This study aims to minimize the noise generated by automobile cooling fans. Fan blade structures with ridged surfaces based on bio-inspired principles are 3D printed and used to replace the conventional fan blades. The effect of the bio-inspired ridge structures on the noise reduction of the cooling fan is demonstrated by orthogonal experiments in a semi-anechoic chamber. Experimental results show that with an increase in the rotational speed, the effect of the surface textures on the acoustic performance of the cooling fan becomes more significant. For example, at a fan speed of 1750 r/min, all the bio-inspired blade designs reduce noise compared with the original fan and, in particular, the sound pressure level is reduced by 3.83 dB(A) for the design with a ridge width of 4 mm and a ridge pitch of 15 mm. Through variance analysis of the measured noise, the ridge pitch distance has the most significant impact on noise reduction under low speed conditions whilst, under high speed conditions, the ridge width has the most significant influence. In addition to the experimental studies, computational fluid dynamics (CFD) simulations of the cooling fan are carried out to explain the mechanism of noise reduction for the ridged fan blades. When the fan runs, the horseshoe vortexes generated by the ridge structures disturb the flow of the boundary layer, reduce the influence of the fluid flow on the boundary layer, and delay the transition of the fan blade laminar flow to turbulence. It is also seen that there is a reduction of the intensity of the fan blade trailing edge vortices and the scale of the secondary vortices, thereby achieving the overall aim of noise reduction. This research has significance in the noise reduction design of automobile cooling fans.

This work makes use of experimental and numerical studies to investigate the reduction of braking noise and vibrations of brake disks by introducing various M-shaped grooves on the brake disk frictional surfaces. Experiments with a brake test dynamometer have been carried out to compare the braking vibrations and noise of the grooved disks with that of the un-grooved disk. The experimental results demonstrate that disks with grooves significantly reduced braking vibrations and noise at both low and high frequencies, and as the initial braking temperature and braking pressure increased, the reduction effect is further enhanced. The investigation also shows the wear rates of both the grooved disks and brake pads are also significantly reduced. Thermo-mechanical coupled finite element models of the brake pads and disks with and without grooves are developed to investigate the mechanisms of the reduction of braking noise and vibrations by the introduction of grooves on the disk frictional surfaces. The numerical results show that the number of grooves plays an important role in reducing the interface surface temperatures, enhancing heat flux, reducing thermal deformation, changing the contact pressure distribution, and stabilizing the coefficients of friction of the braking sliding contacts. The thermal effects contribute to the wear reduction of both the braking disks and pads, and the reduction of braking noise and vibrations. In addition, both the finite element modal analysis and the experimental modal testing results show that surface grooves increase the modal damping ratio of the disks, which certainly plays a role on the reduction of braking noise and vibrations. This study has significance to the surface modification of brake disks in order to reduce braking noise and vibrations, as well as the wear resistance of brake disks and pads.