Numerical Experiments of Isolated Convection under Polynya     

Naosuke?OkadaEmail author  Motoyoshi?Ikeda  Shoshiro?Minobe 《Journal of Oceanography》2004,60(6):927-943

The dense water formation process under polynya or lead is examined by numerical experiments using a three-dimensional non-hydrostatic model. Many numerical experiments on isolated convection in an initially homogeneous fluid have been performed for different sets of external parameters, in order to investigate a relationship between the convection process and the external parameters. The main focus is on the situation in which the horizontal length scale of disk-shaped buoyancy forcing (radius R) is comparable with the total water depth (H). The two dynamical regimes described in previous work—the baroclinically unstable convection and the baroclinically stable convection—are confirmed in the experiments. A horizontal shift of a convective chimney is important to a density anomaly in baroclinically stable convection. For the stable range, as R is reduced R/H < 0.7, a new regime is found, called “single-plume convection”, in which multiple convective plumes do not fully develop, and the density anomaly scale has nearly no dependency on R. This change of dependency on R is consistent with that derived by scaling analysis. The non-hydrostatic component is more significant than the hydrostatic one in the single-plume convection. The information obtained is useful for parameterizing dense water formation under ice cover in a numerical model with a large grid size; i.e., the newly formed water has a density anomaly independent of the polynya size smaller than the water depth, while the anomaly increases as the size exceeds the depth.  相似文献   

Equation of state of neutron star cores and spin down of isolated pulsars     

P. Haensel  J. L. Zdunik 《Astrophysics and Space Science》2007,308(1-4):363-369

We study possible impact of a softening of the equation of state by a phase transition, or appearance of hyperons, on the spin evolution of isolated pulsars. Numerical simulations are performed using exact 2-D simulations in general relativity. The equation of state of dense matter at supranuclear densities is poorly known. Therefore, the accent is put on the general correlations between evolution and equation of state, and mathematical strictness. General conjectures referring to the structure of the one-parameter families of stationary configurations are formulated. The interplay of the back bending phenomenon and stability with respect to axisymmetric perturbations is described. Changes of pulsar parameters in a corequake following instability are discussed, for a broad choice of phase transitions predicted by different theories of dense matter. The energy release in a corequake, at a given initial pressure, is shown to be independent of the angular momentum of collapsing configuration. This result holds for various types of phases transition, with and without metastability. We critically review observations of pulsars that could be relevant for the detection of the signatures of the phase transition in neutron star cores. This work was partially supported by the Polish MNiI Grant no. 1P03D-008-27.  相似文献   

Precession as a probe of the neutron star interior     

Bennett Link 《Astrophysics and Space Science》2007,308(1-4):435-441

Strong evidence that some neutron stars precess (nutate) with long periods (∼1 yr) challenges our current understanding of the neutron star interior. I describe how neutron star precession can be used to constrain the state of the interior in a new way. I argue that the standard picture of the outer core, in which superfluid neutrons coexist with type II, superconducting protons, requires revision. One possible resolution is that the protons are not type II, but type I. Another possibility is that the neutrons are normal in the outer core. I conclude with a brief discussion of the implications for detectable gravitational wave emission from millisecond pulsars. Much of the work described here was supported by the National Science Foundation under Grant AST-00098728.  相似文献   

Mountains on neutron stars     

B. Haskell 《Astrophysics and Space Science》2007,308(1-4):541-543

The aim of this work is to compare a neutron star with an accreted crust and one with a non-accreted crust, and estimate which one is potentially a better source of gravitational waves (i.e. can sustain a larger “mountain”). To do this we present a new formalism, and find that a non-accreted crust can sustain a slightly larger “mountain”. We also discuss the importance of relativistic effects.   相似文献   

Internal heating and thermal emission from old neutron stars     

Andreas Reisenegger  Rodrigo Fernández  Paula Jofré 《Astrophysics and Space Science》2007,308(1-4):413-418

The equilibrium composition of neutron star matter is achieved through weak interactions (direct and inverse beta decays), which proceed on relatively long time scales. If the density of a matter element is perturbed, it will relax to the new chemical equilibrium through non-equilibrium reactions, which produce entropy that is partly released through neutrino emission, while a similar fraction heats the matter and is eventually radiated as thermal photons. We examined two possible mechanisms causing such density perturbations: (1) the reduction in centrifugal force caused by spin-down (particularly in millisecond pulsars), leading to rotochemical heating, and (2) a hypothetical time-variation of the gravitational constant, as predicted by some theories of gravity and current cosmological models, leading to gravitochemical heating. If only slow weak interactions are allowed in the neutron star (modified Urca reactions, with or without Cooper pairing), rotochemical heating can account for the observed ultraviolet emission from the closest millisecond pulsar, PSR J0437-4715, which also provides a constraint on |dG/dt| of the same order as the best available in the literature. This work made use of NASA’s Astrophysics Data System Service, and received financial support from FONDECYT through regular grants 1020840 and 1060644.  相似文献   

Performance of dense digital surface models based on image matching in the estimation of plot-level forest variables     

