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明渠紊流中随水流迁移的涡结构在传质过程中具有重要作用。为观测涡结构在整个生存期内的运动过程,将两套高频粒子图像测速系统的测量范围沿水流方向拼接,实现流场的大范围测量。将上述方法用于测量摩阻雷诺数为490~870的明渠紊流的纵垂面流场,并结合涡结构跟踪算法,得到了涡的生存时间、运动速度、位移及强度变化等参数。分析表明,涡结构的水平运动与水流同步,但整体有向水面抬升的趋势,在低雷诺数条件下,最大生存时间约10倍涡周转时间,最大垂向位移约0.4倍水深;随着雷诺数的增大,涡结构的量纲一运动速度未发生明显变化,但极限强度和强度变化速率均增大,且极限强度的增加幅度相对较小,使得涡结构的生存时间缩短,纵、垂向位移相应减小。研究结果揭示了生存时间是影响涡结构运动规律的关键变量,初步得到了涡结构运动规律随水流条件的变化趋势。 相似文献
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在明渠中开展不同水流条件下低强度均匀沙平衡输沙试验,基于灰度相减方法分析整个床面推移质运动特征,以探寻紊流结构与推移质运动之间的作用机理。结果表明:①受水槽横向方向水流强度分布特征影响,推移质运动概率从水槽中线到两侧壁逐渐变小,且基本呈对称分布;②在紊流相干结构作用下,推移质床面在紊流低速条带区形成凸槽,高速条带区形成凹槽,推移质运动概率沿水槽横向方向存在高低相间的带状分区;③随摩阻雷诺数增大,相邻两推移质运动高概率区域的间距值变化范围为0.13~0.24倍水槽宽度,其值随摩阻雷诺数增大而增大;④不同水流条件下,推移质运动高概率区域间距值约为水深的2倍,这与流向涡模型吻合,表明流向涡是诱导床面出现凹凸相间形态的重要因素。 相似文献
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利用高频粒子图像测速PIV(Particle Image Velocimetry)系统对明渠弯道水流进行了测量,在两种不同试验方式的配合下,不仅提供了大量高精度的平面二维流场数据,还成功重构了弯道水流的三维平均流速场,为深入研究弯道水流复杂的时均流动结构提供了试验依据。试验结果表明,由于弯道曲率不连续,水流主流区在弯道入口位于凸岸附近,随后在离心力的作用下逐渐转移至凹岸,并一直维持至弯道出口;凹岸顶托及环流运动导致不同横向位置处的纵向流速剖面近床面区域速度梯度及最大流速的位置沿程发生改变;弯道曲率越大,最大水深平均纵向流速转移至靠近凹岸一侧的位置离弯道入口越远;受上游直段二次流的影响,环流运动在弯道入口断面呈三涡结构,随后快速衰减为双涡结构;弯道主环流的强度沿程先增加后减小,涡核位置沿程往复摆动。 相似文献
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提出了基于河道管理、河流修复与水资源可持续发展的含植物明渠水动力学研究理念,强调了含植物明渠水动力学研究的重要意义.从含植物水流的时均阻力特性、紊流机理的物理模型试验研究及数值模拟计算研究等几方面,深入介绍了国内外关于含植物明渠水动力学研究进展.指出:必须进一步解决对柔性植物枝叶形状及柔韧度的相似模拟;建立柔性植物的刚度、水流特性及阻力特性之间的定量规律;进一步研究含植物水流各向异性的紊流特性;发展更为精细的大涡数值模型来研究水深较大的复杂的各向异性紊流等有待于深入探讨的问题. 相似文献
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含淹没植物明渠中存在的Kelvin-Helmholtz（KH）涡,对水流能量交换和物质输移具有直接的影响。基于淹没刚性植物条件下的水槽实验,利用粒子图像测速技术（PIV）得到瞬时流速场和涡量场,定量刻画KH涡的垂向几何尺度（上下边界和垂向作用范围）,结合三维声学多普勒流速仪（ADV）测量的流速时间序列和小波变换方法,分析KH涡的频率,研究植物淹没度对KH涡上下边界位置、作用范围和频率的影响。研究发现:本实验确定的KH涡上、下边界与根据能量分布计算得到的混合层上、下边界基本一致;在本实验条件下,KH涡上边界为0.7~0.8倍水深,下边界在植物密度一定时围绕一常数波动,KH涡的频率在0.14~0.20 Hz之间;随着淹没度的增大,KH涡的上边界升高,下边界变化不明显,作用范围和频率增大。 相似文献
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建立三维水动力学模型分析中华鲟产卵栖息地水力因子的时空分布和中华鲟产卵时的适宜水力特性,为设计最佳的生态调度方案提供科技支撑。研究结果表明,中华鲟产卵栖息地的上产卵区水深和流速的变化主要受流量影响,涡量的变化主要受地形影响;中华鲟产卵前需要高流量脉冲刺激产卵。坝下与隔流堤之间流速和涡量值的大小与波动远大于其他区域,均值为2.4 m/s和11 m2/s。产卵栖息地水体表层、中层和底层的流速和水平涡量分布格局相似,均是在上产卵区值较大,空间分布多样性高。水体中层垂向涡量的值远大于底层和表层。产卵栖息地水体表层、中层和底层水力分布特征为中华鲟繁殖提供了有利的水力条件,体现出中华鲟对产卵栖息地不同功能区的自主选择性。 相似文献
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储集层孔隙度和电阻率曲线间微差形态波形的复杂程度隐含着储集层含油气性的信息,可以用混沌动力学系统加以刻划。油层相空间庞加莱截面明显具有自相似结构,呈现混沌特性;而水淹层和水层的相空间庞加莱截面显示,其混沌性呈显著减弱之势,系统趋于简单有序。其功率谱密度不论水淹程度如何,整体为一随频率增大其能量按指数递减连续频谱;随水淹程度逐渐减弱,递减幅度减小,整体能量逐渐降低,某些频率成分被吸收削弱,由光滑状逐渐演变成锯齿起伏状,其功率谱熵逐渐增大。基于测井信息的混沌特征和功率谱熵特征识别水淹层的方法,对利用传统测井解释理论判别比较困难的低阻油层有较强的识别能力。 相似文献
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A number of sensitivity experiments have been conducted to investigate the influence of using synthetic data on cyclone forecasts
