Assessment of aerodynamic noise sources around an automobile-door mirror using a hot-film sensor

The aerodynamic noise sources around an automobile door mirror were assessed using a hot-film sensor. A low-noise wind tunnel was developed to reproduce the aerodynamic noise generated by the door mirror, and velocity fluctuations near the solid wall were measured based on the hot-wire anemometry principle. The pressure fluctuations caused by these velocity fluctuations were expressed as a pressure equation based on the generalized Bernoulli equation. The noise measured outside the cabin reflected the characteristics of the aerodynamic noise generated by pressure fluctuations caused by a solid wall, and was proportional to the sixth power of the mainstream velocity. The velocity fluctuations near the wall around the door mirror, as measured by the hot-film sensor, were larger at the tip of the visor on the mainstream side. These fluctuations increased in the low-frequency band. Computational aeroacoustics (CAA) analysis also confirmed the presence of a strong low-frequency aerodynamic source at the visor tip. The broadband noise measured in the cabin was higher near the door mirror. This broadband noise was attributed to the vibration of the door glass, owing to the turbulence-induced fluctuations near the door mirror within the same frequency domain. The output of the hot-film sensor measured around the door mirror was consistent with the noise characteristics analyzed by the CAA. These results indicate that the hot-film sensor can be used to identify the location of aerodynamic noise sources around door mirrors or to evaluate the relative intensity of the noise sources.