Incident Turbulence Interaction Noise from an Axial Fan

Abstract

The development of analytical methods allowing the prediction of the noise radiated by an airfoil in a turbulent flow is an active research topic. Its extension to the blades row is still in progress. However, their validation by well measurements is missing. In this paper, a prediction method of broadband noise, due to the incident turbulence and radiated in free field by a subsonic low-pressure axial fan, is reviewed then validated. This approach is an extension of the model initially established for an isolated airfoil placed in a turbulent flow to rotating blades. The formulation providing the noise produced by an airfoil in rectilinear motion is used for the calculation of the instantaneous acoustic spectrum generated by a blade segment considering the rotation effects. The acoustic calculation is based on the Amiet formulation allowing the determination of the far field acoustic power spectral density using a statistical description of the upstream turbulence. A low-pressure axial fan without guide vanes is exposed to a range of turbulent flow fields generated by five different inflow arrangements. Turbulence inflow properties and far field sound were both measured allowing comparison with the predicted results. The mean flow velocity distribution in the plane of the rotor and the turbulent intensities are measured using hot wire anemometry. The local spatial correlation lengths in circumferential direction are derived via a crosscorrelation method employing two hot wire probes at various angular distances. The results thus obtained were used to calculate the power spectral density of the inflow velocity fluctuations, which are the input data for the aeroacoustic model. As expected, the inflow arrangements have a significant influence on both the statistical parameters of the flow field in the plane of the fan rotor and the broadband noise radiated by the fan. The directivity feature of the broadband turbulence interaction noise shows that the main radiation lobe is located along the fan axis. The predicted acoustic power spectra show a good agreement with the experimental results and especially for inflow arrangements generating low turbulence intensity.