共查询到20条相似文献,搜索用时 31 毫秒
1.
2.
3.
Numerical study on near wake flows of a flat plate in three kinds of oncoming flows is made by using the discrete vortex model
and improved vorticity creation method. For steady oncoming flow, both gross and detailed features of the wake flow are calculated
and discussed. Then, in harmonic oscillatory oncoming flow two different wake flow patterns withK
c=2,4 and 10 are obtained respectively. Our results present a new wake flow pattern for lowKc numbers (Kc<5) describing vortex shedding, pairing and moving in a period of the oscillatory flow starting from rest. The calculated
drag and inertia force coefficients are closer to experimental data from the U-tube than the previous results of vortex simulation.
For in-line combined oncoming flow the vortex lock-in and dynamic characteristics are simulated. The results are shown to
be in good agreement with experiments.
The project supported by National Natural Science Fundation of China and LNM of Institute of Mechanics. CAS 相似文献
4.
Oscillatory Marangoni convection in silicone oil–liquid bridges with different geometrical aspect ratios is investigated by three‐dimensional and time‐dependent numerical simulations, based on control volume methods in staggered cylindrical non‐uniform grids. The three‐dimensional oscillatory flow regimes are studied and compared with previous experimental and theoretical results. The results show that the critical wavenumber (m), related to the azimuthal spatio‐temporal flow structure, is a monotonically decreasing function of the geometrical aspect ratio of the liquid bridge (defined as the ratio of length to diameter). For this function, a general correlation formula is found, which is in agreement with the previous experimental findings. The critical Marangoni number and the oscillation frequency are decreasing functions of the aspect ratio; however, the critical Marangoni number, based on the axial length of the bridge, does not change much with the aspect ratio. For each aspect ratio investigated, the onset of the instability from the axisymmetric steady state to the three‐dimensional oscillatory one is characterized by the appearance of a standing wave regime that exhibits, after a certain time, a second transition to a travelling wave regime. The standing wave regime is more stable for lower aspect ratios since it lasts for a long time. This behaviour is explained on the basis of the propagation velocity of the disturbances in the liquid phase. For this velocity, a general correlation law is found as a function of the aspect ratio and of the Marangoni number. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
5.
本文研究从上面加热的液桥中不同 pr 数流体的热表面张力驱动对流。在 Ma 数相同的条件下,不同 pr 数流体液桥中的温度分布和流场结构定性相似,但定量结果不同。小pr数(pr<<1)流体液桥中的粘性边界层远小于热边界层,最大流函数所在位置向冷端偏移,有较大的流动速度。结果表明,Ma 数是描述这种流动的敏感参数。 相似文献
6.
7.
Instability of thermocapillary convection in liquid bridges of low Prandtl number fluids is investigated by direct three-dimensional and time-dependent simulation of the problem. The field equations are numerically solved explicit in time and with finite difference methods in a staggered cylindrical grid. The numerical results are analyzed and interpreted in the general context of the bifurcation's theory. According to recent stability analyses the computations show that for semiconductor melts the first bifurcation is characterized by the loss of spatial symmetry rather than by the onset of oscillatory flow. When the basic axisymmetric flow field becomes unstable, after a short transient, a three-dimensional supercritical steady state is obtained. It is shown that the flow field organization, depending on the critical wave number, is related to the geometrical aspect ratio of the liquid bridge and that lower is the aspect ratio, higher is the critical wave number and more complex the thermofluid-dynamic field structure. 相似文献
8.
近二十年来,微重力流体开展了半浮区液桥热毛细对流的不稳定性与转捩的研究.文中给出了热毛细振荡对流发生的临界参数,分析了液桥几何位形(尺度比,体积比)、物理参数及传热参数对临界Maxangoni的影响.报导了有关的地面模拟实验,微重力实验以及本问题的线性稳定性分析、能量分析和数值模拟结果,并介绍了定常轴对称热毛细对流通过非定常振荡热毛细对流到湍流的转捩过程和三种热毛细振荡对流的产生机理. 相似文献
9.
The pattern of Rayleigh-Benard convection of air in a rectangular box heated-from-below is studied by numerically solving
the three-dimensional time-dependent Navier-Stokes equations under the Boussinesq approximation. Slightly supercritical Rayleigh
number was adopted to track the evolutions of flow structure as a function of enclosure's aspect ratio (A=L/H). The flow will asymptotically evolve to different patterns, among which, two possible types of flow pattern are found. One
consists of the pair of straight vortex rolls and the other appears as closed vortex rings. The transition between the flow
patterns indicates that there exists a flow bifurcation with the variation of container's aspect ratio. In addition, both
steady and oscillatory flows have been observed, corresponding to the pair of straight vortex rolls and the vortex ring, respectively.
The complexity of flow structure tends to increase with the increasing aspect ratio of the rectangular enclosure.
The project supported by the National Natural Science Foundation of China (19889210), the National Distinguished Young Fund
(10125210), the Hundred Talents Program of CAS, and the Training Program for the Trans-Century Outstanding Young of MOE 相似文献
10.
