基于LBM-LES的水翼绕流及空化流的并行数值模拟与实验研究

发布时间：2018-10-31 10:10

【摘要】：水翼绕流的水动力特性及其空化流动机理的研究对提高水力机械的空化性能及其运行稳定性具有重要的工程意义及应用价值。本文分别通过实验研究与数值计算的手段对三维水翼绕流流场及空化流动进行了深入研究。首先,通过水翼空化水洞实验成功捕捉了初生空化、片空化、云空化、超空化阶段空化形态及各阶段空穴结构的周期性变化。实验捕捉了初生空化时翼型头部微小空穴及其产生-增长-脱落-压缩-溃灭的发展过程;附于翼型表面的片空化只在小攻角条件产生,随时间逐渐增大,演变为从尾部开始与水翼表面部分脱离形成云空化。随空化数的进一步降低,水翼空化流动进入云空化阶段,小攻角条件下云空化产生于翼型头部,随空化数降低,空穴逐渐增大;大攻角条件下,在翼型头部和尾部同时存在云空化的周期性脱落,且翼型尾部流场出现空化涡街,继续降低空化数,空化区完全覆盖翼型上表面并延伸至流动下游,与主流区域间形成明显分界面,即为超空化现象。针对三维高雷诺数湍流计算,基于格子Boltzmann方法(LBM)Chapman-Enskog多尺度分析和大涡模拟(LES)思想,引入考虑湍流特性的等效松弛时间概念,联合LBM单松弛模型与LES亚格子尺度应力模型,构建了 LBM-LES耦合模型,对雷诺数Re = 2.5×104的三维水翼绕流进行数值计算,不同攻角流场流动特性及旋涡数量、位置和尺度的计算结果与水翼绕流实验结果吻合性较好,且数值方法能捕捉实验中难以捕捉的小尺度涡。并对压力系数和升阻力系数进行了定量定性分析,所有结果均验证了 LBM-LES模型对水翼湍流计算的可行性与准确性。针对气液两相大密度比的特点,基于Carnahan-Starling(C-S)气体状态方程对立方型气体状态方程斥力项修正而具有高计算精度的优势及其采用适当的粒子间相互作用势计算方式,将C-S气体状态方程与Shan-Chen模型耦合,构建了三维空化流SC-CS(Shan-Chen-Carnahan-Starling)模型,并成功预测了不同温度下三维相分离过程,获得超过2×104的气液相密度比,通过Maxwell等面积曲线分布验证了该模型对空化数值研究的适用性,并将空化流SC-CS模型成功应用于三维气核空化的发生发展和收缩溃灭及复现过程,计算结果符合能障理论。温度、气核内外压差和气核初始半径等影响因素研究表明,气核空化过程中温度越高,内外压差越大,空化越容易发生,且气核膨胀速度越快;气核收缩溃灭过程中半径变化规律相似,气核越小,溃灭速度越快。将空化流SC-CS模型进一步应用于水翼空化三维绕流,开展了液体内存在的气核从液体中析出形成空泡的非均质空化和翼型上表面低压区相变引起的均质空化研究。非均质空化模拟了水翼流场中存在的气泡当与液相压差足够克服表面张力作用而发生空化再收缩溃灭的过程。均质空化模拟了翼型前缘附近低压区相变产生的初生空化发生发展脱落溃灭过程。通过相关工况均质空化计算与空化实验结果对比,得到了基本一致的空化发生位置、发展溃灭过程及溃灭位置,验证了三维空化流SC-CS模型对模拟复杂边界条件下复杂流场空化的有效性,拓展了 LBM应用领域。为提高三维水翼绕流及三维空化流动计算效率,采用MPI消息传递接口,通过C++语言程序编写,建立基于三维空化流SC-CS模型的并行算法,通过并行效能分析得到针对本研究的数值模拟采用5个进程进行的并行计算执行时间最短,加速比最大,且具有较高的通信效率。
[Abstract]:The hydrodynamic characteristics of turbulent flow and its cavitation flow mechanism have important engineering significance and application value to improve the cavitation performance and operational stability of hydraulic machinery. In this paper, the three-dimensional turbulent flow field and cavitation flow are studied by means of experimental research and numerical calculation respectively. Firstly, primary cavitation, cavitation, cloud cavitation, cavitation morphology and periodic variation of hole structure at various stages were successfully captured through the experiment of cavitation tunnel. The experimental results show that the microcavitation and the generation-growth-shedding-compression-collapse development of the airfoil at primary cavitation are captured. The cavitation of the blade attached to the airfoil surface is only generated at small angle of attack, and gradually increases with time. Evolving into cloud cavitation from the tail and from the bottom surface portion. With the further reduction of cavitation number, the cavitation flow enters the cavitation stage of the cloud, the cloud cavitation is generated at the head of the airfoil under the condition of small attack angle, the cavity gradually increases along with the cavitation number, and under the condition of large attack angle, the periodic shedding of the cloud cavitation exists at the head part and the tail part of the airfoil at the same time, and the cavitation area completely covers the upper surface of the airfoil and extends to the downstream of the flow. Based on the Lattice Boltzmann method (LBM) Hellman-Enskog multiscale analysis and large eddy simulation (LES), the equivalent relaxation time concept considering the turbulent characteristics is introduced, and the LBM-LES coupling model is constructed by combining the LBM single relaxation model and LES subgrid scale stress model. The numerical calculation of the three-dimensional turbulent flow of Reynolds