Analysis of underwater thruster model with ambient flow velocity using CFD

Thrust force is a very important factor for underwater vehicles. The thrust force that is determined by the pressure gradient between a propeller and a thruster can be represented by the ambient flow velocity introduced as the control volume and the axial flow velocity of a propeller. Because a change in ambient flow velocity triggers a change in the pressure gradient between a propeller and a thruster, a model taking account of the ambient flow velocity is required for an unmanned underwater vehicle (UUV) system. However, the axial flow velocity introduced into a propeller is very difficult to measure without accurate test devices. Therefore, in this study, the axial flow velocity is calculated with the computational fluid dynamics (CFD) method to use it as a basis for estimating the approximate value of the thrust force. As a result, a relatively accurate analysis of the effect of the ambient flow velocity on the thrust force can be obtained with considerable time and cost effectiveness as compared to the existing experimental methods. To evaluate the validity of the data from the CFD analysis results depending on the change in ambient flow velocity and the pressure gradient of a thruster, the resulting CFD values were compared with the thrust forces obtained in the previously performed thrust force experiment of a thruster depending on the ambient flow velocity in a circulating water channel.     

Simulation of bi-directional pedestrian flow by using a cell transmission model

An extended cell transmission model is proposed to simulate bi-directional pedestrian flow in the corridor. In the model, the walking space is discretized into regular hexagonal cells. Three walking preferences of pedestrians are taken into account, including walking on the right-hand side, following front people in the same direction, and avoiding conflicts with ones in the opposite direction. An implementation of the model with periodic boundary condition is then presented. Furthermore, by simulation experiments, we show the effects of the model parameters on flow distributions and fundamental diagrams. The model is also calibrated through comparing the flow-density relationships from empirical data and model simulations. In addition, the model can successfully reproduce typical self-organization phenomena in bi-directional pedestrian flow, e.g., two-lane formation and multi-lane formation, although it is not a microscopic model.     

Performance of a new CFD flow solver using a hybrid programming paradigm

This paper presents several algorithmic innovations and a hybrid programming style that lead to highly scalable performance using shared memory for a new computational fluid dynamics flow solver. This hybrid model is then converted to a strict message-passing implementation, and performance results for the two are compared. Results show that using this hybrid approach our OpenMP implementation is actually marginally faster than the MPI version, with parallel speedups of up to 599 out of 640 using OpenMP and 486 with MPI.     

基于CFD的船舶斜浪三自由度运动仿真研究

针对船舶在斜浪中的多自由度耦合运动问题, 建立了三维粘性流耐波性数值波浪水池, 采用边界条件造波法生成高精度的斜向规则波, 通过同时求解RANS方程和刚体运动学方程, 结合网格整体移动方法和滑移网格方法, 实现了船舶斜浪航行的垂荡、纵摇及横摇三自由度耦合运动数值模拟。给出了DTMB5512船模斜浪中的垂荡、纵摇及横摇的频率响应函数, 与线性切片理论计算结果进行比较, 吻合良好。该方法可为船舶斜浪航行的耐波性预报提供一种有效的途径。     

Simulation of free-surface flow using a boundless grid

Physically based fluid simulation in recent years has been successful for small-scale fluids such as liquid in a cubic cavity.However,for boundless free-surface flow of large scale and irregular area,there is a critical trade-off between simulation efficiency and accuracy because of the restriction of the traditional regular computational grids.This paper introduces boundless computational grids based on hierarchical runlength encoding to simulate large-scale free-surface flow.We first modeled the free-surface flow with a lattice Boltzmann method,and calculated the surface curvature in the update process.We then introduced an effective method with which to calculate the surface curvature according to the surface tension so that the surface detail was enhanced,and obtained the curvature of each surface effectively as it was extracted.Furthermore,we dynamically compressed and indexed the computational cells with the improved hierarchical run-length encoding algorithm,so that the grid expands dynamically according to the fluid flow and the computational resources used were proportional to the volume of the fluid.Finally fluids in different situations were simulated realistically.The proposed method makes the best of the computational resources to perform the simulation with high resolution,and dynamically allocates resources so that the fluid can expand in random directions without boundaries,which is suited to the simulation of large-scale visual scenes.     

