Broadband Noise Reduction by Controlling Vortex Structure in Turbomachinery Blade Row

采用激光多普勒测速技术（LDV）对螺旋桨式通风机内的三维旋涡流场进行了实验分析，并采用大涡模拟（LES）进行了数值分析。研究发现，在动叶尖附近形成了三种涡结构：叶尖涡、前缘分离涡和叶尖泄漏涡。叶尖涡非常强，在叶尖附近的流场中占主导地位。它的形成始于靠近中弦的叶尖吸力侧。叶尖涡几乎沿切向对流，从而冲击相邻叶片的压力面。用大涡模拟研究了涡结构的非定常性质。计算结果表明，叶尖涡对螺旋桨式风机内部旋涡流场的结构和非定常特性起着重要的作用。桨尖涡的螺旋型破裂发生在桨距中点附近。破裂引起叶尖涡的大的周期性运动，使得涡周期性地冲击相邻叶片的压力表面。涡流结构的运动会在转子叶片和叶片通道中产生高压波动，为此，介绍了两种螺旋桨式通风机围带，通过控制通风机转子的叶尖涡流来降低通风机噪音。叶尖涡的切向结构在转子叶尖前缘附近引起向内的径向流动，并在转子叶尖和围带之间引起反向流动，从而对转子内的通流产生较大的阻塞效应。为了减少转子叶尖和围带之间的逆流而设计的围带可以减少由于叶尖涡流引起的阻塞效应，叶尖涡流使转子中的通流减速，从而导致风扇噪声的降低。

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.