Lattice Boltzmann simulation of flows in bifurcate channel at rotating inflow boundary conditions and resulted different outflow fluxes

The Lattice Boltzmann method(LBM) is used to simulate the flow field in a bifurcate channel which is a simplified model of the draft tube of hydraulic turbine machine.According to the simulation results,some qualitative conclusions can be deduced.The reason of uneven flux in different branches of draft tube is given.Not only the vortex rope itself,but also the attenuation of the rotation strength is important in bringing on the uneven flux.The later leads to adverse pressure gradient,and changes the velocity profile.If the outlet contains more than one exit,the one that contains the vortex rope will lose flux because of this adverse pressure gradient.Several possible methods can be used to minimize the adverse pressure gradient domain in order to improve the efficiency of turbine machine.     

单一水平轴风电机组尾迹的模拟方法与流动机理研究综述

为实现碳达峰、碳中和“3060”目标, 风能将在我国能源体系发挥重要作用. 风力机尾迹是影响风电性能和度电成本的关键因素, 需在风力机布置和控制设计中充分考虑. 本文首先介绍风力机尾迹的数值模拟方法, 包括解析模型、低阶模型、大涡模拟和来流湍流生成方法. 解析模型和低阶模型可快速计算风力机尾迹, 但依赖于模型参数, 且不能或不能准确预测尾迹湍流特性. 结合风力机参数化模型的大涡模拟可准确预测尾迹蜿蜒等湍流特征, 是流动机理研究的有力工具, 可为发展快速预测模型提供数据和理论支撑. 接着, 本文介绍了叶尖涡、中心涡和尾迹蜿蜒并讨论其产生机理. 对于湍流来流, 叶尖涡主要存在于近尾迹. 蜿蜒是远尾迹的主要特征, 影响下游风力机的来流特征. 尾迹蜿蜒的产生有两种机制: 来流大尺度涡和剪切层失稳. 数值和观测结果显示两种机制共同存在. 机舱和中心涡对尾迹蜿蜒有重要影响. 采用叶片和机舱的致动面模型可准确预测尾迹蜿蜒. 研究显示不同风力机尾迹间的湍流特征存在相似性, 为发展尾迹湍流的快速预测模型提供了理论依据. 当前研究多关注平坦地形上的风力机尾迹, 复杂地形和海洋环境下的大气湍流和风力机尾迹的机理复杂, 现有工程模型无法准确预测, 有待深入研究.      

Numerical outflow boundary conditions for the shallow-water equations

A simple explanation is given of the occurrence of wiggles in the flow field near outflow boundaries. If the shallow-water equations are solved numerically spurious solutions with an oscillatory character turn out to exist, which can be generated by certain additional numerical boundary conditions on the downstream side. The wiggles usually damp quickly with the distance from the boundary. Some ways of handling the downstream boundary are given which largely avoid the occurrence of wiggles.     

热喷干扰气体模型对飞行器气动特性影响分析

针对不同气体模型对高超声速飞行器喷流反作用控制系统(RCS)热喷干扰流场模拟的计算效率和准确性问题, 基于喷流燃气物理化学模型, 通过数值求解含化学反应源项的三维N-S方程, 建立了飞行器RCS热喷干扰流场数值模拟方法, 分别采用化学反应流、反应冻结流、二元异质流以及空气喷流四种气体模型开展了典型外形热喷干扰流场的数值模拟, 研究了不同气体模型对热喷干扰流场结构、飞行器气动力热特性的影响, 分析了不同马赫数、飞行高度下的变化规律. 研究表明: 化学反应流模型计算精度较高, 计算与风洞试验数据的吻合程度优于其他三种简化模型; 在本文的低空条件下, 采用简化模型进行热喷干扰流场数值模拟, 会低估分离区大小, 使飞行器气动力特性预测出现偏差, 同时也会低估表面热环境, 对防热系统设计不利, 随着马赫数增加, 简化模型对气动力热特性预估的误差进一步增大, 同时不同简化模型之间的差异也进一步增大; 飞行高度较高时, 模型之间的差异减小, 此时可采用简化模型进行计算以提高计算效率. 本文的研究结果可为飞行器热喷干扰流场数值模拟及喷流反作用控制系统设计提供参考.      

