A CFD study on thermo-acoustic instability of methane/air flames in gas turbine combustor

Thermo-acoustic instability of methane/air flames in an industrial gas-turbine combustor is numerically investigated adopting CFD analysis. The combustor has 37 EV burners through which methane and air are mixed and then injected into the chamber. First, steady fuel/air mixing and flow characteristics established by the burner are investigated by numerical analysis with single burner. And then, based on information on the flow data, the burners are modeled numerically via equivalent swirlers, which facilitates the numerical analysis with the whole combustion system including the chamber and numerous burners. Finally, reactive flow fields within the chamber are investigated numerically by unsteady analysis and thereby, spontaneous instability is simulated. Based on the numerical results, scaling analysis is conducted to find out the instability mechanism in the combustor and the passive control method to suppress the instability is proposed and verified numerically.     

Numerical study on the improvement of resistance performance for fast-ferry with transom appendage

In recent, stern wedges and stern flaps are installed for the improvement of propulsion and resistance performance of fast-ferry. For example, U.S. Navy has achieved the development of stern wedges and stern flaps for destroyer to enhance powering performance. It is generally known that stern wave systems as well as bow wave systems play an important role in the wave making resistance performance for fast-ferry. The bow diverging wave system has been usually simulated by an interface tracking method (ITM). However, it is difficult to apply the ITM to the numerical simulation of the stern wave and spray phenomenon because of over-turning wave and wave-breaking. Therefore, to solve this problem an interface capturing method (ICM) is introduced. In the present study, a numerical method with the ICM is developed to evaluate the resistance performance of fast-ferry. Incompressible Navier-Stokes and continuity equations are employed in the present study and the equations are discretized by Finite Difference Method in the general curvilinear coordinate system. CIP (Constrained Interpolated Profile) method is used for the discretization of convection terms, respectively. The free surface location is determined by level set method. In order to validate the numerical method, numerical simulations for Wigley hull are performed and their results are compared with experimental results. Several numerical simulations of ship waves for fast-ferry are performed to find advantages of appendage installation. Through those simulations, the computed results, such as wave profile and resistance coefficient, are compared with the measured results which are achieved from Samsung Ship Model Basin (SSMB). The effects of transom appendage on the resistance performance are discussed with the computed results in this study.     

Finite-element vibration analysis of tapping-mode atomic force microscopy in liquid

Investigation of morphology and mechanical properties of biological specimens using atomic force microscopy (AFM) often requires its operation in liquid environment. Due to the hydrodynamic force, the vibration of AFM cantilevers in liquid shows dramatically different dynamic characteristics from that in air. A good understanding of the dynamics of AFM cantilevers vibrating in liquid is needed for the interpretation of scanning images, selection of AFM operating conditions, and evaluation of sample's mechanical properties. In this study, a finite element (FE) model is used for frequency and transient response analysis of AFM cantilevers in tapping mode (TM) operated in air or liquid. Hydrodynamic force exerted by the fluid on AFM cantilevers is approximated by additional mass and hydrodynamic damping. The additional mass and hydrodynamic damping matrices corresponding to beam elements are derived. With this model, numerical simulations are performed for an AFM cantilever to obtain the frequency and transient responses of the cantilever in air and liquid. The comparison between our simulated results and the experimentally obtained ones shows good agreement. Based on the simulations, different characteristics of cantilever dynamics in air and liquid are discussed.     

Study on numerical methods for transient flow induced by speed-changing impeller of fluid machinery

In order to establish a reliable numerical method for solving the transient rotating flow induced by a speed-changing impeller, two numerical methods based on finite volume method (FVM) were presented and analyzed in this study. Two-dimensional numerical simulations of incompressible transient unsteady flow induced by an impeller during starting process were carried out respectively by using DM and DSR methods. The accuracy and adaptability of the two methods were evaluated by comprehensively comparing the calculation results. Moreover, an intensive study on the application of DSR method was conducted subsequently. The results showed that transient flow structure evolution and transient characteristics of the starting impeller are obviously affected by the starting process. The transient flow can be captured by both two methods, and the DSR method shows a higher computational efficiency. As an application example, the starting process of a mixed-flow pump was simulated by using DSR method. The calculation results were analyzed by comparing with the experiment data.     

Concepts and Limitations of Using Optical Profilometry for Air Bearing Simulations of Hard Disk Sliders

The topography of hard disk sliders is commonly measured with optical profilometers that produce a regularly spaced matrix of topography data corresponding to the pixel array of the CCD camera. This paper explores a new concept that combines optical profilometry and air bearing simulations to achieve an accurate method for predicting fly height early in the manufacturing process. Various sources of error in the topography measurement are analyzed with regards to their impact on the accuracy of air bearing simulations. The findings are illustrated by a feasibility study based on numerical simulations of a slider with a negative pressure air bearing design.     

