Inertia and diurnal oscillations of Ekman layers in atmosphere and ocean     

《Dynamics of Atmospheres and Oceans》2020

A 1D model, including a time variation of eddy viscosity and mixed layer depth, is applied to study Ekman spirals. It simulates a weak velocity in the atmosphere but a jet in the upper oceanic mixed layer during daytime; and a strong velocity in the atmosphere but a weak, uniform velocity in the ocean at night. The mean spirals in both atmosphere and ocean are close to the average spirals at midday and midnight, they are not flat as suggested by previous studies but consistent with the observations of Polton et al (2013). Our results also show shorter length scale for magnitude decay than for rotation of mean velocity as observed in the ocean, which comes from the combined effects of the diurnal variation of PBL and the Coriolis force. The latter becomes more important away from the surface. In the upper oceanic mixed layer, the mean velocity mainly comes from the strong jets in the late afternoon and early evening. Near and below the depth of Ekman depth, the weak velocities change with time and cancel out each other if averaged timing is longer than the inertia period. It results in diminishing of magnitude of the mean velocity, but the amplitude of individual parcel oscillating can still be quite large near the Ekman depth. Meanwhile, the change of velocity angle from the surface is near or less than 90 degree. Hence, shorter length scale for magnitude decay than for rotation of the mean velocity is not controlled by viscosity alone. Meanwhile, the model does not need two viscosities as suggested previously.The results also show that either the diurnal variation of surface stress or eddy viscosity alone can create a diurnal oscillation of velocity in the ocean. The interactions between PBL force and the Coriolis force can create a weak instability in the atmosphere and ocean at 30° and 90°. This weak instability may explain the observed nocturnal LLJ near 30 °N on the lee of the Rocky Mountains and the intensification of mesoscale circulation simulated by Sun and Wu (1992).  相似文献   

Numerical analysis of a cylinder moving through rate-dependent undrained soil     

Hongxia Zhu  Mark F. Randolph 《Ocean Engineering》2011,38(7):943-953

Geo-hazard assessment of the potential damage to a pipeline caused by a submarine landslide requires a quantitative model to evaluate the impact forces on the pipeline. In contrast with typical geotechnical problems, the strain rate within the fast moving, flow-like submarine landslide is typically far higher, which will lead to enhancement of the soil strength and therefore result in larger impact forces. Generally, there are two possible predictive frameworks for strain-rate dependence: a fluid dynamics framework and a geotechnical framework. By comparison of common rheological models adopted in these two different approaches, a unified additive power-law model, a normalised form of the Herschel-Bulkley model from fluid mechanics, is explored in this paper. This model has been used in conjunction with a large deformation finite element approach to investigate the undrained limiting loads on a cylinder moving steadily through inertia-less soft rate-dependent material, in order to quantify the strain-rate effects.The flow mechanism and the effects of the shear-thinning index and Oldroyd number on the shear zones are explored. The calculated resistance factors are compared with the drag coefficients obtained from computational fluid dynamics analysis. The average rate of strain experienced by the soil flowing past the cylinder is estimated for a given flow velocity and an expression in the form of a conventional bearing capacity equation, but with shear strength linked directly to the normalised flow velocity, is proposed to predict the magnitude of the viscous force exerted by the debris flow.  相似文献   

Magma eruption rates of Merapi volcano,Central Java,Indonesia during one century (1890–1992)     

Suparto Siswowidjoyo  Ismangun Suryo  Izumi Yokoyama 《Bulletin of Volcanology》1995,57(2):111-116

The magma eruption rates of Merapi volcano form 1890 to 1992 are re-examined chronologically. For this volcano, movements of extruded lavas and domes as well as their extrusions are important because they control the modes of the subsequent activities and cause nuées ardentes and lahars. The monthly eruption rates varied widely, but the cumulative volume of lavas has increased linearly and is expressed as 0.1x10⁶ m³/month. The magma production rate of this volcano may have been constant for these 100 years. Recurrent excessive effusion of lavas is tentatively interpreted by assuming a magma reservoir. The averaged eruption rate is small in comparison with other volcanoes such as Nyramuragia, Kilauea and Vesuvio. However, it is remarkable that the activity has been continuous for these 100 years and the total amount of lava discharged during this period reached more than 10⁸ m³. A simple model for the formation of the 1992 lava dome is presented. The viscosity of the lavas is probably between 10⁶ and 10⁷ P and the length of the magma conduit is probably less than 10 km.  相似文献   

