Characterization of vortex structures with self-excited oscillations based on Liutex-Omega vortex identification method

The self-excited oscillation effect produces a continuous periodic pulsation without an external excitation source.It is widely used in fluid heat and mass transfer,cavitation and resistance reduction,and other related fields.The self-excited oscillation effect is significantly influenced by the vortex structure created by the jet passing through the specially designed cavity.The flow field in a self-excited oscillation cavity is simulated in this paper using the large eddy simulation(LES)method.The Liutex-Omega((Ω)L)method is used to analyze the vortex structure's evolution inside the cavity and is contrasted with the Q-criterion,the λ2-criterion,and the Omega(Ω)method.The studies indicate that the(Ω)L method is less sensitive to threshold selection compared with other methods,while it is more capable of identifying weak vortices.The change in cavity vortex structure can be devided into the four stages of vortex ring priming,growth and development,wall touch separation,and fragmentation.The turbulent energy generated by shear effect can promote the growth and development of the vortex ring structure and has an important influence on the formation of the vortex ring structure.The vortex strength reveals the interaction mechanism between the shear effect and vortex rings.The vortex core area illustrates that the small-scale vortices are mainly distributed inside the collision walls of the cavity and the downstream flow channel.The Liutex-omega method has unique advantages in analyzing the cavity flow field and revealing the mechanism of self-excited oscillations.     

LES investigation of the tip vortex cavitating flow with special emphasis on the interaction between cavitation and vorticity by a modified cavitation model

Cavitation within the tip vortex(TV)flow remains a challenging issue in the design of high-speed and low-noise hydraulic machinery.In this paper,the TV cavitating flow around an elliptical hydrofoil is calculated by using large eddy simulation(LES)combined with a modified Schnerr-Sauer(S-S)cavitation model.The original S-S cavitation model is modified by taking into account the typical effect of vortex flow.The partial pressure term which can describe the vortex quantitatively and qualitatively is confirmed asρmω2xr2c and is considered into the R-P equation of the modified S-S cavitation model.Comparison between the numerical and experimental results shows good agreement in the form and evolution of cavities,including attached cavities(AC)and tip vortex cavities(TVC).The vorticity transport equation is utilized to investigate the dynamic mechanisms of the vortex development around the TVC.Further analyses indicate that cavitation in the TV flow influences the pressure in the core of the cavity and the local flow patterns.Typical vortex structures in the TV cavitating flow include TV,secondary vortex(SV)and wake vortex(WV).The direction and magnitude of the rotation effect can be described by axial vorticity which is drawn on the iso-surface of Q=1×105s-2.The development of the TV cavitating flow can be divided into two stages:Stage I,the development and fusion of TV,SV,stage Ⅱ,the dissipation of SV.The stretching term dominates the evolution of TV,and the dilatation term is the main reason in the mergence process of SV.     

Hydrodynamic characteristics and noise reduction mechanism of a wave leading hydrofoil placed in the wake of a cylinder

We have investigated the hydrodynamic and acoustic performance of a hydrofoil with a wave leading edge that is being ingested in a cylindrical wake,to explore the interaction and noise reduction mechanism with the use of near flow field and far field noise decoupled prediction methods of large eddy simulation(LES)and Ffowcs Williams-Hawkings(FW-H).Our results indicate that the wave leading edge has minimal effect on the hydrodynamic performance,however,it has demonstrated the ability to significantly improve the acoustic performance.Through the comparison of sound pressure level(SPL)and acoustic directivity,we have observed that the wave leading edge can significantly reduce the broadband noise in the far field.This is due to its ability to break the large-scale structure of the incoming flow,which weakens the direct impact and therefore reduces the tone noise.Additionally,the interaction between the broken vortex and the boundary layer around the hydrofoil surface is weakened,leading to a reduction in surface pressure pulsation and broadband noise intensity.The wave structure primarily affects the flow structure near the leading edge,resulting in a reduction in flow disturbance and sound source intensity,and an improvement in the acoustic feedback loop between the foil and the fore-cylinder.     

