溃坝洪水水面线的简化计算方法

针对溃坝洪水水面线采用非恒定流计算资料要求全、技术要求高,提出了先用溃坝流量衰减公式求出各个断面的流量,再用恒定流计算公式分别由下游到上游计算相应各个断面的水位,由此推求一种较好的溃坝洪水水面线的简化计算方法.结果表明,该方法简便、合理、可靠,有一定的应用价值,可供借鉴.     

基于有限体积法的有压管道水锤计算

在水电站运行过程中,为了避免机组增减负荷产生的水锤威胁输水系统的安全,需要对有压管道中的水锤进行精确的模拟计算。基于有限体积法对有压管道水锤方程分别进行空间和时间尺度上的积分,并采用隐式的Crank-Nicolson格式对偏微分项进行处理,从而得到了具有二阶精度且无条件稳定的水锤方程离散格式。通过算例比较了该离散格式与特征线法的计算结果,结果表明,新推导的离散格式在计算有压管道水锤问题时具有较高的精度且稳定性较好。     

水工消能设施消能率的理论分析及计算方法

由于水工消能设施内水流流态复杂,当水流流态为恒定急变流和非恒定流时,现有消能率计算公式不能适用.为此,基于水工消能设施内流体能量消散的本质,通过能量方程推导出适用于恒定流和非恒定流计算消能率的一般公式,利用重积分数值计算方法和离散函数求导法则处理所提的消能率计算公式,并给出恒定均匀流、恒定急变流、非恒定流条件下消能率的...     

某抽水蓄能电站首次泵工况启动关键因素的数值分析

根据某抽水蓄能电站实际水力布置情况,在分析其上库的实际充水特点和引水隧洞中明满流交替的水力特征的基础上,建立相应数学模型,采用特征隐式格式法求解有压管道非恒定流方程,优化了机组首次泵工况运行时的导叶开关规律及球阀关闭规律等关键因素,并利用优化后的关键因素对泵工况进行了大波动过渡过程计算,结果具有合理性。     

无压引水渠道及前池非恒定流计算模型构建原理与方法

为计算水电站在负荷变动下渠道和压力前池的水位波动过程、分析渠道末端扩散段和压力前池对明渠水位波动的影响,基于明渠和有压管非恒定流理论,采用特征线法求解明渠和压力管道的圣维南方程组、变时步特征线法解决无压渠道和压力管道不同时间步长,构建了“无压引水渠道+扩散段+压力前池+压力钢管”数学模型,并利用Matlab软件编制相应的计算程序。     

基于CFD的蒸汽调节阀流量特性研究之二

汽轮机蒸汽调节阀采用阀后联通的方式可有效减小在低负荷运行、小流量调节时叶片的汽流弯应力,提高机组运行的安全可靠性.对阀后联通结构的调节阀进行了理论推导和计算,结合CFD数值模拟手段得到了阀门的流量特性.运用倒推法对阀门型线做出了改进,得到新的型线并进行了验算,结果表明阀门流量特性曲线光滑、平稳,调节性能良好.     

河网非恒定流计算的牛顿迭代解法

对描述河网非恒定流的圣维南方程组进行差分,利用牛顿迭代的数值计算方法建立迭代方程组,并应用一种特殊的高斯消元法来求解此方程组;给出了一维河网非恒定流的一个通用解法;用一个实例验证了此解法的可行性.     

气波增压器中不定常非等熵流的分析

本文对气波增压器中的不定常非等熵流动建立了较符合实际的数学模型,并利用特征线法对所归纳的方程及其边界条件在计算机上进行数值求解。最后,结合 CB-200气波增压器的三种工况进行数值计算,并与试验结果比较,表明本文提出的处理方法具有数值计算收敛较快、精度较高和实际符合较好的优点。     

CFD技术在管道阀门水击计算中的应用

运用计算流体动力学（CFD）的动网格技术对管道中的阀门关闭进行了动态模拟,针对同一模型,并与特征线法及试验进行了比较。结果表明CED解法能够反映出阀门关闭水击上升的最大压力,说明CFD方法可以应用于水击的计算中。动态模拟还能观察到随着阀门的关闭,阀门处所呈现的复杂涡系,以及阀门受力情况,分析结果为阀门的设计提供数据。     