花岗质熔体的结构特征与其产铀潜力     

翟建平 《铀矿地质》1988,(5)

本文从花岗质熔体结构特征的角度,探讨了熔体成分与其产铀潜力之间的关系。分析了熔体中的K,Na对铀富集的控制作用,八面体、四面体空隙和晶体场效应,O/Si比值对铀活动性的影响。讨论了决定熔体粘度的一些因素,提出了熔体相对粘度值的计算方法,结合实例介绍了熔体相对粘度值的意义及地质应用。总结了有利于产铀的熔体结构特征标志。  相似文献   

Effect of sediment particles on the velocity profile of sediment-water mixtures in open-channel flow     

Lei Zhang  Jianzhao Guan  Deyu Zhong  Yousheng Wang 《国际泥沙研究》2023,38(3):361-373

The presence of sediment particles in open-channel flow has an important effect on turbulence; thus, an empirical, turbulent eddy viscosity formula was established for application in the limit for low concentrations. The current study establishes a theoretical relation for the mixture viscosity based on the two-phase mixture model. The percentage contribution of the three mechanisms of mixture viscosity,namely, fluid turbulence(FT), particle turbulence(PT), and inter-particle collisions(IPCs), w...  相似文献   

Shaking table tests on subgrade reaction of pipe embedded in sandy liquefied subsoil   总被引：3，自引：0，他引：3  

I. Towhata  W. Vargas-Monge  R. P. Orense  M. Yao 《Soil Dynamics and Earthquake Engineering》1999,18(5):1009

An interest in the behavior of liquefied sand during seismic flow failure led the authors to conduct shaking table tests in which an embedded pipe was pulled laterally and the required drag force was monitored. Test results showed that the amplitude of shaking acceleration affected the behavior of sand in both dry and water-saturated conditions. In dry sand, the induced inertia force decreased the shear strength and consequently the magnitude of the drag force. When the sand was saturated, a special consideration was made of the similitude of dilatancy between 1-G model tests and the in-situ situation. This goal was attained by employing very loose sand in model tests. The rate-dependency in which the drag force increased with the rate of pipe movement was focused on, leading to an apparently viscous behavior of sand. This is consistent with what several former studies reported.  相似文献   

西藏南部地堑构造成因的数值模拟   总被引：7，自引：0，他引：7  

张东宁  许忠淮 《中国地震》1997,13(4):349-357

青藏高原上地壳在印度洋板块的挤压作用下发育出大量断层，因此上地壳的破碎已经从整体上降低了其有效粘滞系数。其所承受的水平向压应力逐渐转移到中、下地壳中。随着应力转移的进行，上地壳和上地幔内σ１可能已转为垂直方向，处于拉张减薄状态，并在藏南地区形成了一系列张性地堑构造。而目前在中、下地壳内σ１可能仍为近南北的水平方向。在印度洋板块的强烈挤压下，该深度范围内柔性的中、下地壳物质仍在垂直方向继续膨胀、增厚，并导致青藏高原的继续抬升。本文利用粘弹性蠕变本构关系的有限元方法模拟了上述设想的可能性。  相似文献   

Prediction of magnetite segregation in dense medium cyclone using computational fluid dynamics technique     

