超声速钝体逆向喷流减阻的数值模拟研究

为研究逆向喷流技术对超声速钝体减阻的影响,采用标准k-ε湍流模型,通过求解二维Navier-Stokes方程对超声速球头体逆向冷喷流流场进行了数值模拟,并着重分析了喷口总压、喷口尺寸对流场模态和减阻效果的影响。计算结果显示:随着喷流总压的变化,流场可出现两种流动模态,即长射流穿透模态和短射流穿透模态;喷流能使球头体受到的阻力明显减小;存在最大减阻临界喷流总压值(在所研究参数范围内最大减阻可达51.1%);在其它喷流物理参数不变时,随着喷口尺寸的增大,同一流动模态下的减阻效果下降。本文的研究对超声速钝体减阻技术在工程上的应用具有一定的参考价值。     

管网系统抗震优化设计的正交枚举法

在管网系统总投资限额条件下,以系统抗震可靠度最大为优化目标,进行管网系统的抗震优化设计。根据管网系统的功能要求,设计特点及单元重要性分析,将系统优化变量转化为较少的离散变量,并利用正交枚举法进行简化计算,从而避免了大规模非线性规划求解的困难。     

Effects of unsteady deformation of flapping wing on its aerodynamic forces

Effects of unsteady deformation of a flapping model insect wing on its aerodynamic force production are studied by solving the Navier-Stokes equations on a dynamically deforming grid.Aerodynamic forces on the flapping wing are not much affected by considerable twist,but affected by camber deformation.The effect of combined camber and twist deformation is similar to that of camber deformation.With a deformation of 6% camber and 20°twist(typical values observed for wings of many insects),lift is increased bv 10%～20%and lift-to-drag ratio by around 10%compared with the case of a rigid flat-plate wing.As a result.the deformation can increase the maximum lift coefficient of an insect.and reduce its power requirement for flight.For example,for a hovering bumblebee with dynamically deforming wings(6?mber and 20°twist),aerodynamic power required is reduced by about 16%compared with the case of rigid wings.     

高亚声速运输机后体减阻措施研究

为了降低高亚声速运输机的阻力,改善飞机的气动性能,本文在某一典型运输机机身外形的基础上,研究了后体参数和涡流发生器对机身后体流场及阻力的影响。通过数值模拟计算,得出了不同外形的后体流态分布及阻力特性数据。结果表明,小的上翘角和收缩比、大的扁平度和长细比能够显著地减小压差阻力,而涡流发生器能够有效地减小摩擦阻力。该结果可为后续的型号研制提供参考。     

Robust design and numerical simulation on drag reduction by a mixture film for liquid turbulence in a channel

An important way of increasing the speed and lowering the fuel consumption of ships is by decreasing the frictional drag. One of the most promising techniques for reducing drag is the use of air bubbles. The goal of this investigation is to establish a set of optimum robust parametric levels for drag reduction by a mixture (air–water) film in turbulent channel flow. Based on the conditions laid out by the Taguchi orthogonal array method, turbulent flows, with air bubbles injected into a channel, are simulated using commercial computational fluid dynamics software. The local shear stress on the upper wall is computed to evaluate the efficiency of drag reduction. Many factors can affect drag reduction. The factors investigated in this study are the rate of air injection, bubble size, area of air injection, flow speed, and measured position of the shear stress. These factors have been investigated through the analysis of variance, which has revealed that the rate of air injection and water flow speed dominate the efficiency of drag reduction by a mixture film. According to the results, the drag can be reduced by an average of 83.4%; and when the configuration of the parametric levels is optimum the maximum drag reduction of 88.5% is achieved. Copyright © 2008 John Wiley & Sons, Ltd.     

A novel interface‐tracking method based on Lagrangian particles for deformation analysis of a red blood cell in a capillary

A novel efficient interface‐tracking method is developed to gain an insight into the interface in a multiphase or multifluid system, called the modified particle binary level set (MPBLS) method, in which the binary level set function is defined to distinguish the different phases or fluids and further modified by Lagrangian particles scattered along the interface for achieving higher accuracy. The validation of the MPBLS method is carried out first by simulating the free motion of a red blood cell (RBC) in the rotating, shear and Poiseuille flows, respectively. Subsequently, further validations are performed by comparing with the experimental and numerical results published previously. As one of important applications, the MPBLS method is employed to investigate the deformation behaviors of RBCs with different shapes in a capillary. The simulations show that the healthy RBC gradually changes the geometric shape from a biconcave to a steady parachute shape. It is thus guaranteed that the RBC successfully traverses through the smaller capillaries compared with undeformed RBC. However, the unhealthy RBC with the circular or elliptical shape has different deformation behaviors, in which the steady parachute shape is much less concave at the rear and more convex in the front. Copyright © 2011 John Wiley & Sons, Ltd.     

基于压力迭代修正的大型冷却塔抗风性能分析

目前,大型冷却塔的抗风性能分析主要把得到的风压分布施加到结构上进行结构计算,没有考虑风载引起的结构变形对风压分布的影响,会产生较大的误差。本文结合计算流体力学与有限元分析的特点,建立了考虑结构变形影响的风压迭代修正模型,进行平均风压下大型冷却塔的抗风性能分析,并讨论了内部空气流动及淋水层的影响。计算结果与规范分析结果进...     