I. S. Shivakumara Jinho Lee S. Suresh Kumar N. Devaraju 《Archive of Applied Mechanics (Ingenieur Archiv)》2011,81(11):1697-1715
The linear and nonlinear stability of double diffusive convection in a layer of couple stress fluid–saturated porous medium
is theoretically investigated in this work. Applying the linear stability theory, the criterion for the onset of steady and
oscillatory convection is obtained. Emphasizing the presence of couple stresses, it is shown that their effect is to delay
the onset of convection and oscillatory convection always occurs at a lower value of the Rayleigh number at which steady convection
sets in. The nonlinear stability analysis is carried out by constructing a system of nonlinear autonomous ordinary differential
equations using a truncated representation of Fourier series method and also employing modified perturbation theory with the
help of self-adjoint operator technique. The results obtained from these two methods are found to complement each other. Besides,
heat and mass transport are calculated in terms of Nusselt numbers. In addition, the transient behavior of Nusselt numbers
is analyzed by solving the nonlinear system of ordinary differential equations numerically using the Runge–Kutta–Gill method.
Streamlines, isotherms, and isohalines are also displayed. 相似文献
11.
We investigate the steady state convection amplitude for solutal convection occurring during the solidification of a rotating mushy layer in a binary alloy system for a new Darcy equation formulation. We adopt a large far field temperature and assume that the initial composition is very close to the eutectic composition. The linear stability analysis showed that rotation stabilised solutal convection. The results of the weak non-linear analysis of stationary convection indicates the presence of Hopf bifurcation, associated with the oscillatory mode, developing at Ta = 3. 相似文献
12.
Taishi Yano Koichi Nishino Hiroshi Kawamura Ichiro Ueno Satoshi Matsumoto Mitsuru Ohnishi Masato Sakurai 《Experiments in fluids》2012,53(1):9-20
Microgravity experiments have been conducted on the International Space Station in order to clarify the transition processes of the Marangoni convection in liquid bridges of high Prandtl number fluid. The use of microgravity allows us to generate large liquid bridges, 30?mm in diameter and up to 60?mm in length. Three-dimensional particle tracking velocimetry (3-D PTV) is used to reveal complex flow patterns that appear after the transition of the flow field to oscillatory states. It is found that a standing-wave oscillation having an azimuthal mode number equal to one appears in the long liquid bridges. For the liquid bridge 45?mm in length, the oscillation of the flow field is observed in a meridional plane of the liquid bridge, and the flow field exhibits the presence of multiple vortical structures traveling from the heated disk toward the cooled disk. Such flow behaviors are shown to be associated with the propagation of surface temperature fluctuations visualized with an IR camera. These results indicate that the oscillation of the flow and temperature field is due to the propagation of the hydrothermal waves. Their characteristics are discussed in comparison with some previous results with long liquid bridges. It is shown that the axial wavelength of the hydrothermal wave observed presently is comparable to the length of the liquid bridge and that this result disagrees with the previous linear stability analysis for an infinitely long liquid bridge. 相似文献
13.
Liping Yao Zhong Zeng Yi Zhang Zhouhua Qiu Huan Mei Liangqi Zhang Yongxiang Zhang 《Heat and Mass Transfer》2012,48(12):2103-2111
The effects of rotating magnetic field (RMF) on the three-dimensional thermocapillary flow of semiconductor melt (Pr?=?0.01) in a floating half-zone model under microgravity are investigated numerically by the finite volume method. The results indicate that the thermocapillary flow without magnetic field is a steady three-dimensional convection for Ma?=?40 in a floating half-zone model with As?=?1, and the convection evolves to an oscillatory three-dimensional flow by applying 1–6?mT RMF with 50?Hz rotating frequency. Based on the fast Fourier transform spectrum, the convection is confirmed to be a periodically oscillating flow, the oscillatory main frequency, 1.59?×?10?3?Hz for 1?mT RMF and 5.84?×?10?2?Hz for 6?mT RMF, increases with the magnetic strength. However, with increasing the magnetic field strength up to 7?mT, the three-dimensional thermocapillary flow is effectively controlled and the convection turns into a steady axisymmetrical one. 相似文献
14.
Experimental study of the free surface deformation due to thermal convection in liquid bridges 总被引:1,自引:1,他引:0
Optical imaging was used to measure the free surface deformation due to thermal (Marangoni-buoyant) convection in liquid bridges
of 5-cSt silicone oil. We obtained the free surface position averaged over time in both the steady and oscillatory regimes.
The deviation of the free surface contour from the corresponding equilibrium shape was determined with an uncertainty of about
2 μm. This deviation grew linearly with the applied temperature difference with a proportionality coefficient depending on
the liquid bridge volume at equilibrium. Shrinkage at the upper part of the liquid bridge was slightly greater than bulging
at the lower with the sum of the maximum deviations at both parts being about 30 μm near the onset of oscillations. This sum,
normalized with the radius of the supporting disks, was of the same order of magnitude as the Capillary number. We observed
the influence of thermal expansion, surface tension variation over the free surface, and fluid motion separately. The local
mean curvature was also calculated and compared with its value at equilibrium, showing that the hydrodynamic effects were
important. 相似文献
15.
