Numerical Simulation of the Influence of Water Flow on the Piers of a Bridge for Different Incidence Angles

A two-dimensional mathematical model is used to simulate the influence of water flow on the piers of a bridge for different incidence angles.In particular,a finite volume method is used to discretize the Navier-Stokes control equations and calculate the circumferential pressure coefficient distribution on the bridge piers’surface.The results show that the deflection of the flow is non-monotonic.It first increases and then decreases with an increase in the skew angle.

Investigation of scale efect for the computation of turbulent flow around a circular cylinder

In order to investigate the scale effect of turbulent flow around a circular cylinder, two similarity numbers （criteria） based on turbulent kinetic and dissipation rates associ- ated with the fluctuation characteristics of turbulence wake are deduced by analyzing the Reynolds averaged NavierStokes equations （RANS）. The RNG k-s models and finite volume method are used to solve the governing equations and the second-order implicit time and upwind space discretization algorithms are used to discrete the governing equations. A numerical computation of flow parameters around a two-dimensional circular cylinder with Reynolds numbers ranging from 102 to l07 is accomplished and the result indicates that the fluctuation of turbulence flow along the center line in the wake of circular cylinder can never be changed with increasing Reynolds numbers when Re ≥ 3 × 10^6. This conclusion is useful for controlling the scale of numerical calculations and for applying model test data to engineering practice.

Simulation of Flow Around Cylinder Actuated by DBD Plasma

The electric-static body force model is obtained by solving Maxwell＇s electromagnetic equations. Based on the electro-static model, numerical modeling of flow around a cylinder with a dielectric barrier discharge （DBD） plasma effect is also presented. The flow streamlines between the numerical simulation and the particle image velocimetry （PIV） experiment are consistent. According to the numerical simulation, DBD plasma can reduce the drag coefficient and change the vortex shedding frequencies of flow around tile cylinder.

Stress-Blended Eddy Simulation of the Turbulent Flow Around Circular Cylinder with Dynamic Hybrid RANS/LES Model

In order to verify the effectiveness and superiority of the dynamic hybrid RANS/LES(DHRL)model,the flow around a cylinder with sinusoidal fluctuating velocity at the inlet was used as the test case.The latest computational fluid dynamics(CFD)model can flexibly choose any existing large-eddy simulation(LES)method combined with RANS method to calculate the flow field.In addition,the DLES model and DDES model are selected as typical representatives of the turbulence model to compare the capture ability of the flow field mechanism.The internal flow field including the y+value,velocity distribution,turbulent kinetic energy and vortex structures is comprehensively analyzed.Finally,the results show that the new model has enough sensitivity to capture the information of the flow field and has more consistent velocity distribution with the experimental value,which shows its potential in practical engineering applications to some extent.

NUMERICAL STUDY ON THE FLOW AROUND A CIRCULAR CYLINDER WITH SURFACE SUCTION OR BLOWING USING VORTICITY-VELOCITY METHOD

A vorticity-velocity method was used to study the incompressible viscous fluid flow around a circular cylinder with surface suction or blowing. The resulted high order implicit difference equations were effeciently solved by the modified incomplete LU decomposition conjugate gradient scheme ( MILU-CG). The effects of surface suction or blowing' s position and strength on the vortex structures in the cylinder wake, as well as on the drag and lift forces at Reynoldes number Re = 100 were investigated numerically. The results show that the suction on the shoulder of the cylinder or the blowing on the rear of the cylinder can effeciently suppress the asymmetry of the vortex wake in the transverse direction and greatly reduce the lift force; the suction on the shoulder of the cylinder, when its strength is properly chosen, can reduce the drag force significantly, too.

NUMERICAL SIMULATIONS OF THE VORTEX-SHEDDING IN FLOWS AROUND RECTANGULAR CYLINDERS

Flows around rectangular cylinders with a series of width-to-height ratios are calculated by means of the Improved Finite Analytic Method (IFAM) and the formation, development and shedding of vortices from the cylinders are simulated successfully. According to these results of time-dependent processes the physical phenomena in the flows are investigated in detail, and the discontinuity of Strouhal number is explained in the case of the width-to-height ratio equal to 3.0. The numer- ical solutions here show good agreement with the experimental results. In addition, based on several hundreds of the calculated flow patterns a moving picture is made by the computer image processing technology and recorded on a video tape, and then the vivid pictures of the physical process of vortex-shedding can be replayed later and analysed in detail.