by a global spectral model. Some well known vortices have been used and the generated wind and pressure profiles are compared.
It is found that the Rankine vortex and Holland’s vortex show the best representation of cyclonic circulation. Hence these
two vortices are used in the sensitivity studies to simulate two cyclones, one of May 1979 and the other of August 1979. For
this purpose the FGGE level-III b data set, produced at ECM WF, UK is used. Synthetic temperature and humidity data are also
introduced to make the cyclones more realistic.
With the use of Holland’s vortex the system is found to move faster than with the Rankine vortex. Also, the tracks of the
cyclones simulated with Rankine vortex are found to be on the left side of the observed track while that of Holland’s vortex
is on the right side of the observed track. However, substantial filling up of the systems are noticed with introduction of
diabatic initialization of the mass and velocity fields and the forecasts of both the vortices behave differently. It is suggested
that proper selection of synthetic vortex, initialization scheme and resolution of the model are very important for better
forecast of cyclones. 相似文献
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构建起双向耦合的液固两相流动旋涡动力学模型与数值方法;应用离散涡方法,计算非定常不稳定水流场;采用Lagrange方法模拟颗粒运动,颗粒对流体的反作用通过修正涡泡运动速度来实现。利用所建模型,计算了两种St数的泥沙粒子在圆柱绕流场中的运动。结果证明了液固两相流动中颗粒运动与旋涡存在着明确的相关结构:(1)当水沙混合物中的泥沙颗粒碰上旋涡时,泥沙颗粒被卷入旋涡中,被卷入旋涡中的泥沙颗粒在运动过程中始终分布于旋涡区;(2)均匀水沙混合物绕圆柱流动,由于流体流过圆柱时产生剧烈分离流动,使得在尾迹流内中等St数 (St～o (1))的泥沙颗粒从均匀水沙混合物中分离出来而往旋涡区聚集。 相似文献
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Randhir Singh P. K. Pal C. M. Kishtawal P. C. Joshi 《Journal of Earth System Science》2005,114(4):427-436
The initialization scheme designed to improve the representation of a tropical cyclone in the initial condition is tested
during Orissa super cyclone (1999) over Bay of Bengal using the fifth-generation Pennsylvania State University — National
Center for Atmospheric Research (Penn State — NCAR) Mesoscale Model (MM5). A series of numerical experiments are conducted
to generate initial vortices by assimilating the bogus wind information into MM5. Wind speed and location of the tropical
cyclone obtained from best track data are used to define maximum wind speed, and centre of the storm respectively, in the
initial vortex. The initialization scheme produced an initial vortex that was well adapted to the forecast model and was much
more realistic in size and intensity than the storm structure obtained from the NCEP analysis. Using this scheme, the 24-h,
48-h, and 72-h forecast errors for this case was 63, 58, and 46 km, respectively, compared with 120, 335, and 550 km for the
non-vortex initialized case starting from the NCEP global analysis. When bogus vortices are introduced into initial conditions,
the significant improvements in the storm intensity predictions are also seen.
