It is not uncommon these days for you to build a grid system that contains hundreds if not thousands of boundary condition patches. Likewise models often have hundreds of volume condition regions as well. Many of these boundaries are artifacts of the grid generation process (blocking strategies and construction entities) and not necessarily representative of the physical part. The problem is that each of these entities (BCs and VCs) take up resources which can have a significant impact on the time it takes to read or write a DTF file. Not to mention the complexity of working with a model that has 2000 BCs from which you are trying to pick the single BC that needs to be set to isothermal @ 350K!

This note describes a process that can be performed on most DTF files to "simplify" them. After simplifying the file you will be left with a DTF file that contains a single zone and has only enough BC and VC records needed to describe the model. That is, all BC/VC records with the same information (name and values) will be combined into one record. This can often take a model file with more than 1000 BCs and simplify it to a model with less than 10 BCs. This greatly speeds up file read/writes and makes model setup much more easy and error free.

Notice in the table below the drastic change in the number of entities and file load times for the "simplified" versions of a typical fuel cell or hard disk drive model file.

If you have ever thought it would be nice to reduce the number of BCs or VCs that you have to deal with, then please read on for an example of how to simplify your file.

Note: All files used for this example can be downloaded by clicking here.

Let's look at a simple example of a cylinder geometry where the bottom side of the cylinder must be broken into several unique patches for specifying special boundary conditions, while the outer side of the cylinder will be considered a wall and the top of the cylinder will be an outlet. The entire volume of the cylinder will be considered as a single fluid region.

View from Bottom (cyan=wall, yellow=inlet)

View from Top (cyan=wall, green=outlet)

The easiest way to make this grid system is to grid the bottom (unique) side of the cylinder and then extrude it to make the full cylinder. In this case we have made a prismatic grid system. So 2D unstructured domains were made and then extruded to make the 3D prism grids.

At the completion of the gridding we have 17 volumes (one for each prism domain) and 100 boundaries (tops, bottoms, sides, and internal interfaces between the domains). The picture below gives you some indication of the complexity of the grid system (several surfaces have been removed for clarity).

Now for the trick! Based on the problem spec given above we know that we only need to physically specify 1 volume region (since everything is fluid) and 4 unique boundary conditions:

• INLET on the bottom pieces
• WALL on the bottom pieces
• WALL on the side
• OUTLET on the top

Before we can simplify the model we need to make sure that we have uniquely identified the boundaries. In this case we have used CFD-GEOM's BC/VC editor to set the BC names and types appropriately (e.g., bottom_inlet, bottom_wall, side_wall, and top_outlet).

We can then use the dtf_decompose utility with the -combined option to automatically simplify this file for us.

dtf_decompose -combined -file_out cylinder_simple.DTF cylinder.DTF 1 1

The above command should be run from the ms-dos command prompt or unix shell. (Make sure you are in the directory where the model DTF file is located!)

The syntax can be explained as follows:

dtf_decompose

The utility we are going to use. Included with the ACE_SOLVER package and must be version 3.0.24 or greater (You can check the version you have by running "dtf_decompose -v" from the command line. On MS-Windows systems the version number will be written to a file dtf_decompose.version in the current directory.)

-combined

The -combined option will collect all of the BCs which have the same name and same properties into one record. Likewise for VCs.

-file_out

The next argument will be the output DTF file

cylinder_simple.DTF

This is the new file that will be created from this operation

cylinder.DTF

This is the input file which will be processed

1 1

First number is the sim# to process in the input file (almost always will be 1).
The second number is the number of zones to be created in the output file (must be 1).

When you load the simplified file into CFD-ACE-GUI you will see only 4 boundary conditions to pick (hence the "grouping" has already been done for you).

This was a simple example but this trick can be used on many models to help make your problem setup easier and on more complicated examples this will often cut file read/save by an order of magnitude. Try this process the next time you are faced with a complex model where simplification might make your modeling efforts easier and more efficient.

If you have any questions about this feature or would like us to discuss some other topic in the future, please let us know.

Regards,
Richard Thoms
ESI CFD Customer Support

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