Computational Fluid Dynamics Software That's Free, Fast, and Packed With Features: What's Not to Like about OVERFLOW?

The OVERFLOW code is one of the most respected computational fluid dynamics software packages available anywhere. If you are in the U.S. and you are looking for a CFD solution, you owe it to yourself to check it out.

As one of the longest-lasting U.S. government CFD codes, OVERFLOW has been used extensively within NASA, at other government agencies, in academia, and in industry. Peter Buning at NASA Langley maintains the canonical version, over the years, but many different variants have been developed as different people adapted the code to their needs. Notable among these is OVERFLOW-D, by Bob Meakin at NASA Ames. As the new technology in these variants proves its worth, Dr. Buning often merges it into the main version of the code.

A Highly Capable CFD Solver

As a result, OVERFLOW comes complete with a set of features that is the envy of other CFD code developers (including me...I'd love to grab some of the technology in OVERFLOW for Wind-US). Multi-grid, preconditioning, the latest flux algorithms...whether you want a steady-state solution as fast as possible or a high resolution transient solution, this computational fluid dynamics software can do it.

For my work, OVERFLOW has some built-in capabilities which make it the perfect choice for some of my company's applications:

  • Automatic off-body grid Cartesian generation
  • Built-in hole-cutting and block coupling algorithms
  • Automatic domain decomposition for parallel processing load balancing
  • Fifth-order accuracy WENO schemes for explicit inviscid terms
  • Second-order accurate implicit solver (with Newton iterations)
  • HLLC scheme for inviscid terms
  • Option to output a time-averaged flow field in addition to instantaneous snapshots
  • Built-in moving body capability

On top of all that, OVERFLOW is one of the fastest-running codes I've encountered (in terms of time per iteration).

While I can't show you the specific applications we're running (as I write this) with OVERFLOW, I can show you some of the nifty features of the code that we are using. For example, one of the capabilities that has been imported from OVERFLOW-D is the ability to generate off-body grids automatically.

To do this, the user must first generate a set of computational meshes that surround the solid surfaces of whatever configuration is being modeled. In this example, I grabbed the body grids for the “wingbody” test case that ships with this computational fluid dynamics software package (see the figure below). This mesh has four blocks with a total of 900K points.

Body grids around a generic wing-body configuration

The code can then automatically create a series of Cartesian meshes which surround the body grids and smoothly transition from the fine body grids to the coarse farfield computational meshes. For this case, the full grid set ends up being 36 blocks with a total of 1.6 million points. A snapshot of a cross-section of the full grid is shown below.

Automatically generated off-body grids around a generic wing-body configuration

Using the “X-Ray Cutter” algorithm built into OVERFLOW, the regions where the coarser grids overlap finer grids can be “blanked” and everything is coupled together in CHIMERA fashion.

Sample OVERFLOW Application

This Computational Fluid Dynamics software package comes with several test cases so that users can acclimate themselves to the way it does things, and also to ensure that their installation is behaving properly.

One of these cases is a 2-D jet exhausting into a crossflow. The crossflow is air at Mach 4, while the jet in this case is hydrogen at Mach 1. The chemistry is handled in a very simple fashion—no reactions...just different molecular weights.

The movie below shows the Mach contours as the flow develops from the initial conditions.

The following movie shows a similar development, but this time the vorticity magnitude is portrayed.

A Few Limitations

As much as this computational fluid dynamics software can do, it can't do everything. For one thing, it is limited to structured grids. Also, while it solves multi-block grids, it requires overset (not abutting) block-to-block communication. The code's ability to handle chemistry is also limited. And I still haven't figure out how to get it to make coffee either :)

Finally, one of the biggest problems using this code that I have found is that the documentation is somewhat haphazard. A full user's manual exists for version 1.8aa. A developer's manual is available for version 1.5e. The current version as I write this is version 2.1o. To bridge the gaps, there are various “what's new”, “notes”, and “README” files.

As I write this, Bob Nichols, who works for the U.S. Department of Defense User Productivity Enhancement and Technology Transfer (PET) Program (via the University of Alabama Birmingham) has been putting together an updated manual. I'm anxiously waiting for it to be finished—even the incomplete version is a huge help.

[Update: "User's Manual for OVERFLOW 2.1" was released in August 2008. It is consistent with version 2.1t.]

A Perfect Solution for Some Organizations

From the above, I hope you have gotten a taste of what OVERFLOW can do. If you are interested in this computational fluid dynamics software, you should also look at the associated CHIMERA Grid Tools. This is a set of utilities which allows you to prepare grids for use in OVERFLOW, as well as preparing input files. You can even use it to generate a full grid for external flow simulations.

Using OVERFLOW, the CHIMERA Grid Tools, and one of the freely available flow visualization tools, it is possible (for U.S. residents, at least) to assemble a complete computational fluid dynamics software package with no license costs—although the grid generation capability is somewhat limited. If your applications fit within the scope of this CFD code's capabilities, OVERFLOW is a very good code to work with.

If you have any questions or comments, or you have a favorite code you'd like to see reviewed, let me know. I'd appreciate your feedback on OVERFLOW or any other computational fluid dynamics software.

When you're ready, you can browse through some of the other free CFD solver reviews or return to the Innovative CFD home page

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