Omega-Liutex method for the interaction among wave, current and fixed vertical cylinder

This paper presents a numerical investigation of the interaction among wave,flow and a fixed cylinder using the naoe-FOAM-SJTU solver,which is a viscous solver for hydrodynamics based on OpenFOAM.The simulation considers the strong nonlinear interaction among a regular wave,flow,and a single fixed cylinder due to viscosity,which is common in offshore structures.The study focus on the vortex induced by viscosity and its significant role in hydrodynamic performance computation.A mesh convergence study is conducted prior to the simulation,and a proper set of mesh is chosen.The simultaneous generation of the regular wave and flow is realized using the numerical field boundary,and the wave propagation is validated.The results of wave elevation and pressures captured by wave gauges and probes around the fixed cylinder are compared with experiment results.The structure of vortices is displayed at significant times,and the phenomena of wave elevation and pressure changes around the cylinder are analyzed in detail using the Omega-Liutex identification method.

水泵水轮机尾水管压力脉动特性及涡带特性分析

抽蓄电站是电网的重要组成部分,水泵水轮机长时间偏离最优工况运行会导致水力不稳定问题,其中,尾水管涡带和压力脉动问题备受关注,严重影响机组安全稳定运行。以水泵水轮机为研究对象,分析了其尾水管内流特性与压力脉动时频特性。对比了两种第二代和两种第三代涡识别方法对尾水管涡带演化过程的可视化效果,Omega-Liutex涡识别方法对阈值依赖性小且对涡带形态的识别更清晰完备,因此,采用Omega-Liutex涡识别方法研究了不同负荷工况下尾水管涡带的瞬态变化特性。研究结果可以为水泵水轮机稳定运行提供一定的理论参考。

Extension Omega and Omega-Liutex methods applied to identify vortex structures in viscoelastic turbulent flow

The vortex structure plays a significant role in the investigation of the turbulent drag reduction effect of the viscoelastic turbulent flow.This paper aims to find out an optimal vortex identification method for the viscoelastic turbulent flows,and then studies the turbulent drag reduction mechanism by analyzing the characteristics of the identified vortex structures in the turbulent flows of the viscoelastic fluids.The Q,λ2,Liutex,Omega(Ω)and Omega-Liutex(ΩR)methods are adopted for the identification of vortex structures in the forced homogeneous isotropic turbulence(FHIT)with/without the polymer additive,respectively.The comparison among these five methods shows that the threshold values for the Q,λ2 and Liutex methods should be specially adjusted so as to suitably describe the strong and weak vortex structures in the FHIT of both the Newtonian and viscoelastic fluids,while a fixed threshold value of 0.52 for theΩandΩR methods is effective for both the Newtonian and viscoelastic fluids.The comparison between the identified vortex structures in the FHIT with and without the polymer additive indicates that theΩandΩR methods are more appropriate for the vortex identification because their dimensionless values with a fixed range from 0 to 1 can avoid the effect of the different ranges of the Q,λ2 and∣R∣(for the Liutex method)for the Newtonian and viscoelastic fluids.This also illustrates that theΩandΩR methods can be extended to identify the vortex structures in the turbulent flow of the viscoelastic fluid.Finally,the characteristics of the vortex structures in the FHIT of the viscoelastic fluid are analyzed by utilizing theΩandΩR methods.The results show that both the strong and weak vortex structures are inhibited by increasing the concentration of the polymer solution and by decreasing the Weissenberg number,especially for the weak vortex structures.

横向沟槽减阻中的涡结构识别方法研究

湍流相干结构演化规律的分析是探究沟槽减阻机理的一项重要研究手段。为探究Liutex参数与Omega准则在沟槽减阻机理研究中的适应性问题,采用大涡模拟方法,对设置有横向沟槽结构的槽道流域进行数值模拟,比较了Liutex参数及Omega准则和传统识别方法识别结果的异同。通过与传统涡识别方法综合对比后发现:Omega准则的阈值选取更加方便,且敏感度低,鲁棒性强,但对于壁面附近的小涡结构,相较于传统识别方法,并无突出优势;Liutex线相较于涡量线更加准确且规整,与涡结构的吻合性更佳;Omega准则与Liutex参数在横向沟槽结构流域的减阻机理分析中均具有很好的适应性,对于沟槽减阻机理的研究具有重要意义。

Liutex-涡定义和第三代涡识别方法

本文回顾了涡定义和涡识别方法的发展历史,着重介绍了作者UTA(University of Texas at Arlington)团队及其合作者在涡科学和湍流研究的一些最新学术创新成果。UTA团队发现了可以定量描述流体刚性转动部分的物理量——Liutex向量,其主要思想是把流体刚性转动从流体运动中提取出来,进而用Liutex来定义和识别涡结构,并已在广泛应用中证明了其作为涡识别方法的优越性。基于Liutex向量可以进一步研究涡量分解、速度梯度张量分解、流体运动分解、湍流结构、湍流生成机理以及旋涡的科学识别,为流体运动学的发展开辟了广阔的研究空间。区别于第一代涡识别方法和第二代涡识别方法,Liutex是一个向量,其方向代表当地转轴,大小代表当地流体刚性旋转角速度的二倍。本文详细介绍了基于Liutex向量的第三代涡的定义和识别方法,包括Liutex等值面、Liutex-Omega等值面、Liutex向量线、Liutex涡核线、以及最新发现的中低雷诺数湍流边界层中的Liutex-5/3幂次相似律,其发现很大程度上扩大了传统湍流能谱幂次律的适用范围,对建立湍流模型具有重要意义。

超声速平板边界层中流向涡的生成条件

针对马赫数为4.5的超声速平板边界层,基于线性稳定性理论(LST)选取初始扰动组合,通过直接数值模拟(DNS),计算了第一模态不稳定波扰动组合沿流向演化生成流向涡的过程。采用改进Omega-Liutex旋涡识别方法进行涡识别,结合流向不同位置截面的流线图,分析了流向涡的生成特性。根据流向涡在zy截面内的流线特征,提出了流向涡的生成条件,研究发现:流向涡可以直接通过一对展向对称的第一模态不稳定斜波扰动与基本流叠加得到,不是必须经过非线性作用。