青藏高原及邻近地区低涡系统结构研究进展

青藏高原及邻近地区低涡系统(西南涡、高原涡)是造成我国暴雨等灾害性天气的主要系统之一,也是高原天气学的重点研究对象。过去十多年,关于西南涡、高原涡的研究已取得了大量有意义的成果。因此,本文重点针对高原低涡天气系统,总结了西南涡、高原涡的结构特征及其演变机制研究现状,评述了其取得的主要进展,指出了一些有待于深入研究的科学问题,在此基础上,展望了高原低涡天气系统未来的主要发展方向。     

从二维地转风到三维涡旋运动

给出了大气三维涡旋运动基本态所满足的简洁的偏微分方程. 大气涡旋运动存在低压辐合上升和高压辐散下沉的基本状况，基本态的三维速度场可以用流函数和对流速度势分解，且具有螺旋结构. 当Reynolds数Re→∞时，涡旋运动就化为地转风，涡旋运动近似就化为地转风近似.     

Numerical benchmark studies on drag and lift coefficients of a marine riser at high Reynolds numbers

Numerical benchmark studies on drag and lift coefficients of a marine riser have been organized by the 27th ITTC Ocean Engineering Committee. The purpose of the studies was to benchmark the capabilities of CFD methods through quantitative comparisons and validation studies against the model test results of a circular cylinder by MARIN. Studies were focused on the drag crisis phenomenon for the stationary smooth cylinder in the critical Reynolds number regime. Eight organizations have participated in the studies by using RANS, DES and LES methods. An overview of the model test results, test cases, submissions and comparison results are presented in this paper. Conclusions and recommendations are made for future studies.     

Experimental analysis on the risk of vortex ventilation and the free surface ventilation of marine propellers

The paper presents a discussion of the ventilation inception and air drawing prediction of ships propellers, aiming to predict under what conditions ventilation will happen, and the actual physical mechanism of the ventilation.Three different types of ventilation inception mechanisms are included in our discussion: free surface vortex ventilation, ventilation by sucking down the free surface without forming a vortex as well as ventilation by propeller coming out of the water. Ventilation prediction is based on a series of model tests, where the propeller is tested in different levels of intermittent ventilation. The use of underwater video gives a visual understanding of the ventilation phenomena.Ventilation by vortex formation has analogies with other phenomena, such as the inlet vortex in pump sumps, ground vortex at the inlet of the aircraft engines and the Propeller Hull Vortex Cavitation (PHVC). The paper includes comparison between Propeller Hull Vortex Cavitation (PHVC) and Propeller Free Surface Vortex Ventilation (PFSVV) as well as comparison between PFSVV and vortex formations of aero engines during high power operation near a solid surface.Experimental data based on several different model tests shows the boundary between the vortex forming, non-vortex forming and free surface ventilation flow regimes. For comparison the following parameters, which determined the intensity of the hydrodynamic interaction between the propeller and free surface have been used: propeller load coefficient c_T, tip clearance ratio c/D, propeller submergence ratio h/R, ambient velocity V_i and flow cavitation/ventilation number σ_cav/σ_vent.     

龙卷风的漏斗结构理论

龙卷风是大气中风速和垂直速度极大的小尺度系统.本文用气压梯度力、惯性离心力、黏性力三力平衡的柱坐标(r,θ,z)下的大气动力和热力学方程组,求龙卷风的三维速度场(vr,vθ,vz),从理论上绘制出龙卷风的三维漏斗型结构.充分说明龙卷风由涡旋流和急流这两种流叠加而成,涡旋流是由惯性离心力造成的,急流是由水平辐合辐散而引起的强对流.龙卷风是在极端不稳定的大气层结中形成的.     

不同节点布置下二维涡激振动的无单元法参数化分析

尝试建立1种基于无单元法的涡激振动数值模拟算法,并给出适用于涡激振动分析的前处理自动布点方法.基于无单元法理论,使用动最小二乘法构造形函数,利用无单元伽辽金法,采用速度和压力分离模式,及手工布点和自动布点2种方法所得出的节点布置形式,对流场控制方程进行空间离散,模拟二维涡激振动的流场形态,并对VIV相关参数进行分析,计算不同节点布置情况下的升力系数(Cl)、曳力系数(Cd)及斯特罗哈数(St),并与物理模型实验结果进行对比.计算结果表明,无单元伽辽金法应用于立管VIV分析是可行的,且文中采用2种布点方法均能较好的模拟流场中泻涡脱落的形态,计算结果与传统方法和物理模型实验结果吻合良好.证明文中2种布点方法都能用于固定圆柱的二维VIV分析,但自动布点法能够更好地适用于复杂问题的计算及圆柱体在流场中的VIV动力响应分析.     

On the merger of subsurface isolated vortices

Vortex merger is a phenomenon characterizing the whole class of geophysical vortices, from atmospheric storms and large oceanic eddies up to small scale turbulence. Here we focus on the merger of subsurface oceanic anticyclones in an idealized primitive equations model. This study has been motivated by past and recent observations of colliding lens-like anticyclones off of Gibraltar Strait. The critical conditions for merger (critical merger distance and time needed for merger) are determined. We will show that the predictions of classical two-dimensional merger are not verified for subsurface isolated vortices. For instance, critical merger distances will be reduced because of the vortex potential vorticity (PV) structure. The post-merger characteristics of the vortex (radius, extension and PV), are also determined. Merger-related effects, like production of peripheral filaments and small-scale eddies are also investigated and suggest the contribution of merger in both direct and inverse energy cascades.     

