Aeroacoustics
PowerFLOW is the only CFD tool used productively for aeroacoustics simulations in the automotive industry today. Our customers use PowerFLOW to simulate a wide variety of aeroacoustic phenomena including:
- Windnoise: Hydrodynamic and acoustic pressure fluctuations on the surface of a vehicle
- Buffeting: Low-frequency booming sounds caused by open sunroofs and open side-windows
Key Advantages
Acoustic phenomena are by definition compressible and unsteady – PowerFLOW is uniquely suited for this application: the Lattice-Boltzmann technology on which PowerFLOW is based on is inherently compressible and transient. This allows PowerFLOW to capture all key flow phenomena that are relevant to the aeroacoustic quality of a vehicle. The fact that PowerFLOW can capture both hydrodynamic and acoustic fluctuations – as well as the coupling between these effects – makes it a unique tool in this application area.
PowerFLOW simulations are always transient and time-accurate, and therefore the pressure fluctuations responsible for aeroacoustic phenomena are automatically captured as part of every PowerFLOW simulation, at all points in the flow domain. The timesteps used in PowerFLOW are typically so small that frequencies up to 10KHz and beyond can be captured.
Windnoise study on side mirrors. Model A (left) has a significantly larger aerodynamic wake than Model B (right). Aerodynamic/aeroacoustic simulations can quickly identify areas of wind noise created by components, and model adjustments can be made to make passenger comfort significantly better.
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Windnoise
Windnoise consistently ranks among the top complaints in automotive customer satisfaction surveys, and with improvements that minimize other noise sources such as engine noise and rolling noise in modern vehicles the importance of windnoise will further increase.
With PowerFLOW our customers are able to detect the unwanted aeroacoustic sources of windnoise early in the development process and identify ways to improve them – a long time before this is possible experimentally in the windtunnel.
The types of windnoise that can be simulated with PowerFLOW include:
- Broadband mid to high-frequency fluctuations around the side-windows, windshield and roof of the vehicle. These fluctuations are caused by small and large flow separations driven by geometric details such as side-mirrors, A-Pillars and windshield wipers. PowerFLOW predicts the aeroacoustic noise sources on the outside of the vehicle. By coupling to Statistical Energy Analysis (SEA) tools the transmission of these noise sources into the interior of the vehicle can be computed. Powerful visualization techniques are available to visualize fluctuations for different frequency bands, and to identify and remove their sources.
- Low-frequency fluctuations in the underbody region of the car. Flow separations caused by the complex geometric details on the underbody are responsible for most of the low-frequency noise experienced by passengers at medium and high speeds. PowerFLOW can reliably predict these fluctuations, and then these fluctuations can be used as input for a vibroacoustic analysis in a structural analysis tool to simulate the transmission into the vehicle interior.
- Whistling, howling and hissing sounds. These undesired tonal noises show up as sharp peaks in the frequency spectrum and are caused by period flow separations around small geometry features of the vehicle such as roof racks, windshield wipers, or grille details. These sounds can be clearly identified using the frequency spectra created by PowerFLOW simulations.
Buffeting
Low-frequency buffeting noise caused by open sunroofs and side-windows is among the most vexing aeroacoustic effects and is present at certain velocities in many vehicles. Buffeting noise is caused by a resonant coupling of the pressure fluctuations driven by the periodic flow separation at the leading edge of the sunroof or side-window opening and acoustic modes of the vehicle cabin. Capturing it with a simulation tool requires the coupling between hydrodynamic and acoustic effects – one of the strengths of PowerFLOW.
Using a detailed representation of the sunroof opening and the vehicle interior PowerFLOW simulations at different velocities are performed to determine the onset, peak, and offset velocities of the buffeting behavior. Typical remedies such as deflectors of various shapes and sizes, or more unusual measures which are difficult or impossible to test experimentally, such as variations of size or location of the sunroof, can then be quickly tested in PowerFLOW.
Validation
A combination of validation cases ranging from simple academic cases to full vehicles with full geometric detail has allowed us to thoroughly validate the aeroacoustic capabilities in PowerFLOW. A representative sample of well documented results for both windnoise and buffeting is available to all customers at the Exa User Center.
For more information on Exa PowerFLOW or aeroacoustic applications, please click here.
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