Measuring the non-thermal pressure in early-type galaxy atmospheres: a comparison of X-ray and optical potential profiles in M87 and NGC 1399

We compare the gravitational potential profiles of the elliptical galaxies NGC 4486 (M87) and NGC 1399 (the central galaxy in the Fornax cluster) derived from X-ray and optical data. This comparison suggests that the combined contribution of cosmic rays, magnetic fields and microturbulence to the pressure is ∼10 per cent of the gas thermal pressure in the cores of NGC 1399 and M87, although the uncertainties in our model assumptions (e.g. spherical symmetry) are sufficiently large that the contribution could be consistent with zero. In the absence of any other form of non-thermal pressure support, these upper bounds translate into upper limits on the magnetic field of ∼10–20 μG at a distance of 1–2 arcmin from the centers of NGC 1399 and M87. We show that these results are consistent with the current paradigm of cool cluster cores, based on the assumption that active galactic nuclei regulate the thermal state of the gas by injecting energy into the intracluster medium. The limit of ∼10–20 per cent on the energy density in the form of relativistic protons applies not only to the current state of the gas, but also essentially to the entire history of the intracluster medium, provided that cosmic ray protons evolve adiabatically and that their spatial diffusion is suppressed.     

Interstellar holography

The dynamic spectrum of a radio pulsar is an in-line digital hologram of the ionized interstellar medium. It has previously been demonstrated that such holograms permit image reconstruction, in the sense that one can determine an approximation to the complex electric field values as a function of Doppler shift and delay, but to date the quality of the reconstructions has been poor. Here we report a substantial improvement in the method which we have achieved by simultaneous optimization of the thousands of coefficients that describe the electric field. For our test spectrum of PSR B0834+06 we find that the model provides an accurate representation of the data over the full 63 dB dynamic range of the observations: residual differences between model and data are noise like. The advent of interstellar holography enables detailed quantitative investigation of the interstellar radio-wave propagation paths for a given pulsar at each epoch of observation. We illustrate this using our test data which show the scattering material to be structured and highly anisotropic. The temporal response of the medium exhibits a scattering tail which extends to beyond  100 μs  , and the centroid of the pulse at this frequency and this epoch of observation is delayed by approximately  15 μs  as a result of multipath propagation in the interstellar medium.     

Influence of the anomalous thermal conductivity on the structure of the solar atmosphere transition region

The generalized Wiedemann-Franz law for a nonisothermal quasi-neutral plasma with developed ion-acoustic turbulence and Coulomb collisions has been proven. The results obtained are used to explain the anomalously low thermal conductivity in the chromosphere-corona transition region of the solar atmosphere. Model temperature distributions in the lower corona and the transition region that correspond to well-known experimental data have been determined. The results obtained are useful for explaining the abrupt change in turbulent-plasma temperature at distances smaller than the particle mean free path.     

通风路基通风管管内空气流动特性研究

基于青藏高速公路试验工程的实测资料,利用数值模拟系统分析了通风路基通风管内的空气流动特性和变化规律.研究表明,当外界环境风速一定时,通风管内不同区域流动特征显著不同,表现为较为明显的三段性：在入口扰动区段,管内空气紊流强度最大;中部为完全湍流区,紊流强度最弱;出口射流影响区的紊流强度介于入口扰动区和完全湍流区之间.随着外界环境风速的增加,管内空气流速相对环境风速增加的量值随之增加,总体呈现非线性快速增加的趋势;入口扰动响应的长度和紊流强度增加最显著;尾部射流影响区由于受到空气流动惯性等因素影响,其长度以非线性加速减小,其空气运动轨迹的振动幅度以非线性加速提高.管内各区域空气流动特性的不同是造成通风管不同部位局部换热量差异的根本原因,这些流动特性的存在进一步叠加,或是导致通风管路基阴阳坡效应的重要因素之一.     

Mean and turbulent flow fields in a simulated ice‐covered channel with a gravel bed: some laboratory observations

Northern rivers experience freeze‐up over the winter, creating asymmetric under‐ice flows. Field and laboratory measurements of under‐ice flows typically exhibit flow asymmetry and its characteristics depend on the presence of roughness elements on the ice cover underside. In this study, flume experiments of flows under a simulated ice cover are presented. Open water conditions and simulated rough ice‐covered flows are discussed. Mean flow and turbulent flow statistics were obtained from an Acoustic Doppler Velocimeter (ADV) above a gravel‐bed surface. A central region of faster flow develops in the middle portion of the flow with the addition of a rough cover. The turbulent flow characteristics are unambiguously different when simulated ice covered conditions are used. Two distinct boundary layers (near the bed and in the vicinity of the ice cover, near the water surface) are clearly identified, each being characterized by high turbulent intensity levels. Detailed profile measurements of Reynolds stresses and turbulent kinetic energy indicate that the turbulence structure is strongly influenced by the presence of an ice cover and its roughness characteristics. In general, for y/d > 0·4 (where y is height above bed and d is local flow depth), the addition of cover and its roughening tends to generate higher turbulent kinetic energy values in comparison to open water flows and Reynolds stresses become increasingly negative due to increased turbulence levels in the vicinity of the rough ice cover. The high negative Reynolds stresses not only indicate high turbulence levels created by the rough ice cover but also coherent flow structures where quadrants one and three dominate. Copyright © 2012 John Wiley & Sons, Ltd.     

