低固相含量液流中磨蚀翼型空化特性研究

针对水力机械内部空蚀及泥沙磨损联合作用破坏问题,探索空化促进磨损破坏的方式.在闭式循环装置的收缩-扩散实验段中,采用激光-CCD系统获取低固相含量下空化场中翼型表面多个断面的空穴结构分布,分析了空穴结构空间分布规律及其初生、发展、脱落、溃灭的周期性行为特征.针对空穴结构分布,采用了一种基于统计原理的图像处理方法,获得了空泡云平均值及标准差值的灰度分布,前者为流场中各位置的空化程度,后者为各位置空泡云体积变化率的分布.结合已有的空蚀磨损研究成果认为,水力机械内部空化促进颗粒磨损的方式取决于空穴结构及其体积变化率在流场中的分布规律.     

基于数字图像处理的柔性水翼形变测量

针对振荡水翼式潮流能发电装置中柔性水翼的形变测量问题,设计了一套基于高速相机的非接触式测量系统,可在不影响水翼水动力学特性的前提下,连续采集水翼运动过程中的图像并计算水翼形变。建立了图像坐标系和水翼世界坐标系的坐标转换模型,给出了相关参数的标定方法和形变的计算公式。提出了一种基于Canny算子的水翼迎流面边缘检测方法,给出了边界的多项式拟合算式,从而可计算出水翼迎流面边缘上任一点的形变。设置了不同实验条件进行了验证,识别出的水翼总长度误差在2%以内,形变量符合实际规律。实验结果表明该方法能够有效测量水翼迎流面边缘上任一点的形变,且具有现场布置简单、不影响水动力学特性的优点。     

非对称尾部形状水翼水力阻尼识别方法研究

水力阻尼是影响流激振动幅值预测精度的关键参数,是水力机械流激振动领域研究的热点问题。非对称尾部形状水翼在涡激振动和升力的联合作用下,振动响应的平衡位置具有时变特性,采用传统自由振动衰减法获得的水力阻尼比误差大幅度增加,甚至失效。为了克服传统自由振动衰减法应用局限,本文借助双向流固耦合数值模拟方法获得流激振动响应位移,通过带通滤波结合平衡位置校准,研究了动水环境中对称和非对称尾部形状水翼水力阻尼的识别方法。结果表明,数值模拟可较准确获取低阶结构模态和尾部旋涡脱落频率,相比实验结果,低阶弯曲模态频率、低阶扭曲模态频率和15 m/s流速下脱落涡频率最大偏差分别为7.58%、2.90%和1.42%;带通滤波可消除周期性涡激振动对响应信号的影响,水力阻尼比识别偏差度从7.51%下降到1.92%;平衡位置校准方法可采用多项式拟合法、线性插值法和光滑样条曲线法,所对应的水力阻尼比识别偏差度分别为34.93%、3.53%和0.16%。工程上,可优先推荐滤波结合线性插值法,在需要高精度水力阻尼比的场合,则必须采用滤波结合光滑样条曲线法。     

仿海蟹机器人浮游步态动力学建模与运动控制

为实现足桨耦合推进仿海蟹机器人在未知海流扰动作用下对目标点的跟踪控制,对仿海蟹机器人浮游步态的动力学和运动控制进行研究.综合考虑重力、浮力、游泳足拍动产生的推力以及水动力的影响,建立了仿海蟹机器人水下复杂环境的动力学模型.在此基础上,设计了一种基于指数趋近律的滑模变结构控制器,将游泳足上下拍翼运动和摇翼运动的相位差作为被控量,对机器人的转艏角速率进行控制.通过李亚普诺夫直接法,证明该系统可实现全局渐近稳定.最后进行了单一目标点和多目标点跟踪运动仿真和实验,结果表明:该方法可以使机器人具有良好的目标点跟踪能力,并对系统动力学参数不确定性及外界扰动具有较高鲁棒性.     

水力机械过流表面空蚀程度的图像检测

在闭式循环水洞即空蚀磨损试验系统中,对铝制raf61翼型,在空化数为0.6的条件下进行空蚀程度研究.传统的量化方式难以满足测量要求,提出了数字图像检测的方法.获取翼型表面SEM空蚀图像,针对图像直方图单峰分布进行背景灰度级修正,采用Canny算子检测空蚀坑洞边缘,二值形态学方法统计计算空蚀破坏表面参数.检测结果表明:该表面蚀坑数为1.54个,空蚀破坏表面积为8.3865 ×10~4个像素,图像测量检测有较高精度.     