Development of highly accurate interpolation method for mesh‐free flow simulations II. Application of CIVA method to incompressible flow simulations

This is the second report on the development of a highly accurate interpolation method, which is called cubic interpolation with volume/area (CIVA) co‐ordinates, for mesh‐free flow simulations. In this paper, the method of determining the c‐parameter of CIVA using a constant curvature condition is first considered for the two‐ and three‐dimensional cases. A computation of a three‐dimensional passive scalar advection problem is performed for accuracy verification and for comparison with widely used methods. Then, an application algorithm of the CIVA method respecting incompressible fluid simulation is presented. As the incompressible condition based on Lagrangian approaches causes problems, in this paper we consider the condition based on the conventional Eulerian approach. The CIVA‐based incompressible flow simulation algorithm enables a highly accurate simulation of many kinds of problems that have complicated geometries and involve complicated phenomena. To confirm the facts, numerical analyzes are executed for some benchmark problems, namely flow in a square cavity, free surface sloshing and moving boundary problems in complex geometries. The results show that the method achieves high accuracy and has high flexibility, even for the flows involving high Reynolds number, complicated geometries, moving boundaries and free surfaces. Copyright © 2000 John Wiley & Sons, Ltd.     

Essential consistency of pressure Poisson equation method and projection method on staggered grids

A new pressure Poisson equation method with viscous terms is established on staggered grids. The derivations show that the newly established pressure equation has the identical equation form in the projection method. The results show that the two methods have the same velocity and pressure values except slight differences in the CPU time.     

Optimization of an axial coarse powder separator for low-density lignite based on the optimal seeking method

As a major equipment for thermal power plants, the main function of coarse powder separators is to separate particles into size groups according to particle diameter. With the rising coal prices, power plants are using more low-density lignite. Consequently, the original equipment does not operate under normal conditions. Therefore, to return the equipment to normal operating conditions, the isokinetic sampling method, computational fluid dynamics (CFD), and discrete phase model (DPM) based on the Euler–Lagrange method are applied in this research to analyze the effect of baffle shapes, baffle numbers, and centrifugal blades on the equipment performance index, like R₉₀. The shape of the baffle plate was optimized and improved. Preferential method was applied to determine the optimal number of baffles to ensure normal working conditions. Results show that curved baffles can suppress the axial negative gradient field below themselves better than rectangular baffles. The curved baffles selected by the optimal seeking method make full use of the negative axial gradient field and provide the particles entering the separation zone at tangential velocity in advance with the addition of centrifugal blades. Thus, pre-separation can be realized. The R₉₀ is 5 at the separator outlet and 95 at the powder return port, indicating that the output capacity of the separator returned to a reasonable operating performance.     

An optimal method for the design of a robotic tracked vehicle to operate over fresh concrete under steering motion

The objective of this paper is to find an optimal method for the design of tracked base travel systems for special purpose vehicles and robotic machines that may be required to steer over a light bonded terrain composed of fresh concrete. For the case of a vehicle traveling on a weak fresh concrete during construction, the paper presents detailed comparative studies of the steering performances of a small model tracked test vehicle with alternative amount of steering ratio for various concrete slump values. For these studies a detailed simulation analytical method has been developed. From this work it is proven, in comparison to experiment, that the simulation analytical method is useful for predicting various steering performances of a test tracked vehicle running upon soft fresh concrete of various consistencies.     

Insights on a sign‐preserving numerical method for the advection–diffusion equation

In this paper we explore theoretically and numerically the application of the advection transport algorithm introduced by Smolarkiewicz to the one‐dimensional unsteady advection–diffusion equation. The scheme consists of a sequence of upwind iterations, where the initial iteration is the first‐order accurate upwind scheme, while the subsequent iterations are designed to compensate for the truncation error of the preceding step. Two versions of the method are discussed. One, the classical version of the method, regards the second‐order terms of the truncation error and the other considers additionally the third‐order terms. Stability and convergence are discussed and the theoretical considerations are illustrated through numerical tests. The numerical tests will also indicate in which situations are advantageous to use the numerical methods presented. Copyright © 2008 John Wiley & Sons, Ltd.     

西二线管道腐蚀非开挖检测技术研究

针对埋地铁磁管道在非开挖条件下的腐蚀检测难题,对管道弱磁检测技术进行了工程应用验证.该技术利用弱磁检测仪对管道磁信号进行采集,通过分析磁信号曲线图波形特征,可以识别和定位腐蚀缺陷,同时可以根据腐蚀等级指标F值对腐蚀进行评价.首先根据磁信号dBx出现峰值并产生波动、dBy改变方向且过零点、同时dBz也出现峰值并产生波动的...     

