水翼尾部形状对水翼绕流及水力阻尼特性影响研究

Karman vortex shedding from the blade trailing edges of hydraulic machines may cause strong vibration and blade crack. Engineering practices show that modification of the blade trailing edge geometry may avoid the Karman vortex resonance, but the physical mechanism behind it has not yet well understood. In this experimental investigation, we simplify a blade of hydraulic machines as a hydrofoil, propose three typical trailing edge shapes, and investigate the relationship between the trailing edge geometry with hydrodynamic damping. Firstly, the wake dynamics and the hydrodynamic response characteristics of the hydrofoil should be carefully measured on the different flow rate conditions. The wake dynamics should include the interaction between the shear layer coming from the pressure and suction sides, vortex shedding process, vortex shedding frequency, pressure fluctuation, velocity and pressure distributions, strength of the vortex and vortex core diameter. Hydrodynamic damping factor is identified by fitting the frequency response function gotten form the hydrofoil hydrodynamic response experiments. The relationship between hydrodynamic damping factor and the reduced flow rate is constructed and its applicable conditions are presented. Finally, the intrinsic mechanism of no “lock-in” phenomenon of the Donaldson trailing edge is revealed, and makes clear the effects of trailing edge shape on the hydrofoil hydrodynamic damping. By considering the hydrodynamic damping affection, we provide engineers with the hydrofoil trailing edge optimization method, which may scientifically guide the blade design for the blades of impeller, stay vanes and guide vanes, so as to further improve the operation stability of hydraulic machines.

水力机械叶片尾部的卡门涡会导致机组强烈振动和叶片裂纹。工程实践表明，修改叶片尾部几何形状有可能避免卡门涡共振的发生，但其背后的机理却尚未被揭示。本研究以试验为主要研究手段，针对3种较为典型的水力机械叶片尾部形状，以水翼为主要研究对象，开展尾部形状与其水力阻尼特性关系的研究。首先获取水翼在不同流速条件下的尾流区流场特征（包括尾部正背面剪切层干涉及旋涡脱落过程、脱落旋涡频率、脉动压力场和速度场分布、涡强度和涡核直径等）和水翼水动力响应特性参数；通过不同流速下的水翼振动响应试验识别出水力阻尼参数，构建不同尾部形状水翼的水力阻尼参数与流速的关系，并明确其适用条件；最后揭示Donaldson尾部形状水翼不发生“锁频”现象的内在机理，并给出尾部形状对水力阻尼的影响，最终提出考虑水力阻尼特性的水翼尾部形状优化方法，以期科学地指导水力机械叶片尾部形状设计，为进一步提高水力机组稳定运行提供科学指导。

水力机械叶片尾部的卡门涡会导致机组强烈振动和叶片裂纹。工程实践表明，修改叶片尾部几何形状有可能避免卡门涡共振的发生，但其背后的机理却尚未被揭示。本研究通过试验和数值计算研究了不同尾部形状水翼绕流及其水力阻尼特性。主要研究内容及创新性成果如下：. （1）设计了水翼水力阻尼实验，在水翼表面嵌入激振片给予水翼激振，最大程度的减小了对水翼附近流场的干扰作用，有效的克服了高速水流环境下水声噪音的影响。通过测试获得了不同流速和共振模态下的水力阻尼参数值；. （2）明确了水力阻尼比与流速之间的关系。水翼固有频率随着流量的增加略有增大，在共振发生的流量点之前，水力阻尼比保持定值之后线性增加，该规律与水翼尾部形状无关；. （3）获取了水翼在不同流速条件下的尾流区流场特征（包括尾部正背面剪切层干涉及旋涡脱落过程、脱落旋涡频率、脉动压力场和速度场分布、涡强度和涡核直径等）和水翼水动力响应特性参数；. （4）建立了较为可靠的水力阻尼数值预测方法，分析误差在20%以下。将水翼尾流流场的水动力学参数与结构振动响应联系起来，提炼流场特征参数与水力阻尼参数的关系，提出了一种通过观水翼测流场特征来预判其水动力学响应的方法。