Aeroacoustics is one of the top concerns in the automotive industry. Sound quality requirements are quite demanding. The most severe aero acoustic phenomena in vehicles are the ones with a tonal nature. Parts directly exposed to the airflow can generate broadband noise, a discrete aeolian tone or even strong flow induced sounds. Usually, all these kinds of acoustic phenomena are undesirable, but the challenge is to find out what exactly causes the sound and how it can be avoided as a single, particular solution optimized for one problem will prove less effective in a different design. In the present case a high-pitched whistling sound occurred when driving the car at about 74mph to 80mph (120 to 130kmh). Additionally, the car showed random vibrations of the engine hood at a speed of 105 mph (170kmh). Under these circumstances the car was not saleable.

Inhouse root cause analysis from the customer didn’t yield a clear and concise result. Some field tests were carried out on the test track, but no consistent solution could be found to address the problem. The time was ripe to call in external specialists who could track down the possible cause and confirm it with well-founded calculation results. That`s where we came in. It was our job at BST to investigate the noise mechanisms involved and to find a solution to avoid it. Hence, carrying out a numerical simulation capable to predict the flow and acoustic behavior was of outermost importance as by means of simulations it is possible to detected specific problems and different configurations can be virtually optimized before a first prototype is built. We aimed to understand the aero acoustic mechanism, which would allow us to strongly reduce the noise sources due to geometry changes and material properties.

In a top-down process we started with a thorough high-resolution analysis of the complete vehicle to identify critical areas, pressure fluctuations, turbulences, the leakage flow etc. As a next step we decomposed the complete vehicle field, created a smaller sub-domain, and applied a computational aero acoustic simulation (CAA). A method, which indeed is quite complex, time-consuming, and costly but the precise results justify the expense. We recognized acoustic fluctuations and identified a leakage – a misflow in a certain area. An open gap could induce secondary flow and some separation zones. Therefore, the fluctuation pressure in the critical zone was analyzed by a Fast Fourier-Transformation (FFT). Obviously, somewhere in the presumably critical zone a design change had to be made to reduce the vortex formation and to fight the sound issue. It became clear that passing the engine hood, the airflow led to a vibration excitation of a standing column of air, like a Helmholz resonator. The air column produced a tonal frequency.

By a detailed analysis all unsteady phenomena could be resolved. We applied a high-resolution CAA and a large eddy simulation, localized the critical area, detected a dominant resonant frequency, and confirmed the results. The analysis resolved all resonance and acoustic phenomena. Therefore, an improvement of the flow induced noise situation could be achieved by modifying the design according to our detailed suggestions.