I’m having a tough time understanding how cell size works in Pylith. I currently have a Trelis journal creating a uniform 2-D grid with cell length of 0.001 (Trelis units). In Pylith, I have spatialdb files (with linear search query) specifying certain varations in fault properties over the length of the fault. However, I am currently getting the “Could not find parameters for physical properties at (…)” error. I’m understanding this to mean that I am not specifying fault properties over the entire fault. I am getting various results as I change parameters like the cell size in Trelis, the non-dimensionalizing length in Pylith, or the “to-meters” parameter in the spatialdb files, all without getting an idea for how Pylith determins the length of my unit cell in Trelis. I’d greatly appreciate a detailed explanation for this as it would help me create grids in Trelis more systematically as opposed to my current trial-and-error method, especially since I cannot find an explanation for this in the manual.
The cell size you use in Trelis should be the same as the cell sizes you’re using in your spatialdb files and elsewhere. Unless you specify otherwise, the cell size of 0.001 means your cells are only 1 mm in length, which seems extremely small. Furthermore, the dimensions you use in Trelis will affect how you set up your spatialdb. For example, if you have a rectangular mesh and do not move it downward (so the upper surface is at z=0), there is a good chance that your spatialdb files do not cover the proper range. One thing you can do when creating your mesh is to use SI units. See, for example, examples/2d/subduction in the PyLith examples (look at geometry.jou). That way, you can specify mesh dimensions in km if you want. The output mesh will be in default units of meters, which is what PyLith uses by default.
Thanks for your reply, Charles. 1mm is actually what I need for my problem, so perhaps it’s the location of the origin point I am confused about. I am working off of where the origin is in Trelis, which I find with the command “draw location position 0 0
0”. Is the origin in Pylith also defined at the same point?
PyLith always uses the coordinate system given in the mesh file. It never translates or rotates the coordinate system. However, the finite-element mesh and all spatial databases specify quantities using dimensioned values (meters, kilometer, etc). Internally, PyLith nondimensionalizes the computational domain. The default length scale is kilometers. If your cell size is 1 mm, then a length scale of 1 mm is likely appropriate for the nondimensionalization of the problem.
The origin for PyLith is also (0,0). To check the dimensions of your mesh, you can open the .exo file in Paraview and then look in the ‘Information’ panel. Your spatial database files should be consistent with these dimensions.