河口底边界层湍流观测后处理技术方法分析

河口底边界层过程是河口海岸研究与工程应用中的重要内容。三维点式高频流速仪(ADV)已经成为湍流现场观测的最有效的工具之一,然而受测量状态、复杂的波流环境、底床几何结构等因素的影响,湍流观测的后处理目前还不成熟。在前人工作的基础上,提出了河口底边界层湍流观测后处理的综合技术方法,包括测量状态判断、数据质量检测、坐标系旋转、去除毛刺及滤波,探讨了这些处理方法中的某些步骤及处理顺序对湍流参数估算可能产生的影响,提出了综合后处理技术的准确性评估方法。该研究对于近岸海洋湍流混合、泥沙输运等重要问题的解决可以提供较为扎实的技术支持。     

应用切割单元法对海底管道局部冲刷数值模拟

采用SIMPLEC算法的有限体积法,求解非定常流动的N-S方程,采用k-ω紊流模型,通过模拟均匀无粘性推移质的冲刷和淤积,得到海底管道由搁置在海床上的状态变为悬跨状态这一过程的管道周围局部海床冲刷情况,建立了海底管道局部冲刷的二维数值模型。海底管道从搁置在底床上到冲刷悬空的过程中,管道周围的空间产生了拓扑变化,这给采用贴体网格并在计算过程中进行网格重构的传统方法带来了很大困难,而采用切割单元法,把物体轮廓从静止的背景直角坐标结构化网格中切割出去,计算过程中不需要传统意义的网格重构过程。数值模型预测的海底管道周围局部冲刷结果与Mao的物理模型实验实测结果及Liang和Cheng等的数值模拟结果符合较好,验证了模型的准确性。     

Effects of shear in the convective boundary layer: analysis of the turbulent kinetic energy budget

Effects of convective and mechanical turbulence at the entrainment zone are studied through the use of systematic Large-Eddy Simulation (LES) experiments. Five LES experiments with different shear characteristics in the quasi-steady barotropic boundary layer were conducted by increasing the value of the constant geostrophic wind by 5 m s^-1 until the geostrophic wind was equal to 20 m s^-1. The main result of this sensitivity analysis is that the convective boundary layer deepens with increasing wind speed due to the enhancement of the entrainment heat flux by the presence of shear. Regarding the evolution of the turbulence kinetic energy (TKE) budget for the studied cases, the following conclusions are drawn: (i) dissipation increases with shear, (ii) the transport and pressure terms decrease with increasing shear and can become a destruction term at the entrainment zone, and (iii) the time tendency of TKE remains small in all analyzed cases. Convective and local scaling arguments are applied to parameterize the TKE budget terms. Depending on the physical properties of each TKE budget contribution, two types of scaling parameters have been identified. For the processes influenced by mixed-layer properties, boundary layer depth and convective velocity have been used as scaling variables. On the contrary, if the physical processes are restricted to the entrainment zone, the inversion layer depth, the modulus of the horizontal velocity jump and the momentum fluxes at the inversion appear to be the natural choices for scaling these processes. A good fit of the TKE budget terms is obtained with the scaling, especially for shear contribution.     

Stable-boundary-layer regimes from the perspective of the low-level jet

This paper reviews results from two field studies of the nocturnal stable atmospheric boundary layer (SBL) over the Great Plains of the United States. Data from a scanning remote-sensing system, a High-Resolution Doppler Lidar (HRDL), provided measurements of mean and turbulent wind components at high spatial and temporal resolution through the lowest 500–1000 m of the atmosphere. This data set has allowed the characteristics of the low-level jet (LLJ) maximum (speed, height, direction) to be documented through entire nights. LLJs form after sunset and produce strong shear in the layer below the LLJ maximum or nose, which is a source of turbulence and mixing in the SBL. Simultaneous HRDL measurements of turbulence quantities related to turbulence kinetic energy (TKE) has allowed the turbulence in the subjet layer to be related to LLJ properties. Turbulence structure was found to be a function of the bulk stability of the subjet layer. For the strong-LLJ (> 15 m s⁻¹), weakly stable cases the strength of the turbulence is proportional to the strength of the LLJ. For these cases with nearly continuous turbulence in the subjet layer, low-level jet scaling, in which lengths are scaled by the LLJ height and velocity variables are scaled by the LLJ speed, was found to be appropriate. For the weak-wind (< 5 m s⁻¹ in the lowest 200 m), very stable boundary layer (vSBL), the boundary layer was found to be very shallow (sometimes < 10 m deep), and turbulent fluxes between the earth’s surface and the atmosphere were found to be essentially shut down. For more intermediate wind speeds and stabilities, the SBL shows varying degrees of intermittency due to various mechanisms, including shearinstability and other gravity waves, density currents, and other mesoscale disturbances.     

Advances in the Study of Moving Sediments and Evolving Seabeds

Sands and mud are continually being transported around the world’s coastal seas due to the action of tides, wind and waves. The transport of these sediments modifies the boundary between the land and the sea, changing and reshaping its form. Sometimes the nearshore bathymetry evolves slowly over long time periods, at other times more rapidly due to natural episodic events or the introduction of manmade structures at the shoreline. For over half a century we have been trying to understand the physics of sediment transport processes and formulate predictive models. Although significant progress has been made, our capability to forecast the future behaviour of the coastal zone from basic principles is still relatively poor. However, innovative acoustic techniques for studying the fundamentals of sediment movement experimentally are now providing new insights, and it is expected that such observations, coupled with developing theoretical works, will allow us to take further steps towards the goal of predicting the evolution of coastlines and coastal bathymetry. This paper presents an overview of our existing predictive capabilities, primarily in the field of non-cohesive sediment transport, and highlights how new acoustic techniques are enabling our modelling efforts to achieve greater sophistication and accuracy. The paper is aimed at coastal scientists and managers seeking to understand how detailed physical studies can contribute to the improvement of coastal area models and, hence, inform coastal zone management strategies.