波流耦合作用下紊流边界层水沙数学模型

在河口地区,波流相互作用下紊流边界层的泥沙起动输移规律非常复杂,深入探讨其泥沙运动规律很有必要。该文基于波浪边界层的控制方程,引入了时均水流压力,通过波流相互作用下的摩阻流速反映波浪与水流的夹角,用标准k-ε模型封闭方程,建立了波流叠加的紊流边界层内的弱耦合水流模型,分别讨论了波浪速度振幅、时均流速等的分布情况,与实测数据对比得到了较好的模拟结果。其次,基于水流模型建立了泥沙模型,采用对称振荡流和波流相互作用两种情况下的泥沙实测数据验证了泥沙模型的正确性,讨论了不同分布规律的涡黏系数对于模型的影响,为实际工程提供理论参考,结果具有一定的指导意义。     

三方柱绕流的大涡模拟及频谱分析

高雷诺数条件下,无论来流是均匀还是非均匀,多方柱绕流情况下方柱受力及尾流的相互干扰都是相当复杂的.为了研究方柱受力及下游尾流的相互干扰,基于大涡模拟紊流模型对后品字等边布置的三方柱绕流进行了数值模拟,数值模拟结果表明,上游两个方柱的阻力系数要明显小于下游方柱,而上游两方柱升力系数远大于下游方柱,且下游方柱的升力系数基本...     

NUMERICAL SIMULATION OF TURBULENT FREE SURFACE FLOW OVER OBSTRUCTION

A two-dimensional hybrid numerical model, FEM-LES-VOF, for free surface flows is proposed in this study, which is a combination of three-step Taylor-Galerkin finite element method, large eddy simulation with the Smagorinsky sub-grid model and Computational Lagrangian-Eulerian Advection Remap Volume of Fluid （CLEAR-VOF） method. The present FEM-LES-VOF model allows the fluid flows involving violent free surface and turbulence subject to complex boundary configuration to be simulated in a straightforward manner with unstructured grids in the context of finite element method. Numerical simulation of a benchmark problem of dam breaking is conducted to verify the present model. Comparisons with experimental data show that the proposed model works well and is capable of producing reliable predictions for free surface flows. Using the FEM-LES-VOF model, the free surface flow over a semi-circular obstruction is investigated. The simulation results are compared with available experimental and numerical results. Good performance of the FEM-LES-VOF model is demonstrated again. Moreover, the numerical studies show that the turbulence plays an important role in the evolution of free surface when the reflected wave propagates upstream during the fluid flow passing the submerged obstacle.     

Some notes on numerical simulation and error analyses of the attached turbulent cavitating flow by LES

In this letter, the attached turbulent cavitating flow around the Clark-Y hydrofoil is investigated by the numerical simulation with special emphasis on error analysis of large eddy simulation(LES) for the unsteady cavitation simulation. The numerical results indicate that the present simulation can capture the periodic cavity shedding behavior and show a fairly good agreement with the available experimental data. Further analysis demonstrates that the cavitation has a great influence on LES numerical error and modeling error. The modeling error and numerical error are almost on the same order of magnitude, while the modeling error often shows a little bit larger magnitude than numerical error. The numerical error and modeling error sometimes can partially offset each other if they have the opposite sign. Besides, our results show that cavitation can extend the magnitudes and oscillation levels of numerical error and modeling error.     

明渠均匀流流速横向分布的涡黏性模型

为了研究明渠水流的运动特性,开展了明渠水流流速横向分布数学模型的研究。本文根据量纲分析方法和实验资料,得到水流横向剪切作用下横向涡黏性系数的关系式;基于均匀流的水流运动方程,按照有限差分方法,建立流速横向分布计算模型。该模型具有计算简单、快捷和精度较高的优点,且数值计算不受横向涡黏性系数的结构形式和断面形态的影响。采用明渠水流实验资料验证数学模型,数学模型计算的流速横向分布与实验资料较一致,具有较高的理论意义和应用价值。     

弄另水电站1~#发电机定子铁损试验分析

大中型水轮发电机的定子,因外形尺寸大,受到运输条件的限制,其定子机座采用分瓣制造运输,在安装现场进行组装焊接、铁芯堆积及定子下线。定子铁损试验是在定子铁芯冲片叠装、压紧后进行,通过测量铁损、温升及温差的方法检查定子铁芯冲片的制造及叠装整体质量,定子铁芯制造及安装质量好坏关系着机组的长期安全运行。文章对弄令电站1#发电机定子铁损试验的实施进行了研究分析。