基于凹槽管道内流体速度矢量图的减阻特性分析

横向凹槽表面减阻特性广泛的表现在自然界中,文章阐述了目前已有的几种非光滑表面减阻机理,计算分析了十种不同凹槽管道的阻力,并根据计算结果和仿真速度矢量图分析了不同凹槽管道的减阻特性:凹槽管道减阻主要和来流速度,凹槽内漩涡的丰满度和管道径向的湍流区厚度有关。在一定速度范围内,横向凹槽管道较相同管径的直管道有减阻效果,且减阻率出现先增后减,即存在减阻率最佳速度;管道凹槽内漩涡的丰满度越高,湍流区厚度越小,凹槽管道减阻率越大。     

速度对脊状表面减阻影响的数值仿真研究

在管道运输中,流动处湍流状态时,对V型脊状表面和光滑表咏在多个速度下进行了数值仿真计算,发现了脊状表面微观流场的特点和速度对脊状表面减阻效果的影响规律。针对回转体脊状表面的流场特性,为控制减阻,进行数值仿真时合理选择了计算模型、计算流域、计算刚格及边界条件。仿真结果表明：在同一速度下,相对光滑表面而言,脊状表面所受的压差阻力略有增大,但占总阻力份额80％以上的粘性阻力显著降低,从而形成减阻效果;对于同一个脊状表面,速度越大,其湍流边界层能量的交换和转化越强烈,使得低速下的减阻效果明显优于高速。     

旋翼翼型动态失速特性的数值仿真研究

旋翼翼型动态失速具有很强的非定常性与非线性,为研究动态失速发生时的气动和流场特性,基于CFD软件FLU-ENT,采用单方程S-A湍流模型,对NACA0012翼型的动态失速特性进行数值仿真。仿真结果显示,轻失速是后缘分离,旋涡影响范围小;深失速则形成了很强的前缘分离涡,结构复杂,影响范围大。参数研究显示,较大的湍流强度能减缓动态失速的发生,并且动态失速状态下的分离涡影响范围较小。通过与实验的对比证明,用S-A湍流模型的方法可以较好地仿真动态失速过程中旋涡的形成与发展过程。     

输油管道中缺陷快速定位的应力波技术

为解决输油管道中裂纹缺陷难于快速定位的问题,基于应力波的特性研究应力波无损检测技术在管道缺陷检测中的应用。借助ANSYS AUTODYN建立多组动态仿真模型,计算并分析仿真结果数据,对比光滑无裂纹缺陷的管道与含有裂纹缺陷管道的应力波传播规律。经过分析应力波的传播变化过程,得出管道裂纹对应力波的干扰规律,进而得到2种利用应力波进行无损检测的方法：一是沿着管道轴向布置测点的检测方法;二是在管道截面布置测点的检测方法。     

旋风分离器减阻杆减阻的数值研究

文中首次利用数值方法对旋风分离器进行了减阻杆减阻的研究。应用CFD计算流体力学软件FLUENT，对某种型号的Stairmand高效型旋风分离器的三维有旋流场速度和压力进行了数值模拟，计算了减阻杆的减阻效率。流场和减阻效率的数值结果与实验数据吻合较好，证明对旋风分离器减阻杆减阻研究时所采用的网格划分方法、RSM湍流模型和边界条件是可靠的，为数字设计高效率的减阻杆提供了简便可靠的办法。     

湍流效应退化图像的数值模拟和仿真计算

大气湍流干扰空中目标的成像探测,使得观测到的目标图像是严重抖动和模糊的.为了具体分析湍流效应退化图像的特征,总结了光学成像系统的传输特性和湍流流场的统计特性,用随机相位屏对大气湍流效应进行了数值模拟,建立了湍流形成畸变波前过程的仿真模型,对点源目标发出的光波经过湍流流场后,在成像探测器焦面上形成的点源目标图像,计算了短曝光像和长曝光像的光强分布,定义了描述像点弥散和抖动的指标,并进行了定量分析和仿真计算.通过对计算结果进行分析,验证了仿真模型的合理性,并对湍流效应造成的图像退化给出了定量的描述,对进一步进行湍流效应退化图像校正算法研究具有重要意义.     

