Reducing the noise generated by car roof racks.

Abstract

The aerodynamic mechanisms responsible for the production of noise present in the flow field around the Hubco A-bar roof rack have been investigated experimentally. The frequency spectra of sound emissions were determined using sound measurement equipment in the field and in a specially designed low noise wind tunnel. Flow visualisation and hotwire anemometry techniques were also used to establish a model of the flow around the roof rack. Correlation was drawn between flow phenomena and sound emissions. The source of the annoying tonal emission was established to be a periodic vortex shedding from the trailing edge of the aerofoil shaped profile. Treatments to reduce or eliminate this sound emission were developed, based upon changing the flow pattern across the surface of the roof rack. The generation of chordwise trailing vortices near the leading edge of the A-bar was found to be effective in noise abatement by disturbing the formation of the regular vortex street. These chordwise vortices may also break up the spanwise correlation of the periodic wake reducing the strength of the noise emission. Chordwise vortices are generated by installation of cross flow inducing obstructions such as cylinders, hemispheres, serrations and diagonal steps near the leading edge. The treatments have been developed to meet the design requirements for robustness, durability and aesthetic appeal without sacrificing the strength or life of the roof rack itself. The noise treatment techniques chosen for the A-bar were trialled successfully on competitor roof racks.