In 1954, two crashes involving the world’s first commercial airliner, the de
Havilland Comet, brought the words “metal fatigue” to newspaper headlines
and into long-lasting public consciousness. The aircraft, also one of the first to
have a pressurized cabin, had square windows. Pressurization combined with
repeated flight loads caused cracks to form in the corners of the windows, and
those cracks widened over time until the cabins fell apart. As well as being a
human tragedy in which 68 people died, the Comet disasters were a wake-up
call to engineers trying to create safe, strong designs.
Since then, fatigue has been found at the root of failure of many mechanical
components such as turbines and other rotating equipment operating under
intense, repeated cyclical loads.
The primary tool for both understanding and being able to predict and avoid
fatigue has proven to be finite element analysis (FEA).

Computational Fluid Dynamics is a perfect tool for studying rotating components. A glance at such disparate machines as pumps, table fans, axial fans for electronics cooling, and hair dryers, shows that they all have one thing in common: rotating components.

Engineers who design equipment with rotating components need to analyze and understand the behavior of those components if they want to improve performance. For example, if the blades of a table fan are the wrong shape, or if they’re incorrectly orientated, the fan may generate little or no air.

CFD (computational fluid dynamics) helps engineers study many of the issues involved in rotating component behavior. It provides a way to save a great deal of time and money in obtaining the necessary information, and assists engineers in designing better quality rotating equipment.

The use of CFD makes it possible to eliminate expensive physical prototypes, and find serious flaws much earlier in the design process. Starting with some CFD basics, this article will give engineers an overview of how CFD can simulate rotational flow, and will then take a closer look at how users of COSMOSFloWorks can solve typical problems.

Just about every product consists of multiple parts connected by bolts, screws, pins, or springs to create assemblies. Different kinds of assemblies present different simulation challenges. Common to all of them, however, is the need to simulate the connectors that join assembly components – simulation that traditionally took a great deal of analysis knowledge and a lot of time. Dedicated analysts find assemblies difficult and time consuming to analyze, despite their specialized education and experience. For example, to simulate a pin connection – that is, a pair of mated cylinders connected by a dowel pin that permits or restricts rotation between the parts, such as might be used for a pair of pliers – a professional analyst used to have to model the pin that goes through the hinge cylinder and define the gap contact between the pin and the cylinder surface before starting the actual analysis. He also needed to know what size pin to use.

Design engineers, for whom the most important part of the day’s work is product design rather than simulation, aren’t specialist analysts. They are very busy people who don’t have the time to simulate connectors in the traditional way.

What if they don’t have to? What if the software they use is smart enough to do the tough parts for them?

That’s just what COSMOSWorks and COSMOSDesignSTAR can do. The programs contain virtual connectors that make it much easier and faster to analyze assemblies containing pin, spring, bolt and screw connectors. The concepts behind these virtual connectors are the same as those used by generations of dedicated analysts. COSMOS takes no shortcuts in accuracy. It offers a simple interface for the user, calling for straightforward input, while putting many of the tasks formerly performed by analysts into the software – providing complex, accurate results.