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1.
结合格子玻尔兹曼方法(Lattice Boltzmann Method,简称LBM)和多块网格(Multi-Block Grid)技术,数值研究较小雷诺数(Re=100)下均匀来流绕串列对转双圆柱问题,综合分析圆柱中心间距比S/D和圆柱无量纲旋转速度α对流场结构的影响,考察前、后圆柱的涡脱落形态和升阻力特性.结果表明:当间距比为1.2时存在一临界旋转比αc,转速超过这一值后前圆柱产生负Magnus效应;间距比为2时,流场出现类似单旋转圆柱时的第二不稳定模态;当间距比较大(S/D=4、6)时,前、后柱之间存在涡脱落,后柱尾涡中出现2S、2S*、2P、P+S等多种形态. 相似文献
2.
纵向受迫振荡圆柱绕流问题的数值模拟 总被引:2,自引:0,他引:2
用有限体积法对平行于均匀来流方向受迫振荡的圆柱绕流问题进行了二维数值模拟.雷诺数选取Re=200、855、4000等几种亚临界雷诺数情况.通过研究不同振幅和振动频率下的流场结构和一些重要流动参数如升阻力系数、Strouhal数等随Re数、KC数、Stokes数的变化关系,验证了实验中观察到的一定条件下发生的"频率锁定"现象,并将涡脱落方式划分为三种主要模态.文中引入网格速度,对常用的处理速度与压力耦合的SIMPLE算法作了适当的补充和修改,以适应随时间变化的网格坐标. 相似文献
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本文研究雷诺数(Re)对圆形渐缩喷嘴湍流射流的影响.实验在射流出口雷诺数为 Re = 4050—20100 的范围内进行,分别测量了射流出口、中心线的平均及湍流流场以及部分径向剖面速度分布.所有测量均采用单热线恒温热线风速仪进行高频采样,所测流场范围在轴向上为 0—30d(这里d为射流出口直径).虽然出口速度分布均为"平顶帽"形,但测量结果依然反映出Re对射流出口以及下游流场有强烈的影响.当Re小于临界值(~10000)时
关键词:
雷诺数
圆形射流
热线风速仪 相似文献
4.
通过结合格子Boltzmann方法(LBM)和虚拟区域(Fictitiou sDomain)思想,建立格子Boltzmann-虚拟区域(LB-DF/FD)方法.采用两套网格系统,欧拉网格用于流体,拉格朗日网格用于固体.原有的LBM在计算运动固体的受力方面存在数据振荡,LB-DF/FD方法改进了此缺陷.为验证该方法,模拟圆柱绕流、圆形颗粒在无限长通道中平动及在无限大流场中转动三种情况,结果与其他数值解及理论解符合得很好.利用该方法模拟低雷诺数下通道中串列旋转圆柱周围的流场,分析圆柱间距(g)及雷诺数(Re)对流场结构的影响.给出Re=0.001,0.1和10下,0.2≤g≤8.0的流线结构、圆柱升力、阻力以及力矩等数值结果.结果表明,g对流场的结构及圆柱的受力有显著影响,Re对圆柱阻力及Stokes单元数目的影响较大. 相似文献
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采用实验和大涡模拟对喷嘴出口雷诺数(Re= U0 hρ/μ, 其中 U0为出口平均速度, h为平面喷嘴出口狭缝高度, ρ和 μ分别为流体的密度与动力黏度)为25–10000, 喷嘴间距 L为4h–40h范围内的平面撞击流偏斜振荡特性进行了研究. 通过对平面撞击流模拟和实验的结果进行比较, 验证了数值模拟的可靠性, 并对平面撞击流发生偏斜振荡的无因次参数(喷嘴间距 L/h与出口雷诺数 Re)范围进行划分, 重点考察了湍流平面撞击流的偏斜振荡周期及速度-压力变化特征. 研究结果表明大涡模拟能对平面撞击流的偏斜振荡进行有效预报; 当平面撞击流发生周期性偏斜振荡时, 特定点的压力与速度也发生周期性变化, 且变化周期与偏斜振荡周期一致, 偏斜振荡本质上是由速度-压力的周期性变化和转换引起的.
关键词:
平面撞击流
偏斜振荡
大涡模拟 相似文献
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本文对传统的光滑粒子动力学方法进行了改进, 改进的光滑粒子动力学方法对传统粒子方法中的核梯度进行了修正, 采用了一种新型的核函数和离散格式, 应用改进的光滑粒子动力学方法对微液滴振荡过程进行了数值研究. 研究了不同纵横比和雷诺数(Re)下振荡阻尼与振荡的周期、振幅与Re数的关系. 研究表明: 对于纵横比λ≤ 4时的微液滴振荡过程, 其他参数恒定不变的前提下, Re数越大, 液滴形状变化越剧烈, 波的阻尼作用越弱, 液滴振荡周期变长; 在Re数一定的前提下, 随着液滴初始的纵横比的增大, 液滴振动的振幅增大, 液滴振荡的周期变长. 相似文献
10.
