Grid Method for
Project: Geotrans CAU Site Refinement
The method of Design is
as follows:
2D Template
Create a 300 meter spaced quad grid,
refine the corrective action unit (CAU) site areas and rotate 5 degrees.
The point distribution used to represent the Bechtel Geologic model
consists of a rectangle with the corners NW (542909.12, 4132671.15),
NE (563530.36, 4130867.03), SE (561856.97, 4111740.08), SW (541235.73,
4113544.2) with 300 meter spacing. The entire set of points is a rectangle
translated 5 degrees off the x-axis.
There are two areas of refinement;
Scotch is represented with the x,y polygon with corners at (554833., 4124400.),
(556626., 4124243.),
(554990., 4126193.), (556783., 4126036.).
Serena has corners at (548714., 4126742.), (550507., 4126585.), (548897.,
4128834.), (550690.,
4128677.). Each CAU
site is refined so that each has elements of edge size 75 meters, surrounded
by a transitional buffer with elements of size 150. The remaining elements
have edge size of 300 meters.
/scratch/rigel3/tam/nts_geotrans_9904/points_cau.in
This image shows a close up of the
two CAU sites, Serena on the left and Scotch on the right. Spacing for
the grid is 300 meters, 150 meters around the sites and 75 meters within
the sites.
Resample Surfaces
The input surface files for the Bechtel
model are created using the CAU triangulated template, a ray-shooting technique,
and contoured surface
files from NTS. To create each new
surface, the CAU template is positioned above a HSU surface and rays are
projected through each x,y point on
to the HSU surface to find the elevation
for each point. This is done with each HSU surface to create the x,y,z
coordinate points for each
surface. The points are then connected
into triangular elements, creating a Triangular Irregular Network (TIN)
for each surface of the model.
Each surface is represented by a
TIN sheet of 5275 x,y,z coordinates and the connectivity for 10282 triangles.
/scratch/rigel3/tam/nts_geotrans_9904/mksurf.in
This image illustrates the ray-shooting
technique. The CAU point distribution (black points) is used to shoot rays
through a HSU surface, surface braq is shown here colored by elevation.
A ray is shot through each CAU ray point, on to the braq surface to find
the braq elevation coinciding with the CAU ray point. The x,y,z points
are then connected into a TIN sheet
The image below shows
the new surface representing braq colored with site and buffer values.
Stack-to-Tet
The three-dimensional model is developed
by effectively stacking all the contoured surfaces and populating the volume
between the layers with CAU attributes. The TIN sheets are stacked from
lowest elevation to highest elevation. The volumes between the surfaces
are converted to prism elements (6 nodes, 2 triangle faces, 4 quadrilateral
faces) with vertical connections between adjacent layers. Each prism is
converted to three tetrahedra so that the final representation is in the
form of a 3D tetrahedral mesh.
In preparation of a computational
grid, thick units are subdivided vertically to provide a more gradual transition
to the thinner layers, and to keep the horizontal edges in proportion to
the vertical edges. The interfaces are buffered by a lower and upper surface
of a distance of 15 meters to capture the unit interfaces with voronoi
cells after the computational grid is created. A minimum unit thickness
of 14 meters is chosen so that large aspect ratio tetrahedral and triangular
elements are avoided. The grid is checked to ensure there are no holes,
and that the geometry correctly represents the Bechtel Geologic model.
/scratch/fourier1/tam/geotrans_trilayers_refv3/tet.in
This image shows the tetrahedral
mesh. A region has been cut out to better show the unit volume elements.
The refinement within Serena site can be seen.
Connect Delaunay
Once a tetrahedral mesh is formed, the
points are copied to a clean CMO. Duplicate points are dudded with filter/1,0,/.01
The points are then connected into a Delaunay mesh. Numerical solution
techniques for flow and transport calculations with finite volume and integrated
finite difference methods place geometric constraints on the quality
of a mesh. To optimize the mesh reconnection algorithms enhance the quality
while preserving the geometry. Reconnection can be done without adding
points by allowing connections to flip. In the final mesh, points may be
added or removed as needed to create a Delaunay grid with positive coupling
coefficients.
/scratch/fourier1/tam/geotrans_trilayers_refv3/connect.in
Final FEHMN input files
The Fehm flow and transport code uses
finite volume control volumes for solution of flow and transport equations.
Part of the grid generation process is to calculate the Voronoi control
volume associated with each node in the grid and the area of the polygonal
faces of the Voronoi control volumes. In addition to control volumes, lists
are created containing the surface area of each node on the surface of
the grid, for use in scaling constant flux boundary conditions. Node sets
for each material property are also written, including outside, top and
bottom locations for each node.
The final Delaunay computational
grid for three-dimensional calculations contains 138680 nodes and 839289
tetrahedral elements and accurately represents the geometry of the Bechtal
Model.
/scratch/fourier1/tam/geotrans_trilayers_refv3/fehm.in
Recolor with Fault - Unit
relationship
To find the elements that are located in fault regions, a new routine developed
by Bogdan A Pathak was used. This routine provides the capability to find
the intersection of a grid containing one-dimensional and two-dimensional
elements with a grid containing two-dimensional and tetrahedral elements
and note which elements (if any) intersected. By using the fence diagram
grid and stratigraphic grid as input to intersectelements, a grid that
has fault information, as well as the original stratigraphy, is produced
as output.
Once the elements that are within any of the three faults are found,
the material colors are rewritten. The material units not in a fault region
are kept with the values 1 to 23. For each unit, if the points fall within
a fault region, the color is the unit value + 50. So that a fault in material
unit 2 has a value of 52.
/scratch/fourier1/tam/geotrans_trilayers_refv3_flt/fault_zone.in
/scratch/fourier1/tam/geotrans_trilayers_refv3_flt/ck_toppts.in