Broadband Noise Reduction by Controlling Vortex Structure in Turbomachinery Blade Row

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Three-dimensional structure of the vortical flow field in a propeller fan has been investigated by experimental analysis using laser Doppler velocimetry (LDV) measurements and by numerical analysis using a large eddy simulation (LES). It is found that three vortex structures are formed near the rotor tip : the tip vortex, the leading edge separation vortex and the tip leakage vortex. The tip vortex is so strong that it dominates the flow field near the tip. Its formation starts from the blade tip suction side near the midchord. The tip vortex convects nearly in the tangential direction, thus impinging on the pressure surface of the adjacent blade. The unsteady nature of vortex structures has been investigated by LES. The simulation shows that the tip vortex plays a major role in the structure and unsteady behavior of the vortical flow in the propeller fan. The spiral-type breakdown of the tip vortex occurs near the midpitch. The breakdown gives rise to large and cyclic movements of the tip vortex, so that the vortex impinges cyclically on the pressure surface of the adjacent blade. The movements of the vortex structures induce high pressure fluctuations on the rotor blade and in the blade passage.Two types of shroud for propeller fans were introduced to reduce the fan noise by controlling the tip vortex of the fan rotor. The tangential structure of the tip vortex induces an inward radial flow near the leading edge of the rotor tip and a reverse flow between the rotor tip and the shroud, causing a large blockage effect on the through flow in the rotor. The shroud designed so as to diminish the reverse flow between the rotor tip and the shroud can decrease the blockage effect due to the tip vortex, which decelerates the through flow in the rotor, thus leading to the reduction in the fan noise.