基于EMMS曳力的湍动床气固流动CFD模拟

湍动流化床已广泛用于费托合成、FCC催化裂化等工业过程。其主要流动特点是由底部密相鼓泡区与顶部稀相分散区两者构成。由于湍动流化的复杂性,导致目前对该类体系内非均匀结构形成与演化的研究相对较少。近年来,快速发展的计算流体力学方法为深入研究此类流化提供了一种有效的手段,然而传统的双流体模型往往不能成功捕获到湍动流化的复杂结构。为了考虑非均匀结构对曳力的影响,本文提出了一个改进的曳力模型,即将湍动流化床分为3个区域:底部鼓泡区、过渡区和顶部稀相区。其中,底部区采用气泡EMMS曳力模型进行修正,过渡区采用Wen-Yu曳力模型,顶部区采用Schiller-Naumann曳力模型。修正的曳力模型与双流体模型耦合后,再通过FLUENT商业软件平台对湍动流化床内的气固两相流动进行CFD模拟。模拟结果显示,修正的曳力模型可以较好的预测湍动流化床内的流体动力学特征。该曳力模型能够模拟底部密相鼓泡区和顶部稀相分散区的两相共存结构,颗粒浓度在径向分布上为"环-核"结构,模拟的轴向颗粒浓度分布与实验数据吻合较好。     

A sub-grid air entrainment model for breaking bow waves and naval surface ships

Direct numerical simulations (DNS) of air entrainment by the breaking bow waves of naval surface ships are outside of the computational reach of the most powerful computers in the foreseeable future. This creates a need for physically-based models of air entrainment for applications in numerical simulations for ship design. Due to the non-linear dependence of the terminal bubble velocity with diameter most air entrainment flows have a high void fraction region immediately below the free surface. We present a model that locates this region employing the local liquid velocity and the distance to the interface. Using this model and the bubble size distributions measured by Deane and Stokes [Deane BD, Stokes MD. Scale dependence of bubble creation mechanisms in breaking waves. Nature 2002;418:839-44] we have simulated the air entrainment in the breaking wave experiments of Wanieski et al. [Waniewski TA, Brennen CE, Raichlen F. Measurement of air entrainment by bow waves. J Fluids Eng 2001;123:57-63]. Comparison against these experimental data is good. We then apply this model to simulate the flow around naval combatant DTMB 5415 and the research vessel Athena. The model predicts air entrainment in regions where it was actually observed at sea, namely the breaking bow wave, along the water/air/hull contact line and in the near-wake. To the best of our knowledge this is the first model of air entrainment that compares favorably with data at laboratory scale and also presents the right trends at full-scale.     

微机械气体红外探测器的CFD模拟

概要地介绍了一种可工作于室温环境下的微型气体红外探测器,它基于气体吸收红外辐射后以无辐射去激励方式产生一系列热效应的物理基础,可获得包含红外辐射源信息的信号.为深入研究热效应所产生的微热信号对器件整体性能的控制和影响,建立了相应的流体力学(CFD)模型,通过数值分析方法,获得了气体物质微观行为随红外辐射源辐射特性的变化关系.     

Nonlinear modelling and control of the flow over aerofoils using CFD simulations

A simulation based approach for nonlinear dynamical modelling and feedback control of the drag to lift ratio for aerofoils is investigated through case studies involving NACA 23012, ag13 and b737a aerofoils. The flow around the aerofoils is studied via numerical solutions of the 2D Navier–Stokes (NS) equations. A standard computational fluid dynamics (CFD) solver is extended to be able to measure desired feedback values and to apply a control input to the flow field. The proposed modelling and control approach is based on first determining the measurement points and injection points on the aerofoil for the control input. Then, to estimate the dynamical model, some input–output data are collected by injecting a chirp input flow to the field and saving the measurement data. Next a Hammerstein–Wiener (HW) type nonlinear dynamical model of the flow field is estimated using system identification. For control design, the nonlinear part of the model is eliminated by means of inverse functions, followed by the application of automated tuning methods to the linear part to obtain the closed-loop system. The results show that the designed feedback control system can reduce the drag to lift ratio considerably as compared to the unactuated case.     

Valuation by using a fuzzy discounted cash flow model

The objective of this paper is to extend the classical discounted cash flow (DCF) model by developing a fuzzy logic system that takes vague cash flow and imprecise discount rate into account. In order to explicitly discuss a more appropriate valuation model, uncertain information will be fuzzified as triangular fuzzy numbers to quantify and evaluate the intrinsic value of a company's financial asset under the framework of DCF approach. We will find that the fuzzy discounted cash flow (FDCF) model proposed in this paper is one extension of classical (crisp) model and should be more suitable to capture the elements of valuation than non-fuzzy models.     

Pulsatile flow of blood using a modified second-grade fluid model

We study the unsteady pulsatile flow of blood in an artery, where the effects of body acceleration are included. The blood is modeled as a modified second-grade fluid where the viscosity and the normal stress coefficients depend on the shear rate. It is assumed that the blood near the wall behaves as a Newtonian fluid, and in the core as a non-Newtonian fluid. This phenomenon is also known as the Fahraeus–Lindqvist effect. The equations are made dimensionless and solved numerically.     

