共查询到20条相似文献,搜索用时 46 毫秒
1.
Vorobieff P Ecke RE 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,60(3):2953-2956
We present an experimental characterization of cylinder wakes in flowing soap films. From instantaneous velocity and thickness fields, we find the vortex-shedding frequency, mean-flow velocity, and mean-film thickness. Using the empirical relationship between the Reynolds and Strouhal numbers obtained for cylinder wakes in three dimensions, we estimate the effective soap-film viscosity and its dependence on film thickness. We also compare the decay of vorticity with that in a simple Rankine vortex model with a dissipative term to account for air drag. 相似文献
2.
D. S.-K. Ting 《实验传热》2013,26(4):357-370
The effect of a 6.35-mm-diameter cylindrical rod on the transitional flow in a 50.8-mm square channel has been investigated experimentally. A Chromel-Constanian thermocouple with 12,7-fim-diameter leads was used to measure the temperature fluctuations of slightly heated air (5°C) flowing in the channel, over the range of Reynolds number Re = 1,500-3,500. Without the rod, laminar-to-turbulent transition occurred at Re ≈ 2,500-3,500 in the empty square channel The rod destabilized the flow and advanced the onset of fluctuations profoundly. Within the wake of the rod, large fluctuations were detected. Temperature measurements also reviewed the existence of regular vortex shedding at Re = 1,500, and the development of the wake downstream of the rod. This study confirmed the feasibility of using a fine thermocouple for studying obstructed channel flow. 相似文献
3.
We measure, using a lattice gas algorithm, the time development of a wake behind a cylinder at Reynolds numbers equal to 30 and 60. We measure also drag at these values and determine Re90 to be the Reynolds number where vortex shedding starts. Agreement with experimental results is good. We also measure vortex shedding frequency at Re = 108 from transverse velocity and at Re = 180 from lift on the cylinder. The corresponding Strouhal numbers are obtained and discussed. 相似文献
4.
Rapidly flowing soap films provide a simple and attractive system to study two-dimensional hydrodynamics and turbulence. By measuring the rapid fluctuations of the thickness of the film in the turbulent regime, we find that the statistics of these fluctuations closely resemble those of a passive scalar field in a turbulent flow. The scalar spectra are well described by Kolmogorov-like scaling while the high-order moments show clear deviations from regular scaling just like dye or temperature fluctuations in 3D turbulent flows. 相似文献
5.
Lattice Boltzmann Simulation of the Cross Flow Over a Cantilevered and Longitudinally Vibrating Circular Cylinder
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The multiple-relaxation-time lattice Boltzmann method (MRT-LBM) is implemented to numerically simulate the cross flow over a longitudinal vibrating circular cylinder. This research is carried out on a three-dimensional (3D) finite cantilevered cylinder to investigate the effect of forced vibration on the wake characteristics and the 3D effect of a cantilevered cylinder. To meet the accuracy of this method, the present calculation is carried out at a low Reynolds number Re =100, as well as to make the vibration obvious, we make the vibration strong enough. The calculation results indicate that the vibration has significant influence on the wake characteristics. When the vibrating is big enough, our early works show that the 2D vortex shedding would be locked up by vibration. Contrarily, this phenomenon would not appear in the present 3D case because of the end effect of the cantilevered cylinder. 相似文献
6.
The fully 3D turbulent incompressible flow around a cylinder and in its wake at a Reynolds number Re = = 9×104 based on the cylinder diameter and Mach number M = 0.1 is calculated using Large Eddy Simulations (LES). Encouraging results are found in comparison to experimental data for the fluctuating lift and drag forces. The acoustic pressure in far-field is commutated through the surface integral formulation of the Ffowcs Williams and Hawkings (FWH) equation in acoustic analogy. Five different sound sources, the cylinder wall and four permeable surfaces in the flow fields, are employed. The spectra of the sound pressure are generally in quantitative agreement with the measured one though the acoustic sources are pseudo-sound regarding the incompressible flow simulation. The acoustic component at the Strouhal number related to vortex shedding has been predicted accurately. For the broad band sound, the permeable surfaces in the near wake region give qualitative enough accuracy level of predictions, while the cylinder wall surface shows a noticeable under-prediction. The sound radiation of the volumetric sources based on Lighthill tensors at vortex shedding is also studied. Its far-field directivity is of lateral quadrupoles with the weak radiations in the flow and cross-flow directions. 相似文献
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8.