The impact of the vortex size on the structure of the initial vortex is also evaluated. We found that when the radius of maximum
wind (RMW) of the specified vortex is smaller than that of which can be resolved by the model, the specified vortex is not
well adapted by the model. In contrast, when the vortex is sufficiently large for it to be resolved on horizontal grid, but
not so large to be unrealistic, more accurate storm structure is obtained. 相似文献
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现今二维粒子图像测速（PIV）广泛应用,为进一步提高PIV测量结果对紊流及水流和泥沙相互作用机理的理解,分析了二维PIV中三维涡旋的可测性。结合涡旋识别的旋转强度方法、坐标旋转定理和紊流经典Burgers涡模型,推导得出:二维PIV能否检测出三维涡旋,主要受涡旋环量和旋涡与测量平面之间的倾角两个因素影响,且仅当涡旋环量大于某一特定值（为倾角的函数）时,三维涡旋才可以被测量出。利用广泛使用的直接数值模拟（DNS）槽道紊流数据统计分析,得出了该条件下倾角和环量的相关曲线与拟合方程;结合环量概率密度累积分布可知,在常规二维PIV实验中,涡旋被测量出的概率越大,相应的倾角也越大,其中以90%、50%和10%的概率被测量出的三维涡旋与测量平面间分别呈20°、11°和5°的倾角。 相似文献
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Most tropical cyclones have very few observations in their vicinity. Hence either they go undetected in standard analyses
or are analyzed very poorly, with ill defined centres and locations. Such initial errors obviously have major impact on the
forecast of cyclone tracks using numerical models. One way of overcoming the above difficulty is to remove the weak initial
vortex and replace it with a synthetic vortex (with the correct size, intensity and location) in the initial analysis. The
objective of this study is to investigate the impact of introducing NCAR–AFWA synthetic vortex scheme in the regional model
MM5 on the simulation of a tropical cyclone formed over the Arabian Sea during November 2003. Two sets of numerical experiments
are conducted in this study. While the first set utilizes the NCEP reanalysis as the initial and lateral boundary conditions,
the second set utilizes the NCAR–AFWA synthetic vortex scheme. The results of the two sets of MM5 simulations are compared
with one another as well as with the observations and the NCEP reanalysis. It is found that inclusion of the synthetic vortex
has resulted in improvements in the simulation of wind asymmetries, warm temperature anomalies, stronger vertical velocity
fields and consequently in the overall structure of the tropical cyclone. The time series of the minimum sea level pressure
and maximum wind speed reveal that the model simulations are closer to observations when synthetic vortex was introduced in
the model. The central minimum pressure reduces by 17 hPa while the maximum wind speed associated with the tropical cyclone
enhances by 17 m s −1 with the introduction of the synthetic vortex. While the lowest central pressure estimated from the satellite image is 988 hPa,
the corresponding value in the synthetic vortex simulated cyclone is 993 hPa. Improvements in the overall structure and initial
location of the center of the system have contributed to considerable reduction in the vector track prediction errors ie.
642 km in 24 h, 788 km in 48 h and 1145 km in 72 h. Further, simulation with the synthetic vortex shows realistic spatial
distribution of the precipitation associated with the tropical cyclone. 相似文献
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The present work is concerned with the study of intensification of tropical disturbances with a view to improve prediction
and early warning. The tropical disturbances are known to come in sizes (radii) ranging from 100–400 kms. Since the vortices
of different sizes give rise to different initial convergence fields and since the subsequent development of the tropical
depressions is very sensitive to the initial convergence fields, we argue that the size of the incipient vortex is likely
to be an important factor in determining the subsequent development of a tropical disturbance.
We have examined the above hypothesis using an axisymmetric model of tropical cyclone. The incipient vortex is introduced
by prescribing an initial temperature perturbation with wind in gradient balance. The results show a fairly sharp selection
of scale at about 250 km radius. This implies that out of a number of initial disturbances of varying sizes and embedded in
the same large scale environment, it is the vortex with about 250 km radius size that will develop to the most severe system.
The sensitivity of this selective intensification at this incipient vortex radius to initial perturbation field and the mean
thermodynamic state is investigated. Finally, the importance of such a selective scale of intensification for prediction,
tracking and early warning of tropical cyclones is emphasized. 相似文献
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采用三维湍流模型及其混合有限分析解法进行横流中湍射流这一复杂三维流动的研究。在利用实验数据对模型及其计算方法验证的基础上,对多种喷口形式和流速比工况下的流速场和涡量场进行了数值计算,模拟得到了其旋涡结构发展演化特性。在射流初始阶段,横流在射流背流面形成绕流分离旋涡,其结构与射流喷口形式和流速比有关。射流主轴沿流向布置的窄缝射流产生的旋涡最为特殊和复杂,在喷口侧面存在4个分离点,从而形成4个旋涡。在射流远区,流动主要由反向旋转涡对所控制,并诱导出二次涡对。分析得出了射流喷口形式及流速比对反向旋转涡对涡核位置和旋转强度的影响。 相似文献