Influence of Unsteady Vortex Structures on Noise Reduction of Windscreens

The effects of windscreens on low-frequency wind noise reduction were previously investigated using a steady-state computational fluid dynamics model. The current concentration is on higher frequencies where the wind noise reduction is no longer independent of frequencies, and unsteady fluid dynamics is required to provide pressure fluctuation information on the windscreen surface. Flow across an oscillating cylinder is studied as a model problem. An immersed boundary method has been developed to compute the fluid flow. Pressure fluctuations on the surface of a rigid, impermeable windscreen are obtained from the flow computation. Noise reduction effects inside of the windscreen are then calculated based on the integration of surface pressure distributions caused by unsteady vortex structures. The results show that for a cylinder oscillating at a frequency close to the natural vortex shedding frequency, the peak noise sensed at the center of the cylinder is at twice of the oscillation frequency and its second and third harmonics. For a non-oscillating cylinder, the peak noise sensed at the center is at the vortex shedding frequency itself and its second harmonic.