搅拌槽内粘稠物系中气液相间氧传递

以发酵罐中气液相间氧传递为背景，考察了搅拌槽内搅拌器形式、物系流变性质、通气搅拌操作条件等对假塑性粘稠物系中氧传递过程的影响。结果表明，这些因素主要通过改变气体分散状态和相间传质面积来影响氧传递速率。根据气泡在搅拌槽内不均匀分布现象，多层搅拌下气液相间传质过程可以用气泡运动分区分布模型来描述。它说明了采用轴向流桨和涡轮桨组合的搅拌形式在氧传递方面的优越性，为强化发酵罐中供氧指明一条有效途径     

Multi-point design optimization of hydrofoil for marine current turbine

The comprehensive performance of the marine current turbine is an important issue in the ocean energy development. Its key is the performance of the hydrofoil, which is used to form the turbine blade. A multi-point optimization method of the hydrofoil is proposed in this paper. In this method, the Bezier curve is used to parameterize the hydrofoil. The geometrical parameters are used as variables while the lift-drag ratio and the cavitation performance of the hydrofoil are used as the objective functions. The NSGA-II algorithm is chosen as the optimization algorithm. In order to resolve the difficulty of this high-dimensional multi-objective optimi- zation problem, the conception of the distance metric in the metric space is introduced to unify the lift-drag ratio and the cavitation performance under different working conditions. And then, the above optimization method is applied in the NACA63-815 hydro- foil＇s optimal design under three typical conditions. Finally, the results from the performance comparison of the original and optimi- zed hydrofoils obtained by using the CFD simulation are analyzed in detail. It is indicated that the optimized hydrofoils enjoy a better hydrodynamic performance than the original ones under the three conditions. The feasibility and the theoretical validity of this optimization method are confirmed by the results.     

水力机械叶片尾流机理研究

建立了基于弱可压缩流体基础之上的非恒定、粘性流数学模型,采用大涡模拟计算方法,固壁边界条件采用＂部分滑移条件＂,成功地对水力机械叶片尾流现象进行了模拟计算.计算结果表明,水力机械在非设计工况下,叶片绕流流态十分复杂,在负压面存在回流、脱流旋涡,以及叶片尾流.叶片正压面流速与叶片负压面流速差较大,加上负压面上的脱流旋涡,是形成叶片尾流的原因.尾流内存在周期性很好的类似卡门涡列的流动现象,其频率与叶片绕流状况有关,随着叶片入流角增加而增大,且属于低频范畴,因此,不能简单地采用圆柱绕流卡门涡街的频率公式进行计算.     

水扑翼枢轴位置优化及实验验证

为解决平原河网水动力不足的问题,利用复合谐波运动的仿生水翼在超低扬程工况下进行推水作业,并基于有限体积法（FVM）和重叠动网格技术,通过数值仿真与实验相结合的方法,确定了扑动水翼推水作业时的最佳枢轴位置。结果表明：扑动水翼尾迹流场结构呈现反卡门涡街状态,其尾迹涡街偏斜程度、前缘涡的强度以及每个时刻前缘涡的状态变化均受枢轴位置影响;当枢轴位置向水翼尾部移动时,推水效率先增后减,L=0.2c时效率最佳,相较枢轴位于水翼前缘时,效率可提升2%～5%;扑动水翼的水力特性曲线与传统水泵相似,其最大扬程小于1 m,且随着枢轴位置向水翼尾部移动,其最大扬程呈现减小的趋势,进一步满足了超低扬程工况需求。     

Numerical simulation of unsteady cavitating flows around a transient pitching hydrofoil

The objective of this paper is to improve the understanding of the influence of multiphase flow on the turbulent closure model,the interplay between vorticity fields and cavity dynamics around a pitching hydrofoil.The effects of pitching rate on the subcavitating and cavitating response of the pitching hydrofoil are also investigated.In particular,we focus on the interactions between cavity inception,growth,and shedding and the vortex flow structures,and their impacts on the hydrofoil performance.The calculations are 2-D and performed by solving the incompressible,multiphase Unsteady Reynolds Averaged Navier Stokes(URANS)equations via the commercial CFD code CFX.The k-SST(Shear Stress Transport)turbulence model is used along with the transport equation-based cavitation models.The density correction function is considered to reduce the eddy viscosity according to the computed local fluid mixture density.The calculation results are validated with experiments conducted by Ducoin et al.(see Computational and experimental investigation of flow over a transient pitching hydrofoil,Eur J Mech/B Fluids,2009,28:728–743 and An experimental analysis of fluid structure interaction of a flexible hydrofoil in various flow regimes including cavitating flow,Eur J Mech B/fluids,2012,36:63–74).Results are shown for a NACA66 hydrofoil subject to slow(quasi static,6°/s,*0.18)and fast(dynamic,63°/s,*1.89)pitching motions from=0°to=15°.Both subcavitaing(=8.0)and cavitating(=3.0)flows are considered.For subcavitating flow(=8.0),low frequency fluctuations have been observed when the leading edge vortex shedding occurs during stall,and delay of stall is observed with increasing pitching velocity.For cavitating flow(=3.0),small leading edge cavities are observed with the slow pitching case,which significantly modified the vortex dynamics at high angles of attack,leading to high frequency fluctuations of the hydrodynamic coefficients and different stall behaviors compared to the subcavitating flow at the same pitching rate.On the other hand,for the fast pitching case at=3.0,large-scale sheet/cloud cavitation is observed,the cavity behavior is unsteady and has a strong impact on the hydrodynamic response,which leads to high amplitude fluctuations of the hydrodynamic coefficients,as well as significant changes in the stall and post-stall behavior.The numerical results also show that the local density modification helps to reduce turbulent eddy viscosity in the cavitating region,which significantly modifies the cavity lengths and shedding frequencies,particularly for the fast pitching case.In general,compared with the experimental visualizations,the numerical results with local density correction have been found to agree well with experimental measurements and observations for both slow and fast transient pitching cases.