TRANSIENT EFFECT OF A BOGER FLUID FLOWING AROUND A CONFINED CYLINDER

This paper is concerned with the numerical simulation of the transient effect of an inertialess Boger flow past a confined circular cylinder and the comparison of predictions with particle image velocimetry (PIV) measurements given by Shiang et al.. Dynamic simulation based on the Oldroyd-B constitutive model was carried out using a Lagrangian-Eulerian algorithm. The evolution of velocity field was obtained for the flow at two Deborah (De) numbers, i.e. De = 1.2 and 3.0. At low De, the flow reached steady state rapidly, and showed a symmetric flow regime. However, at high De, the time required to reach steady flow behind the cylinder increased significantly, and the distribution of the velocity field appears to be asymmetric with respect to the stagnation line. Fairly good agreement between the numerical results and the experimental observations is reported. It can be concluded that both the experimental measurements and the present simulations indicate that the elasticity of the polymeric flow strongly affect the flow regime of viscoelastic flow around a confined cylinder.

Viscous Fluid Flowing around the Plate: Turbulence

Diffusion of momentum gives rise to viscosity. This article presents a solution in the explicit form of the equation of the momentum diffusion for a viscous fluid flowing around a plate taking into account deceleration. Three characteristic regions of a viscous flow have been described: the mantle, the body of the boundary layer, the viscous sublayer. In the mantle, the effect of viscosity is significant at a considerable distance from the plate. The momentum diffusion is focused in the body of the boundary layer. The diffusion force that produces the momentum of force giving rise to eddies is localized in the viscous sublayer. At the beginning of the plate, a moment of force twists the liquid along the flow, creating eddies that roll along the plate. For this reason, they are pressed against the surface of the plate. But at some distance from the beginning of the plate, the moment of force changes its orientation to the opposite and twists the vortices in the opposite direction, causing the vortices to roll along the plate against the flow. This causes the liquid to detach from the surface of the plate. This is the beginning of turbulence. The diameter of the vortex produced in the viscous sublayer is small being of the order of the thickness of the viscous sublayer. The vortex possesses a large angular velocity. Due to the momentum diffusion and the effect of the eddies combined in passing along the plate, its diameter increases up to the size of the thickness of the boundary layer and even more, whereas its angular velocity decreases down to the values really observed. The value of the critical Reynolds number of the transition from the laminar flow to the turbulent one has been found, and it agrees with the experimental data. The value of the shear stress produced by the viscous fluid on the plate surface has also been obtained. The way of measurement of the friction coefficient characterizing the effect of the plate on the flow has been proposed. It has been shown that the boundary condition of adhesion to the surface of a body flown around, that is applied in the estimation of viscous flows, contradicts the real processes of the flow.

LARGE EDDY SIMULATION AND SPECTRUM ANALYSIS OF FLOW AROUND TWO SQUARE CYLINDERS ARRANGED SIDE BY SIDE

Large eddy simulation cooperated with the second order full extension ETG(Euler-Taylor-Galerkin) finite element method was applied to simulate the flow around two square cylinders arranged side by side at a spacing ratio of (1.5.) The second order full extension ETG finite element method was developed by Wang and He. By means of Taylor expansion of terms containing time derivative, time derivative is replaced by space derivative. The function of it is equal to introducing an artificial viscosity term. The streamlines of the flow at different moments were obtained. The time history of drag coefficient, lift coefficient and the streamwise velocity on the symmetrical points were presented. Furthermore, the symmetrical problem of the frequency spectrum of flow around two square cylinders arranged side by side were studied by using the spectral analysis technology. The data obtained at the initial stage are excluded in order to avoid the influence of initial condition on the results. The power spectrums of drag coefficient, lift coefficient, the streamwise velocity on the symmetrical points were analyzed respectively. The results show that although the time domain process of dynamic parameters is non-symmetrical, the frequency domain process of them is symmetrical under the symmetrical boundary conditions.

