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Solid Mechanics: Rigid domain + applied moment rotating cylinder in agar block — final rotation vs torque ~10× off from expected

Posted 26 févr. 2026, 18:01 UTC−5 Modeling Workflow, Material Models Version 5.5 0 Replies

Yoan Rossy

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Hello,

I’m looking for advice on whether my modeling approach is correct and what I can do to improve it. To preface this, I have no FEM experience and I am trying to learn COMSOL on the fly to get this working.

The Goal: I’m trying to reproduce an experimental torque-rotation curve. The setup is a 3D agar gel block with a rigid body cylinder embedded inside it. I apply a moment to the cylinder so that it rotates within the gel, and I want to plot the final equilibrium rotation angle of the cylinder measured from its initial horizontal position versus the applied torque. The cylinder is rotating about an axis that is orthogonal to its main axis (so it rotates like a baton about its center, not spinning about its length).

Conceptually, a moment is applied, the cylinder rotates, the gel resists the motion, and the system reaches an equilibrium rotation angle.

The issue I’m having is that the final rotation of the cylinder is about an order of magnitude off (~10x) from the experimental results, and I’m not sure why. The current rotation is very small compared to what is expected.

Geometry: I created a 3D block with a cylinder centered inside it. The geometry is finalized using Form Union. I have tried using Form Assembly but am struggling to get the model to converge. I believe Form Union may be a reasonable approximation since the gel is relatively compliant, so I assumed continuity between the cylinder and surrounding gel.

Materials: The gel block is modeled as a custom material representing agar, with Young’s modulus, Poisson’s ratio, and density taken from experimental values. The cylinder is modeled as a rigid body so I just list its density in the rigid domain settings.

Physics: I am using Solid Mechanics. The gel is modeled as a linear elastic material (I also tried a hyperelastic model, but it did not significantly change the magnitude of the results). The cylinder is modeled as a rigid domain (I have tried using a rigid connector, but I get the same results). I apply a moment to the cylinder about an axis orthogonal to its main axis (again, it rotates like a baton about its center, not spinning about its length). The rigid domain constraints and settings are: * Zero translation in all directions * Rotation constrained about z and x axis * Free rotation only about the axis where the moment is applied (y axis) * Rotation defined about the center of gravity Boundary conditions on the gel block: * Bottom face fixed * Side faces are rollers * Top face is free

Mesh: Currently using a coarse mesh, but refining the mesh does not appear to significantly affect the results.

Study: I have tried both stationary and time-dependent studies, and both give roughly the same final rotation.

Results: I am computing the rigid body rotation using a global variable evaluation. I have also plotted rigid body rotation versus applied torque and the rotation is way smaller in the simulation for all values of applied torque.

Any help as to how I can improve my model would be appreciated. The main questions I have are: is modeling the cylinder as a rigid domain inside a linear elastic (or hyperelastic) gel appropriate for capturing this behavior? What could cause the rotation to be off by ~10x? What should I check first to improve accuracy (mesh, boundary conditions, material model, constraints, etc.)?

I have attached my model and I can provide any additional information if needed.

Thanks.

Attachments:

cylinder_block_model.mph

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