Test The Limits

Don't let NVH Issues Ruin a Roof-down Cruise

By Exa

December 02 2015

Convertibles made a bit of ‘noise’ at the Los Angeles Auto Show last week, with the unveiling of the Range Rover Evoque Convertible, Volkswagen Beetle Dune and Fiat 124 Spider. You could say that with temperatures hitting 75°F in November, LA was the perfect place for new convertibles to raise the roof. But was anyone thinking about NVH challenges with the roof down?


Dropping your top is bad news for emissions because it creates an ugly wake structure that increases aerodynamic drag. Exa’s market-leading PowerFLOW CFD software provides unrivaled ability to deliver low-drag vehicles, but style-conscious convertible buyers tend to be more worried about the comfort issues generated by lowering the roof than a higher fuel bill. In a poorly designed convertible, the wind will recirculate back from the rear on to the occupants’ heads. Clip-in mesh screens behind the seats can help to alleviate the problem, but new simulation software could provide a more elegant solution.


Exa has been helping OEMs address such challenges for many years, and PowerFLOW has long been the established leader in aeroacoustic simulations. Now the company has launched new software: its patent-pending FIND (Flow-Induced Noise Detection) capability within PowerACOUSTICS, Exa’s state-of-the-art solution for flow-induced noise analysis.


What makes FIND special is its ability to analyze the fluid flow of a design and rank the different noise sources in order of importance, providing engineers with the insight they need to target design changes in the right areas. The noise generated can also be listened to before and after design modifications, so that the impact of any proposed improvements can be aurally assessed. As always with Exa's solutions, this empowers engineers to understand issues early in the development process. Product designs can then be improved in a digital environment, long before costly physical prototypes – that may provide no insight – have been built.


BMW Group has already been using FIND to great effect in the aeroacoustic development of heating, ventilation and air conditioning (HVAC) systems, the noise from which is one of the top complaints for automotive passengers. BMW had previously found that analysis of the flow through standalone components such as ducts or blowers gave incomplete information on the acoustic performance of the whole system. Worse still, it had been found that it could be misleading to decide on which sub-system to improve, based solely on the sounds it generated.


“We initially used PowerFLOW and PowerACOUSTICS FIND for the BMW 7 Series and Rolls-Royce Phantom,” says BMW’s Group Leader for Overall Vehicle Development, Dr.-Ing. Michael Spickenreuther. “We were so impressed with the noise sourcing capability that we have now implemented FIND across all our platforms. FIND is enabling us to deliver quieter cars.”


BMW applied FIND to the analysis of HVAC systems, but the technology is equally applicable to any aeroacoustic simulation task, including the buffeting generated by dropping the roof on a convertible.


“This capability applies to all aeroacoustics applications across multiple industries, such as wind noise and HVAC systems noise in automotive, landing-gear noise in aerospace, or applications in the rail industry,” says Dr. Franck Pérot, Sr. Director of Acoustic Applications for Exa. “FIND provides significant additional value to our clients through these breakthrough capabilities that were not available anywhere, until now.”


For more on PowerACOUSTICS FIND, click here.