M. Narasimha  Mathew Brennan  P.N. Holtham 《International Journal of Mineral Processing》2007

Multi-phase simulations of turbulent driven flow in a dense medium cyclone with magnetite medium have been conducted in Fluent, using the Algebraic Slip Mixture model to model the dispersed phases and the air-core, and both the Large Eddy Simulation turbulence model (LES) and Reynolds Stress Models (RSM) for turbulence closure. The predicted air-core shape and diameter were found to be close to the experimental results measured by gamma ray tomography. It is possible to use the LES turbulence model with ASM multi-phase model to predict the air/slurry interface accurately. Multi-phase simulations (air/water/medium) show appropriate medium segregation effects but over-predict the level of segregation compared to that measured by gamma ray tomography near the wall. This is believed to be because of unaccounted back-mixing of the dispersed phase due to turbulence in the basic Algebraic Slip Mixture model. The predictions of accurate axial segregation of magnetite medium are investigated using the LES turbulence model in conjunction with the multi-phase mixture model and viscosity corrections according to the feed particle loading factor. At higher feed densities the agreement between the Dunglison [Dunglison, M.E., 1999, A general model of the dense medium cylone, PhD thesis, JKMRC, University of Queensland] correlations and experimental measurements and the CFD is reasonably good, but the overflow density is lower than the model predictions. It is believed that the excessive underflow volumetric flow rates are responsible for under prediction of the overflow density. The effect of size distribution of the magnetite has been fully studied. As expected, the ultra-fine magnetite sizes (i.e. 2 and 7 microns) are distributed uniformly throughout the cyclone. As the size of magnetite increases, more segregation of magnetite occurs close to the wall.  相似文献   

Viscous fluid characteristics of liquefied soils and behavior of piles subjected to flow of liquefied soils   总被引：5，自引：0，他引：5  

Jae-Ik Hwang  Chang-Yeop Kim  Choong-Ki Chung  Myoung-Mo Kim   《Soil Dynamics and Earthquake Engineering》2006,26(2-4):313-323

Lateral movement of sloping ground due to flow liquefaction has caused many pile foundations to fail, especially those in ports and harbor structures. Several researchers have found and verified that the behavior of liquefied soils can be simulated appropriately by modeling the liquefied soils as viscous fluid. In this study, the influence of the lateral movement of liquefied sloping ground on the behavior of piles was analyzed on the assumption that the flow of liquefied soils can be treated as viscous fluid flow. Sinking ball tests and pulling bar tests were performed to measure the viscosity of liquefied Jumoonjin sand. Then, the behavior of a single pile installed in liquefiable infinite slopes consisting of sand was investigated by numerical analyses. The liquefied sand behaved as non-Newtonian fluid, whose viscosity decreased with increasing shear strain rate. Furthermore, the flow of liquefied soils had a crucial effect on the stability of piles installed in the sloping ground.  相似文献   

钙长石成分熔体粘滞度和自扩散系数压力效应的分子动力学研究     

赵亚娟张毅刚  郭光军Keith REFSON 《岩石学报》2004,20(3):737-746

用分子动力学方法,研究了1999 K下,压力由23 MPa上升到15183 MPa的过程中,CaAl_2Si_2O_8成分熔体的微观结构、剪切粘滞度和粒子自扩散系数的压力效应。在此基础上,探讨压力对剪切粘滞度与粒子自扩散系数之间关系的影响,并将它同微观结构的变化联系起来。结果表明,粒子自扩散系数的压力效应与熔体结构有很强的相关性;压力的挤压效应阻碍了粒子的扩散,而Si-O和Al-O 5次配位体的形成又加速了扩散过程,两种相反的作用相互抵消,造成的结果是在0～5 GPa范围内,Si~(4 ),O~(2-)和Al~(3 )等网架形成粒子的自扩散系数随压力变化不明显;当压力继续增大时,挤压效应占了主导,导致自扩散系数值快速减小。Ca~(2 )作为网架修饰粒子,自扩散系数随压力升高单调下降。压力小于5 GPa时,粒子自扩散系数的大小关系是:D_(Ca)>D_(Al)>D_O>D_(Si)。系统粘滞度随压力的变化与熔体中BO的含量密切相关:BO含量小于域值时,一定范围内BO含量的变化不会对粘滞度产生很大的影响,超过域值,BO含量的微小增加会导致粘滞度值迅速增大。有效应用Eyring方程的关键是方程中粒子跳跃距离的确定,本研究发现,Si~(4 )和O~(2-)的跳跃距离可以通过系统中非桥氧的百分含量来获得。这一发现使得我们能够利用系统中NBO的含量,结合Eyring方程有效进行不同压  相似文献