流场涡旋中颗粒碰撞黏附机制的CFD-DEM(XDLVO)模拟

混凝装备中流场的涡旋特征是影响煤泥水等微细颗粒絮凝效果的关键因素之一。为解析流场涡旋影响颗粒混凝碰撞与黏附机制,以圆柱绕流产生的涡街为代表性涡旋流场,采用CFD方法模拟了流速和圆柱直径对流场涡旋强度和尺度的影响;引入XDLVO理论辅助描述离散元方法中颗粒间的相互作用力,对粒径为25~100μm颗粒在上述流场中的碰撞与黏附过程进行CFD-DEM模拟,并试验验证。结果表明,在流速0.06~0.12 m/s、圆柱直径2~6 mm、雷诺数为120~720条件下,圆柱绕流场中的涡旋半径(r)与圆柱直径(D)的关系近似为r=0.133 3D+0.421 4,与流速无显著相关性;涡旋中心的最大涡量与流速成正比,涡旋强度与圆柱直径的2次方正相关。CFD-DEM(XDLVO)模拟方法获得的黏附聚集体的特征参数与试验结果吻合较好,说明其可较准确地描述圆柱绕流场中颗粒的碰撞与黏附过程。对流场涡旋分布特征和聚集体在流场分区中分布特征的分析表明,流场中的涡旋通过惯性离心力使颗粒远离涡旋中心向涡旋周边的黏性剪切区富集,在改变颗粒运动方向的同时,增加了颗粒在黏性区的局部浓度,从而提高了颗粒的碰撞概率,有效促进了颗粒的黏附并大。当涡旋尺度为物料粒径的10倍左右时最有利于颗粒的碰撞黏附,因此可根据应用场景的物料粒度来优化设计绕流圆柱直径,使其达到最好的黏附效果。

非恒定流下桩群绕流特性试验

非恒定流下圆柱绕流产生的尾涡会对结构疲劳损伤、水流挟沙能力等产生较大影响。基于非恒定流桩群绕流水槽试验,采用声学多普勒流速仪(acoustic Doppler velocimeter,ADV)测量了桩群周围的底层水流速度,分析了单峰洪水型非恒定流下,桩群绕流对底层流场、横纵向时均流速分布、横纵向紊动强度等的影响规律。研究表明:横、纵时均流速大小均与流量变化相关,纵向时均流速均为正值,横向时均流速在左右岸对称测点上大小一致。纵向流速相对紊动强度和横向流速紊动强度仅与流量大小有关,与流量所处的上升阶段与下降阶段无关,桩群绕流对附近流场的影响范围有限,且不同区域受到桩群绕流的影响程度存在一定差异。

高桩码头群桩绕流水动力特性数值分析

为探究高桩码头桩基础形式下的群桩水动力特征,基于Flow-3D软件对来流速度为0.25~1.50 m/s的3×3群桩阵列绕流的湍流流场进行三维数值模拟。研究结果表明:桩柱的两侧均形成高速区,随着来流速度的增加,高速区范围显示出逐渐扩大的趋势。上游桩后方的回流区范围较大,而中游桩和下游桩的回流区大小基本相同,均小于上游桩的回流区。随着来流速度的增加,剪切力和流体力逐渐增大,并随来流速度呈非线性增长趋势,且上游桩受力最大,下游桩次之,中游桩受力最小。随着来流速度的增加,群桩尾涡核心区的涡量随之增大,中游桩和下游桩后方的尾涡强度明显比上游桩的弱,中游桩和下游桩的尾迹涡长度保持稳定。随着来流速度的增加,漩涡强度显著提高,前桩与后桩之间的漩涡相互纠缠。研究成果对于理解群桩绕流的水动力特性具有重要意义,可为工程设计和实际应用提供参考。

