Questions on target force functions

[Oscar Jin]

Hi,

I’m a newbie to IBAMR and I’m trying to simulate a “fish” with an active revolute joint using the IBFE method. I’m stuck on defining the target force function. I think I need the X-position of a specific node (at the joint position) to rotate the nodes around it. But I don’t know how to get that X-position during the solving process. Maybe the ‘void* ctx’ variable could help, but I’m not sure how to use it.

Can anyone give me some pointers? I’d really appreciate it!

[Boyce Griffith]

On Mar 31, 2025, at 5:16 PM, Oscar Jin <oscarj...@gmail.com> wrote:

Hi,

I’m a newbie to IBAMR and I’m trying to simulate a “fish” with an active revolute joint using the IBFE method. I’m stuck on defining the target force function. I think I need the X-position of a specific node (at the joint position) to rotate the nodes around it. But I don’t know how to get that X-position during the solving process. Maybe the ‘void* ctx’ variable could help, but I’m not sure how to use it.

Can you model the joint using an active strain? You can sort of view this as a way of changing the reference configuration that the swimmer wants to achieve. If you did this with a neo-Hookean material model and let the stiffness become very large, then it becomes a (penalty method for) prescribed deformational kinematics. It is hard to see how using target points will avoid prescribing the actual swimming kinematics — which I’m guessing that you don’t want to do?

Can anyone give me some pointers? I’d really appreciate it!

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