than 0. This will only show regions with reverse flow for clearer observation.

Streamlines do not support filtering for negative values alone, but if backflow is significant, it will naturally appear in the streamline visualization. I recommend first identifying backflow zones using the thresholded velocity field, then examining streamlines within those specific areas to confirm the behavior. 

Regarding voxel resolution:

• Reducing the number of voxels while decreasing voxel size shrinks the total sample volume. This may limit your view to a single fiber instead of multiple ones, potentially compromising representativeness.
• Smaller voxels improve accuracy only until geometric features (e.g., pore throats or fiber diameters) are sufficiently resolved. Beyond ~10 voxels per feature, further refinement yields negligible simulation result improvements but increases runtimes.
• Only reduce voxel count if necessary to save computational resources or runtime. Always ensure the remaining volume remains statistically representative of your sample geometry for reliable results.

Best regards,
Jonas

[1. Physical Reasonableness]

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

[Diffusivity]

Hello Shunsuke,

you can compute a wide variety of material properties with GeoDict. Below I have listed the modules that are directly related to the properties you mentioned and added brief notes on other modules that might interest you for fuel cell applications. I recommend reviewing the corresponding sections of our user guide to get a full picture of what is possible.

Diffusivity
Diffusion can be simulated with our module DiffuDict (located under Predict). In general this allows you to simulate the net movement of molecules or atoms from a region of higher to lower concentration.
More information: https://geodict-userguide.math2market.de/2026/diffudict.html

Tortuosity
For the computation of the tortuosity there are many different approaches based on geometric or physical properties of the sample. All available approaches for calculating tortuosity are combined in the Compute Tortuosity GeoApp (located under GeoApp → General GeoApps → Geometric Properties → Compute Tortuosity).
More information: https://geodict-userguide.math2market.de/2026/geoapp_computetortuosity.html

Bottleneck Diameter & Pore Size Distribution
These properties are computed with PoroDict (located under Analyze), which offers several options for pore size distributions, percolation paths, and more.
More information: https://geodict-userguide.math2market.de/2026/porodict.html

Other structural parameters

• AddiDict: Simulate particle, molecule, or concentration transport and adsorption
  https://geodict-userguide.math2market.de/2026/addidict.html
  
• MatDict: Compute material properties of the solid phase such as surface areas
  https://geodict-userguide.math2market.de/2026/matdict_materialcharacterization.html
  
• FiberFind: Identify fibers and obtain statistics such as orientation, diameter, or length
  https://geodict-userguide.math2market.de/2026/fiberfind.html
  
• ConductoDict: Compute effective conductivities
  https://geodict-userguide.math2market.de/2026/conductodict.html
  
• FlowDict: Simulate fluid flow and obtain properties like permeability or pressure drop
  https://geodict-userguide.math2market.de/2026/flowdict_link.html
  
• ElastoDict: Compute mechanical properties and simulate deformations
  https://geodict-userguide.math2market.de/2026/elastodict.html
  
• SatuDict: Simulate two phase fluid systems, saturation, and related properties
  https://geodict-userguide.math2market.de/2026/satudict.html
  

Accuracy of calculated properties
A comparison of simulated and measured permeabilities and diffusivities for a gas diffusion layer is presented in the following publication:
Becker, J., Flückiger, R., Reum, M., Büchi, F. N., Marone, F., & Stampanoni, M. (2009). Determination of Material Properties of Gas Diffusion Layers: Experiments and Simulations Using Phase Contrast Tomographic Microscopy. Journal of the Electrochemical Society, 156, B1175–B1181. https://doi.org/10.1149/1.3176876

Additional studies on fuel cells that employ GeoDict can be found here: https://www.math2market.com/showroom/scientific-publications.html?industry=fuel-cells

Additional resources

• Our website: https://www.math2market.com/industrial-solutions/fuel-cells.html
• Our YouTube channel, in particular:
  
  • https://www.youtube.com/watch?v=tr7OW2rVN4w
  • https://www.youtube.com/watch?v=D2ohp-wEKXU
• Tutorial in our learning center: https://www.math2market.com/service-support/learning-center/digital-analysis-of-fibers-and-binder-content-of-four-carbon-paper-gdls.html

Best regards,
Jonas

[Color & Visibility]

Hello Shunsuke,

You can change the appearance of any material (including the pore space) directly from the Color & Visibility panel that is located on the right‑hand side of the GeoDict user interface. Below is a quick walk‑through of the most common options you mentioned.