基于高速公路客流数据的重庆与毗邻地区联系网络空间结构研究

基于高速公路交通客流数据,采用社会网络与GIS空间分析法,以客流联系视角探讨重庆市与毗邻地区的城镇网络空间结构。结果表明:1)重庆中心城区为网络的绝对核心,长寿、江津、璧山为网络中的重要节点城市,涪陵、璧山、长寿在网络中通达性明显,涪陵、黔江、江津、万州的中介作用突出;2)重庆与毗邻地区的客流网络呈现三级轴线联系,整体网络呈现明显的放射性特征,部分地区联系突破省级行政边界,网络层级按照“4+15+18”分布;3)重庆与毗邻地区的凝聚子群表现出高度的地理邻近性,形成“核心―边缘”结构。未来重庆中心城区需进一步加强与四川东部地级市的辐射能力与交通联系,进一步培育与成都双城联系的发展轴,在远期发展中考虑纳入遵义市等地区更好融入重庆都市圈。

非恒定流下桩群绕流局部冲刷试验

水流经过桩群时,桩前会出现下降水流和马蹄形漩涡,显著增加水流的挟沙能力与输移能力。本文基于非恒定流桩群绕流局部冲刷水槽试验,研究非恒定流下桩群绕流的局部冲刷分布以及最大冲刷深度;通过计算流体力学软件Flow 3D对水槽试验进行模拟计算仿真,研究非恒定流下桩群绕流局部冲刷发展过程。研究表明:上游单桩的冲刷范围为桩前2D(桩径)至桩后1D,桩外侧2.5D至桩内侧1D;下游单桩的冲刷范围为桩前1D至桩后1D,桩外侧1.5D至桩内侧1D,且桩群各个区域的横向冲刷断面也有差异;最大冲刷深度和冲刷体积随时间呈对数形式增加,直至达到冲刷平衡状态,且20%的时间局部冲刷可完成冲刷平衡时的50%,50%的时间可达到冲刷平衡的80%。

近壁旋转圆柱流场特性数值模拟分析

[目的]为探讨近壁旋转圆柱尾流及流体力特性,对典型间隙比下旋转圆柱绕流进行研究。[方法]对雷诺数Re=200下3种典型间隙比(G/D=0.2,0.8,1.4)的旋转圆柱绕流展开数值模拟,对比不同间隙比和转速比下的圆柱尾流及流体力特性。[结果]结果显示:当G/D=0.2时,圆柱表面脱涡会受到显著抑制,圆柱表面升阻力无波动;当G/D=0.8和1.4且转速比较低时,会发生“尾流涡”脱落现象,其结构与2S模式相似,升阻力系数呈正弦周期性波动,振幅较小;当正旋转速较大时,圆柱表面无漩涡脱落,形成稳定的D模式尾流(随转速比增大由D+模式变为D-模式),“尾流涡层”与“壁面涡层”发生分离,“壁面涡”呈现多周期性脱落现象,升阻力系数呈多周期波动,振幅显著增大;当反旋转速较大时,圆柱表面被一层正涡量的涡层包裹,漩涡脱落受到显著抑制,升阻力无波动。[结论]所得结论可为高效流动控制技术发展提供参考。

分流板对圆柱绕流湍流结构演化影响的实验研究

采用粒子图像测速(Particle Image Velocimetry,PIV)技术实验研究了带分流板的圆柱绕流流动特性,分析了分流板对圆柱绕流湍流结构演化特性的影响。雷诺数为3.9×10^(3),分流板长度L与圆柱直径D之比L/D为0~2.50。实验结果表明,随着L/D从0增大至1.00,圆柱后方回流区长度和回流区面积显著增大;当L/D>1.00时,L/D对圆柱后方回流特性的影响较小。分流板影响圆柱绕流湍流结构的演化特性:当L/D<1.00时,分流板抑制尾涡脱落,斯特劳哈尔数Sr减小,L/D从0增大至1.00,Sr减小了26.34%。此外,圆柱尾涡在分流板后缘诱导形成旋转方向相反的二次涡,二次涡沿分流板向上游运动。1.00≤L/D<2.00时,分流板作用于圆柱尾涡,尾涡发生破碎,形成一些小尺度尾涡。L/D≥2.00时,尾涡附着于分流板上,二次涡主要分布于圆柱后方。随着L/D增大,分流板减小了圆柱后方的湍流脉动强度。