采用界面跟踪法FTM(Front-Tracking Method), 研究点热源流场中由Marangoni效应引起的液滴运动。模拟不同的Marangoni(Ma)数下液滴的运动。研究发现液滴运动速度先迅速增大到稳定迁移速度, 而后下降, 在t=1.2时出现反转, 速度随Ma数的增加而增加。液滴内部存在与Hill球涡相同的回流。随着Ma数的增大, Hill球涡进行分裂, 且涡旋中心有轻微的移动。同时温度场末端拓扑结构出现两次分叉。第一次分叉出现在下临界Ma数, 最低温度点由滞止点跳进液滴内部; 第二次分叉出现在上临界Ma数, 内部的壳型冷却区从中心点破裂, 出现一个环面型冷却区。 相似文献
11.
The wake of a streamwise oscillating cylinder is presently investigated. The Reynolds number investigated is 300, based on
the cylinder diameterd. The cylinder oscillates at an amplitude of 0.5d and a frequencyf
e/fs=1.8, wheref
e is the cylinder oscillating frequency andf
s is the natural vortex shedding frequency of a stationary cylinder. Under these conditions the flow is essentially two dimensional.
A two-dimensional direct numerical simulation (DNS) scheme has been developed to calculate the flow. The DNS results display
a street of binary vortices, each containing two counter-rotating vortical structures, symmetrical about the centerline, which
is in excellent agreement with measurements. The drag and lift on the cylinder have been examined. The time averaged drag
and lift are 1.4 and 0, respectively, which are the same as those on a stationary cylinder at the sameRe. However, the fluctuating drag was high, about 2.68. It has been found that, being symmetrically formed about the centerline,
the binary vortices induce an essentially zero fluctuating lift, which may have a profound implication in flow control and
engineering. 相似文献
12.
The flow field around a rotationally oscillating circular cylinder in a uniform flow is studied by using a particle image
velocimetry to understand the mechanism of drag reduction and the corresponding suppression of vortex shedding in the cylinder
wake at low Reynolds number. Experiments are conducted on the flow around the circular cylinder under rotational oscillation
at forcing Strouhal number 1, rotational amplitude 2 and Reynolds number 2,000. It is found from the flow measurement by PIV
that the width of the wake is narrowed and the velocity fluctuations are reduced by the rotational oscillation of the cylinder,
which results in the drag reduction rate of 30%. The mechanism of drag reduction is studied by phase-averaged PIV measurement,
which indicates the formation of periodic small-scale vortices from both sides of the cylinder. It is found from the cross-correlation
measurement between the velocity fluctuations that the large-scale structure of vortex shedding is almost removed in the cylinder
wake, when the small-scale vortices are generated at the unstable frequency of shear layer by the influence of rotational
oscillation. 相似文献
13.
M. Rodríguez y Domínguez R. Romero-Méndez M. Ramos-Paláu F. G. Pérez-Gutiérrez 《显形杂志》2006,9(3):309-318
A flow visualization experiment was performed in order to characterize the laminar horseshoe vortex system that appears upstream
of the junction of a short cylinder and a pair of flat parallel plates. The experiments were performed in a water tunnel and
the technique used for flow visualization was laser illumination of seeded particles whose traces were captured using long
exposure photography. Geometrical and flow parameters, such as Reynolds number and height-to-diameter ratio of the cylinders,
are varied during the experiments and the flow regimes are analyzed as a function of these parameters. The behavior of vortex
systems is reported. For low Reynolds number cases, the vortices stay in a fixed position, as the Reynolds number is increased
the number of vortices grows and for larger Reynolds numbers the vortex system becomes oscillatory and for further increases
it becomes periodic. As for the dimensionless height of the cylinders, the vortex system is weak for short cylinders and increases
its strength and number of vortices as the cylinder height-to-diameter ratio is increased. For further increases in height
the vortex system do not change, which shows that the flow becomes independent of the height-to-diameter ratio for sufficiently
tall cylinders. Information of the frequency of appearance of periodic vortices is also included. 相似文献
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R. M. Arutyunyan V. L. Ginzburg G. F. Zharkov 《Journal of Experimental and Theoretical Physics》1997,84(6):1186-1196
The question of a surface barrier which determines the behavior of a vortex in a hollow superconducting cylinder of finite
thickness in an external magnetic field is discussed, taking into account magnetic flux quantization in the cavity. The behavior
of magnetic vortices in a hollow superconductor in the presence of a thermoelectric current is also considered. Pairs of magnetic
vortices with opposite magnetic field orientations (vortex-antivortex pairs) are generated by this current near T
c. The thermoelectric current drives the antivortex (the vortex with oppositely directed field) out of the cylinder, whereas
the vortex is ejected into the cavity and remains on the inside cylinder surface as a current. The number of magnetic flux
quanta trapped inside the cylinder increases by one. The relation of this mechanism to the “giant” thermoelectric effect in
hollow superconductors is discussed.
