亚临界雷诺数下波浪型圆柱绕流的数值模拟及减阻研究

采用大涡模拟数值方法对亚临界雷诺数Re=3000下波浪型圆柱的绕流现象进行研究分析。研究结果表明,波浪型圆柱的三维尾迹涡结构能得到很好的控制,它在轴向方向呈现周期性正负涡的交替分布特性。随着幅值的增大,波浪型圆柱表面的自由剪切层得以延展,使得旋涡的脱落发生在波浪型圆柱下游较远处,从而达到减阻的目的。波浪型圆柱与普通直圆柱相比,其幅值对平均阻力系数及脉动升力系数的减少及尾迹控制有着更重要的影响,最大减阻可达16%。     

低雷诺数下并列双波浪柱尾迹干扰效应数值模拟

该文针对雷诺数Re=100下不同波长参数(/mλD=2、4、6和8)、不同间距比(1.5/3.5m≤L D≤)的并列双波浪柱后尾迹复杂的三维流动结构及干扰效应进行了数值研究。研究表明:三维并列双波浪柱在较小的间距比/1.5mL D=时,波峰对波谷排列的并列波浪柱后尾迹呈现明显偏置双稳流动结构,而波峰对波峰排列工况下基本呈现单一波浪型钝体后尾迹流动结构(尤其是在大波长/mλD=6和8时);当间距比/3mL D≥时,随着波长/mλD的增大,上下波浪柱各自呈现单波浪柱后尾迹流动结构;较大波长的三维波浪形状更好地组织了后尾迹三维流动结构,减少了并列双波浪柱后尾迹的相互干扰,从而减阻减振明显,最高减阻可达14.5%,脉动升力系数上下游波浪柱均趋于0.1。该研究可为近海工程的多立管结构及悬索桥的拉索振动抑制等提供理论支持。     

FORCE REDUCTION OF FLOW AROUND A SINUSOIDAL WAVY CYLINDER

A large eddy simulation of cross-flow around a sinusoidal wavy cylinder at Re=3000 was performed and the load cell measurement was introduced for the validation test. The mean flow field and the near wake flow structures were presented and compared with those for a circular cylinder at the same Reynolds number. The mean drag coefficient for the wavy cylinder is smaller than that for a corresponding circular cylinder due to the formation of a longer wake vortex generated by the wavy cylinder. The fluctuating lift coefficient of the wavy cylinder is also greatly reduced. This kind of wavy surface leads to the formation of 3-D free shear layers which are more stable than purely 2-D free shear layers. Such free shear layers only roll up into mature vortices at further downstream position and significantly modify the near wake structures and the pressure distributions around the wavy cylinder. Moreover, the simulations in laminar flow condition were also performed to investigate the effect of Reynolds number on force reduction control.     

不同长细比圆柱绕流的大涡模拟

自从人们对层流圆柱绕流现象有了系列研究及清楚的认识后,逐渐把目光投向湍流的圆柱绕流问题,但高雷诺数下的圆柱绕流具有很强的三维特性,因此在数值计算时模型轴向长度的选取是大家关心的问题。该文基于开源软件OpenFOAM运用LES计算了4种不同长细比圆柱的静止圆柱绕流(包括一个二维算例),重点分析和对比了结果中的一些基础参数如St数、平均阻力系数等,与实验结果吻合良好。文中还进一步分析了时均顺流向速度U在流场中沿流向和横向的分布,发现长细比为π的圆柱算例中,计算域的轴向尺度已经能满足圆柱下游靠近圆柱处的流场中的三维结构的发展。     

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...     

近底水平圆柱振荡绕流的三维数值模拟

在Honji和Sarpkaya等人的实验研究中发现，绕圆柱的振荡流并不是简单的二维流动，而是存在复杂的三维流态。本文用有限元方法求解原始变量的Navier-Stokes方程，模拟了Re数为392、Kc数为2时正弦振荡流绕平面附近圆柱的流动。分析了三维流场结构的特点，并分析了平面边界对尾流结构的影响。     

Vanishing of three-dimensionality in the wake behind a rotationally oscillating circular cylinder

The flow behind a three-dimensional rotationally oscillating circular cylinder was studied by a numerical method. The computations were performed at a Reynolds number of 260, which is at a level that the flow wake has developed into a three-dimensional state called Mode-B. The purpose of this paper is to examine the influence of various rotational amplitudes (0.1-0.7) on the wake instability of the flow, while the oscillation frequency is fixed to the value of that measured in the wake of a stationary cylinder. The results show that the rotation with sufficiently high amplitude brings the flow back to its nominal two-dimensional state. Moreover, it is found that the value of the time-averaged drag and the RMS value of the lift are larger than those of a stationary circular cylinder.     