Models of interacting pairs of thin,quasi-geostrophic vortices: steady-state solutions and nonlinear stability

We study pairwise interactions of elliptical quasi-geostrophic (QG) vortices as the limiting case of vanishingly thin uniform potential vorticity ellipsoids. In this limit, the product of the vertical extent of the ellipsoid and the potential vorticity within it is held fixed to a finite non-zero constant. Such elliptical “lenses” inherit the property that, in isolation, they steadily rotate without changing shape. Here, we use this property to extend both standard moment models and Hamiltonian ellipsoidal models to approximate the dynamical interaction of such elliptical lenses. By neglecting non-elliptical deformations, the simplified models reduce the dynamics to just four degrees of freedom per vortex. For simplicity, we focus on pairwise interactions between identical elliptical vortices initially separated in both the horizontal and vertical directions. The dynamics of the simplified models are compared with the full QG dynamics of the system, and show good agreement as expected for sufficiently distant lenses. The results reveal the existence of families of steadily rotating equilibria in the initial horizontal and vertical separation parameter space. For sufficiently large vertical separations, equilibria with varying shape exist for all horizontal separations. Below a critical vertical separation (stretched by the constant ratio of buoyancy to Coriolis frequencies N / f), comparable to the mean radius of either vortex, a gap opens in horizontal separation where no equilibria are possible. Solutions near the edge of this gap are unstable. In the full QG system, equilibria at the edge of the gap exhibit corners (infinite curvature) along their boundaries. Comparisons of the model results with the full nonlinear QG evolution show that the early stages of the instability are captured by the Hamiltonian elliptical model but not by the moment model that inaccurately estimates shorter-range interactions.     

On the nonlinear evolution of columnar vortices in a rotating environment

We examine the three-dimensional, nonlinear evolution of columnar vortices in a rotating environment. As the initial vorticity distribution, a wavetrain of finite amplitude Kelvin-Helmholtz vortices in shear is employed. Through direct numerical simulation of the Navier-Stokes equations we seek to better understand the process of maturation of the various three-dimensional modes of instability to which such vortical flows are subject, especially those which exist as a consequence of the action of the Coriolis force. In the absence of rotational influence, we thereby demonstrate that the nonlinear evolution of columnar vortices is most strongly controlled by one or the other of two mechanisms. One mechanism of instability is identifiable as a so-called elliptical instability, which promotes the initial bending of vortex tubes in a sinusoidal fashion, while the other is a hyperbolic mode, which is responsible for the development of streamwise vortex streaks in the "braids" between adjacent vortex cores. In the rotating case, anticyclonic vortices are strongly destabilized by weak background rotation, while rapid rotation stabilizes both the cyclones and anticyclones. The strong anticyclones are subject to two distinct forms of instability, namely a Coriolis force modified elliptical instability and an inertial (centrifugal) instability. The former instability is very similar to the nonrotating form of the elliptical instability as it promotes bending of vortex tubes, while the latter instability grows on the edge of the vortex core and generates streaks of vorticity, which surround the vortex core itself. These results of direct numerical simulation fully verify the results of previous linear stability analyses. Taken together, they provide a simple explanation for the broken symmetry that is often observed to be characteristic of the von Karman vortex streets that develop in the atmospheric lee of oceanic islands.     

Lagrangian modeling of water circulation in the Lofoten Basin

Lagrangian particle tracking is implemented for the Lofoten Basin of the Norwegian Sea. The ocean dynamic fields are obtained from the GLORYS 12V1 reanalysis available by the Copernicus Marine Environment Monitoring Service. Spatial distributions of the Lagrangian particles during May-November 2014 are analyzed for two depth layers: the sea-surface (0.5 m) and 266 m. The results show a significant impact of the Norwegian Coastal Current (NCC) on the thermohaline structure of the upper Lofoten Basin, underestimated previously. The NCC penetrates deep into the central Lofoten basin as far as the longitude 0°. In the subsurface layer, the area over which the NCC influences water structure is comparable to the area of the Norwegian Atlantic Slope Current (NASC), as well as to that of the Norwegian Atlantic Frontal Current (NAFC). The NCC maximum influence on the surface water of the Basin is reached in August. The inflow of the NCC is associated with relatively fresh water intrusions (0.5–2‰ fresher than the surrounding waters) moving from the coast to the central part of the Basin. The NASC and NAFC form two main sources of the Atlantic Water in the Lofoten Basin. At 266 m level, the NASC and NAFC waters dominate water structure in the basin. Herewith the NASC influence prevails over that of the NAFC, the latter being limited to the western periphery of the Basin. At this level, the NCC is observed only along a narrow band following the eastern coast. During summer, the core of the Lofoten Vortex (LV) at 266 m is mainly composed of the NAFC water. This fact contradicts the previous point of view of the dominance of the NASC in the LV core at all depth levels. Using two types of Lagrangian maps, we highlighted the summer and the autumn periods in the LV annual lifecycle. The summer LV is characterized by high orbital velocities, which are several times higher than those of the currents along the basin boundaries. The monthly mean orbital velocities in the LV reach 35 сm s⁻¹. To the end of autumn, the LV weakens with the monthly mean orbital velocities below 10 cm s⁻¹.