Numerical simulation of flow past twin near-wall circular cylinders in tandem arrangement at low Reynolds number

Fluid flow past twin circular cylinders in a tandem arrangement placed near a plane wall was investigated by means of numerical simulations. The two-dimensional Navier-Stokes equations were solved with a three-step finite element method at a relatively low Reynolds number of Re = 200 for various dimensionless ratios of 0.25 ≤ G/D ≤ 2.0 and 1.0 ≤ L/D ≤ 4.0, where D is the cylinder diameter, L is the center-to-center distance between the two cylinders, and G is the gap between the lowest surface of the twin cylinders and the plane wall. The influences of G/D and L/D on the hydrodynamic force coefficients, Strouhal numbers, and vortex shedding modes were examined. Three different vortex shedding modes of the near wake were identified according to the numerical results. It was found that the hydrodynamic force coefficients and vortex shedding modes are quite different with respect to various combinations of G/D and L/D. For very small values of G/D, the vortex shedding is completely suppressed, resulting in the root mean square (RMS) values of drag and lift coefficients of both cylinders and the Strouhal number for the downstream cylinder being almost zero. The mean drag coefficient of the upstream cylinder is larger than that of the downstream cylinder for the same combination of G/D and L/D. It is also observed that change in the vortex shedding modes leads to a significant increase in the RMS values of drag and lift coefficients.     

A STUDY OF OSCILLATING FLOWS OVER PROPAGATING RIPPLES:PART Ⅱ. VORTICITY DYNAMICS

1 .　ＩＮＴＲＯＤＵＣＴＩＯＮＡｈｏｒｉｚｏｎｔａｌｂａｃｋ ａｎｄ ｆｏｒｔｈｍｏｔｉｏｎｐｒｏｄｕｃｅｄｎｅａｒａｓａｎｄｂｅｄｂｙ ｐｒｏｇｒｅｓｓｉｖｅｇｒａｖｉｔｙｗａｖｅｓｐｒｏｐａｇａｔｉｏｎｉｎｓｈａｌｌｏｗｗａｔｅｒｕｓｕａｌｌｙｇｉｖｅｓｒｉｓｅｔｏｎｅａｒｌｙｓｙｍｍｅｔｒｉｃａｌｂｅｄ ｆｏｒｍｓｃｏｍ ｍｏｎｌｙｋｎｏｗｎａｓｒｉｐｐｌｅｓ .Ｏｓｃｉｌｌａｔｉｎｇｆｌｏｗｓｏｖｅｒｐｅｒｉｏｄｉｃｒｉｐｐｌｅｓａｒｅｏｆｐｒａｃｔｉｃａｌａｓｗｅｌｌａｓｓｃｉ ｅｎｔｉｆｉｃ…     

NUMERICAL SIMULATION OF AN OSCILLATING FLOW PAST A CIRCULAR CYLINDER IN THE VICINITY OF A PLANE WALL

Oscillating flow around a circular cylinder in the vicinity of a plane wall was investigated by solving the two-dimensional incompressible Navier-Stokes equations with a finite element Galarkin residual method. The effect of the gap G/D between the cylinder surface and the wall on the flow behavior was studied. For the case of G/D 〈 0.25, the periodicity in the flow is attributed to both the outer shear layer instability and the oscillating frequency. As G/D 〉 0.25, vortex shedding occurs and the periodicity in the flow is mainly due to the competition of the oscillating frcqucncy and the vortex shedding frequency from an isolated stationary cylinder.     

STUDIES ON THE NUMERICAl SIMULATION OF UNSTEADY FLOWS

ＳＴＵＤＩＥＳＯＮＴＨＥＮＵＭＥＲＩＣＡｌＳＩＭＵＬＡＴＩＯＮＯＦＵＮＳＴＥＡＤＹＦＬＯＷＳ￥ＬｕＸｉ－ｙｕｎ（ＵｎｉｖｅｒｓｉｔｙｏｆＳｃｉｅｎｃｅａｎｄＴｅｃｈｎｏｌｏｇｙｏｆＣｈｉｎａ，Ｈｅｆｅｉ２３００２６，Ｐ．Ｒ．Ｃｈｉｎａ）Ｓｕｐｅｒｖｉ...     

