Sedimentation of one particle

[Henderson_C]

Hello everyone,

I ran the dkt2d file (drafting kissing and tumbling of two particles) and it worked fine. but when I removed the second particle and wanted to validate the sedimentation of one particle, the particle never stopped accelerating and it never reaches a constant terminal velocity. Why is that? I tried to play with the parameters but nothing worked!

Your reply is much appreciated.

Thanks,

[Nicolas]

Dear Henderson_C,

assuming you are using version 1.5, there should be no general issues preventing you from investigating a single-sphere sedimentation setup. This has been done in 3D e.g. in [Trunk21](https://www.mdpi.com/2079-3197/9/2/11). As the terminal settling velocity results from an equilibrium between the respective forces acting on the particle, it can most likely not be achieved by “playing” with the parameters in a given domain, but requires a consistent parameter setup. It is worth noting at this point, however, that some setups will lead to large settling distances before hitting the terminal velocity. The default domain size of the dkt2d case will most likely not suffice in most cases and needs to be enlarged significantly.

Best regards,

Nicolas

[Henderson_C]

Dear Nicolas,

Thank you for your reply. I am aware that the domain must be large enough for the particle to achieve the maximum velocity. But I did not understand the point regarding using consistent parameters. The paper you attached, validated the one-particle sedimentation, however, using the same inputs (Particle radius, particle density, …) I obtain a different output.

[Nicolas]

Dear Henderson_C,

I referenced the article in order to point out that the respective scenario can in general be investigated with OpenLB. A different output is to be expected for your 2D version (compared to the referenced 3D version) as the surface force to volume force ratio is significantly different for the same parameter setup. You need to be aware that transferring a real-world physical problem, which is inherently three-dimensional, into a 2D version is not straight forward (as it can be reasonable to have a 2D circle both represent a Cylinder or a Sphere in 3D depending on the assumptions). If you want to use the exact parameters used in the publication, you need to switch to 3D instead. If you want to stick to 2D you may want to look for a published 2D version for respective reference parameters.

Best regards,

Nicolas

[Henderson_C]

Dear Nicolas,

As per your recommendation, I have switched to a 3D particle sedimentation but I still obtain incorrect results. Can you advise me on which parameters are of importance regarding this simulation? I am keeping the relaxation time above 0.5 and the spacing seems adequate. I apologize if my questions are still as I am new to this topic. Thank you very much.

Best regards,

[Nicolas]

Dear Henderson_C,

first, please verify that you are meeting the general LBM stability requirements (according to e.g. [Krueger17](https://link.springer.com/book/10.1007/978-3-319-44649-3). You should then set your resolution in such a way that your particle is represented by a sufficient number of grid cells (e.g. 10 cells in diameter). You can find out the specific minimum requirement for your setup by conducting a grid convergence study and evaluating the respective error of e.g. the hydrodynamic force in the sedimentation direction. For a more detailed description of sensible parameters, I would like to again refer to [Trunk21](https://www.mdpi.com/2079-3197/9/2/11) or similar literature.

In case you want to gain some more in-depth insights, you should consider joining the next occurrence of our annual [Spring School](https://www.openlb.net/spring-school-2023/).

[Henderson_C]

Dear Nicolas,

Thank you for your valuable comments and patience with my questions. I now understand it clearly. I am excited to come to the Spring school but unfortunately, I can not due to some circumstances. However, I have one more question if you do not mind. How can I employ certain physics in the particle such as the DD enthalpy-based scheme?

Best regards,

[Author]

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