This paper studies vibrissa shaped cylinder as a passive control method for reducing the aerodynamic noise of the flow past a cylinder. One elliptical cylinder is also investigated for comparison. The far-field noise results show that compared with the cylinder case, the tonal peak almost disappears in the vibrissa shaped cylinder case and the sound pressure level (SPL) could be reduced by up to 30dB within the velocity range from 8m/s to 35m/s. The elliptical cylinder only shows a slight SPL reduction by about 4 dB with a shift in the tonal peak frequency from St=0.2 to St=0.39. Furthermore, some noticeable variations occur in the noise directivity pattern for the vibrissa shaped cylinder case. The shift in the tonal peak frequency, the reduction in noise and modifications in the noise directivity is related to the changing and breakdown of the vortex shedding caused by the interference with the elliptical and vibrissa shaped cylinder surfaces. The LES is conducted to the elliptical and vibrissa cylinder case for the fluid field information to further understand the mechanism of the noise reduction due to the vibrissa shaped cylinder and elliptical cylinder.
A numerical investigation is conducted on aerodynamic noise of flow past a circular cylinder with different spanwise lengths (0.5πD, πD, 2πD, 4πD) at Re = 10, 000, where D is the diameter of the cylinder. The near-field pressure and velocity fields are predicted through Large Eddy Simulation (LES), then the acoustic analogy is used to obtain the far-field noise. The results show good agreements for both the near and far-field with the data from in-house experiments and the literature. Though the spanwise length has limited influence on the power spectral density of the near-field velocity and pressure fluctuations at different spanwise locations, substantial differences are observed for the spanwise pressure coherence and near-wake structures. The 0.5πD case shows primarily two-dimensional flow features immediately behind the cylinder compared to the other three cases, resulting in the overprediction of the spanwise pressure coherence, which has strong implications for the far-field noise prediction. With the spanwise length correction, the differences in overall noise magnitudes of the different cases diminish. Nevertheless, the 2πD and 4πD cases better capture the first and second harmonics of the vortex shedding and its associated directivities than the other two cases, showing the importance of sufficient spanwise lengths in predicting noise from flow past a cylinder.
Understanding of the turbulent flow generated by the flow past a bluff body, such as cylinder, is very pertinent to many modern engineering applications. This paper aims to investigate the effect of spanwise length of flow past a circular cylinder on the near-field aerodynamic characteristics at Re=10,000, using Large Eddy Simulation with the WALE subgrid-scale model. Four different spanwise lengths are considered for the computational domain, namely Lz=0.5πD, πD, 2πD and 4πD. For all cases, the pressure coefficient 𝑪 𝒑 , drag coefficient 𝑪 𝒅 , tone frequencies of PSD are predicted with a reasonable accuracy compared to experiments. The results have shown that changing the spanwise length of the LES case has very limited impact on the pressure coefficient 𝑪 𝒑 , the velocity PSD and the pressure PSD at different spanwise locations. The 𝑪 𝒑 𝒓𝒎𝒔 is overvalued by the LES, and the growth of the spanwise length Lz improves the agreement of the simulated 𝑪 𝒑 𝒓𝒎𝒔 with the experimental data. The spanwise pressure coherence is also analyzed. The spanwise pressure coherence value is observed to decrease with larger spanwise distances between probes. The LES cases with 2πD and 4πD spanwise lengths show relatively smaller coherence value and higher coherence decay rates, indicating they capture more three-dimensional flow features which is important for accurate prediction of the radiated noise of the flow.
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