Broadband Noise Reduction by Controlling Vortex Structure in Turbomachinery Blade Row

采用激光多谱勒测速仪(LDV)的实验分析和大涡模拟(LES)的数值分析，研究了螺旋桨风机内部涡流的三维结构。结果表明，在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。叶尖涡很强，在叶尖附近的流场中占据主导地位。它的形成从靠近中弦的叶尖吸力侧开始。叶尖涡近于切向对流，冲击到相邻叶片的压力面。大涡模拟研究了涡旋结构的非定常性质。数值模拟结果表明，螺旋桨叶尖涡对螺旋桨风机内涡流的结构和非定常行为起着重要的作用。叶尖涡的螺旋型破裂发生在中桨距附近。破裂导致叶尖涡的大范围周期性运动，从而使其周期性地冲击相邻叶片的压力面。旋涡结构的运动会在转子叶片上和叶片通道内引起高压波动，为了降低风机噪声，引入了两种螺旋桨风机护罩，通过控制风机转子的叶尖涡流来降低噪声。叶尖涡的切向结构在转子叶尖前缘附近引起径向内流，并在叶尖和护罩之间产生反向流动，对转子内的通流产生很大的堵塞作用。为减小叶尖和叶冠之间的反向流动而设计的护罩可以减小叶尖涡流的堵塞效应，从而减慢转子内的通流速度，从而降低风扇的噪声。

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.