Numerical investigation of aerofoil noise

With the sustained increase in air travel, noise from aeroplanes remains a significant environmental problem. Due to the considerable reduction in jet noise that has been achieved by designing turbofan jet engines with increasingly large bypass ratios, for modern aircraft in approach, fan noise and airframe noise are among the most important contributors to the perceived sound on the ground. At the same time, aerofoil noise from onshore wind turbines considerably limits their public acceptance despite the economical and political need for renewable energy production. Therefore, a detailed understanding of the physical mechanisms responsible for aerofoil noise and accurate prediction methods would be highly beneficial for a wide range of applications. A large percentage of the overall aerofoil noise can be attributed to aerofoil self-noise, i.e. noise produced by the interaction between the aerofoil with its own boundary layers and wake, with trailing edge noise being the dominant noise source. For that reason, most currently used noise prediction models consider trailing edge noise only.A preliminary fundamental study has shown that for cases where separation events occur on aerofoils, the current noise prediction models are not adequate because noise sources other than trailing edge noise exist. Therefore, there clearly is a need for a detailed investigation of noise generation mechanisms on aerofoils at moderate Reynolds number. For this type of research, it is paramount to perform Direct Numerical Simulations (DNS) to eliminate the disadvantages encountered when using Large Eddy Simulations, such as uncertainties with modelling small scale turbulence, and problems with predicting laminar-turbulent transition. Until recently, DNS of flow separation events were only possible for simplified geometries, such as flat plates, and could not include a trailing edge or the interaction of the separation with the potential flow. However, with the current and future generations of supercomputers, DNS can now be performed of entire aerofoil configurations.