基于时域仿真的暂态稳定判据及在事故扫描中的应用

为解决基于时域仿真的电力系统暂态稳定判断时,传统的角度判别准则引起大量的数值计算的问题,将单机等效法(SIME)辨识方法应用于电力系统时域仿真计算中。通过对该方法下EAC法则对减少电力系统暂态稳定时域仿真时间的研究,提出了一种暂态稳定判据,称为EAC判据。其基本思想是将多机系统等效为单机无穷大(OMIB)系统,随着时域仿真的计算,逐步更新OMIB系统,若满足EAC法则,则停止计算。最后将EAC判据和角度法准则分别应用于基于时域仿真算法的快速事故扫描器中,对EAC判据的有效性和准确性进行了评价。基于两个实例的计算结果比较,结果证明,与传统的角度判别准则相比,EAC判据能够大大减少电力系统暂态稳定时域仿真时间。     

含间隙弹性约束系统的Hopf分岔与混沌研究

用变步长四阶Runge Kutta法 ,通过对一类单自由度含间隙弹性约束系统的数值积分 ,研究了系统周期运动的Hopf分岔及其通向混沌的拟周期道路。选择碰撞界面作为Poincar啨截面 ,通过数值计算 ,首次证明在单自由度系统中也存在Hopf分岔。对分段线性系统分岔与混沌行为的研究 ,为工业生产中含间隙机械系统和冲击振动系统的优化设计提供了理论依据。     

气液耦合激振与超声空化数值模拟运动学分析

提出了一种将外加气体与液压油结合起来的气液耦合激振方式,通过控制气脉冲与液压油形成的空化泡发生生长和破灭,从而产生空化作用对液压系统内部污染物进行分散与剥离;开发了以气液耦合波动为激振方式的气液激振试验系统,建立了波动发生器作用下的气泡运动学方程;数值模拟了波动发生器与超声波作用下空化泡的运动学过程,并进行了对比分析,发现低频的波动发生器作为激振源使气泡产生的空化作用是显著的,为气液耦合激振作用下气泡空化过程的可控性研究提供了部分理论依据。     

Optimal Motion Planning for Differentially Flat Underactuated Mechanical Systems

Underactuated mechanical system has less independent inputs than the degrees of freedom(DOF) of the mechanism. The energy efficiency of this class of mechanical systems is an essential problem in practice. On the basis of the sufficient and necessary condition that concludes a single input nonlinear system is differentially flat, it is shown that the flat output of the single input tmderactuated mechanical system can be obtained by finding a smooth output function such that the relative degree of the system equals to the dimension of the state space. If the fiat output of the underactuated system can be solved explicitly, and by constructing a smooth curve with satisfying given boundary conditions in flat output space, an energy efficiency optimization method is proposed for the motion planning of the differentially fiat underactuated mechanical systems. The inertia wheel pendulum is used to verify the proposed optimization method, and some numerical simulations show that the presented optimal motion planning method can efficaciously reduce the energy cost for given control tasks.     

地下车库诱导通风系统的数值模拟与优化

结合南京市某人防地下车库诱导通风系统的实测数据，利用计算流体动力学(CFD)技术对地下车库的气流分布现状作了数值模拟与分析，提出改进的气流分布方案，并对其CO浓度场作了进一步的模拟计算，确定了地下车库诱导通风系统的最优化方案。     

Thermal investigation of an infrared reflow oven with a convection fan

A two-dimensional numerical model for an infrared reflow soldering with a convection fan is used my modifying the Eftychiou’s numerical modeling. The two-dimensional tunnel model which predicts convective conditions within the reflow oven are solved using the finite volume method with the SIMPLER algorithm. The card model solves the transient two-dimensional heat conduction equation in conjunction with a radiative heat transfer analysis. We also performed an experiment to validate the numerical modeling. The numerical result shows excellent agreement with experimental data. Based on the capability of this model, parametric simulations are performed to determine the thermal response of the solder to variations in the oven operating conditions and heat transfer conditions. This study shows that radiation and conveyor velocity are important factors in the preheat region.     

高维含间隙振动系统的分岔与混沌研究

通过用解析法和变步长四阶Runge-Kutta数值法相结合，对一类三自由度含间隙弹性约束系统进行分析与仿真，证明三自由度含间隙系统通向混沌的道路不仅有倍周期道路和拟周期道路，而且还有包含Neimark-sacke，分岔的倍周期道路、包含叉式分岔的倍周期道路等复杂的混沌演化过程。对该系统分岔与混沌行为的研究，为工业实际中含间隙机械系统和冲击振动系统的优化设计提供理论依据。     

Stiffness and damping coefficients for journal bearing using the 3D transient flow calculation

The dynamic coefficients of journal bearing are necessary components in the analysis of linear stability and response of rotating dynamic systems. We propose a new method for the numerical identification of bearing support force coefficients in flexible rotor-bearing systems based on the 3D transient flow calculation. The CFD commercial software FLUENT is mainly used in this simulation, which employs a finite volume method for the discretization of the Navier-Stokes equations. To determine the dynamic coefficients, a new mesh movement approach is presented to update the volume mesh when the journal moves during the 3D transient flow calculation of a journal bearing. Existing dynamic mesh models provided by FLUENT are not suitable for the transient oil flow in journal bearings. Measurements and identification are performed on a test rotor supported on a pair of identical two-lobe fluid film bearings, and the results obtained from the CFD methods agree well with experimental results. The results indicate that the methods proposed in this paper can predict the dynamic coefficients of the journal bearing in a rotor-bearing system effectively, and provide a further tool for stability analysis.     