Hydrodynamic stability,the Chebyshev tau method and spurious eigenvalues

The Chebyshev tau method is examined; a numerical technique which in recent years has been successfully applied to many hydrodynamic stability problems. The orthogonality of Chebyshev functions is used to rewrite the differential equations as a generalized eigenvalue problem. Although a very efficient technique, the occurrence of spurious eigenvalues, which are not always easy to identify, may lead one to believe that a system is unstable when it is not. Thus, the elimination of spurious eigenvalues is of great importance. Boundary conditions are included as rows in the matrices of the generalized eigenvalue problem and these have been observed to be one cause of spurious eigenvalues. Removing boundary condition rows can be difficult. This problem is addressed here, in application to the Bénard convection problem, and to the Orr-Sommerfeld equation which describes parallel flow. The procedure given here can be applied to a wide range of hydrodynamic stability problems.Received: 4 July 2002, Accepted: 13 September 2002, Published online: 27 June 2003     

A three-dimensional yield-criterion-based flow model for avalanches

In this paper, a flow model for avalanches based on the three-dimensional yield criterion is presented in an attempt to allow the relaxation of the assumption of lateral confinement pressure that is adopted in the traditional three-dimensional Savage–Hutter model (S–H model). One of the advantages of this model is that a simplified constitutive relationship for granular flow, which could reveal the internal mechanism of avalanches, is adopted. Additionally, another advantage is that the strength parameters used in the proposed model are readily available for natural materials. The flow properties of avalanches are influenced by the generalized friction coefficient, which is a parameter that can be assessed by introducing the three-dimensional yield criterion. By comparing the results obtained by numerical simulations using the model proposed in this paper and laboratory experiments, a reasonably good agreement can be reached with regard to the prediction of the moving process of avalanches.     

Gas–liquid two phase flow measurement method based on combination instrument of turbine flowmeter and conductance sensor

In order to solve the flowrate measurement problem of gas–liquid two phase flow widely existing in gas wells of Daqing oil field in China, a new method has been developed, which is based on the combination instrument of turbine flowmeter and conductance sensor with petal type concentrating flow diverter. The turbine and conductance signals under 104 different flow conditions have been acquired through oil–gas–water three phase flow loop experimental facility. To determine the flow pattern in measurement channel, attractor morphologic characteristics are extracted from the conductance signals. For the total flowrate measurement, based on the turbine fluctuant signals of gas–liquid two phase flow, a statistical model with the average error of 7.9% is set up. With regard to the water cut measurement, the characteristics in time and frequency domains are extracted from the fluctuant conductance signals, and then employing the Support Vector Machine (SVM) soft measurement model used in high-dimension data fitting, the water cut prediction is realized with the average error of 0.038. The results show that the combination instrument of turbine flowmeter and conductance sensor with petal type concentrating flow diverter would be useful in measuring the total flowrate and water cut of gas–liquid two phase flow in gas production wells.     

Effects of 3-D channel blockage and turbulent wake mixing on the limit of power extraction by tidal turbines

Three-dimensional incompressible Reynolds-averaged Navier–Stokes (RANS) computations are performed for water flow past an actuator disk model (representing a tidal turbine) placed in a rectangular channel of various blockages and aspect ratios. The study focuses on the effects of turbulent mixing behind the disk, as well as on the effects of channel blockage and aspect ratio on the prediction of the hydrodynamic limit of power extraction. To qualitatively account for the effect of turbulence generated by the turbine (rather than by the shear flow behind the turbine), we propose a new approach, called a blade-induced turbulence model, which does not use any additional model coefficients other than those used in the original RANS turbulence model. Results demonstrate that the power removed from the mean flow by the disk increases as the strength of turbulent mixing behind the disk increases, being consistent with the turbulent shear stress on the interface between the bypass and core flow passages acting in such a way as to decelerate the bypass flow and accelerate the core flow. The channel aspect ratio also affects the flow downstream of the disk but has less influence upstream of the disk; hence its effect on the limit of power extraction is relatively minor compared to that of the channel blockage, which is shown to be significant but satisfactorily estimated using one-dimensional inviscid theory previously reported in the literature.     

