Aero-acoustics is a study of sound generated by air flows during its interaction with solid bodies. The sound generated by jet engines or the tones produced by blowing wind across the bottle opening are some of the examples of a flow-induced sound. The aero-acoustics as a science began five decades ago, due to the development of aircrafts and the need to reduce noise levels.
However, the development of simulation technologies has opened the possibilities of conducting aero-acoustics study on vehicle applications as well. The Computational Aero Acoustics (CAA) is commonly practiced by vehicle designers to identify design parameters that give rise to noise during vehicle motion.
Recent achievements in reducing the noise produced by engine and tires have resulted in higher contribution of the wind noise, making it one of the top complaints from customers. As such, vehicle manufacturers conduct wind tunnel tests to determine the noise frequencies generated by different vehicle speeds.
However, the ever-growing demand from the market forces manufacturers to shorten the time-to-market the product, making prototype tests time consuming and costly. Thus, much of the design analysis is done numerically, making CAA one of the important design tools for engineers.
Factors Contributing Wind Noise
It has been found experimentally that the wind noise is dominant for the vehicles moving at a speed more than 60 miles per hour, decreasing passenger comfort sitting inside the cabin. Major factors that contribute wind noise are small openings or gaps around doors and windows, windshield rain gutter and outer body components such as door mirrors & roof racks. However, the most prominent contributor of wind noise is the rain gutter provided near the A-pillar. It is a long channel that extends along the A-pillar, provided for the purpose of collecting and draining rain water. Without the gutter, the rain water can flow past the pillar along the side windows, reducing the visibility.
The computational technique for aero acoustic analysis involves implantation of two separate numerical solvers. First, a dedicated CFD solver is utilized to solve the flow equations using RANS, DES or LES models depending on the accuracy of the results required. The flow field solutions obtained through the CFD simulation is then used to calculate acoustical propagation using an acoustic solver.
The most common acoustic solving method is Lighthill’s analogy, also known as ‘Acoustic Analogy’, wherein the Navier-Stokes equations are rearranged. The left hand side of the equation is considered as a wave operator, applied as pressure perturbation; while the right hand side is identified as acoustic sources in a fluid flow. The far-field sound pressure is then given in terms of volume integral over the domain containing sound source.
The CAA study will allow design engineers in identifying exterior profile of the vehicle considering the frequency spectra obtained at different air flow velocity. The optimized profile can then be assessed further using CAA, and the results can be compared against physical prototype experiments.
Image Credit: http://www.computationalfluiddynamics.com.au/using-cfd-to-predict-flow-generated-noise/