阀门的流量特性对系统瞬变流分析的影响

阀门在水利水电工程中应用广泛且型式多样,不同的流量特性影响设置阀门系统的瞬变流分析与控制。结合设置阀门的水库—有压管道—阀门系统,分别考虑具有典型流量特性的阀门,在分析相应过流特性的基础上,建立了阀门节点的水力模型,进一步结合压力管道瞬变流分析的特征线法,详细开展了阀门流量特性对系统瞬变流分析及其控制的影响。结果表明,对于有压输供水系统中设置的阀门,应在充分掌握阀门流量特性的基础上,开展必要的系统瞬变流分析及阀门的调节控制分析,以保证系统的安全稳定运行,同时也为工程阀门的选型提供了参考依据。     

Aerodynamic analysis of HAWTs operating in unsteady conditions

This article presents a numerical method for predicting unsteady aerodynamics of horizontal axis wind turbines (HAWTs). In this method the flow field is described by the unsteady incompressible Navier–Stokes equations. The rotor and tower are idealized respectively as actuator disc and flat plate permeable surfaces on which external normal surficial forces are balanced by fluid pressure discontinuities. The external forces exerted by the rotor and tower on the flow are prescribed according to blade element theory. Dynamic behaviour of the rotor aerodynamic characteristics is simulated using either the Gormont or the Beddoes–Leishman model. The resulting mathematical formulation is solved using a control volume finite element method. The fully implicit scheme is used for time discretization. In general, the proposed method has demonstrated its capability to adequately represent the field data. It has been demonstrated that the accuracy of the predicted results depends primarily on the dynamic stall model as well as on the turbulence model employed. Copyright © 2001 John Wiley & Sons, Ltd.     

A Quasi-Implicit Time-Advancing Scheme for Flow in a Three-Dimensional Curved Duct

A quasi-implicit time-marching scheme for solving unsteady incompressible three-dimensional flows on cell-centered unstructured meshes is developed. The finite-volume formulation is used for the spatial derivatives, and the flow variables at the cell face are obtained using the pressure correction. The nonlinear equations resulting from the fully implicit scheme are linearized without deterioration of the overall super-linear time accuracy. The system matrices are solved using the CG iterative method, known as the P-BiCGSTAB method for the momentum equation and the P-CG method for the pressure Poisson equation. The model is applied to simulate fully developed laminar flow in both a 90° curved 3-D circular duct and a 90° curved 3-D square duct. Steady solution is obtained in an unsteady time-marching manner. Computed results compare well with experimental data and other numerical results. It is demonstrated that the present method can be applied to unsteady incompressible laminar 3-D flow with a complex geometry on the unstructured grid system.     

A LOCALLY IMPLICIT SCHEME FOR TURBULENT FLOWS ON DYNAMIC MESHES

A locally implicit scheme with an anisotropic dissipation model is developed on dynamic quadrilateral-triangular meshes. The unsteady Favre-averaged Navier-Stokes equations with moving domain effects and a low-Reynolds-number k  ? ε turbulence model are solved to study turbulent flows over vibrating blades with negative interblade phase angle. A treatment of viscous flux on quadrilateral-triangular mesh is also presented. To assess the accuracy of the locally implicit scheme with anisotropic dissipation model on quadrilateral-triangular mesh, the turbulent flow around an NACA 0012 airfoil is investigated. Based on the comparison with the experimental data, the accuracy of the present approach is confirmed. From the distributions of magnitude of the first harmonic dynamic pressure difference coefficient which includes the present solution and the related experimental and numerical results, it is found that the present solution approach is reliable and acceptable. The unsteady flow behaviors for turbulent flows over vibrating blades with negative interblade phase angle are demonstrated.     