Zhao Jianfu Xie Jingchang Lin Hai Hu Wenrui A. V. Ivanov A. Yu. Belyaev 《Acta Mechanica Sinica》2001,17(2):151-159
A first experimental study on two-phase flow patterns at a long-term, steady microgravity condition was conducted on board
the Russian Space Station “MIR” in August 1999. Carbogal and air are used as the liquid and the gas phase, respectively. Bubble,
slug, slug-annular transitional, and annular flows are observed. A new region of annular flow with lower liquid superficial
velocity is discovered, and the region of the slug-annular transitional flow is wider than that observed by experiments on
board the parabolic aircraft. The main patterns are bubble, slug-annular transitional and annular flows based on the experiments
on board MIR space station. Some influences on the two-phase flow patterns in the present experiments are discussed.
The project supported by the National Natural Science Foundation of China (19789201), the Ministry of Science and Technology
of China (95-Yu-34), and the Post-doctoral Science Foundation of China 相似文献
16.
The equilibrium of a liquid heated from below is stable only for small values of the vertical temperature gradient. With increase of the temperature gradient a critical equilibrium situation occurs, as a result of which convection develops. If the liquid fills a closed cavity, then there is a discrete sequence of critical temperature gradients (Rayleigh numbers) for which the equilibrium loses stability with respect to small characteristic disturbances. This sequence of critical gradients and motions may be found from the solution of the linear problem of equilibrium stability relative to small disturbances.
If the temperature gradient exceeds the lower critical value, then (for steady-state heating conditions) there is established in the liquid a steady convective motion of a definite amplitude which depends on the magnitude of the temperature gradient. Naturally, the amplitude of the steady convective motion cannot be determined from linear stability theory; to find this amplitude we must solve the problem of convection with heating from below in the nonlinear formulation. A nonlinear study of the steady motion of a liquid in a closed cavity with heating from below was made in [1]. In that study it was shown that for Rayleigh numbers R which are less than the lower critical value Rc steady-state motions of the liquid are not possible. With R>Rc a steady convection arises, whose amplitude near the threshold is small and proportional to (R–Rc)1/2 (the so-called soft instability)-this is in complete agreement with the results of the phenom-enological theory of Landau [2, 3].Primarily, various versions of the method of expansion in powers of the amplitude [4–8] have been used, and, consequently, the results obtained in those studies are valid only for values of R which are close to Rc, i. e., near the convection threshold.It is apparent that the study of developed convective motion far from the threshold can be carried out only numerically, with the use of digital computers. In [9, 10] the numerical methods have been successfully used for the study of developed convection in an infinite plane horizontal liquid layer.The present paper undertakes the numerical study of plane convective motions of a liquid in a closed cavity of square section. The complete nonlinear system of convection equations is solved by the method of finite differences on a digital computer for various values of the Rayleigh number, the maximal value exceeding by a factor of 40 the minimal critical value Rc. The numerical solution permits following the development of the steady motion which arises with R>Rc in the course of increase of the Rayleigh number and permits study of the oscillatory motions which occur at some value of the parameter R. The heat transfer through the cavity is studied. The corresponding linear problem on equilibrium stability is solved approximately by the Galerkin method. 相似文献
17.
Rayleigh-Marangoni-Bénard instability in a system of two-layer fluids is studied numerically. The convective instabilities
in the system of Silicon Oil (10cSt) and Fluorinert (FC70) liquids have been analyzed. The critical parameters at onset of
convection are presented in a large range of two-layer depth ratios from 0.2 to 5.0. Numerical results show that the instability
of the two-layer system depends strongly on its depth ratio. When the depth ratio increases, the instability mode changes
from mechanical coupling to thermal coupling. Between these two typical coupling modes, a time-dependent oscillation is detected.
Nevertheless, traveling wave states are found in the case of oscillatory instability. The oscillation mode results from the
competition between Rayleigh instability and Marangoni effect.
The project supported by the National Natural Science Foundation of China (10372105) and the Knowledge Innovation Program
of Chinese Academy of Sciences (KJCX2-SW-L05) 相似文献
18.
Rachid Khiri 《International Journal of Non》2004,39(4):593-604
The problem of finite-amplitude thermal convection in a horizontal layer of a low Prandtl number heated from below and rotating about a vertical axis is studied. Linear stability and weak non-linear theories are used to investigate analytically the Coriolis effect on gravity-driven convection. The non-linear steady problem is solved by perturbation techniques, and the preferred mode of convection is determined by a stability analysis. Finite-amplitude results, obtained by using a weak amplitude, correspond to both stationary and oscillatory convections. These amplitude equations permit to identify from the post-transient conditions that the fluid is subject to Pitchfork bifurcation in the stationary convection and Hopf bifurcation in the oscillatory convection. Thereafter, in the small perturbations hypothesis, an amplitude solution is evaluated and drawn in time and space scales. 相似文献
19.