基于数值模拟的某烘干系统排风管路结构改进

烘干系统广泛应用于工农业生产中,其工作性能的优劣对产品质量及系统耗能影响较大。针对某烘干系统存在的阻力大、排风能力低、流量不均等问题,利用流场数值模拟方法,对排风管路进行了建模计算及分析,找出了排风管路所存在的结构问题。对管路进行了改进设计及仿真计算,通过与原有结构的对比分析,发现改进后的管路结构具有阻力小、各路排风流量均衡的优点,同时,可降低运行成本。最后,简单探讨了关于此烘干系统排风管路设计的几个关键问题。为生产实际中排风管路的设计提供了理论依据。     

空调器室外机流场和噪声特性

针对分体空调室外机空气侧流场和声场的特点,采用计算流体动力学(CFD)的方法对该系统的气体流动进行数值模拟。详细分析了轴流风扇流道内部和出口的速度分布,并利用激光微粒图像测速(PIV)手段,验证数值模拟结果,模拟结果与PIV测量值吻合较好。在CFD数值模拟的基础上,运用Lowson模型预估系统的离散频率噪声,预估值与测量值吻合较好。CFD技术能够有效地分析室外机空气侧流场特性,并为进行气动声学分析提供依据。     

Uncertainty estimation of reliability redundancy in complex systems based on the Cross-Entropy method

The article aims to estimate the uncertainty of possible failure events of redundancy systems based on the cross-entropy (CE) method. Failure events of subsystems and components always result in the incomplete or complete failure of engineering systems, yet optimal condition monitoring of a complex system is heavily dependent on the accuracy analysis of all the failure events of subsystems and components and their interaction effects. The CE method is a versatile tool for estimating probabilities of rare events in complex systems with the least bias beyond conditional constraints. In this paper we introduce the CE method for analyzing the system reliability with the highest uncertainty among all possibilities satisfying supplied moment constraints, and developed numerical CE algorithms capable of estimating the uncertainty of failure modes in an M-dimensional redundancy system domain with moment constraints of order up to N. A general computational framework of event estimation and condition monitoring of redundancy systems is illustrated in which the Monte Carlo simulations and CE optimization algorithms are combined. Numerical results indicate potential improvements in the measure of the uncertainty of redundancy systems that would lead to the best-fit analysis of all the complete or incomplete failure events.     

基于ANN和等值发电机模型的快速暂态稳定计算

暂态稳定分析对于电力系统运行具有重要的意义,针对暂态稳定时域仿真方法计算速度过慢的缺点,首先提出了应用于快速暂态计算的发电机参数等值方法,这种方法可以避免迭代解网络方程,能在保证计算精度的基础上显著减少暂态稳定计算时间,每个迭代步对发电机功角初值进行预测后则能够进一步减少解网络方程次数。算例仿真证明,粒子群算法优化得到的等值参数和基于神经网络的预测功角,在不同的系统运行方式下,能显著减少解网络方程次数和判定系统所处的稳定状态。算法具有计算精度高和收敛性良好的特点,功角预测和等值参数则有望应用于不同规模的系统中。     

A numerical prediction and flight test of the transient fuel temperatures in an aircraft

A numerical prediction of the transient fuel temperature in an aircraft was made and verified with a flight test. The analysis was studied with the finite difference method. Numerical calculation was performed by an explicit method of the modified Dufort-Frankel scheme. Convective heat transfer coefficients were used in calculating heat transfer between the aircraft surface and the ambient air. For an aircraft on the ground, an empirical equation represented as a function of free-stream air velocity was used. And, the heat transfer coefficient for flat plate turbulent flow suggested by Eckert was employed for in-flight phases. The governing equations used in this analysis are the mass and energy conservation equations on fuel and oils. The analysis was verified with the flight test data of a fuel system with additional fuel supplies and return concept. As a result of the verification, the difference of the fuel temperatures obtained by the analysis from those of the flight test data was relatively small with a tendency to increase in the later phases of the flight.     

Numerical efficiency of CUDA based parallel programming for dynamic analysis of multi-body systems with multi-joints and multi-force elements

The graphic processor unit (GPU) is an ideal solution to problems involving parallel data computations. A serial CPU-based program of dynamic analysis for multi-body systems is rebuilt as a parallel program that uses the GPU’s advantages. We developed an analysis code named GMAP to investigate how the dynamic analysis algorithm of multi-body systems is implemented in the GPU parallel programming. The numerical accuracy of GMAP is compared with the commercial program MSC/ADAMS. The numerical efficiency of GMAP is compared with the sequential CPU-based program. Multiple pendulums with bodies and joints and the net-shape system with bodies and spring-dampers are employed for computer simulations. The simulation results indicate that the accuracy of GMAP’s solution is the same as that of ADAMS. In the net type system that has 2370 spring-dampers, GMAP indicates an improved efficiency of about 566.7 seconds (24.7% improvement). It is noted that the larger the size of the system, the better the time efficiency.