耐腐蚀性油气储运管道内部应力流场模型仿真

油气储运管道的内部应力流场模型构建分析是刻画管道耐腐蚀性的重要指标。提出一种基于管道内流数值离散处理和黏性通量控制的耐腐蚀性油气储运管道内部应力流场模型构建方法,并进行数值仿真。根据低雷诺数湍流模型设计流湍流度多块结构化网格,求得载荷变化引起管道的内部流场规定的最小屈服强度,通过黏性通量控制,进行湍流动能生成曲率修正,实现油气管道内部应力流场模型构建。仿真结果表明,该模型能准确分析温度、压力对不同管径管道的变化规律,准确描述温度和压力变化对油气储运管道平均应力比率变化关系,实现对油气管道内部应力流场的定量分析,对提高管道设计的耐腐蚀性和耐高温高压性具有指导意义。     

大气激光通信中的光斑特性模拟和验证

根据空-地大气激光通信信道的特性,基于惠更斯-菲涅耳原理,应用夫琅禾费圆孔衍射的算法,对光波强度和光斑图像在光束截面内的分布进行分析及计算,研究了传输距离、发射孔径、激光波长以及湍流结构常数等对光斑形状及光强分布的影响。利用对数正态分布建立了大气湍流信道模型,通过对不同传输距离条件下光斑上不同点的光强分布进行计算统计和仿真分析,实验和仿真结果表明：光斑的光强分布图样服从贝塞尔分布。因此,在相同的大气湍流条件下,选择较长波长激光和较大孔径的发射器,可以有效降低光强闪烁引起的衰减,从而提高大气激光的通信质量。     

微细管道内壁三维测量技术研究

提出了一种适于管道内壁缺陷检测和三维测量的计算模型和测量方法．形貌检测器在驱动机构的带动下沿着管道轴线行进时,按照控制指令对管道内壁任意截面进行图像拍摄．通过对图像的处理,提取出截面上离散点的图像坐标,再结合对CCD摄像机标定得到的参数,计算出截面环上各离散点在三维坐标系中的位置,从而重构出该截面的形状,完成管道内壁的检测,并为后续的管道内表面三维重建提供数据信息．     

平板边界层转捩的仿真

对计算流体力学(CFD)的难点问题--零压力梯度平板边界层转捩进行了成功仿真.利用商业软件FLUENT的二次开发功能,一种符合现代CFD技术要求的先进转捩模型--Gamma-Theta被用于仿真中,并采用了设置来流粘性比的新方法来进行基于SST k-omega湍流模型的仿真.仿真结果表明,在不同的来流湍流水平下,Gamma-Theta转捩模型可以准确仿真包含自然转捩和bypass转捩现象的平板边界层流动,而设置来流粘性比的新方法对于成功的仿真也起到了关键作用.     

涡激振动行波动力学特征的数值模拟

采用数值模拟分析了水中大长径比结构物涡激振动行波动力学特征,以及水动力学阻尼和模型初始张力对于行波动力学特征的影响.数值模拟模型采用改进的尾流振子模型,该模型考虑了涡激振动横向运动和顺流向运动之间的耦合,考虑了张力沿模型长度的变化和流体的耗散.给出了用于评估行波在整个振动波中所占比例的方法,该方法采用行波椭圆来定量表征行波所占的比例,即行波比.数值模拟结果显示,行波比总体上呈现随流速增加的趋势,但在模态阶数变化的临界流速上突然降低.水动力学阻尼显著影响着行波比,阻尼比越大,行波比越大,行波对于整个振动波的贡献就越大.不同张力情况下,行波比突然下降的临界速度不同.初始张力影响模型的固有频率,影响模态阶数发生转变的流速,从而影响了行波比突然下降的临界流速.     