The numerical and experimental investigations of the near wake behind a modified square stay-cable

The near wake structure, the wake-flow characteristics and the drag coefficients behind a modified square stay-cable(MSC) with sinusoidal variations of the cross-section area along the spanwise direction are investigated experimentally and numerically. The Reynolds numbers are chosen as 100 and 500 for the laminar flow and Re= 6 000 and 22 000 for the turbulent flow. The detailed near wake structures, the velocity fields and the force coefficients for the MSC are captured, the effect of the Reynolds number on the flow structure for the MSC is studied. The numerical and experimental investigations show that the free shear layers from the leading edge are widened and prolonged and then roll up into vortices further downstream the MSC, unlike a straight square stay-cable(SSC) under the same flow conditions. As a result, the distinct mean drag reduction and the fluctuating lift suppression are observed for all Reynolds numbers, a drag reduction of at least 15.8% and the rms lift coefficient reduction of up to 95% are observed, as compared with the case of a straight square stay-cable at Re= 500.     

Experimental investigation of flow past a circular cylinder with hydrophobic coating

A series of experiments are performed in a gravitational low-speed water tunnel to investigate the influence of the hydrophobic coating on the flow past a circular cylinder. The mean velocity and turbulence intensity profiles behind the cylinders are measured using the hot-film anemometer while the separation angles are obtained with the flow visualization technology. For the Reynolds number lower than 3 800, the hydrophobic coatings are in the Cassie state, the velocity deficit and the turbulence intensity in the wake of the hydrophobic cylinders are lower than those of the smooth cylinders which implies the drag reduction effect of the hydrophobic coatings. When the Reynolds number becomes higher than 6 600, the hydrophobic coatings turn into the Wenzel state. Through decomposing the velocity data in the turbulent wake into different scales based on the orthogonal wavelet transform, it is found that the total turbulence intensity in the wake of the hydrophobic cylinders becomes almost the same as in the wake of the smooth cylinders while the intensity of the large scales of vortex components in the wake of the hydrophobic cylinders is still lower. Furthermore, the separation angles show the same trend as a function of the Reynolds number but always take smaller values for the hydrophobic cylinders.     

浅水圆柱绕流流动模式探讨

数值求解了二维浅水方程和RNG k-ε流模型,模拟了不同稳定性系数S和圆柱中心间距G下的单圆柱、两圆柱和三圆柱的尾流流动模式,研究了底部摩擦力对尾流结构的影响。结果表明:随着S增大,尾流在底部摩擦力的作用下趋于稳定,大尺度涡结构逐渐消失。单圆柱尾流区依次出现了涡街（VS）尾流、不稳定漩涡（UB）尾流和稳定漩涡（SB）尾流。多圆柱尾流除了受S数影响外,还与G密切相关。随着G的增大,两圆柱背后依次出现了单钝体绕流、偏转尾流和对称尾流,三圆柱背后则依次出现了单钝体绕流、偏转尾流、对称尾流和非对称尾流。此外,数值结果还表明多圆柱尾流结构不同于单圆柱,多圆柱并排使得尾流更加不稳定。     

近壁圆柱绕流的壁面积沙现象机理分析

针对近壁圆柱绕流过程中出现的在平板壁面积沙现象用数值模拟方法进行了机理分析。应用标准k-ε湍流模型和流函数-涡量法分析了不同间隙比（圆柱与壁面间的距离对圆柱直径之比）下的尾流特性，得到圆柱表面升阻力和平板壁面压强脉动。结合流动显示的染色粒子图方法，进行了详细的分析并提出近壁圆柱绕流过程中在圆柱下游平板壁面形成沙粒线的可能机理。     

NUMERICAL SIMULATION FOR EARLY DEVELOPMENT OF WAKE VORTICES BEHIND A SEMICIRCULAR-SECTION CYLINDER

o.INThoDUCTIONThenumericlmodellingoffluidflowproblemswithwakesisveryimportantinhydrthdynamicsandaeredynamics.Thefiniteelementmethod,withtheflexibilityofavariablemesh,issuperiortothefinitedifferencemethodinsolvingtheseproblems.However,thefi-nitedifferencemethedismuchlesscomplicatedthanthefiniteelementmethodatleastinre-spectofcomputerprogramming.Theboundary-fittedcoordinatesystem,whichhasboththeadvantagesofthegeometricflexibilityofthefiniteelementmethedandthesimplicityofthefinitedifferenceme…     