Chakravarthy SR Shreenivasan OJ Boehm B Dreizler A Janicka J 《The Journal of the Acoustical Society of America》2007,122(1):120-127
This paper reports work on a nonpremixed half-dump combustor, in which methane is injected at the backward-facing step, and mixes and burns with the air flowing past the step in the unsteady recirculation zone. The flow and geometric parameters are widely varied, to gradually change from conditions of low-amplitude noise to excitation of high-amplitude discrete tones. The purpose of the work is to focus on the transition from the former condition to the latter, and to mark the onset of instability. Dimensionless groups such as the Helmholtz and Strouhal numbers are formed based on the observed dominant frequencies, whose variation with the air flow Reynolds number is used to identify the oscillations as those due to the natural acoustic modes or the vortex shedding process. High-speed chemiluminescence imaging reveals shedding of vortical structures in the flame zone. With variation in the conditions, flow-acoustic lock-on and transition from one vortex shedding mode to another is marked by nonlinearity in the corresponding amplitude variations. Such conditions are identified as the onset of instability in terms of the ratio of the flow time scale to the acoustic time scale and mapped against the operating fuel-air equivalence ratio of the combustor. 相似文献
9.
本文利用基于复合网格系统的计算方法,对Re=50~1200的近壁插入圆柱流场进行数值模拟,研究过渡流状态下在壁面附近插入圆柱对下游壁面传热强化的影响。并基于低速循环水槽流动实验台,采用粒子成像测试法(PIV)对Re=100~500的近壁插入圆柱流场进行可视化实验研究,验证了数值模拟方法的可靠性。研究结果表明:近壁插入圆柱流场在Re=100时进入过渡流状态;Re直接影响圆柱尾流中周期性涡脱和壁面涡岛的发生位置及其洗刷效应的大小,随着Re的增大,洗刷效应明显增强,因而,过渡流范围内Re越大,圆柱下游壁面传热强化越大。 相似文献
10.
The interaction of resonant sounds with the flow past a thick, blunt, flat plate in a rigid walled square duct has been examined. Sound pressure levels of up to 146 dB (re 20 μPa) have been recorded. It has been established that the resonant sound can initially be excited at a harmonic of the normal vortex shedding frequency. In some cases, the sound “feeds back” on the vortex shedding process causing a step change in the shedding frequency, increasing the Strouhal number for the plate by up to twice the normal value. This excited vortex shedding and associated resonances can be suppressed by locating the plate at incidence to the air flow direction. Complex duct modes can be generated by the vortex shedding resulting in different regions of the plate shedding at different Strouhal numbers. 相似文献
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The passive oscillations of flexible filaments in a flowing soap film were investigated to learn the serial interaction between them. When arranged in tandem, the downstream filament flaps at the same frequency as that of the upstream one, but with a larger amplitude, whereas the upstream one is almost unaffected compared to the single filament case. The data analysis shows the downstream filament indeed extracts energy from the vortex street and receives greater force than the upstream one or a single filament in a uniform flow. 相似文献
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14.
Agáta Dujavová Michaela Valeriánová Vladimír Štrbík Zuzana Matkovičová Gustav Plesch Ivan Kostič 《Central European Journal of Physics》2007,5(2):229-235
Rhenium avoids air degradation of the target as well as of precursor thin films during the preparation of high temperature
Tl-based superconductors. Addition of Re has been used during synthesis of oriented Tl(Re)-Ba-Ca-Cu-O films. High temperature
superconducting Tl-based thin films were prepared by a two step method combining RF magnetron sputtering and ex-situ thallination. Precursors with a composition of Re0.1Ba2Ca2Cu3Ox were deposited on a CeO2 buffered R-plane sapphire substrate. The sputtered films were prepared at room temperature in an Ar atmosphere. The thallination
of the precursor films was performed in a one zone configuration, where both the pellet and precursor film were kept at the
same temperature. The thallination temperature varied in the range of 850–880°C and samples were held for 30, 45 and 60 min
at this temperature in a flowing oxygen atmosphere. Besides Tl-2212, X-ray diffraction reveals the possibility of also preparing
the Tl-2223 phase, which was until now not reported in a Re doped form.
Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia 相似文献
15.
The influence of Reynolds number and blockage ratio on the vortex dynamics of a trapezoidal bluff body placed inside a circular pipe is studied experimentally and numerically. Low aspect ratio, high blockage ratio, curved end conditions (junction of pipe and bluff body), axisymmetric upstream flow with shear and turbulence are some of the intrinsic features of this class of bluff body flows which have been scarcely addressed in the literature. A large range (200:200,000) of Reynolds number (ReD) is covered in this study, encompassing all the three pipe flow regimes (laminar, transition and turbulent). Four different flow regimes are defined based on the distinct features of Strouhal number (St)–ReD relation: steady, laminar irregular, transition and turbulent. The wake in the steady regime is stationary with no oscillations in the shear layer. The laminar regime is termed as irregular owing to irregular vortex shedding. The vortex shedding in this regime is observed to be symmetric. The emergence of separation bubble downstream of the bluff body on either side is another interesting feature of this regime, which is further observed to be symmetric. Two pairs of mean streamwise vortices are noticed in the near-wake regime, which are termed as reverse dipole-type wake topology. Beyond the irregular laminar regime, the Strouhal number falls gradually and vortex shedding becomes more periodic. This regime is named transition and occurs close to the Reynolds number at which transition to turbulence takes place in a fully developed pipe. The turbulent regime is characterised by a nearly constant Strouhal number. Typical Karman-type vortex shedding is noticed in this regime. The convection velocity, wake width formation length and irrecoverable pressure loss are quantified to highlight the influence of blockage ratio. These results will be useful to develop basic understanding of vortex dynamics of confined bluff body flow for several practical applications. 相似文献
16.
