Welcome to the Getting Started with GeoStudio video series.
This tutorial video will discuss how to fine tune the mesh using the finite element meshing options in GeoStudio Products.
This tutorial video will also explain how to remove mesh constraints from the finite element
mesh.
A summary of practical guidelines for good meshing practice is also outlined in the product-specific engineering books.
It is recommended that these chapters are also reviewed for the user's specific product of interest.
Let's start by looking at a two-dimensional steady- state model in the SEEP/W product.
Here, we can see the model domain consists of an embankment
with a reservoir level of 11 m on the upstream side of the embankment.
There is currently also a drain along the downstream side of the embankment below the toe and
two lines representing a potential core material in the middle of the embankment
We will first turn on our current mesh by going to view, preferences.
In this window, we can turn on and off many of the visualization parameters,
but we will simply turn on the mesh.
Now, we can see how our current mesh looks with no constraints added.
Next, we will open the mesh properties window
by going to draw mesh properties.
We can see that the current global element size is set to 1 m.
This element size has been drawn by default and can be modified to alter the size of the elements at a global level for the entire mesh.
Generally, it is recommended that the mesh is not altered too much
and only necessary alterations should be made at a few limited locations where finer or coarser discretization is needed.
First, we will look at the different finite element mesh pattern options available in the GeoStudio Products.
Each of the products currently allows automatic mesh patterns to be created
based on an unstructured mesh type.
The unstructured meshing pattern will generate a well behaved pattern of either a mixed quadrilateral
and triangular element pattern or a triangular element pattern.
The triangular mesh pattern can be used
if the user prefers a mesh that is automatically created
using the Delaunay triangulation techniques.
The unstructured mesh type is recommended as the algorithm creates a well behaved pattern, especially for odd-shaped regions.
There are also two structured mesh option types available.
The first is the triangular grid of quads and triangles and has been included for triangular regions.
This meshing type creates a mix of square, rectangle,
trapezoid, and triangle elements.
The second structured mesh uses a rectangular grid of quad elements
and is only recommended for rectangular regions.
If used on other regions, this mesh type can be more difficult to control.
In our SEEP/W example,
we will simply keep the quads and triangles unstructured mesh.
We will also keep the default global element size of 1 m.
Given the drain under the toe of the embankment,
we know that there will be a phreatic zone located somewhere along this line.
To ensure that we have a more accurate representation on the location of this phreatic zone,
we will change the mesh discretization
along this line to 0.5 m.
To do this, we will click on the line
representing the drain and choose the "ratio of default size" option from the drop down menu.
By entering 0.5 for our ratio, this creates element sizes of 0.5 m along the line.
The global mesh automatically adapts the quad and triangle elements to accommodate this constraint.
The other option would have been to simply choose the "length of" option and enter in the 0.5 m constraint.
The "number of divisions" option defines the number of elements the user desires along the line or edge.
In this case, to obtain the desired 0.5 m element size, we would simply use the line length, which is 7 m, and divide the length by 0.5
giving us 14 divisions along the line.
Now let's assume we have a geo-fabric located along a line in our embankment.
Interface elements are an option in some of the GeoStudio Products
and are given specific material properties using the KeyIn materials window.
These interface elements can be used for creating geo-fabric materials
or no flow barriers in SEEP/W,
thin insulation layers in TEMP/W,
or to assign friction properties between soils or between a soil and a structure in SIGMA/W.
There are two methods that can be used to create interface elements.
The first method is to create the material in the KeyIn materials window
that will represent the geo-fabric or no flow barrier.
Then, when I use Draw materials
I can choose the line option and click on the location that I wish to simulate my barrier.
This will automatically create my interface elements.
Now when I return the Draw mesh properties window,
I can choose the line again and see that there are interface elements with a thickness of 0.2 m.
We can zoom in closer to see the barrier material color, which is green,
is applied only to the interface elements along one side of the line.
It is important to note that
you cannot remove interface elements from a mesh if there are material properties still applied to them.
In order to remove the interface elements, we must go to Draw materials,
click on remove from lines
and remove the barrier material from the interface elements.
Now we can return to the Draw mesh properties window
and remove the interface elements.
To add the elements from the Draw mesh properties window
you can simply highlight the line and click on the "generate interface elements" toggle and define the desired thickness.
Lastly, we will take a look at secondary node options in the draw mesh properties window.
The number of sample or integration points required in an element depends on the number of nodes and element shape.
For example, a triangular element that has a 1 point integration results in a constant gradient throughout the element.
When secondary nodes are present, however,
the interpolating functions are nonlinear
and require a higher integration order.
The table shows acceptable integration orders for GeoStudio Products.
For quadrilateral elements,
there is the option of four or nine point integration with elements with or without secondary nodes.
In this case, however,
the benefits are marginal and higher computing efforts are required when nine-point integration is used.
Generally, the use of 3 point integration for triangular elements and 4 point integration for quadrilateral elements
is sufficient for most simulations.
Secondary nodes are typically used in stress-strain analyses.
If, at any time, the user wants to restart the fine tuning of the mesh or decrease complexity,
the "remove constraints" button can be used to return to the global element size with no mesh constraints or discretization.
Remember, interface elements must not have materials applied to them
prior to removal or else the interface elements will remain within the mesh.
To summarize this video tutorial,
there are two meshing types available:
structured and unstructured.
It is recommended that the unstructured mesh is used for most simulations, especially those with odd-shaped regions.
Interface elements can be used for simulating thin materials along lines within the domain, such as geo-fabrics, thin insulating layers,
or for material behaviour, such as friction behaviour between a soil and a structure.
Remember, to remove interface elements, you must first remove the materials that are assigned to the lines representing these thin materials
and then return to the draw mesh properties window to toggle off the interface elements.
Lastly, secondary nodes and integration orders can be used to increase the performance of the model.
It is recommended that the three point integration is used for triangular elements
and 4 point integration is used for quadrilateral elements.
Secondary nodes are often used in stress-strain analyses.
It is recommended that the user starts with a mesh that is less than 1000 elements at the start of an analysis.
Once a good understanding has been developed on the critical mechanisms in the particular problem
that is being simulated, the user can increase the discretization of the mesh as required.
If too many elements are used initially,
the solution can take a long time to compute
or may become unattainable or results may become difficult to interpret.
Remember, it is better to keep the problem as simple as possible first
and then increase complexity only as necessary
as the main engineering issues become well understood.
We have now reached the end of this tutorial.
Note that not all of the powerful features of version 9 of GeoStudio have been used or discussed here.
Further information on each command can be found in the online help,
supporting documentation,
as well as in other tutorial videos.
Thank you for watching.