一种风力机气动计算的全自由涡尾迹模型

采用全自由方式建立风力机尾流场的涡尾迹模型,引入“虚拟周期”的概念,并发展一种自适应松弛因子方法,从而改善了自由尾迹迭代的稳定性,提高了迭代收敛速度。利用建立的自由涡尾迹模型,计算了风力机叶片的尾流场结构、气动性能及叶片载荷,并与实验结果进行了对比分析。结果表明,尖速比越大,自适应松弛因子方法对缩小模型计算时间越有效;全自由涡尾迹模型能准确给出风力机尾流场的结构,包括尾迹的扩张以及叶尖涡和叶根涡的产生、发展和耗散的过程,风轮扭矩与实验数据吻合;叶片载荷分布的计算结果在低风速下与实验值基本一致,但是在大风速下差别较大,说明需要一个准确的失速模型。     

Modal analysis of a spinning disk in a dense fluid as a model for high head hydraulic turbines

In high head Francis turbines and pump-turbines in particular, Rotor Stator Interaction (RSI) is an unavoidable source of excitation that needs to be predicted accurately. Precise knowledge of turbine dynamic characteristics, notably the variation of the rotor natural frequencies with rotation speed and added mass of the surrounding water, is essential to assess potential resonance and resulting amplification of vibrations. In these machines, the disk-like structures of the runner crown and band as well as the head cover and bottom ring give rise to the emergence of diametrical modes and a mode split phenomenon for which no efficient prediction method exists to date. Fully coupled Fluid–Structure Interaction (FSI) methods are too computationally expensive; hence, we seek a simplified modeling tool for the design and the expected-life prediction of these turbines.We present the development of both an analytical modal analysis based on the assumed mode approach and potential flow theory, and a modal force Computational Fluid Dynamics (CFD) approach for rotating disks in dense fluid. Both methods accurately predict the natural frequency split as well as the natural frequency drift within 7.9% of the values measured experimentally. The analytical model explains how mode split and drift are respectively caused by linear and quadratic dependence of the added mass with relative circumferential velocity between flexural waves and fluid rotation.     

The Effect of Partial Premixing and Heat Loss on the Reacting Flow Field Prediction of a Swirl Stabilized Gas Turbine Model Combustor

This work addresses the prediction of the reacting flow field in a swirl stabilized gas turbine model combustor using large-eddy simulation. The modeling of the combustion chemistry is based on laminar premixed flamelets and the effect of turbulence-chemistry interaction is considered by a presumed shape probability density function. The prediction capabilities of the presented combustion model for perfectly premixed and partially premixed conditions are demonstrated. The effect of partial premixing for the prediction of the reacting flow field is assessed by comparison of a perfectly premixed and partially premixed simulation. Even though significant mixture fraction fluctuations are observed, only small impact of the non-perfect premixing is found on the flow field and flame dynamics. Subsequently, the effect of heat loss to the walls is assessed assuming perfectly premixing. The adiabatic baseline case is compared to heat loss simulations with adiabatic and non-adiabatic chemistry tabulation. The results highlight the importance of considering the effect of heat loss on the chemical kinetics for an accurate prediction of the flow features. Both heat loss simulations significantly improve the temperature prediction, but the non-adiabatic chemistry tabulation is required to accurately capture the chemical composition in the reacting layers.     

An Energy Based Fatigue Life Prediction Framework for In-Service Structural Components

An energy based fatigue life prediction framework has been developed for calculation of remaining fatigue life of in service gas turbine materials. The purpose of the life prediction framework is to account aging effect caused by cyclic loadings on fatigue strength of gas turbine engines structural components which are usually designed for very long life. Previous studies indicate the total strain energy dissipated during a monotonic fracture process and a cyclic process is a material property that can be determined by measuring the area underneath the monotonic true stress-strain curve and the sum of the area within each hysteresis loop in the cyclic process, respectively. The energy-based fatigue life prediction framework consists of the following entities: (1) development of a testing procedure to achieve plastic energy dissipation per life cycle and (2) incorporation of an energy-based fatigue life calculation scheme to determine the remaining fatigue life of in-service gas turbine materials. The accuracy of the remaining fatigue life prediction method was verified by comparison between model approximation and experimental results of Aluminum 6061-T6. The comparison shows promising agreement, thus validating the capability of the framework to produce accurate fatigue life prediction.     