Chunlong Yu Haibo Huang & Xiyun Lu 《advances in applied mathematics and mechanics.》2014,6(4):461-477
Interaction of vortex rings with solid is an important research
topic of hydrodynamic. In this study, a multiple-relaxation time
(MRT) lattice Boltzmann method (LBM) is used to investigate the flow
of a vortex ring impacting spheroidal particles. The MRT-LBM is
validated through the cases of vortex ring impacting a flat wall.
The vortex evolution due to particle size, the aspect ratio of a
prolate particle, as well as Reynolds $(Re)$ number are discussed in
detail. When the vortex ring impacting a stationary sphere, the
primary and secondary vortex rings wrap around each other,
which is different from the situation of the vortex ring
impacting a plate. For the vortex ring impacting with a prolate
spheroid, the secondary vortex ring stretches mainly along the long
axis of the ellipsoid particle. However, it is found that after the
vortex wrapping stage, the primary vortex recovers along the short
axis of the particle faster than that in the long axis,
i.e., the primary vortex ring stretches mainly along the short axis
of the particle. That has never been addressed in the literature. 相似文献
17.
利用粒子成像测速技术(particle image velocimetry,PIV),在水槽中探究缝隙对圆柱流场结构的影响,应用频谱分析和本征正交分解(proper orthogonal decomposition,POD)方法,研究了开缝圆柱流场相干结构.实验Reynolds数范围内,缝隙的"吹吸"作用从根本上改变了圆柱绕流近区尾流结构,前6阶模态形态是流场中最主要的相干结构.第1,2阶模态形态控制着圆柱绕流流场涡街相继脱落过程,1或2阶模态系数为尾迹涡的固有频率;第3,4阶模态形态控制着脱落旋涡沿流向方向能量运输;第5,6阶模态形态中的同向涡旋结构作用于旋涡缓慢脱离柱体这一过程,并对旋涡能量起着衰减作用. 相似文献
18.
Strouhal-Reynolds number relationship for vortex streets 总被引:1,自引:0,他引:1
A rationale for the empirically observed Strouhal-Reynolds number relation for vortex shedding in the wake of a cylinder is provided. This rationale derives from a mechanism of vortex formation observed in numerical simulations of two-dimensional vortex shedding coupled with an order of magnitude estimate of the terms in the vorticity transport equation based on this mechanism. 相似文献
19.
纳米尺度圆柱绕流现象的分子动力学模拟研究 总被引:1,自引:0,他引:1
本文采用分子动力学模拟方法,利用Lennard-Jones(L-J)势能模型,模拟了Re为28时Ar流体流过Pt纳米圆柱的绕流现象.采用小步长时均值作为涡的瞬时值的方法,在纳米尺度、纳秒量级下得到了涡的周期性产生、组合、发展和脱落现象;在大步长时均条件下,得到了稳定的对称涡,表现出了绕流现象在不同时间范围内的不同特征.绕流过程还体现了流体的密度变化,圆柱上游密度大于下游密度、对称的两侧离轴线越远密度越大.结果表明,分子动力学模拟方法可以有效地细节刻画圆柱绕流现象. 相似文献
20.
Corrugated pipes are commonly used because of their local rigidity combined with global flexibility. The flow through such a pipe can induce strong whistling tones, which is an environmental nuisance and can be a threat to the mechanical integrity of the system. This paper considers the use of a composite pipe: a shorter corrugated pipe segment embedded between smooth pipe segments. Such a pipe retains some flexibility, while the acoustical damping in the smooth pipe reduces whistling tones. Whistling is the result of coherent vortex shedding at the cavities in the wall. This vortex shedding is synchronized by longitudinal acoustic waves traveling along the pipe. The acoustic waves trigger the vortex shedding, which reinforces the acoustic field for a critical range of the Strouhal number values. A linear theory for plane wave propagation and the sound production is proposed, which allows a prediction of the Mach number at the threshold of whistling in such pipes. A semi-empirical approach is chosen to determine the sound source in this model. This source corresponds to a fluctuating force acting on the fluid as a consequence of the vortex shedding. The functional form of the Strouhal number dependency of the dimensionless sound source amplitude is based on numerical simulations. The magnitude of the source and the Strouhal number range in which it can drive whistling are determined by matching the model to results for a specific corrugated pipe segment length. This semi-empirical source model is then applied to composite pipes with different corrugated segment lengths. In addition, the effect of inlet acoustical convective losses due to flow separation is considered. The Mach number at the threshold of whistling is predicted within a factor 2. 相似文献