Creating Rectilinear Meshes for LS-DYNA
=======================================

Most of the meshes used in first-pass FEM simulations of acoustic
radiation force excitations are large rectangular solids with either
uniform cubic 3D elements, or elements with a slight aspect ratio. This
document provides an overview of the process of generating a 3D mesh
using tools in this repository (``fem/mesh``).

Syntax
------

``mesh/GenMesh.py`` will generate a the rectilinear mesh with
user-specified properties. You can specify two opposing corner vertices
of the mesh (``--n1`` and ``--n2``), the element density on each edge
(x, y, z), and the names of the output files for node and element
definitions.

Run: ``python GenMesh.py --help`` for detailed input syntax.

Coordinate System
-----------------

Meshes in this code base utilize different conventions for the FEM
portion of the code compared to the Field II simulation component.
LS-DYNA mesh conventions are:

-  x - elevation
-  y - lateral
-  z - axial (negative, with transducer face @ z = 0)

Maintaining a right-hand rule convention, for a quarter-symmetry model,
x would span a negative range of values for positive 7 values.

Units
-----

The typical unit system for the mesh is CGS, and this will be assumed in
the code that passes data to/from Field II.

Next Steps
----------

1. Your mesh will be used as an input to Field II to simulate the
   acoustic radiation force at nodes. This is done using
   ``field/field2dyna``.

2. You will need to generate boundary conditions for your mesh
   (``mesh/bc.py``)

3. You may want to create structures in your mesh with different
   material properties (``mesh/CreateStructure.py``)