金牛T星在慢引力收缩阶段抛射物质和引力收缩作用下质量和半径的演化时标及其对自转角速度变化的影响

研究了带有引力能源和抛射物质的引力收缩星如金牛T星处于慢引力收缩阶段后,在抛射物质和引力收缩联合作用下对质量和半径的改变及其对自转角速度变化产生的影响。给出了质量和半径随时间演变的联立微分方程组及其解。利用解计算了金牛T星的质量和半径的演化时标以及对自转角速度变化产生的影响。给出数值结果,并讨论了理论和数值结果。

The effect of sex and performance level on pacing in cross-country skiers:Vasaloppet 2004-2017

Background： Pacing, defined as percentage changes of speed between successive splits, has been extensively studied in running and cycling endurance sports; however, less information about the trends in change of speed during cross-country （XC） ski racing is available. Therefore, the aim of the present study was to examine the effect of performance （quartiles of race time （Q）, with Q1 the fastest and Q4 the slowest） level on pacing in the Vasaloppet ski race, the largest XC skiing race in the world. Methods： For this purpose, we analyzed female （n = 19,465） and male （n = 164,454） finishers in the Vasaloppet ski race from 2004 to 2017 using a one-way （2 sexes） analysis of variance with repeated measures to examine percentage changes of speed between 2 successive splits. Overall, the race consisted of 8 splits. Results： The race speeds of Q1, Q2, Q3, and Q4 were 13.6 ± 1.8, 10.6 ± 0.5, 9.2 ± 0.3, and 8.1 ±- 0.4 km/h, respectively, among females and 16.7 ± 1.7, 13.1 ± 0.7, 10.9 ± 0.6, and 8.9 ± 0.7 km/h, respectively, among males. The overall pacing strategy of finishers was variable. A small sex × split interaction on speed was observed （η^2= 0.016, p 〈 0.001）, with speed difference between sexes ranging from 14.9% （Split 7） to 27.0% （Split 1） and larger changes in speed between 2 successive splits being shown for females （p 〈 0.001, η^2=0.004）. A large performance × split interaction on speed, with Q1 presenting the smallest changes of speed between splits, was shown for females （η^2= 0.149, p 〈 0.001） and males （η^2 = 0.169,p 〈 0.001）. Conclusion： Male and fast XC skiers are more even pacers. Coaches and athletes should develop tailored sex- and performance-level pacing strategies; for instance, they should advise fast XC skiers to start fast and maintain their speed, rather than starting slowly and trying to make up time by going faster at times during the race.2018 Published by Elsevier B.V. on behalf of Shanghai University of Sport. This is an open access article under the CC BY-NC-ND license. （http：//creativecommons.org/licenses/by-nc-nd/4.0/）.

Effi cient modeling of the gravity anomaly caused by a sedimentary basin with lateral variable density contrast and its application in basement relief estimation

The forward calculation of gravity anomalies is a non-negligible aspect contributing to the time consumption of the entire process of basement relief estimation.In this study,we develop a fast hybrid computing scheme to compute the gravity anomaly of a basement.We use the vertical prism source equation in a given region R centered at a certain gravity observation point and the vertical line source equation outside R to derive the gravity anomaly.We observe that the computation with the vertical line source equation is much faster than that of the vertical prism source equation,but the former is slightly inaccurate.Therefore,our method is highly effi cient and able to avoid the errors caused by the low accuracy of the vertical line source equation near the observation point.We then derive the general principle of choosing the size of R via a series of prism model tests.Our tests on the gravity anomaly over the Los Angeles Basin confirm the correctness of our proposed forward strategy.We modify Bott’s method with an accelerating factor to expedite the inversion procedure and presume that the density contrast between the sediments and the basement in a sedimentary basin varies laterally and can be obtained using the equivalent equation.Synthetic data and real data applications in the Weihe Basin illustrate that our proposed method can accurately and effi ciently estimate the basement relief of sedimentary basins.

Eddy generation and evolution in the North Pacific Subtropical Countercurrent(NPSC) zone

The seasonal generation and evolution of eddies in the region of the North Pacific Subtropical Countercurrent remain poorly understood due to the scarcity of available data.We used TOPEX/POSEIDON altimetry data from 1992 to 2007 to study the eddy field in this zone.We found that velocity shear between this region and the neighboring North Equatorial Current contributes greatly to the eddy generation.Furthermore,the eddy kinetic energy level(EKE) shows an annual cycle,maximum in April/May and minimum in December/January.Analyses of the temporal and spatial distributions of the eddy field revealed clearly that the velocity shear closely related to baroclinic instability processes.The eddy field seems to be more zonal than meridional,and the energy containing length scale shows a surprising lag of 2-3 months in comparison with the 1-D and 2-D EKE level.A similar phenomenon is observed in individual eddies in this zone.The results show that in this eddy field band,the velocity shear may drive the EKE level change so that the eddy field takes another 2-3 months to grow and interact to reach a relatively stable state.This explains the seasonal evolution of identifiable eddies.

Flow stress behavior of high-purity Al-Cu-Mg alloy and microstructure evolution

The flow stress behavior of high-purity Al-Cu-Mg alloy under hot deformation conditions was studied by Gleeble-1500,with the deformation temperature range from 300 to 500 °C and the strain rate range from 0.01 to 10 s-1. From the true stress-true strain curve, the flow stress increases with the increasing of strain and tends to be constant after a peak value, showing dynamic recover, and the peak value of flow stress increases with the decreasing of deformation temperature and the increasing of strain rate.When the strain rate is 10 s-1 and the deformation temperature is higher than 400 °C, the flow stress shows dynamic recrystallization characteristic. TEM micrographs were used to reveal the evolution of microstructures. According to the processing map at true strain of 0.7, the feasible deformation conditions are high strain rate(>0.5 s-1) or 440-500 °C and 0.01-0.02 s-1.

零售业态演变规律的综合模型构建

本文通过考察已有的零售业态演变理论或模型和零售业态的过去、现状,从零售企业或政府的决策考虑入手,力图构建一个零售业态演变的综合模型,对零售业态演变规律给出一个系统性、全方位的回答。本文追述了零售业态演变方面的相关文献,指出该综合模型构建的必要性和立足点;构建了一个零售业态演变的综合模型,并加以具体说明该模型中的各部分以及各要素之间的关系;也指出该模型的不足以及后续研究的重点。

Evolution of Unsteady Flow near Rotor Tip during Stall Inception

Previously the features of circumferential propagation of self-induced tip leakage flow unsteadiness for a low speed isolated axial compressor rotor in the authors' laboratory were discovered and investigated via numerical simulation,which only occurs below a critical stable flow point that is close to but not yet at the stall limit.Further in this paper,the detailed investigation on evolution of tip leakage flow during the throttling process into spike rotating stall was conducted by adopting the valve-throttling model.During this process,the development of the circumferential propagation of tip leakage flow unsteadiness was especially focused on.According to the unsteady characteristics of pressure signals,the evolvement of compressor flow field can be classified into four stages.As compressor throttled,the oscillation frequency of self-induced unsteady tip leakage flow decreased gradually,and thus resulted in the decrease of its circumferential propagation speed.The circumferential propagation of self-induced tip leakage flow unsteadiness is closely related with rotating instability.When the forward spillage of tip leakage flow at the leading edge occurred,the spike type rotating stall was initiated.Its flow struc-tures were given in the paper.