The holy grail of microfluidics: sub-laminar drag by layout of periodically embedded microgrooves

The sub-laminar drag effect of microgroove surfaces was studied numerically in a steady two-dimensional channel flow at subcritical Reynolds numbers. Considerations are restricted to grooves of a few viscous length scales in depth, which are assumed not to promote the laminar to turbulent transition process. It was found that the drag reduction effect is due to the layout of grooves with respect to the flow direction and contour geometry. Results of computations show that for grooves of curved contour placed normal to the flow direction, drag arising from viscous and pressure forces is modulated due to the functional dependence of forces on the surface area projected in the flow direction. Such a groove layout leads to a large skin-friction reduction, but a comparable increase in pressure drag results in sub-laminar drag if drag over flat surface is considered as a reference. For a curved groove contour, the drag reduction increases with increasing Reynolds number and reaches about 5 % at Reynolds numbers approaching critical.     

Ethernet based time synchronization for Raspberry Pi network improving network model verification for distributed active turbulent flow control

Friction drag primarily determines the total drag of transport systems. A promising approach to reduce drag at high Reynolds numbers(> 104) are active transversal surface waves in combination with passive methods like a riblet surface. For the application in transportation systems with large surfaces such as airplanes, ships or trains, a large scale distributed real-time actuator and sensor network is required. This network is responsible for providing connections between a global flow control and distributed actuators and sensors. For the development of this network we established at first a small scale network model based on Simulink and True Time. To determine timescales for network events on different package sizes we set up a Raspberry Pi based testbed as a physical representation of our first model. These timescales are reduced to time differences between the deterministic network events to verify the behavior of our model. Experimental results were improved by synchronizing the testbed with sufficient precision. With this approach we assure a link between the large scale model and the later constructed microcontroller based real-time actuator and sensor network for distributed active turbulent flow control.     

Waves and spatially localized structures in the turbulent flows of a viscous fluid: Calculation results

A direct numerical simulation was performed of intermittent and turbulent flows of viscous incompressible fluid in an infinite circular pipe. The Navier-Stokes equations were integrated at the Reynolds numbers of 1800 ≤ Re ≤ 4000 calculated from the mean velocity and a pipe diameter of D = 2R. The numerically obtained solutions belong to the class of mean streamwise periodic solutions with a very large period λ_max = 16πR. The Fourier harmonic components of the velocity fluctuations corresponding to very low longitudinal wavenumbers are shown to be the most energetic. A detailed study was carried out of the structures of the calculated turbulent and intermittent flows. The accuracy and the very possibility of the approximation of the turbulent velocity field by the superposition of traveling and standing waves are analyzed. It is shown that the parameters of such a representation (wave amplitudes, phase velocities, and the position of the wave front) are strongly dependent on whether or not very low longitudinal wavenumbers are included in the mathematical model of the flow The numerical solutions at Re = 2200 and 2350 describe the intermittent type of the flow, for which the localized turbulent structures (turbulent puffs) propagate downstream while retaining their spatial dimensions. The space-time structure of the calculated turbulent puffs is compared with the available experimental data. The main statistical characteristics of the turbulence inside and outside the turbulent puff are calculated and the convective rate of the puffs downstream expansion is determined.     

Pseudospectral simulation of turbulent viscoelastic channel flow

The methodology and validation of direct numerical simulations of viscoelastic turbulent channel flow are presented here. Using differential constitutive models derived from kinetic and network theories, numerical simulations have demonstrated drag reduction for various values of the parameters, under conditions where there is a substantial increase in the extensional viscosity compared to the shear viscosity (Sureshkumar, Beris, Handler, Direct numerical simulation of turbulent channel flow of a polymer solution, Phys. Fluids 9 (1997) 743–755 and Dimitropoulos, Sureshkumar, Beris, Direct numerical simulation of viscoelastic turbulent channel flow exhibiting drag reduction: effect of the variation of rheological parameters, J. Non-Newtonian Fluid Mech. 79 (1998) 433–468). In this work, new results pertaining to the Reynolds stress and the pressure are presented, and the convergence of the pseudospectral algorithm utilized in the simulations, as well as its parallel implementation, are discussed in detail. It is shown that the lack of mesh refinement, or the use of a larger value for the artificial stress diffusivity used to stabilize the conformation tensor evolution equations, introduce small quantitative errors which qualitatively have the effect of lowering the drag reduction capability of the simulated fluid. However, an insufficient size of the periodic computational domain can also introduce errors in certain cases, which albeit usually small, can qualitatively alter various features of the solution.     