Zh. éksp. Teor. Fiz. 111, 2175–2193 (June 1997) 相似文献
16.
Through visualization and measurement on the cylinder-plate junction flow, we show the horseshoe vortices can be significantly
modified by altering the section shape of the cylinder. Both smoke-wire and Laser-Induced-Fluorescence (LIF) are employed
to visualize the vortex structures. Laser Doppler velocimeter is used to measure the velocity field in the symmetry plane
upstream of the cylinder. Electrical pressure-scanning valve is applied to acquire the pressure on the plate. It is found
that, the sharper the frontal shape of the cylinder, the closer the vortex shedding position and the primary horseshoe vortex
location to the cylinder. We quantitatively show the variation of the scale and strength of the primary horseshoe vortex,
as well as the maximum wall shear stress, when the section shape of the cylinder is varied. The reduced streamwise adverse
pressure gradient explains why the horseshoe vortices are significantly suppressed when the frontal shape of the cylinder
becomes sharper. At last, we present a swept thin cylinder installed in front of the primary cylinder can be used to suppress
the horseshoe vortices, which is greatly effective and easy to implement. 相似文献
17.
Interference is investigated between a stationary cylinder wake and that of a downstream streamwise oscillating cylinder.
Experiments were carried out in a water tunnel. A laser-induced fluorescence technique was used to visualize the flow structure
behind two inline circular cylinders of identical diameterd. The downstream cylinder was forced to oscillate harmonically at the amplitude of 0.5d and the frequency ratiof
e
f
s=1.8, wheref
e is the oscillation frequency of the downstream cylinder andf
s is the vortex shedding frequency from an isolated stationary cylinder. The investigation was conducted for the cylinder center-to-center
spacingL/d=2.5 ∼ 4.5. Two flow regimes have been identified, i.e. the ‘single-cylinder shedding regime’ atL/d<-3.5 and the ‘two-cylinder shedding regime’ atL/d>3.5. At smallL/d, the upstream cylinder does not appear to shed vortices; vortices are symmetrically formed behind the downstream cylinder
as a result of interactions between the shear layers separated from the upstream cylinder and the oscillation of the downstream
cylinder. This is drastically different from that behind two stationary cylinders atL/d<-3.5, where vortices are shed alternately from the downstream cylinder only. AtL/d=4.5, both upstream and downstream cylinders shed vortices. This is true with or without the oscillation of the downstream
cylinder. The flow structure is now totally different from that atL/d=3.5. The vortices are shed alternately from the upstream cylinder; a staggered spatial arrangement of vortices occurs behind
the downstream cylinder. 相似文献
18.
本文继Zheng-huan Teng[1]中介绍的解Navier-stokes方程的椭圆涡团法,研究了一种新的变形涡团法,用以模拟不可压粘性流体绕圆柱的不定常流动。圆柱在静止流体中突然起动并做匀速直线运动。对整个流动区域构造完全Navier-Stokes方程的解并不容易,近十年出现很多数值研究,本文对算法有所推进。把本文的方法称作变形涡团法是因为圆柱边界附近的流体中用椭圆涡团,远离边界时用圆形涡团。计算圆柱绕流比平板绕流在满足附着条件上更为困难,本文分析了怎样在圆柱边界上给出适当的附着条件的数值方法。在算例中雷诺数分别取200、550、3000,得到了不定常边界层分离,二次涡等复杂的物理现象,这些数值结果与近年实验结果[2]是一致的。 相似文献
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The problem of motion of the vortices around an oscillating cylinder in the presence of a uniform flow is considered. The Hamiltonian for vortex motion for the case with no uniform flow and stationary cylinder is constructed, reduced, and constant Hamiltonian (energy) curves are plotted when the system is shown to be integrable according to Liouville. By adding uniform flow to the system and by allowing the cylinder to vibrate, we model the natural vibration of the cylinder in the flow field, which has applications in ocean engineering involving tethers or pipelines in a flow field. We conclude that in the chaotic case forces on the cylinder may be considerably larger than those on the integrable case depending on the initial positions of vortices and that complex phenomena such as chaotic capture and escape occur when the initial positions lie in a certain region. 相似文献