Numerical investigation of the concentric annulus flow around a cylindrical body with contrasted effecting factors

This paper studies the wall-bounded flow around a cylindrical at a high Reynolds numbers body in a determined computational domain, with simulations of the 3-D, turbulent concentric annulus flow in a straight pipe. Numerical results show that a reversing zone, appearing as a tongue zone with nested velocities higher than the surrounding area, exists behind the cylindrical body. The annulus space is a region of high velocity and low pressure. The zero velocity, of combined the X- velocity and the Y- velocity, exists in the cross sections and no vortex shedding is formed behind the attaching cylinders. Among all investigated effecting factors, the diameters of the attaching and the main cylinders affect the wake feature behind the cylindrical body while the main cylinder length does not affect the distribution tendency of the flow field. The diameters of the main cylinder and the pipe affect the pressure values and the distribution tendencies on the main cylinder surface. Obviously, the increase of the pipe diameter reduces the drag coefficient of the cylindrical body and the increase of the diameter of the main cylinder increases the drag coefficient greatly. The numerical investigation of the concentric annulus flow provides foundations for further improvements of the intricate flow studies.     

A numerical study on dispersion of particles from the surface of a circular cylinder placed in a gas flow using discrete vortex method

The dispersion of particles emitted from the surface of a circular cylinder placed in a gas flow at the Reynolds number of 200 000 is numerically investigated using the discrete vortex method coupled with a Lagrangian approach for solid particle tracking. The wake vortex patterns, the temporal-spatial distributions and trajectories as well as the dispersion functions for particles with various Stokes numbers（St） ranging from 0.001 to 1.0 are obtained. The numerical results reveal that：（1） Solid particles on the cylinder surface are picked up and then transported away from the cylinder by the wake vortex flow.（2） Solid particles emitted from the cylinder surface always follow the vortices in the cylinder wake, and the response of particles to wake vortices is directly related to their Stokes numbers（particles with St= 0.001, 0.0038, 0.01 can distribute both in the vortex core and around the vortex periphery, whereas those with St= 0.1, 1.0 can not enter the vortex core and congregate mainly around the vortex periphery）.（3） The particles move in rolling state in the wake region, and the dispersion intensity of particles in the lateral direction decreases remarkably as the Stokes number of particles is increased from 0.001 to 1.0.     

AN EXPERIMENTAL STUDY OF FLOW AROUND A BIO-INSPIRED AIR- FOIL AT REYNOLDS NUMBER 2.0×10^3.

The fluid flow around a bio-inspired airfoil with corrugated surfaces and its smooth counterpart at chord Reynolds number 3 Re = 2.0 10and different Angle-Of-Attack ( AOA = 0°, 4°, 8°and 12°) were measured by using Particle Image Velocimetry (PIV). The global characteristics of the fluid flow around two airfoils were analyzed by ensemble-averaged velocity field, distribution of reverse flow intermittency, and time-series flow visualizations. At AOA = 0° , no significant variation of the global flow patterns was recognized for both configurations. The statistical results of reverse flow intermittency results demonstrated that the protruding peaks of the corrugated airfoil delay flow separation occur at AOA = 4°. At large AOAs (8°and 12°), however, the flow is massively separated in both configurations, the combination of large separation bubble above the corrugated airfoil and small recirculation zones in the upstream upper valley results in earlier separation of the flow. At AOA=8° , the wake region behind the corrugated airfoil is considerably shortened in comparison to the smooth one, indicating a remarkable reduction of the time-mean lift and drag forces, however, at AOA = 12°, the wake region behind the corrugated one is slightly larger than that behind the smooth one. For the case of AOA = 8°and 12°, the time-series flow visualizations demonstrate the intensified vortex shedding process of the corrugated airfoil, which would give rise to enhanced dynamic loading. Due to the fact that dragonfly wing is practically flexible, it is speculated that the wing structure of a gliding dragonfly might be sophisticatedly deformed in response to the periodic loading to reduce the drag.     