The computation of flow in a spirally fluted tube

The paper describes the development and application of a computational procedure for calculating the velocity field in fully-developed flow through spirally fluted tubes. Topics covered include the choice of coordinate system, the resultant form of the governing equations, the validation of the accuracy of the procedure by comparison with known analytical solutions and a discussion of the results of applying the procedure to the spirally fluted tube flow itself. The computations indicate that the effectiveness of the flutes in imparting swirl to the flow increases markedly with the number of flutes. The swirling motion and the turning over of fluid within the flute region produces very little increase in drag; indeed, when friction factor and Reynolds number are based on hydraulic diameter, the drag is less than for a smooth tube.     

跨声速机翼阻力分解的一种改进方法

提出黏性区域探测器的一种改进形式，并用于捕捉激波和翼梢涡的熵增阻力；给出尾迹平面的可压缩涡动力学诱导阻力表达式，并与基于热力学的诱导阻力对比。在跨声速来流状态下，对ONERA M6和某民用飞机巡航状态下的机翼阻力进行分解，同时分析该民用飞机机翼安装翼梢小翼前、后的远场阻力构成。结果表明：新的区域探测器合理可靠，黏性阻力与伪熵阻力的计算结果更加准确；2种诱导阻力计算方式的计算结果一致，但基于涡动力学的诱导阻力计算方法受积分平面位置的影响更小；安装翼梢小翼基本不影响整个流场的黏性阻力，减阻的主要效果体现为诱导阻力的减小。     

预失真线性化器的仿真研究

根据级联网络理论,基于行波管的非线性特性推导出预失真器模型,并在Matlab仿真平台上从三个角度验证了该模型的可行性和有效性,包括增益特性、相移特性和三阶交调特性,其中三阶交调系数的提取采用了计算精度高的自相关函数法。仿真结果显示,在饱和输入功率条件下加入预失真器后的系统相比行波管的三阶交调改善了6．947dBc。     

Towered waves and anti-waves in the generalized Degasperis-Procesi equation

New traveling wave solutions of the generalized Degasperis-Procesi equation are investigated. The solutions are characterized by three parameters. Using an improved qualitative method, abundant traveling wave solutions, such as smooth waves, peaked waves, cusped waves, compacted waves, looped waves and fractal-like waves, are obtained. Especially, some strange composite wave solutions such as towered waves and their anti-waves are first given. We also study the limiting behavior of all periodic solutions as the parameters trend to some special values.     

A fixed-grid model for simulation of a moving body in free surface flows

A two-dimensional computer model is developed to simulate free surface flow interaction with a moving body. The model is based on the cut-cell technique in a fixed-grid system. In this model, a body is approximated by the partial cell treatment (PCT), in which an irregular body is represented by the volumetric fraction of solid in Cartesian cells. The body motion is tracked by Lagrangian method whereas the fluid motion around the body is solved by Eulerian method. The concept of “locally relative stationary (LRS)” is introduced in this study. In the LRS method, a source term is added locally to the conventional continuity equation on body surfaces to take account of body motions, which subsequently affects the computational results of fluid pressure and flow velocity around the body. The LRS method is incorporated into an earlier Reynolds averaged Navier-Stokes (RANS) equations model developed by Lin and Liu [A numerical study of breaking waves in the surf zone. J Fluid Mech 1998;359:239-64]. The new model is capable of simulating generic turbulent free surface flows and their interaction with a moving body or multiple moving bodies. A series of numerical experiments have been conducted to verify the accuracy of the model for simulation of moving body interaction with a free surface flow. These tests include the generation of a solitary wave with the prescribed wave paddle movements, water exit and water impact and entry of a horizontal circular cylinder, fluid sloshing in a horizontally excited tank, and the acceleration/deceleration of an elliptical cylinder near a water surface. Excellent agreements are obtained when numerical results are compared to available analytical, experimental, and other numerical results. The model is a simple-to-implement computational tool for simulating a moving body in turbulent free surface flows.