Aeroacoustics of Heavy Duty Truck Side Mirrors - An Experimental Study

Side mirrors are a known source of aerodynamically generated noise in vehicles. In this work we focus on mirrors for heavy duty trucks, they are large, often not designed with main focus on aero-acoustics and are located in a cumbersome position on the up-right A-pillar of European trucks. First the test method itself is discussed. To allow fast and cost effective design loops a bespoke vehicle, where the powertrain is separated from the cab, is developed. This vehicle can be run on a standard test track. While running the tests the wind speed is monitored, any variations are then compensated for in the post processing allowing averaging over longer time periods. For the mirror tests the door of the vehicle was especially trimmed to reduce other transmission paths into the cab than the side window. Additionally other possible aeroacoustic sources were reduced as much as practically possible. The generated wind noise was monitored with surface microphones both on the mirror (in the wake) and on the window. Additionally arrays of microphones were placed inside the cab and also accelerometers on the window. First the method is evaluated using a dummy mirror that basically is a Strouhal tone generator. Then actual mirrors were tested. It is seen that although the hydrodynamic turbulence noise dominates at the surface microphones on the window, the noise that actually is seen inside the cab is the acoustic sources generated by the separation around the mirror and A-pillar and convected at the speed of sound to the window.