MULTIGRID SOLUTION OF UNSTEADY NAVIER-STOKES EQUATIONS USING A PRESSURE METHOD

Abstract

A multigrid relaxation method is applied to a pressure-based implicit procedure to solve the unsteady, incompressible Navier-Stokes equations. The present multigrid method is a correction scheme according to Brandt. This method is used to solve the scalar matrices resulting from the finite-volume formulation and uses flux averaging as the restriction operator. The accuracy and computational efficiency are demonstrated with a steady-state driven cavity flow and an unsteady flow over a circular cylinder case. The results are compared with single-grid results using the OrthoMin conjugate gradient method and experimental data     

用时间相关法求解定常粘性流场的加速收敛法

作者详细分析了显、隐格式的CFL数后,指出影响CFL数大小的关键原因在于离散方式,要提高用时间相关法求解定常流场的收敛速度,就必须加大CFL数,而加大CFL数的最直接,最有效的方法是选用隐式方法,作者在兼顾CFL数和精度的情况下,用Taylor展开法结合TVD格式给出了一类加速收敛的隐式方法。     

Numerical Computation of Compressible Viscous Internal Flows

Some implicit time-marching finite-difference solutions of time-averaged Navier-Stokes equations for two-dimensional compressible internal flows are presented.Five numerical examples including subsonic,transonic,supersonic and hypersonic flow fields with steady and unsteady phenomena show validity and flexibility of thepresent calculation code.The TVD scheme suggested by Harten et al is used to improve the shock resolution,and an algebraic turbulence model suggested by Baldwin and Lomax is introduced to evaluate the viscous effectin the turbulent flows.It is found that the computational results show fairly good agreement with the experi-mental data.     

Simulation of Unsteady Flows with Fluctuating Inflow or Outflow Boundary Conditions

This paper describes the numerical simulation of unsteady flows due to incoming wakes and/or varying back pressure. The solution method is based upon the one-step finite-volume TVD Lax-Wendroff scheme. Dual time-step approach and multigrid algorithm are adopted to improve the computational efficiency of the baseline scheme. Numerical results for the transonic unsteady flow in a channel bump and the unsteady flow in a flat plate cascade and the VKI cascade are presented.     

Numerical method for interfacial heat transfer in an oil-water displacement flow

A numerical study of interfacial heat transfer is performed in an oil-water displacement flow. The process of oil-water displacement is one of the key technologies for the underwater tank in offshore oil exploitation. There is a need to investigate the interfacial heat transfer between oil and water as the wax precipitation and solidification of high pour point crude oil at low temperature will particularly affect the flow of oil. Before the numerical simulation is performed, it is necessary to find out the optimal numerical method. For the volume of fluid method, the property parameters in the temperature equation are mixture ones weighted by each fluid. When the unsteady term, convective term, and diffusion term are considered, there are three property parameters that are the coefficient of each term. For each coefficient, whether an algebraic scheme or a harmonic scheme could be used. When the three coefficients are considered together, there would be eight combinations of weighting form. A numerical method is developed by exploring an optimal combination of the weighting form. The results are demonstrated with an analytical solution. It is found that the combination of three harmonic schemes will significantly improve the error. The maximum and total error from this combination are reduced by 62% and 79% compared to the second-best one and by 86% and 93% compared to the worst one. Meanwhile, the clock time only increases by 1.6% and 0.4%. The combinations of three harmonic schemes will result in the largest order of accuracy which is as large as 1.60. The harmonic scheme should be used for the property parameter of all the three terms in the vertical oil-water flow.     

Unsteady flow across a stretching surface

In this paper, the problem of unsteady uniform flow across a stretching surface in an arbitrary direction is studied theoretically, where the unsteadiness is caused by the impulsive motion of the stretching surface. Numerical results of the governing partial differential equations are obtained using an implicit finite-difference scheme for the whole transient from the early or initial unsteady-state flow to the final steady-state flow. The early unsteady-state flow is solved analytically. The numerical solution obtained for the reduced skin friction coefficient is compared with previously reported results and the results for velocity profiles, h and g profiles are also presented in this paper. It is found that there is a smooth transition from the small-time solution (initial unsteady flow) to the large-time solution (final steady-state flow).     

Unsteady free convection MHD flow past a vertical cylinder with heat and mass transfer

Heat and mass transfer characteristics and the flow behavior on the MHD flow past a vertical cylinder are studied. The equations of conservation of mass, momentum, energy and concentration which govern the flow and heat and mass transfer are obtained. The nondimensional governing equations are solved by an efficient, more accurate, unconditionally stable and fast converging implicit scheme. The unsteady effects of material parameters such as Prandtl number, Schmidt number, buoyancy ratio parameter and magnetic parameter on the velocity, temperature and concentration are discussed. The local and average skin-friction, Nusselt number and Sherwood number are also presented graphically. The numerical predictions have been compared with the existing information in the literature and good agreement is obtained.