Flow characteristics of the two tandem wavy cylinders and drag reduction phenomenon

This paper presents an extensive numerical study of 3-D laminar flow around two wavy cylinders in the tandem arrangement for spacing ratios (L/Dm ) ranging from 1.5 to 5.5 at a low Reynolds number of 1...     

Numerical simulation of low-Reynolds number flows past two tandem cylinders of different diameters

The flow past two tandem circular cylinders of different diameters was simulated using the finite volume method. The diameter of the downstream main cylinder （D） was kept constant, and the diameter of the upstream control cylinder （d） varied from 0.1D to D. The studied Reynolds numbers based on the diameter of the downstream main cylinder were 100 and 150. The gap between the control cylinder and the main cylinder （G） ranged from 0.1D to 4D. It is concluded that the gap-to-diameter ratio （G/D） and the diameter ratio between the two cylinders （d/D） have important effects on the drag and lift coefficients, pressure distributions around the cylinders, vortex shedding frequencies from the two cylinders, and flow characteristics.     

并列旋转双圆柱流动特性的数值模拟

该文基于k - ε湍流模型,采用Galerkin有限元法对并列旋转双圆柱的绕流特性进行了数值模拟,计算的雷诺数为 1550.为了考查两圆柱旋转和间距的相互作用,文中采用三种间距比分别是T/D = 1.2,1.6和3.0 (T 为两圆柱中心之间的距离,D为圆柱直径) 和一系列不同的旋转速度比 (|α| ≤ 2).计算显示,当 |α| = 0,即圆柱不转动时,对应三种间距有三种典型的流型,单钝体流型对应小间距、偏流对应中等间距和对称流对应大间距;当 |α| 达到临界值时,涡脱落得到了有效的抑制,流动趋于稳定,升力系数和阻力系数的脉动值趋于零;平均升力系数和阻力系数随着 |α| 的增大分别增大和减小.     

应用PIV进行二维后向台阶流流动特性的研究(1)二维后向台阶流的旋涡结构的研究

利用PIV技术，对二维后向台阶定常流的瞬态速度场进行了测量，得到层流至湍流情况下定常流流场中旋涡的生成、发展和演化的现象和规律。本文分析了各种情况下旋涡结构特点、讨论了与时均流场和自由剪切层的区别，还给出了二维后向台阶流的拓扑结构。     

Three-dimensional numerical simulation of flow around a circular cylinder under combined steady and oscillatory flow

Combined steady and oscillatory flow past a circular cylinder is investigated numerically by three-dimensional Direct Numerical Simulation (DNS). The incompressible Navier-Stokes equations are solved by finite element method (FEM). The aim of this study is to investigate influence of the existence of steady current on the flow regime and hydrodynamics forces. The computational results of pure oscillatory flow past a circular cylinder agree well with the experimental data. The flow ratios (ratio of current velocity to oscillatory velocity amplitude) are 0.0, 0.2 0.5 and 1.0 in the study. The influence of the flow ratio on the vortex shedding regime and hydrodynamic forces on the cylinder are investigated numerically.

     

LARGE-EDDY SIMULATION OF FLOW AROUND CYLINDER ARRAYS AT A SUBCRITICAL REYNOLDS NUMBER

The complex three-dimensional turbulent flows around a cylinder array with four cylinders in an in-line square configuration at a subcritical Reynolds number of 1.5 × 10^4 with the spacing ratio at L/D = 1.5 and 3.5 were investigated using the Large Eddy Simulation （LES）. The full field vorticity and velocity distributions as well as turbulent quantities were calculated in detail and the near wake structures were presented. The results show that the bi-stable flow nature was observed at L/D = 1.5 and distinct vortex shedding of the upstream cylinders occurred at L/D = 3.5 at Re = 1.5 × 10^4. The techniques of Laser Doppler Anemometry （LDA） and Digital Particle Image Velocimetry （DPIV） are also employed to validate the present LES method. The results show that the numerical predictions are in excellent agreement with the experimental measurements. Therefore, the full field instantaneous and mean quantities of the flow field, velocity field and vorticity field can be extracted from the LES results for further study of the complex flow characteristics.     

