Seiji Nakashima

Aerodynamic Noise Simulation of Propeller Fan by Large Eddy Simulation

In this paper, unsteady flow and aerodynamic noise are numerically investigated for a half-open type propeller fan used for outdoor air conditioner components. The flow field is calculated by Front Flow/Blue, which is based on Large Eddy Simulation (LES). The Standard Smagorinsky Model (SSM) and Dynamic Smagorinsky Model (DSM) were used as sub-grid scale models. Aerodynamic noise was calculated by Curle’s equation based on the pressure fluctuation on the blade surface computed by LES. The computed static pressure rise of the fan showed reasonable agreement with the measured equivalent. The time-averaged distributions of the three velocity components downstream of the blades were also compared with those measured by hotwire anemometry, which showed satisfactory agreement between the computed and measured velocity profiles. But the tip vortex passage which was detached from the blade surface predicted by LES was not stable as measured by the experiment. Finally, the predicted far-field sound spectrum agrees reasonably well with measurements in a frequency range of 100 to 1000 Hz although the sound pressure level was underpredicted in the lower frequency range.© 2007 ASME

Grazing Flow Effect on Surface Impedance with Circular Perforations of Small Diameters

It is well known that introduction of perforations in the walls of flow surface causes not only significant sound attenuation but also reduction of aerodynamic sound radiation from a rotating wing. In order to enjoy the largest advantage of perforated surface, the optimal distribution of finite impedance may be realized, which means the impedance characteristics of the surface must be predicted under the grazing flow condition. In this paper grazing flow effect on the surface impedance was investigated by conducting a numerical simulation, where the diameter is 1mm and the Reynolds number based on the diameter ranges from 44 to 93. At the same time, the acoustic impedance without grazing effect was investigated by using the linear one-dimensional flow model, which is utilized as the limit impedance value resulted from zero flow speed. The complex surface impedance ratio marks far larger magnitude than the conventional impedance for larger diameters and higher Reynolds number, which reflects the effect of high viscosity of the flow.