Rotating flow past a short,sliced cylinder at finite Rossby number     

K. A. Polzin  M. Angelats I Coll  M. R. Foster 《地球物理与天体物理流体动力学》2013,107(1-2):67-103

Abstract

Flow past a short obstacle in a rotating reference frame generates a wake that is crucial to the overall flow structure if the Rossby number is of the order of the quarter power of the Ekman number. We present here a theory for such flows for the case when the obstacle's top is an oblique, planar surface. The results arise from a combination of asymptotic analysis and numerical computation, and show that even weak asymmetry generates significant global effect on the entire flow-field. Comparisons with the experiments reported by Foster and Davies (1996) are generally good when the high edge is at 90° to the oncoming flow.  相似文献   

The propagation of torsion along flux tubes subject to dynamical nonequilibrium     

E. N. Parker 《地球物理与天体物理流体动力学》2013,107(4):245-272

Abstract

The dynamical nonequilibrium of close-packed flux tubes is driven by the torsion in the individual tubes so that, wherever tubes with the same sense of twisting come into contact, there is reconnection of their azimuthal field components. The reconnection consumes the local torsion, causing the propagation of torsional Alfven waves into the region from elsewhere along the tubes.

The formal problem of the propagation of the torsion along twisted flux tubes is presented and some of the basic physical properties worked out in the limit of small torsion.

It is pointed out that in tubes with finite twisting the propagation of torsional Alfven waves can be a more complicated phenomenon.

Application to the sun suggests that the propagation of torsion from below the visible surface up into the corona is an important energy supply to the corona for a period of perhaps 10–20 hours after the emergence of the flux tubes through the surface of the sun, bringing up torsion from depths of 10⁴km or more. Torsion is continually supplied by the manipulation and shuffling of the field by the convection, of course.  相似文献   

Resistive instabilities in rapidly rotating fluids: Linear theory of the convective modes     

Weijia Kuang  Paul H. Roberts 《地球物理与天体物理流体动力学》2013,107(1-4):129-162

Abstract

This paper analyzes the linear stability of a rapidly-rotating, stratified sheet pinch in a gravitational field, g, perpendicular to the sheet. The sheet pinch is a layer (O ? z ? d) of inviscid, Boussinesq fluid of electrical conductivity σ, magnetic permeability μ, and almost uniform density ρ _o; z is height. The prevailing magnetic field. B _o(z), is horizontal at each z level, but varies in direction with z. The angular velocity, Ω, is vertical and large (Ω ? V_A/d, where V_A = B₀√(μρ₀) is the Alfvén velocity). The Elsasser number, Λ = σB² ₀/2Ωρ₀, measures σ. A (modified) Rayleigh number, R = gβd²/ρ₀V² _A, measures the buoyancy force, where β is the imposed density gradient, antiparallel to g. A Prandtl number, P_K = μσK, measures the diffusivity, k, of density differences.  相似文献   

Convection in a rotating,laterally heated annulus transition to lower symmetry     

E. R. Lewis  E. L. Koschmieder 《地球物理与天体物理流体动力学》2013,107(1-2):37-47

Abstract

The radial temperature differences at which the transition from lower symmetry to the wave regime and the transition from the wave regime to lower symmetry occur have been measured for rotation rates ≦2rad/sec. It was found that the temperature differences at which the transitions occur differ for a fixed rotation rate, depending on whether the radial temperature difference is either increased or decreased with time. There is hysteresis in the transition at lower symmetry.  相似文献   

An experimental investigation of convection in a rotating sphere subject to time varying thermal boundary conditions     

J. A. Chamberlain  C. R. Carrigan 《地球物理与天体物理流体动力学》2013,107(1-4):303-327

Abstract

A series of experiments has been undertaken to investigate the onset of convection in a rapidly rotating fluid filled sphere. The boundary is subjected to a time varying temperature allowing the simulation of radial temperature profiles associated with internal heating. The system is similar to that treated theoretically by Roberts (1968), Busse (1970) and Soward (1977). It is found that Busse's modification of Roberts' linear analysis, taking into account velocity perturbations which are antisymmetric about the equatorial plane, provides a good estimate of the temperature gradient required to initiate convection. As observed in the experiments of Carrigan and Busse (1983) and predicted by linear theory, convection appears in the form of rolls or columns, aligned parallel to the rotation axis. As in earlier experiments, observed azimuthal wavenumbers are consistently smaller than predicted which we postulate to be a consequence of nonlinear effects. Owing to the presence of a centrifugally driven thermal wind, the predicted azimuthal drift of the rolls has not been observed.  相似文献   

小城镇建设的模式选择与布局     

那声润  李双秋 《地理信息世界》2000,(3)

随着经济的发展我国城市建设的发展日益繁荣，其中小城镇建设问题尤显突出，因而其模式选择与布局统一规划，应注重生态和环境保护，培养自身独特的文化特色等方面，使小城镇建设朝着健康的方向发展。  相似文献   

Deep neural network for complex open-water wetland mapping using high-resolution WorldView-3 and airborne LiDAR data     

