Reduction of aerodynamic noise from square bars by introducing spanwise waviness

This paper presents an investigation, using both numerical and experimental methods, of the application of spanwise waviness to reduce aerodynamic noise from square bars. The numerical simulations are performed using the Delayed Detached-Eddy Simulation approach to obtain the near-field unsteady flow properties, which are then used to calculate the equivalent source terms in the Ffowcs Williams-Hawkings equation for far-field noise prediction. For a straight square bar in cross-flow, which produces strong tonal noise associated with the vortex shedding, a benchmark study shows good agreement between numerical simulations and measurements in terms of far-field noise spectra. Waviness is then introduced along the bar span and the influence of the amplitude and wavelength of the waviness is studied. When the wave amplitude is nearly half the bar width, a large noise reduction of as much as 30 dB is found from both numerical simulations and measurements, including a 10 dB reduction in the broadband level. The influence of the wavelength is much smaller. Analysis of the flow features show that, with increased wave amplitudes, the spanwise flow becomes significant and strong crossflow vortices develop in the near wake which effectively suppress the primary vortex shedding. This reduces the noise level significantly, especially the tonal noise associated with the vortex shedding.