Subgrid Model Influence in Large Eddy Simulations of Non-reacting Flow in a Gas Turbine Combustor

Fuel efficiency improvement and harmful emission reduction are the paramount driving forces for development of gas turbine combustors. Lean-burn combustors can accomplish these goals, but require specific flow topologies to overcome their sensitivity to combustion instabilities. Large Eddy Simulations (LES) can accurately capture these complex and intrinsically unsteady flow fields, but estimating the appropriate numerical resolution and subgrid model(s) still remain challenges. This paper discusses the prediction of non-reacting flow fields in the DLR gas turbine model combustor using LES. Several important features of modern gas turbine combustors are present in this model combustor: multiple air swirlers and recirculation zones for flame stabilisation. Good overall agreement is obtained between LES outcomes and experimental results, both in terms of time-averaged and temporal RMS values. Findings of this study include a strong dependence of the opening angle of the swirling jet inside the combustion chamber on the subgrid viscosity, which acts mainly through the air mass flow split between the two swirlers in the DLR model combustor. This paper illustrates the ability of LES to obtain accurate flow field predictions in complex gas turbine combustors making use of open-source software and computational resources available to industry.     

Turbine vane film cooling: Heat transfer evaluation using high-order discontinuous Galerkin RANS computations

A high-order accurate CFD solver, based on the Discontinuous Galerkin (DG) finite element method, is here employed to compute the heat transfer, with and without film coolant injection, around a turbine vane extensively tested in a wind tunnel. The numerical solution makes also use of a high-order polynomial representation of the airfoil curved boundary in order to minimize the numerical sources of error, leaving possibly only those related to the physical model adopted. The objective of the work is therefore twofold: on the one hand to provide a detailed investigation, often beyond the reach of the experiments, of the complex flow field arising in a film-cooled gas turbine cascade, on the other hand to ascertain the limits of the Reynolds-averaged Navier-Stokes (RANS) approach and its associated turbulence model when using high-order accurate methods. The DG formulation is briefly reviewed, as well as the experimental apparatus and the measuring technique, and then the code is applied to the computation of various test cases characterized by different reference Reynolds and Mach numbers. Two-dimensional results (up to seventh-order accurate) obtained both with the high- and low-Reynolds version of the k-ω model employed are presented. Reasonably good agreement between experimental and numerical results is obtained, even though the outcomes are far from being completely satisfactory especially for flow regimes in the low Reynolds number range. This is due to the lack of suitable modeling of the laminar-turbulent transition process taking place around the blade leading edge. Such a complex phenomenon is out of reach of the modeling capabilities of the high-Re k-ω model, while can be roughly mimicked by the low-Re version of the model, which is able to provide a delayed onset of the turbulence quantities along the blade surface.Third-order accurate computation of the three-dimensional turbine vane are also presented in this work and compared with available measurements to investigate the relevant fluid flow phenomena occurring and to discuss significant issues related to an accurate prediction of the turbine wall heat transfer.     

Fourier analysis of an equal‐order incompressible flow solver stabilized by pressure gradient projection

Fourier analysis techniques are applied to the stabilized finite element method (FEM) recently proposed by Codina and Blasco for the approximation of the incompressible Navier–Stokes equations, here denoted by pressure gradient projection (SPGP) method. The analysis is motivated by spurious waves that pollute the computed pressure in start‐up flow simulation. An example of this spurious phenomenon is reported. It is shown that Fourier techniques can predict the numerical behaviour of stabilized methods with remarkable accuracy, even though the original Navier–Stokes setting must be significantly simplified to apply them. In the steady state case, good estimates for the stabilization parameters are obtained. In the transient case, spurious long waves are shown to be persistent when the element Reynolds number is large and the Courant number is small. This can be avoided by treating the pressure gradient projection implicitly, though this implies additional computing effort. Standard extrapolation variants are unfortunately unstable. Comparisons with Galerkin–least‐squares (GLS) method and Chorin's projection method are also addressed. Copyright © 2000 John Wiley & Sons, Ltd.     