NUMERICAL STUDY OF THE INFLUENCE OF SURFACE ROUGHNESS OF CYLINDER ON FLOW STRUCTURE

1.　INTRODUCTIONThe quietness is an important mark for the behavior of the submarine,so to decreasethe noise of the submarine becomes more and more important.As engine noise and othernoises have been reduced in these days,the interior noise levels in many submarine applica-tions are significantly affected by the water noise in the case of a submarine athigh cruisingspeed.The main sources of water noise are as follows:( 1 ) pressure fluctuations in the tur-bulent boundary layer;( 2 ) turbul…     

Vortex shedding in the flow around two side-by-side circular cylinders of different diameters

In this paper,the 3-D turbulent flow around two side-by-side circular cylinders of different diameters,at sub-critical Reynolds number(Re=3 900),is numerically simulated by the large eddy simulation(LES).The spacing ratios(T/D)between the two cylinders are considered in four cases(T/D=1.2,1.5,1.8 and 2.7)to study the vortex shedding and turbulent properties in the flow field.The main results are focused on the drag and lift coefficients,the vortex shedding frequency,the coherent structure,and the scale properties.It is shown that when T/D is equal to 1.2,the vortex shedding of the main cylinder is strongly suppressed by the small cylinder,the drag and lift coefficients of the main cylinder are smaller than those in other three cases.While T/D is equal to 1.5,the vortex shedding of the main cylinder can be improved,the drag and lift coefficients of the main cylinder are larger than those in other three cases.The empirical mode decomposition(EMD)method is applied to decompose the velocity signals traced by the LES.It is shown that there is a linear relationship between the mean period and the mode in the semi-log coordinates.The vortex shedding period of the main cylinder is consistent with the period of the restructured coherent structures quantitatively.     

二维矩形突起物绕流流动结构的实验研究

流场中旋涡的生成、演化与二维矩形突起物上游的流动分离有着密切的关系,该文利用PIV技术,对在均匀来流条件下无压流流道内底壁矩形突起物流动的二维瞬时流速场进行了测量,得到了湍流情况下,不同台阶比（h1/h）为2和4时流场中旋涡的生成发展与演化的一般规律。文中分析了两种工况下流场中的旋涡结构的特点和突起物上、下游及顶部旋涡的运动特性和细部特征。     

螺旋侧板截面形状对Spar平台涡激运动的影响研究

处于深水中的Spar平台两侧不断产生周期性漩涡脱落,极易诱发涡激运动,缩短平台疲劳寿命。为探究不同截面形状螺旋侧板对漂浮式风力机Spar平台涡激运动的影响,采用大涡模拟(LES)方法,对亚临界雷诺数流动状态下附加不同截面形状螺旋侧板的平台进行研究,并与光滑平台进行对比,分析其水动力系数及尾涡发放特点。研究表明:螺旋侧板在有效抑制平台升力系数的同时,还会增加阻力系数,圆形截面侧板对平台升力系数的降幅最大,阻力系数增幅最小;螺旋侧板的存在使漩涡脱落频率整体减小,从单一主频发放变为主频和次频发放;圆形截面侧板的漩涡脱落点距离平台最远,发放最为迟缓;在研究范围内,发现圆形截面侧板抑涡效果最佳。     

Circulation characteristics of horseshoe vortex in scour region around circular piers

This paper presents an experimental investigation of the circulation of the horseshoe vortex system within the equilibrium scour hole at a circular pier, with the data measured by an acoustic Doppler velocimeter (ADV). Velocity vector plots and vorticity contours of the flow field on the upstream plane of symmetry (y = 0 cm) and on the planes ±3 cm away from the plane of symmetry (y = ±3 cm) are presented. The vorticity and circulation of the horseshoe vortices were determined using the forward difference technique and Stokes theorem, respectively. The results show that the magnitudes of circulations are similar on the planes y = 3 cm and y = -3 cm, which are less than those on the plane y = 0 cm. The circulation decreases with the increase of flow shallowness, and increases with the densimetric Froude number. It also increases with the pier Reynolds number at a constant densimetric Froude number, or at a constant flow shallowness. The relative vortex strength (dimensionless circulation) decreases with the increase of the pier Reynolds number. Some empirical equations are proposed based on the results. The predicted circulation values with these equations match the measured data, which indicates that these equations can be used to estimate the circulation in future studies.     