Hide the Pore Material Completely

• Open the Color & Visibility panel (second entry in the sidebar).
• Find the row that corresponds to your pore phase. In your case it is ID00 Pore.
• Uncheck the Visible checkbox next to that row. The pore volume will no longer be shown, so you will see only the fibers.

Tip: If you still see a gray background after doing this, you're probably looking at the CT images volume field that GeoDict keeps after segmentation. In the View Controls → Volume Field section, uncheck the Volume Field box to turn it off.




Make the Pore Material Transparent

• In the View Controls panel, expand the Structure tab.
• Choose Transparency Mode → either Volume (entire materials) or Surface (only the material boundaries).
• Click the Edit button below.
• In the dialog, set the transparency slider for the Pore phase to the desired value (0% = fully transparent, 100% = fully opaque). Click Apply.

Additional Visualization Options

• Shadows: Go to Camera (in View Controls) → enable Shadows.
• There are also other lighting options under the same tab.
• Background: You can change the Background Color in the Color & Visibility panel.

More Help

For a deeper dive into all available visualization features, consult our GeoDict User Guide:

https://geodict-userguide.math2market.de/2025/visualization.html

Best regards,
Jonas

[Vary]

Hello Mark,

there are two ways to control the random seed value in your macros.

You can add a variable for the random seed and set its value when running the macro. This can either be a fixed value or a range of values using Vary.

Alternatively, you can let Python generate a random value for the random seed. For example, you can add the following to your macro:

Code Select Expand

import random

'RandomSeed': random.randint(0, 100000),

This way, a new random seed is used each time the macro is executed.

Best regards,
Jonas

Hello Jack,

Thank you for clarifying the issue.

If you still have the same GeoDict instance open in which you performed the dilation, you can use the Undo button (or press Ctrl+Z) in the toolbar to revert to the previous state. Please note that this is only possible within the same session. If GeoDict has been closed and you reopened the saved, already dilated structure, reverting is unfortunately no longer possible.

In that case, your only option would be to check whether you performed any intermediate computations that generated a result file. Each computation that produces a result file also creates a corresponding result folder, which always contains the input structure. If such a folder is available, you may be able to retrieve the intermediate structure from there.

Best regards,
Jonas

Hello Jack,

did you overwrite the structure file that was created by the segmentation when saving the dilation step?

If not you can find the segmented structure in the result folder of your image segmentation.

Best regards,
Jonas

[How do I start a new board at the front page?]

How do I start a new board at the front page?

• It is not intended to change the boards on the forums starting page. The starting page is read only.

[Spatial Intensity Histogram]

Hello Jack,

If I understand correctly, you're currently using the Spatial Intensity Histogram within Compute Metrics. Unfortunately, this function does not meet your requirements, as it generates a histogram of all gray values combined and only displays the probability - separate histograms for each material cannot be obtained this way.

To generate the histogram you need, I recommend following the approach using Python scripting previously outlined by my colleague Janine:

Hello Jack,

there are many options to analyze volume fields with the Compute Metrics command, as described here: https://geodict-userguide.math2market.de/2025/geodict_computemetrics.html

For example, you can compute the mean value and standard deviation for a specific material with it. However, the option to compute the histogram for only one material is not included yet.

But if needed, you can use Python to create your own custom histogram GeoApp.

For example, you get a 3D numpy array of your structure using the command gd.getStructure() as described here:
https://geodict-userguide.math2market.de/2025/automation_api_structure.html?anchor=getstructure

and for your volume field using gd.getVolumefield(volume field index), as described here:
https://geodict-userguide.math2market.de/2025/automation_api-results.html?anchor=getvolumefield

Then, you can for example use numpy methods to get a histogram for only one material ID.
https://numpy.org/doc/stable/index.html

Best regards,
Janine

Best regards,
Jonas

This topic has been moved to Automation.

https://forum.math2market.de/index.php?topic=124.0

[ImportGeo-Vol]

Hello Jack,

from your description, it sounds like you're currently viewing the segmented structure with three materials, rather than the original gray value volume field.