Modeling of lifting‐device aerodynamics using the actuator surface concept

The actuator surface (AS) concept and its implementation within a differential, Navier–Stokes control volume finite‐element method (CVFEM) are presented in this article. Inspired by vortex and actuator disk methods, the AS concept consists of using porous surfaces carrying velocity and pressure discontinuities to model the action of lifting surfaces on the flow. The underlying principles and mathematics associated with AS are first reviewed, as well as their implementation in a CVFEM. Results are presented for idealized 2D cases with analytical solutions, as well as for the 3D cases of a finite wing and an experimental wind turbine. In the case of the finite wing, wake induction is well handled by the model with accurate predictions of induced angles and drag when compared with the Prandtl lifting line model. Comparisons with volume force approaches, often used to model the action of propellers or wind turbine blades in a simplified analysis, show that the AS concept has some interesting advantages in terms of accuracy and respect of flow physics. This new approach is easy and rapid to embed in most computational fluid dynamics (CFD) methods. It is applicable to a wide range of problems involving thin lifting devices like finite wings, propellers, helicopter or wind turbine blades. Copyright © 2009 John Wiley & Sons, Ltd.     

Internal hydraulic analysis of impeller rounding in centrifugal pumps as turbines

The use of pumps as turbines in different applications has been gaining importance in the recent years, but the subject of hydraulic optimization still remains an open research problem. One of these optimization techniques that include rounding of the sharp edges at the impeller periphery (or turbine inlet) has shown tendencies of performance enhancement.In order to understand the effect of this hydraulic optimization, the paper introduces an analytical model in the pump as turbine control volume and brings out the functionalities of the internal variables classified under control variables consisting of the system loss coefficient and exit relative flow direction and under dependent variables consisting of net tangential flow velocity, net head and efficiency.The paper studies the effects of impeller rounding on a combination of radial flow and mixed flow pumps as turbines using experimental data. The impeller rounding is seen to have positive impact on the overall efficiency in different operating regions with an improvement in the range of 1-3%. The behaviour of the two control variables have been elaborately studied in which it is found that the system loss coefficient has reduced drastically due to rounding effects, while the extent of changes to the exit relative flow direction seems to be limited in comparison. The reasons for changes to these control variables have been physically interpreted and attributed to the behaviour of the wake zone at the turbine inlet and circulation within the impeller control volume.The larger picture of impeller rounding has been discussed in comparison with performance prediction models in pumps as turbines. The possible limitations of the analytical model as well as the test setup are also presented. The paper concludes that the impeller rounding technique is very important for performance optimization and recommends its application on all pump as turbine projects. It also recommends the standardization of the rounding effects over wide range of pump shapes including axial pumps.     

基于深度神经网络的横流转捩预测

基于深度神经网络DNN构建了从层流流场无量纲速度梯度、流向涡强度等物理量到横流转捩模态下间歇因子间的映射关系,获得一种新的数据驱动转捩模型.通过将数据驱动转捩模型与SST k-ω湍流模型耦合,有效简化了转捩模型输运方程求解,实现高效、准确的亚音速三维边界层横流转捩流场计算. DNN训练数据来自变雷诺数的NLF(2)-0415无限展长后掠翼计算结果,并以两种工况进行测试,数据驱动转捩模型预测精度与γ-Reθ转捩模型近似.将数据驱动转捩模型用于其他典型横流转捩算例的计算,以验证其泛化能力.对于变后掠角的NLF(2)-0415后掠翼,数据驱动转捩模型与γ-Reθt-CF模型预测的转捩位置几乎一致,并且能够预测出后掠角从45°增长到65°的过程中,转捩位置先向前再向后移动的现象;对于标准椭球体,使用低分辨率网格进行计算,数据驱动转捩模型依然能够实现转捩位置预测,对椭球体表面Cf的计算结果与多个平台的横流转捩模型、实验结果基本一致.研究表明,以横流转捩相关物理量作为输入对DNN进行训练,并将获得的数据驱动转捩模型与SST k-ω湍流模型耦合,可以实现对横流转捩的有效预测,且具有较强的泛化能力.数...