《ISPRS Journal of Photogrammetry and Remote Sensing》2013

Recent research results have shown that the performance of digital surface model extraction using novel high-quality photogrammetric images and image matching is a highly competitive alternative to laser scanning. In this article, we proceed to compare the performance of these two methods in the estimation of plot-level forest variables. Dense point clouds extracted from aerial frame images were used to estimate the plot-level forest variables needed in a forest inventory covering 89 plots. We analyzed images with 60% and 80% forward overlaps and used test plots with off-nadir angles of between 0° and 20°. When compared to reference ground measurements, the airborne laser scanning (ALS) data proved to be the most accurate: it yielded root mean square error (RMSE) values of 6.55% for mean height, 11.42% for mean diameter, and 20.72% for volume. When we applied a forward overlap of 80%, the corresponding results from aerial images were 6.77% for mean height, 12.00% for mean diameter, and 22.62% for volume. A forward overlap of 60% resulted in slightly deteriorated RMSE values of 7.55% for mean height, 12.20% for mean diameter, and 22.77% for volume. According to our results, the use of higher forward overlap produced only slightly better results in the estimation of these forest variables. Additionally, we found that the estimation accuracy was not significantly impacted by the increase in the off-nadir angle. Our results confirmed that digital aerial photographs were about as accurate as ALS in forest resources estimation as long as a terrain model was available.  相似文献   

稠油注蒸汽热采过程中的水化学问题     

李雨新 《地球科学与环境学报》1993,(4)

本文简要论述了稠油注蒸汽采过程中所引发出的水化学问题,指出:在注蒸汽热采过程中,随着蒸汽的注入,将在储层内自注热井向四周逐渐形成一些连续过渡的温、压、pH值和水相状态不同的汽水浸润带,即蒸汽饱和带→蒸汽过热水混合带→过热水带→与储层温压条件相当的热水带,由于各带内状态条件的不同,故各带内所引发出的水-岩作用(包括矿物的溶解与沉淀作用)将是有所不同的,因而对各带内储层孔隙结构的影响程度也将是不同的。文中还扼要地介绍了对此进行水化学研究的方法原理和研究途径。  相似文献   

江苏一次冬季强浓雾天气持续和消散诊断分析   总被引：3，自引：3，他引：0  

马明明  潘婧茹  张雪蓉  濮梅娟 《气象科学》2016,36(3):396-402

本文利用常规气象观测资料、NCEP再分析资料及加密自动气象站资料对2006年12月24—27日江苏省持续了4 d的大雾天气进行分析,重点对强浓雾天气的持续和消散特征进行了诊断研究。结果表明:此次雾在辐射降温条件下形成,在暖平流作用下辐射雾转为平流雾并且增强;暖湿平流为雾的发展提供了有利的水汽和热力条件;中高层辐合下沉增温和低层弱上升运动,使得逆温层稳定维持,是浓雾持续不消有利的动力条件;雾消散阶段,低层正涡度平流南下,使大气层结出现位势不稳定,伴随着冷空气入侵,辐散下沉,垂直运动相应增大,逆温层彻底被破坏,动力和热力条件相配合最终导致雾消散。  相似文献   

Wave equation analyses of tapered FRP–concrete piles in dense sand     

Mohammed Sakr  M. Hesham El Naggar  Moncef Nehdi 《Soil Dynamics and Earthquake Engineering》2007,27(2):166-182

Fiber-reinforced polymers (FRP)–concrete composites provide an attractive alternative to conventional pile materials such as steel, concrete and wood by improving the durability of deep foundations. In the current study, FRP tubes with different taper angles are filled with self-consolidating concrete (SCC) and driven into dense sand that is enclosed in a large pressurized soil chamber. Driving tests are conducted on FRP–SCC composite piles to determine how the pile material and geometric configuration affect its driving performance. Dynamic data is employed to determine the soil parameters in the TNO model (i.e., soil quake and damping constant) using the DLTWAVE signal-matching program. The driveability of FRP–SCC and traditional pile materials is compared using the wave equation analysis program PDPWAVE. The experimental data and the wave equation analyses indicate that the taper shape has a favourable effect on the driveability and static resistance of piles. It is also found that the driveability of FRP–SCC composite piles is similar to that of conventional prestressed concrete and steel piles. However, empty FRP tubes required a much higher driving energy. Their low flexural resistance along with risk of buckling can hinder their driveability in different soil conditions.  相似文献   

近10年武汉市大雾变化特征及2006年一次大雾个例分析   总被引：1，自引：0，他引：1  

李才媛  韦惠红  王东阡 《湖北气象》2007,26(3):241-245

为了了解近一个时期武汉市大雾的时空变化特征、大气环流背景及有关物理要素场的分布情况,根据1995～2005年武汉市大雾观测记录,统计分析了这一时期武汉市大雾的变化特点,并对2006年3月10日出现在武汉市的一次大雾天气过程进行了分析。结果表明:近11年来,武汉市大雾日数继续下降,冬季大雾出现频率最高,春、秋季大雾频率次之,大雾持续时间明显延长;高空弱的偏西气流,中低层弱的暖湿气流,近地面有逆温层,地面为均压场等条件,是大雾形成的有利环流形势。  相似文献