The aerodynamic sources acting on a car side window, generally defined as the small wavelength sources such as turbulent eddies and boundary layer structures, are generally much larger in amplitude than the acoustic sources, those with longer wavelengths. So naturally you would expect that the aerodynamic sources have a much greater contribution on vehicle interior noise, right?
But frequently that’s not that case. The reason is that body structures are typically trimmed to maximize sound absorption and insulation so very little sound enters the cabin through them. This means that most of the aerodynamic and acoustic noise entering the cabin comes through the windows and they are particularly sensitive to acoustic sources. In fact, one group of researchers determined that in some situations modifying the flow structure to reduce aerodynamic pressure fluctuations can have minimal impact on the noise reaching the passenger’s ears.
The approach of using compressible computational fluid dynamics (CFD) to solve the fluid dynamics and acoustics in a single solution is not capable of distinguishing between aerodynamic and acoustic sources. The alternative is using incompressible CFD to determine the fluid dynamic loading and an aeroacoustic code to determine the acoustic loading.
The Actran aeroacoustics code now offers two methods – wave number decomposition and pellicular mode decomposition — to identify the acoustic contribution to the pressure fluctuations on a car side window or other surface. A team of MSC engineers recently used both of these methods on compressible CFD results applied to the Hyundai Simplified Model (HSM). The two methods provided basically consistent results, although there were some variations that will be investigated in a future study.
Visit www.mscsoftware.com/product/actran-acoustics to obtain more information on Actran acoustic simulation software.