基于弯矩修正法的浅水区海底直铺管道允许悬跨长度计算

海底管道悬空问题是管道工程建设中经常遇到的重要问题,当管道的悬跨长度达到一定值时,管道较容易失稳,而悬空油气管道失稳破坏导致的后果往往非常严重,因此需要对悬空的海底管道进行定期的维护和处理,但由于海上作业难度大,针对悬空管道的治理成本非常高,因此可考虑对处于悬空但在安全允许悬空范围内的管道暂时不进行加固,此时就需要计算管道的允许悬跨长度,为管道的治理提供依据。由于我国油田大多建设在近海地区,因此在考虑管道的悬空问题时需考虑浅水区波浪的特点。本文考虑浅水区波浪的非线性影响,推导得到浅水区波浪场内任意一水质点的水平流速和竖向流速计算公式,并根据该公式和Morison方程,进一步建立了管道悬跨水动力载荷的表达式,最后基于弯矩修正法,提出浅水区平铺海底管道允许悬跨长度计算公式。根据工程实例,结合本文研究,计算得到该工程所在海区平铺管道允许悬跨长度。     

Evaluation of an energy relaxation method for the simulation of unsteady,viscous, real gas flows

This study investigates a new energy relaxation method designed to capture the dynamics of unsteady, viscous, real gas flows governed by the compressible Navier–Stokes equations. We focus on real gas models accounting for inelastic molecular collisions and yielding temperature‐dependent heat capacities. The relaxed Navier–Stokes equations are discretized using a mixed finite volume/finite element method and a high‐order time integration scheme. The accuracy of the energy relaxation method is investigated on three test problems of increasing complexity: the advection of a periodic set of vortices, the interaction of a temperature spot with a weak shock, and finally, the interaction of a reflected shock with its trailing boundary layer in a shock tube. In all cases, the method is validated against benchmark solutions and the numerical errors resulting from both discretization and energy relaxation are assessed independently. Copyright © 2004 John Wiley & Sons, Ltd.     

A study on the behaviour of high-order flux reconstruction method with different low-dissipation numerical fluxes for large eddy simulation

This work focuses on the numerical dissipation features of high-order flux reconstruction (FR) method combined with different numerical fluxes in turbulence flows. The famous Roe and AUSM+ numerical fluxes together with their corresponding low-dissipation enhanced versions (LMRoe, SLAU2) and higher resolution variants (HR-LMRoe, HR-SLAU2) are incorporated into FR framework, and the dissipation interplay of these combinations is investigated in implicit large eddy simulation. The numerical dissipation stemming from these convective numerical fluxes is quantified by simulating the inviscid Gresho vortex, the transitional Taylor–Green vortex and the homogenous decaying isotropic turbulence. The results suggest that low-dissipation enhanced versions are preferential both in high-order and low-order cases to their original forms, while the use of HR-SLAU2 has marginal improvements and the HR-LMRoe leads to degenerated solution with high-order. In high-order the effects of numerical fluxes are reduced, and their viscosity may not be dissipative enough to provide physically consistent turbulence when under-resolved.     

Modelling of inflow-conditions for vortex particle methods to simulate atmospheric turbulence and its induced aerodynamic admittance on line-like bluff bodies

ABSTRACT

This paper presents a novel method for the simulation of aerodynamic admittance of turbulent wind on bluff line-like structures using a pseudo 3D model of Vortex Particle Method (VPM). The method is a computationally efficient extension of the 2D VPM, where a coupled set of simulation slices accounts for the 3D nature of the oncoming wind flow. Pre-computed vortex particles are seeded in each of the parallel 2D simulation slices in order to model the turbulent velocity perturbations. Here, the modelling of the inflow seeding particles is enhanced, reducing the computational cost and allowing extendibility into quasi 3D domain. This a priori computation of the seeding vortex particles is based on modelling the atmospheric turbulence characteristics. The method is applied to simulate turbulent flow around an infinitesimally thin flat-plate, to asses its validity at the viscous-rotational boundary layer, which is important for accurate fluid-structure Interaction simulations. Furthermore, sensitivity analysis to different attributes is assessed     

Resolution sensitivity of momentum‐exchange and immersed boundary methods for solid–fluid interaction in the lattice Boltzmann method

In the lattice Boltzmann method (LBM), the mechanism of fluid–solid interaction can be effectively captured by appropriately enforcing the no‐slip conditions in shear direction, and bounce‐back of the non‐equilibrium distribution portion in the normal direction at fluid–solid interfaces. Among various solid–fluid interaction schemes being proposed for LBM in recent decades, two simple fluid–solid interaction methods—the momentum exchange algorithm (MEA) and the immersed boundary scheme (IBS)—were developed based on the above concept. In this paper, MEA and IBS are implemented in a D2Q9 LBGK system and applied to measure the wall correction factors of drag force upon a stationary circular particle midway in the Poiseuille channel flow at very low Reynolds number and drag coefficients at low to moderate Reynolds numbers. MEA and IBS are also employed to compare the fluid‐induced torque over the cylinder in the Taylor–Couette flow, and the steady velocity of a particle settling under the influence of gravity inside a tube. The above experiments show that IBS seems to be more accurate and less demanding on lattice resolution. Copyright © 2010 John Wiley & Sons, Ltd.