How to disable calculating net moments

[Oscar Jin]

Hi all,

I'm not simulating the self-propelled swimming of a fish, and found that the fish was not swimming straight forward, i.e. it had a deviation in the y direction. I would suppose the total moments on the fish were not zero when it started, when the motion was unstable. My friend told me there's a way to force the moments to zero in IBAMR. How can I do that? Thanks.

[Amneet Bhalla]

You can set the y translational momentum to zero in the input file, e.g.,  change this line

https://github.com/IBAMR/IBAMR/blob/master/examples/ConstraintIB/eel3d/input3d#L147

to 

calculate_translational_momentum = 1,0,0

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[Oscar Jin]

Hi Amneet,

I'm using the IBFE method. Are there any similar flag options? Thanks!

[Boyce Griffith]

On May 27, 2025, at 10:52 AM, Oscar Jin <oscarj...@gmail.com> wrote:

Hi Amneet,

I'm using the IBFE method. Are there any similar flag options? Thanks!

I don’t know of a canned way to do this in IBAMR, but it is easy to add point wise forces to IBFE models. In this case, I think that you should be able to add an additional uniform body force that acts to constrain unwanted y-displacements of the COM. Let me know if it is unclear how to get started. I think that the main difficulty will be in computing the position of the COM.

To view this discussion visit https://groups.google.com/d/msgid/ibamr-users/340ffe11-b0ff-4fd0-a672-3b785bbcc136n%40googlegroups.com.