The Myths of CFD

Dec. 13, 2007
Computational fluid dynamics works better, and is easier than you think.

Ivo Weinhold
Flomerics Inc.
Marlborough, Mass.

Computational fluid dynamics (CFD) has long had the reputation of being too difficult, too slow, and too expensive for everyday bench-level engineers. A decade ago, this was true. But CFD has changed. Today, it is easier, faster, and less expensive. The disadvantages of earlier CFD programs, however, persist as myths and stand in the way of engineers using CFD in the early phases of design. These myths help explain why only about 30,000 out of over 1 million mechanical-design engineers worldwide use CFD to simulate fluid flows inside and around their products.

Myth #1: CFD is too difficult
A recent survey of mechanical design engineers found that 58% believed that most engineers don’t have the expertise or knowledge to use CFD. This is most likely because CFD programs of a decade ago, and many still today, require that users understand the computational aspects of fluid dynamics to get accurate results. For example, they need to know how to translate models into the CFD environment, then reverse the model so the software models the empty flow space instead of the solid product. They must also create a proper mesh, determine the right boundary conditions, then tweak the solver settings to ensure convergence. These tasks made CFD the province of skilled specialists which could not be performed as part of everyday design.

But in the last few years, CFD programs have become easier to run. For example, most CFD packages use native 3D CAD data, automatically grid the flow space, and manage flow parameters such as object-based features. They also automatically ensure convergence in almost every application without manual tuning. And the software controls mesh quality to avoid one of the biggest reasons for run divergence. In short, CFD software eliminates the need for engineers to understand the computational part of CFD and lets them focus on the fluid dynamics.

The skills needed to use CFD software are simply knowledge of the CAD program and physics of the device being designed, both of which design engineers should already have. Engineers are thus able to focus their time and attention on improving the design as opposed to operating the software.

Myth #2: CFD takes too long
CFD has long been primarily used as a validation tool for completed designs. That’s because it took so long and couldn’t be run on initial designs without throwing the project way behind schedule.

Creating a good mesh, for example, seemed to take forever. CAD geometry had to be manually translated into a format the CFD package understood. Then cavities had to be extracted and meshed. Even though automatic meshers have been around for a long time, they usually took a considerable amount of manual intervention to maintain mesh quality. Users had to eliminate gaps and overlaps and maintain the required skewness, aspect ratio, warpage, and volume of individual cells. And these manual processes had to be repeated for every design change.

Newer CFD programs shorten analyses by automating these steps. Native 3D CAD data is used directly for fluid-flow simulations without translations or copies. Ancillary data for flow simulation, such as material properties and boundary conditions, is associatively linked to the CAD model and carried along with all design changes. The CFD software analyzes the CAD model, identifies fluid and solid regions, and creates a mesh that doesn’t need to be fixed. So new parts and design changes can be meshed in a matter of minutes.

Myth #3: CFD is too expensive
The cost of CFD software once limited its use. It could cost $25,000 for a one-year lease. And it cost even more as companies had to hire experts to run the software. And until recently, CFD software needed specialized hardware, such as supercomputers, to provide timely results.

The latest CFD software brings down costs. The software, for example, has fallen to about $25,000 for a perpetual license, with about $4,500 in annual maintenance fees. And users do not need to be math experts to run today’s CFD programs, just typical engineers with minimal training. The software runs inside the same CAD environment engineers are already familiar with and there is no need to understand the math behind CFD. Finally, the latest generation of CFD software runs on PCs and laptops, which have the power of yesteryear’s supercomputers but cost only a few thousand dollars.

Myth #4: You can’t use CAD models for CFD analysis
In the past, users had to copy or translate CAD models to a different program to create a CFD model. That’s because the tools needed to let CAD models work in CFD programs took a considerable amount of manual intervention. Translations might only work for 80% of a model. The rest had to be recreated or simplified by hand. Many engineers found it more efficient to start from scratch, recreating the entire model in the CFD program despite the extra time it took.

Current CFD software starts with 3D-CAD data to simulate flow, not translations or copies. All design changes are done in the CAD program using familiar modeling tools. There’s no need to create phantom objects in the feature tree to represent fluid spaces. Flow conditions are defined on the CAD model and organized much like other data in the feature tree. As a result, original CAD models can be used for CFD analysis without modification.

Myth #5: Most products don’t need CFD
In the past, CFD was concentrated in a few industries such as automotive, aerospace, and power generation, industries making high-value products in which fluid flow was an obvious and major component. This led to the belief that CFD only adds value to high-value designs. Actually, fluid flow affects performance for a wide range of products, performance that could be improved by CFD analysis. This explains why CFD is being used to design products as diverse as swimming pools, plumbing supplies, sprinklers, gas meters, printers, disk drives, and oil filters.

Many engineers still think CFD software is difficult, time consuming, and expensive. But the latest generation of software has addressed all these concerns. The new software adds CFD to design, so engineers can focus on problems and solutions. CFD should be helping engineers get reliable answers and insights, and deliver better performance, lower costs, and get products on the shelves quicker.



CFD is often thought of as an exotic method of looking at air and fluid flows in designs of high-end products, such as in this analysis of a racing car from Ceemo Engineering in the Netherlands. Advances in software, however, have made CFD available to designers of everything from nuts and bolts to nuclear reactors.

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