Let me walk you through the full workflow I use, which might help clarify things:

1. Import the dataset via ImportGeo-Vol. Enter all necessary parameters (like voxel size), then click on Image Processing.

2. This opens the 3D Image Processing dialog, where you can process the images (e.g., crop, rotate, or filter), and then segment them into different materials. At this point, the gray value data is also loaded into the main GeoDict window as a Volume Field.


3. Segment the structure using tools like Global Thresholding or AI Segmentation. After clicking Create Segmentation, the segmented structure is loaded into GeoDict and visible alongside the gray value volume field.


4. With both the segmented structure and gray value data loaded, you can now compute gray value statistics for each material using Compute Metrics. To do this, right-click the Gray Value volume field in the Project Status section and select Compute Metrics.


5. In the dialog, ensure the Gray Value field is selected. Choose which metrics to compute (e.g., Minimum, Maximum, Mean Value, Standard Deviation and Variance), then under Values for, select Chosen Material IDs and specify the material of interest. Click Run to generate the statistics for the gray values of belonging to this material only.


Let me know if anything remains unclear or if you need help with a specific step.

Best regards,
Jonas

[Gray Values]

Hello Jack,

thank you for the clarification. Could you please specify how exactly you created the Gray Values volume field?
From what you describe, it seems that your volume field might be empty and does not actually contain any values.

The easiest way to obtain the Gray Values volume field is by importing your dataset using ImportGeo-Vol.
This automatically loads the dataset into the import dialog and, at the same time, into the main GeoDict GUI as a volume field containing the gray values for each voxel.

Let me know if this helps or if you run into any issues!

Best regards,
Jonas

[Compute Metrics]

Hi Jack,

Thanks for your message - great to hear the previous response was helpful!

Regarding the issue you're seeing, could you specify your workflow and settings for using Compute Metrics on your gray-value dataset?

As an example, I applied Compute Metrics to one of the gray-value datasets that we deliver with GeoDict (Long Fiber Composite):

First, I loaded the dataset and segmented it into solid fibers and pore space using Global Thresholding with the Otsu method.
[attachimg=1]

Now with both the segmented structure and Gray Values volume field loaded, I selected Compute Metrics on this volume field.
[attachimg=2]

In the Compute Metrics dialog I chose to compute the Minimum, Maximum, Mean Value, Standard Deviation and Variance. I computed three cases:

• For All Materials
• For Chosen Material IDs -> 0 to get the metrics only for the pore space gray values
• For Chosen Material IDs -> 1 to get the metrics only for the fiber gray values

This gave me the following results:
[attachimg=3]

You can see the difference, e.g. between the mean gray values for pore space (16.776), fibers (82.6702), and the complete dataset (27.5684).

Please let me know if this explanation helped you or if you have any questions.

Best regards,
Jonas

[Boundary Conditions in Flow Direction]

Hi Danny,

the Boundary Conditions in Flow Direction define how the flow enters and leaves your structure depending on the chosen flow directions.

The Periodic boundary condition repeats the structure periodically before the inflow and after the outflow plane. In the example shown below this option does not work since there is no connected flow path. This option works best for periodic structures and structures with high porosity. We recommend using periodic boundary condition when possible, as the computational memory requirements are comparatively low leading to shorter runtimes.


The Symmetric boundary condition mirrors the structure at the inflow and outflow plane. This option works well for low porosity structures because the pores remain connected due to the mirroring.


The Velocity inlet, Pressure outlet boundary conditions apply a constant flow velocity in the inlet and a constant pressure drop in the outlet.


All the options described above do not change the thickness of your model. The option Add implicit region adds voxel before the inflow plane and after the outflow plane. This option changes the thickness of the structure and influences the final simulation result.


I hope this clarifies your question.
Please let me know if anything is unclear or if you have any further questions.

Kind regards,
Jonas