亚临界区圆柱绕流相干结构壁面模化混合RANS/LES模型

本文发展了一种具有壁面模化大涡模拟能力的雷诺平均纳维-斯托克斯(RANS)和大涡模拟(LES)方法的混合模型(简称WM-HRL模型),致力于对亚临界区雷诺数钝体绕流相干结构这类复杂流动现象进行高置信度的CFD解析模拟研究.该方法通过一个仅与当地网格空间分布尺寸有关的湍动能解析度指标参数rk即可实现从RANS到LES的无缝快速转换,并且RANS/LES混合转换区的边界位置及其各个分区(包括RANS区、LES区及RANS/LES混合转换区)对湍动能的解析能力均可通过两个指标参数nrk1-Q和nrk2-Q准则进行预先设定.通过对雷诺数Re=3900下圆柱绕流场的系列数值模拟研究,获得了能够高置信度解析并捕捉其绕流场中三维时空瞬态发展相干结构特性的湍动能解析度指标参数nrk1-Q和nrk2-Q准则的组合条件.研究表明,该WM-HRL模型不仅能够准确获取圆柱绕流场中剪切层小尺度K-H不稳定性结构的精细谱结构,而且在同一套网格系统下通过变化湍动能解析度指标参数nrk2-Q和nrk1-Q准则的组合条件,还可以精细解析圆柱绕流场中两类不同回流区的长度结构特征,及其对应的圆柱尾部近壁面处V和U形两个平均流向速度剖面的分支结构特性.

极地环境下圆管液滴撞击特性分析

覆冰是威胁极地环境下海洋工程装备上部设施安全运行的潜在危险,而液滴收集系数反映了结冰对象捕获过冷却液滴的有效程度,因而是覆冰生长预测的关键参数。本文采用欧拉法对颗粒两相圆柱绕流进行数值仿真,通过研究液滴的斯托克斯数(St)和液滴雷诺数(Rew)两个关键因素,对液滴运动轨迹的影响规律进行分析。研究发现,斯托克斯数对液滴运动轨迹影响明显,在St较大(St>1)时,收集系数完全取决于St,而Rew的影响较弱,随着Rew的增加,整体收集系数和局部收集系数缓慢减小;在St较小(0.26<St<1)时,随着Rew的增加,局部和整体收集系数显著减小;由于流体在圆柱下游形成漩涡,在St<0.26时具有更大的随流性,驱使液滴卷吸入圆柱尾部近壁区,甚至与圆柱后壁发生碰撞,使得局部和整体收集系数随Rew增加而变大。

基于动力学模态分解的方柱绕流非定常流场分析

具有方形断面的结构或构件广泛应用于实际工程中,针对方柱绕流风荷载和风致振动特性开展研究至关重要。方柱绕流的尾流效应往往涉及多种复杂流动现象,捕捉其流动主要模态对深入分析流场特性以及流动控制具有重要意义。该文基于动力学模态分解(Dynamic Mode Decomposition,DMD)方法建立了能够提取流场主要模态并进行流场重构的降阶模型。基于数值模拟获取非定常流场快照;利用DMD进行主要模态提取;基于提取的主要模态对非定常流场进行重构。结果表明:DMD方法提取的前4阶模态能够准确重构原始流场,为流动控制和结构设计提供参考。

高雷诺数下倒虹吸闸墩绕流水力特性

为研究倒虹吸出口闸墩绕流特性,以南水北调某典型倒虹吸为例,基于Flow3D软件建立倒虹吸模型,根据现场原型实测水位、流速进行模型验证;利用该模型模拟了雷诺数6.17×10^(6)~9.38×10^(6)范围下闸墩绕流特性。结果表明,高雷诺数条件下,随着雷诺数的增加,闸墩绕流造成的液面波动幅度增加,中墩液面波动显著大于边墩;闸墩后的尾涡量持续增加,雷诺数对斯特劳哈尔数影响不大;边侧闸墩受力的非对称性明显,边墩闸室间特征流速偏流比存在先增大再减小的变化规律,偏流效应造成的离心距不断增加。本研究可针对性改善输水工程水流流态。