单向流作用下近壁面圆柱的流向振动

在单向流水槽中对近壁面圆柱体的流向振动进行了实验模拟。分析了圆柱流向振动位移时程曲线,探讨了圆柱振动频率随水流速度的变化特征。研究了圆柱振动幅值和频率的变化特性,结合圆柱尾迹涡的变化规律,分析了流向振动的起因。实验结果表明:随着流速的增加,圆柱流向振动经历了发生、发展和消失的变化过程,振动频率呈现出缓慢增长的趋势;在流向振动触发初期和后期衰退时,流向振动频率存在多值现象,频谱中呈现多个谱峰;圆柱流向振动所对应的约减速度范围明显小于横向涡激振动所对应的范围。     

NEAR WAKE OF A MODEL HORIZONTAL-AXIS WIND TURBINE

An experimental investigation on the properties of the near wake behind the rotor of a Horizontal-Axis Wind Turbine (HAWT) was carried out at model scale. Measurements were made with a stationary slanted hot-wire anemometer using the technique of phase-locked averaging. The primary aim is to study the formation and development of the three-dimensional wake. Five axial locations were chosen within four chord lengths of the blades over a range of tip speed ratios. The results show that during the downstream development of the wake, the wake centre traces a helical curve with its rotation direction opposite to that of the rotor. The distribution of mean velocity behind the HAWT rotor reveals an expansion and a decay of the three-dimensional wake. The shapes of the mean velocity distribution are similar along the blades span at the same downstream axial location. It is shown that the turbulence levels in the wake are higher than those in the non-wake region. The circumferential component and the radial component of the turbulence intensity are higher than the axial component. Our study offers some food of thought for better understanding of the physical features of the flow field as well as the performance of HAWT.     

NUMERICAL STUDY OF VORTEX SHEDDING FROM A SQUARE CYLINDER AND ITS EFFECTS ON HEAT TRANSFER OF MIXED CONVECTION

1 .　INTRODUCTIONForseveraldecades ,thenumericalandexperi mentalinvestigationofvortexsheddingfromaheated cooledbluntbodyinthemixednaturalandforcedconvectionregimeshasbeenasubjectofgreatinterestamongappliedmathematicians,fluiddynamicistsandheattransferanalysistowingtoitsnumerousengineeringapplicationssuchasthecoolingofelectroniccomponents ,thecoolingofhotwireanemometersandtheliftenhancementasattributedtotheMagnuseffects .Literatureonvortexsheddingfromcircularcylinderinthemixedconvectionregi…     

基于粒子图像测速技术的淹没植被斑时均尾流结构研究

水生植被对维持生态系统健康稳定具有重要意义，而绕有限尺寸淹没植被群落的流动又对物质输移和植被演化具有重要影响。本研究将淹没植被群落概化为圆形刚性圆柱阵列，并基于实验室水槽实验，采用粒子图像测速技术对不同密度植被群落的尾流时均流场进行测量，以探究植被密度变化对其尾流结构的影响。结果表明：植被斑内的顺流向出流强度随植被密度的增大而增大，同时植被斑直径的大小也对出流强度有影响；当植被密度大于0.1时，植被斑的群体行为变得十分明显，在时均流场中植被斑下游会产生类似于淹没实心圆柱的三维“拱”形回流涡，其与植被斑之间的距离随植被密度的增大而减小；稳定尾流区长度与拱形涡到植被斑后缘的距离成近似正线性相关关系。研究成果对深入理解植被群落的绕流结构具有促进意义。     

Receptivity of plane Poiseuille flow to local micro-vibration disturbance on wall

The receptivity of plane Poiseuille flow to local single-period micro-vibration disturbances with different phases at the top and bottom walls was investigated through direct numerical simulation of three-dimensional incompressible Navier-Stokes equations. Results show that the disturbance presents a symmetrical distribution in the spanwise direction when the micro-vibration on the wall ends, and the initial disturbance velocities and spatial distribution of the disturbance structure are different at the top and bottom walls. The disturbance’s velocity, amplitude, and high- and low-speed streaks increase with time, and the amplitude of streamwise disturbance velocity is larger than those of spanwise and vertical disturbance velocities. However, no significant Tollmien-Schlichting wave was found in the flow field. The number of disturbance vortex cores gradually increases with the disturbance area. High-speed disturbance fluid concentrates near the wall and its normal velocity largely points to the wall, while low-speed disturbance fluid largely deviates from the wall. Furthermore, the streamwise velocity profiles near the top and bottom walls both become plump because of the existence of the disturbances, and the streamwise velocity profiles show a trend of evolving into turbulent velocity profiles. The shear stress near the wall increases significantly. The local micro-vibration disturbance on the wall in plane Poiseuille flow can induce the development of a structure similar to turbulent spots.