Synchronous machine control using a two level model follower

In this paper a controller is developed for a synchronous machine using a hierarchical approach to model following. A linear model is defined which incorporates the desired response characteristics for the non-linear machine and the controller attempts to reduce the error between the system and model states while not utilising excessive control effort. This latter problem is solved by decomposition using an off-line two level structure where on level one two one point boundary value problems are solved whereas on level 2 a prediction algorithm is used. Simulation results show that the algorithm converages rapidly giving a control which reduces the error satisfactorily. The control calculation is very efficient in computer storage and takes less than half the computation time of the global optimal control calculation.     

油水两相流流型的混沌时间序列分析

采用一种基于互信息第一极小值计算法确定了相空间重建延迟时间,用Mackey-Glass方程及Lorenz奇异吸引子数据作算例检验了该计算方法的可靠性．通过计算的混沌吸引子关联维对油水两相流流型进行了表征,结果表明对含水率为61％～91％及总流量为10～60(m^3／d)的水包油流型,油水两相流电导波动信号的关联维与含水率及总流量有着良好的相关关系,而对含水率为51％的油水两相流过渡流型,关联维随总流量呈现不规则突变,表明了混沌吸引子关联维数对油水两相流流型变化具有敏感的“指示器”特性．     

Collocation method using quintic B-spline and sinc functions for solving a model of squeezing flow between two infinite plates

An axisymmetric viscous flow, generated by two large parallel plates slowly approaching each other is investigated. The steady nonlinear governing equations are converted into a fourth-order nonlinear differential equation using integrability condition. The resulting nonlinear boundary value problem is solved using quintic B-spline collocation and sinc-collocation methods. The approach consists of reducing the problem to a set of algebraic equations. Numerical examples are included to demonstrate the validity and applicability of the techniques and a comparison is made with existing results in the literature.     

Cross-stream diffusion under pressure-driven flow in microchannels with arbitrary aspect ratios: a phase diagram study using a three-dimensional analytical model

This article presents a three-dimensional analytical model to investigate cross-stream diffusion transport in rectangular microchannels with arbitrary aspect ratios under pressure-driven flow. The Fourier series solution to the three-dimensional convection-diffusion equation is obtained using a double integral transformation method and associated eigensystem calculation. A phase diagram derived from the dimensional analysis is presented to thoroughly interrogate the characteristics in various transport regimes and examine the validity of the model. The analytical model is verified against both experimental and numerical models in terms of the concentration profile, diffusion scaling law, and mixing efficiency with excellent agreement (with <0.5% relative error). Quantitative comparison against other prior analytical models in extensive parameter space is also performed, which demonstrates that the present model accommodates much broader transport regimes with significantly enhanced applicability.     

Simulation of fluid flow in granular microstructure using a non-staggered grid scheme

A finite difference numerical scheme has been utilized to simulate fluid flow in granular microstructures. The pixels of their digital images represent the granular microstructure in the finite difference grid. The scheme utilizes a non-staggered grid arrangement, which requires only one finite difference mesh to solve the governing fluid flow equations. As such, the scheme is more efficient when it comes to dealing with non-orthogonal coordinates and complex geometry of boundary conditions such as that of granular microstructure.The numerical scheme is verified by comparing the permeability values of a medium of packed columns to a closed form solution. It is then used to evaluate the permeability coefficients of idealized and natural granular microstructures. It has been found that as the directional aspect ratio increases, the resistance of a particle to fluid flow increases, which results in a decrease in the permeability coefficient. A medium of elliptical particles has higher permeability coefficient than a medium of rectangular particles for the same porosity because of its lower surface area. The permeability anisotropy has been found to increase with an increase in the aspect ratio or a decrease in porosity. Spherical glass beads have been found to have higher permeability coefficients than Ottawa sand and Silica.     

基于模型的海浪模拟仿真

在计算机图形学领域,自然景物的模拟一直都是一项研究重点.由于不规则性和随机性,要对水波特别是波浪进行逼真地模拟较为困难.研究了借助几何模型模拟自然界中海浪的运动,通过图形学中的真实感处理,得到逼真的海浪波动效果.在研究过程中首先是建立简单的海浪模型,通过辅助衰减函数将其改进,以符合海浪运动的物理规律,同时加入了动态变化的影响因子以增加海浪运动的随机性.结果表明,该模型可以较为真实的模拟海浪的基本运动.     

变工况下微通道两相换热器器性能模拟

利用均相模型对微通道两相换热器(蒸发器、冷凝器)进行模拟,通过制冷工质R134a热力状态及参数的计算模拟换热器设计工况下的稳态情况.首先建立换热器仿真模型,得出设计工况下的换热器设计结果,然后此基础上,着重分析当外界温度、进出口状态以及质量流量发生变化时,相应的换热器性能参数变化情况,以及相关影响因素.