**Model Geometry**
**Test Mass**
I found the dimension of the test mass flat in the drawings of the mock test mass design here: LIGO-D080687.
**Fibers**
I modelled the fibers with the profile described in LIGO-D080751, fig 3.7.
**Ears**
I grabbed the d-values from LIGO-T1000545, but since the d-value is defined as the distance from the center of mass (of the penultimate mass (PUM) or the E/ITM) to the bend point (BP) of the fiber (I believe the point on the fiber with maximal flexure in the fundamental mode), I did not go through the effort of figuring out where the bend point is but rather grabbed the horn-CM distance from LIGO-T1100407
I wanted to get the real aLIGO parameters for the first version of this model, and have parametrized the model in such a way that I can define all of the parameters that need to change (surface area of the ear-TM bond, length of the fibers, thickness and profile of fibers, d-value, etc) and scale them with mass in some way for future iterations on this design.
I need to pare down the number of parameters, because I started by fully defining the ears and now am importing a 3D model of the ears and planning to scale these with mass.
**Materials**
For the material of the entire test mass and suspension, I used the fused silica that is specified as [solid,NIST SRM 739 - Type I]. I wasn't sure the difference between the types of silica, but this one said SRM so I thought it might have been defined on my distribution of COMSOL by a LIGO person. A quick google search showed me that person may have been rana?? https://labcit.ligo.caltech.edu/~rana/research/etm.html
**Physics**
I'm using a solid mechanics model.
**Fixed Boundary Constraint**
I fixed the position of the bonding surfaces for the PUM ears, so it is as if they are contacted to a completely fixed PUM (the PUM is not included in the model, but the upper ears are included, so the constraint is on the ears not the fiber. See drawing).
**Gravity**
I added gravity to all parts of the model. Apparently, it is not trivial to use gravity in a frequency domain study in COMSOL, as described in this presentation here. Fortunately, the presentation in the link is interested in the transfer function for a mass on a string also, so I follow the simulation steps they describe below.
**Boundary Load**
I add a boundary load that will vary sinusoidally for the frequency domain study.
**Mesh**
I have not yet messed with the meshing for these models. Obviously the points with more flexture and smaller parts (like at the horns of the ears, the tapering parts of the fibers, etc) will require a finer mesh.
**Study**
I need to incorporate the advice on how to build this study described in the link above. The following might also be useful, though I haven't looked through them yet:
https://www.comsol.com/model/dynamics-of-double-pendulum-14021
https://www.comsol.se/forum/thread/4843/pendulum-response?last=2010-04-27T01:48:26Z |