Test The Limits

Engineers Exhaust Valuable Time and Resources on a Noisy Challenge

By Franck Pérot

June 03 2015

Why is my exhaust system noisy? Why isn’t it noisy enough? It doesn’t matter which manufacturer they work for: engineers are always looking to optimize exhaust noises… and simulation-driven design has an important role to play in getting them just right.

Exa used its acoustic simulation tool to help one exhaust supplier understand why a system they’d designed for a major European OEM emitted a loud whistle – about 15dB above the broadband levels – at a specific engine speed.

It was the engineers’ first time using computational fluid dynamics [CFD] for this type of job, and they asked us to investigate the rpm level at which the noise appeared. They provided Exa with the geometry of the exhaust and airflow rate that allowed us to run it through a full simulation.

Not only did we go back to them with the specific engine speed, but we also pinpointed the source of the noise. PowerFLOW figured out that the unwanted whistle was being caused by the shape of the perforations inside the exhaust, creating specific vortices resonating inside the system.

And therein lies the beauty of simulation-driven design: physical prototype testing gives little or no feedback to solve the problems identified. It tells you the ‘what’, but not the ‘why’. Exa PowerFLOW, on the other hand, provides the ability to gain powerful insights that lead design and engineering teams to create better solutions…faster.

We were able to demonstrate that, by changing the shape of these perforations from round to more rectangular, we could kill the unwanted whistling experienced by this one manufacturer. This video with audio shows exactly what can be achieved using simulation-driven design.

Adversely, some premium carmakers like exhaust noise, as their models feature an exhaust signature sound that customers love. We are working with one luxury brand to better understand how to create their ideal noise from the exhaust – what should be the size of the cavities, the effect of the perforations, the shape of the exhaust line? These all play a part in pinning down the perfect and most appropriate exhaust note.

Get it right and that noise signature is a powerful tool to help sell cars. Get it wrong, and it can just as easily put buyers off.