《International Journal of Applied Earth Observation and Geoinformation》2020

Wetland inventory maps are essential information for the conservation and management of natural wetland areas. The classification framework is crucial for successful mapping of complex wetlands, including the model selection, input variables and training procedures. In this context, deep neural network (DNN) is a powerful technique for remote sensing image classification, but this model application for wetland mapping has not been discussed in the previous literature, especially using commercial WorldView-3 data. This study developed a new framework for wetland mapping using DNN algorithm and WorldView-3 image in the Millrace Flats Wildlife Management Area, Iowa, USA. The study area has several wetlands with a variety of shapes and sizes, and the minimum mapping unit was defined as 20 m² (0.002 ha). A set of potential variables was derived from WorldView-3 and auxiliary LiDAR data, and a feature selection procedure using principal components analysis (PCA) was used to identify the most important variables for wetland classification. Furthermore, traditional machine learning methods (support vector machine, random forest and k-nearest neighbor) were also implemented for the comparison of results. In general, the results show that DNN achieved satisfactory results in the study area (overall accuracy = 93.33 %), and we observed a high spatial overlap between reference and classified wetland polygons (Jaccard index ∼0.8). Our results confirm that PCA-based feature selection was effective in the optimization of DNN performance, and vegetation and textural indices were the most informative variables. In addition, the comparison of results indicated that DNN classification achieved relatively similar accuracies to other methods. The total classification errors vary from 0.104 to 0.111 among the methods, and the overlapped areas between reference and classified polygons range between 87.93 and 93.33 %. Finally, the findings of this study have three main implications. First, the integration of DNN model and WorldView-3 image is useful for wetland mapping at 1.2-m, but DNN results did not outperform other methods in this study area. Second, the feature selection was important for model performance, and the combination of most relevant input parameters contributes to the success of all tested models. Third, the spatial resolution of WorldView-3 is appropriate to preserve the shape and extent of small wetlands, while the application of medium resolution image (30-m) has a negative impact on the accurate delineation of these areas. Since commercial satellite data are becoming more affordable for remote sensing users, this study provides a framework that can be utilized to integrate very high-resolution imagery and deep learning in the classification of complex wetland areas.  相似文献   

多波束成像声纳系统低功耗设计     

崔海英  董琎琎  夏伟杰  窦法旺  蒋鹏飞  金雪 《海洋测绘》2015,35(3):71-74

针对多波束成像声纳系统降低系统功耗提高续航能力的要求,从硬件和算法两个方面展开了对系统的低功耗设计。硬件方面通过采用单电源运算放大器(OPA)电路架构对系统调理模块的放大、滤波链路进行了重新设计,实际测试结果表明实现了14.37W的系统功耗降低。算法方面通过提出一种基于圆阵的旋转多波束形成方法,在完成系统多波束设计的基础上实现了85倍的资源优化效果,为信号处理模块的FPGA芯片低功耗选型提供了技术支撑。  相似文献   

1.2m望远镜激光测距系统同步旋转快门的设计与实现     

何少辉  郑向明 《天文研究与技术》2009,6(4)

介绍了一种对2台步进电机相互间采取等差角同步旋转的控制方案.根据这个方案设计和制作的机电控制电路实现了同步旋转快门功能,最终的同步精度控制在步进电机的性能1.8度/步的范围内,应用在激光测距系统中起到了安全保护单光子探测器的作用.  相似文献   

Dirac spinor scattering states with positive-energy in rotating spheroid models     

Zhi-Fu Gao  Ci-Xing Chen  Na Wang  Xin-Jun Zhao  Zhao-Jun Wang 《Astronomische Nachrichten》2024,345(2-3):e240012

There are many rotating spheroids in the universe, and many astronomers and physicists have used theoretical methods to study the characteristics of stellar gravity since Newton's time. This paper derives the solutions of eight scattering states $({\varphi }^{(0)},{\chi }^{(0)},{\varphi }^{(1)},{\chi }^{(1)},{\varphi }^{(2)}$, ${\chi }^{(2)},{\varphi }^{(3)}$, and ${\chi }^{(3)})$ for the Dirac equation with positive-energy $E=\mathit{im}$, and establishes the relationship between the differential scattering cross section ${\sigma }_i(p,\theta ,\phi )$ and the stellar density $\mu$. It is found that: (1) For the eight scattering states, their average scattering cross-sections $\overline{{\sigma }_i}$ are proportional to ${\mu }^2$, and depend on the star's radius, and the higher the stellar density $\mu$, the greater the sensitivity of $\overline{\sigma \mathit{i}}$ to the change of $\mu$; (2) For the four scattering states ${\chi }^{(i)},i=0,1,2,3$, their average scattering amplitudes $\bar{f}(p,\theta )$ and $\bar{\sigma }(p,\theta )$ depend on the mass $m$ of the particles; while for the other four scattering states ${\varphi }^{(i)}$, $i=0,1,2,3$, then $\bar{f}$ and $\bar{\sigma }$ are independent of $m$. This study links the gravitational characteristics of stars with the scattering cross section, creating a new method for studying the gravitational characteristics, which helps to reveal the mystery of the gravity of rotating ellipsoidal stars.  相似文献