Large eddy simulation of turbulent attached cavitating flow with special emphasis on large scale structures of the hydrofoil wake and turbulence-cavitation interactions

In this paper, the turbulent attached cavitating flow around a Clark-Y hydrofoil is investigated by the large eddy simulation(LES) method coupled with a homogeneous cavitation model. The predicted lift coefficient and the cavity volume show a distinctly quasi-periodic process with cavitation shedding and the results agree fairly well with the available experimental data. The present simulation accurately captures the main features of the unsteady cavitation transient behavior including the attached cavity growth, the sheet/cloud cavitation transition and the cloud cavitation collapse. The vortex shedding structure from a hydrofoil cavitating wake is identified by the Q- criterion, which implies that the large scale structures might slide and roll down along the suction side of the hydrofoil while being further developed at the downstream. Further analysis demonstrates that the turbulence level of the flow is clearly related to the cavitation and the turbulence velocity fluctuation is much influenced by the cavity shedding.     

FORCE CHARACTERISTICS AND VORTEX SHEDDING OF A PITCHING FOIL IN SHEAR FLOWS

The objective of this study is to deal with unsteady force acting on a pitching foil in shear flow and to study the relation of the force characteristics with vortex shedding near the foil. The two-dimensional incompressible Navier-Stokes equations in the vorticity and stream-function formulation were solved with the fourth-order essentially compact finite difference schemes for the space derivatives and a fourthorder Runge-Kutta scheme for the time advancement. The force characteristics and vortex shedding of the pitching foil in shear flows were investigated. The effects of some typical factors, including the incoming flow shear, the oscillating frequency and amplitude, on the vortex shedding and force behavior were analyzed and discussed.     

PROPULSIVE PERFORMANCE AND VORTEX SHEDDING OF A FOIL IN FLAPPING MOTION

1 .　INTRODUCTIONInsectsandfishhaveexperiencedabillions yearprocessofevolutionwithnaturalselectionfortheirsurvivalandhavedevelopedtheirsuperiorandcomplete performanceofflightandswim ming .Usually ,aflappingmotionisabasicmodeoflocomotionforinsects ,birds ,andfish .Thrustandliftaregeneratedwhentheflappingwingsortailsinteractwiththesurroundingfluids .Becauseofthehighlyunsteadynatureofviscousflowaroundaflappingwing ,itisfarfromsatisfactorytounderstandthephysicalmechanismandvortexsheddinginuns…     

LES NUMERICAL SIMULATION OF CAVITATION BUBBLE SHEDDING ON ALE 25 AND ALE 15 HYDROFOILS

A cavitation calculation scheme is developed and applied to ALE 15 and ALE 25 hydrofoils, based on the Bubble Two-phase Flow (BTF) cavity model with a Large Eddy Simulation (LES) methodology. The Navier-Stokes equations including cavitation bubble clusters are solved through the finite-volume approach with a time-marching scheme. Simulations are carried out in a 3-D field with a hydrofoil ALE 15 or ALE 25 at an angle of attack of 8° and cavitation number σ = 2.3 with a 2 × 10⁶, meshing system. With the time-marching, the cavitation bubble gradually grows to a steady lump shape and then produces an irregular small bubble behind the main cavitation bubble, finally shedding from the leading edge of the cloud cavitation structure. The calculated results including velocity field and pressure field are consistent with experiment data at the same Reynolds number and cavitation number. The vortex and reverse flow are observed on the hydrofoil surface.     

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF FREE SURFACE VORTEX

An experimental model was set up to investigate the formation and evolution of the free surface vortex. A Particle Image Velocimetry （PIV） was used to measure the free surface vortex flow field at different development stages. Flow visualization was used to locate the vortex position and find its structure. Empirical formulas about the critical submergence and the whole field structure were obtained. It is found that the tangential velocity distribution is similar to that of the Rankine vortex and the radial velocity changes little in the vortex functional scope. Vortex starts from the free surface and gradually intensifies to air entrainment vortex. The vortex core moves during the formation and evolution of the free surface vortex. Based on the experimental model, the vortex position and structure were predicted by numerical simulation combined with a vortex model and compared with that of the experiments, which shows satisfactory agreement.