number Re = 2.5 Mt. 104, the flow characteristics of different attack angle flow field and the number, position and scale of vortex flow are good, and the numerical method can capture the small-scale vortex which is difficult to capture in the experiment. Based on the quantitative analysis of the pressure coefficient and the lift resistance coefficient, all the results verify the feasibility and accuracy of the LBM-LES model on the turbulent flow calculation. According to the characteristics of the two-phase bulk density ratio of gas liquids, based on Carnahan-Starling (C-S) gas state equation, the advantages of high calculation accuracy and the suitable inter-particle interaction potential calculation method are adopted to couple the C-S gas state equation to the Shan- Chen model. A three-dimensional cavitating flow SC-CS (Shaan-Chen-Carnahan-Starling) model is constructed, and the three-dimensional phase separation process at different temperatures is successfully predicted, and the gas-liquid density ratio of more than 2 ppmw 104 is obtained, and the applicability of the model to the cavitation numerical research is verified through an area curve distribution such as a neutron source and the like. and the cavitation flow SC-CS model is successfully applied to the development and contraction collapse and reproduction process of the three-dimensional gas core cavitation, and the calculation result is consistent with the barrier theory. It is shown that the higher the temperature, the greater the internal and external pressure difference, the easier the cavitation and the faster the gas-core expansion velocity, the smaller the radius of the gas core during the collapse of the gas core, the smaller the gas core, the higher the temperature, the internal and external pressure difference of the gas core and the initial radius of the gas core. the faster the collapse speed is. In this paper, the cavitation flow SC-CS model is further applied to the cavitation three-dimensional swirling flow, and the non-homogeneous cavitation of the gas core in the liquid and the homogeneous cavitation caused by the phase change of the low pressure zone on the upper surface of the airfoil are developed. The non-homogeneous cavitation simulates the process of cavitation and collapse when the pressure difference between the bubbles and the liquid phase is sufficient to overcome the surface tension. Homogeneous cavitation simulates the development of primary cavitation in the low-pressure zone near the leading edge of the airfoil. Compared with the experimental results of cavitation, the equivalent cavitation location, the development collapse process and the collapse position are obtained, and the effectiveness of the three-dimensional cavitation flow SC-CS model on complex flow field cavitation under complex boundary conditions is verified, and the application of LBM is extended. in ord to improve that computational efficiency of three-dimensional cavitation flow and three-dimensional cavitation flow, a parallel algorithm based on the three-dimensional cavitation flow SC-CS model is established by the MPI message passing interface and written by C ++ language program. The parallel performance analysis results in the shortest execution time, maximum acceleration ratio and higher communication efficiency for the numerical simulation of this study.
【学位授予单位】：中国农业大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O35

【参考文献】