天文学研究新成果(1) 美国爱荷华大学(University of Iowa)的研究人员提供了一种理论来解释宇宙是如何成为充满光亮的,他们的研究结果已经在《皇家天文学会月纪》( Monthly Notices of the Royal Astronomical Society )杂志网站发表—— P. Kaaret, M. Brorby, L. Casella, A. H. Prestwich. Resolving the X-ray emission from the Lyman-continuum emitting galaxy Tol 1247-232. Monthly Notices of the Royal Astronomical Society, 2017, 471(4): 4234–4238 . DOI: 10.1093/mnras/stx1945 . Abstract Chandra observations of the nearby, Lyman-continuum (LyC) emitting galaxy Tol 1247-232 resolve the X-ray emission and show that it is dominated by a point-like source with a hard spectrum (Γ = 1.6 ± 0.5) and a high luminosity . Comparison with an earlier XMM–Newton observation shows flux variation of a factor of 2. Hence, the X-ray emission likely arises from an accreting X-ray source: a low-luminosity active galactic nucleus or one or a few X-ray binaries. The Chandra X-ray source is similar to the point-like, hard spectrum (Γ = 1.2 ± 0.2), high-luminosity (10 41 erg s − 1 ) source seen in Haro 11, which is the only other confirmed LyC-emitting galaxy that has been resolved in X-rays. We discuss the possibility that accreting X-ray sources contribute to LyC escape. 更多信息请注意浏览原文或者相关报道 ; Researchers propose how the universe became filled with light August 30, 2017 Credit: CC0 Public Domain Soon after the Big Bang, the universe went completely dark. The intense, seminal event that created the cosmos churned up so much hot, thick gas that light was completely trapped. Much later—perhaps as many as one billion years after the Big Bang—the universe expanded, became more transparent, and eventually filled up with galaxies, planets, stars, and other objects that give off visible light. That's the universe we know today. How it emerged from the cosmic dark ages to a clearer, light-filled state remains a mystery. In a new study, researchers at the University of Iowa offer a theory of how that happened. They think black holes that dwell in the center of galaxies fling out matter so violently that the ejected material pierces its cloudy surroundings, allowing light to escape. The researchers arrived at their theory after observing a nearby galaxy from which ultraviolet light is escaping. The observations show the presence of very bright X-ray sources that are likely accreting black holes, says Philip Kaaret, professor in the UI Department of Physics and Astronomy and corresponding author on the study. It's possible the black hole is creating winds that help the ionizing radiation from the stars escape. Thus, black holes may have helped make the universe transparent. Kaaret and his team focused on a galaxy called Tol 1247-232, located some 600 million light years from Earth, one of only three nearby galaxies from which ultraviolet light has been found to escape. In May 2016, using an Earth-orbiting telescope called Chandra, the researchers saw a single X-ray source whose brightness waxed and waned and was located within a vigorous star-forming region of Tol 1247-232. The team determined it was something other than a star. Stars don't have changes in brightness, Kaaret says. Our sun is a good example of that. To change in brightness, you have to be a small object, and that really narrows it down to a black hole, he says. But how would a black hole, whose intense gravitational pull sucks in everything around it, also eject matter? The quick answer is no one knows for sure. Black holes, after all, are hard to study, in part because their immense gravitational pull allows no light to escape and because they're embedded deep within galaxies. Recently, however, astronomers have offered an explanation: The jets of escaping matter are tapping into the accelerated rotational energy of the black hole itself. Imagine a figure skater twirling with outstretched arms. As the skater folds her arms closer to her body, she spins faster. Black holes operate much the same way: As gravity pulls matter inward toward a black hole, the black hole likewise spins faster. As the black hole's gravitational pull increases, the speed also creates energy. As matter falls into a black hole, it starts to spin and the rapid rotation pushes some fraction of the matter out, Kaaret says. They're producing these strong winds that could be opening an escape route for ultraviolet light. That could be what happened with the early galaxies. Kaaret plans to study Tol 1247-232 more closely and find other nearby galaxies that are leaking ultraviolet light, which would help corroborate his theory. Explore further: Image: Computer simulation of a supermassive black hole 天文学研究新成果(2) 遥远的星系发出15种高能无线电脉冲 Distant galaxy sends out 15 high-energy radio bursts August 30, 2017 by Robert Sanders A sequence of 14 of the 15 detected bursts illustrate their dispersed spectrum and extreme variability. The streaks across the colored energy plot are the bursts appearing at different times and different energies because of dispersion caused by 3 … more Breakthrough Listen, an initiative to find signs of intelligent life in the universe, has detected 15 brief but powerful radio pulses emanating from a mysterious and repeating source – FRB 121102 – far across the universe. Fast radio bursts are brief, bright pulses of radio emission from distant but largely unknown sources, and FRB 121102 is the only one known to repeat: more than 150 high-energy bursts have been observed coming from the object, which was identified last year as a dwarf galaxy about 3 billion light years from Earth. Possible explanations for the repeating bursts range from outbursts from rotating neutron stars with extremely strong magnetic fields – so-called magnetars – to a more speculative idea: They are directed energy sources, powerful laser bursts used by extraterrestrial civilizations to power spacecraft, akin to Breakthrough Starshot's plan to use powerful laser pulses to propel nano-spacecraft to Earth's nearest star, Proxima Centauri. Bursts from this source have never been seen at this high a frequency, said Andrew Siemion, director of the Berkeley SETI Research Center and of the Breakthrough Listen program. As astronomers around the globe try to understand the mechanism generating fast radio bursts, they have repeatedly turned their radio telescopes on FRB 121102. Siemion and his team alerted the astronomical community to the high-frequency activity via an Astronomer's Telegram on Monday evening, Aug. 28. As well as confirming that the source is in a newly active state, the high resolution of the data obtained by the Listen instrument will allow measurement of the properties of these mysterious bursts at a higher precision than ever possible before, said Breakthrough Listen postdoctoral researcher Vishal Gajjar, who discovered the increased activity. First detected with the Parkes Telescope in Australia, fast radio bursts have now been seen by several radio telescopes around the world. FRB 121102 was discovered on Nov. 2, 2012, (hence its name) and in 2015 it was the first fast radio burst seen to repeat, ruling out theories of bursts' origins that involved the catastrophic destruction of the progenitor, at least in this instance. Regardless of FRB 121102's ultimate source, when the recently detected pulses left their host galaxy, our solar system was less than 2 billion years old, noted Steve Croft, a Breakthrough Listen astronomer at UC Berkeley. Life on Earth consisted only of single-celled organisms; it would be another billion years before even the simplest multi-cellular life began to evolve. As part of Breakthrough Listen's program to observe nearby stars and galaxies for signatures of extraterrestrial technology, the project science team at UC Berkeley added FRB 121102 to its list of targets. In the early hours of Saturday, Aug. 26, Gajjar observed that area of the sky using the Breakthrough Listen backend instrument at the Green Bank Telescope in West Virginia. The instrument accumulated 400 terabytes (a million million bytes) of data over a five-hour period, observing across the entire 4 to 8 GHz frequency band. This large dataset was searched for signatures of short pulses from the source over a broad range of frequencies, with a characteristic dispersion, or delay as a function of frequency, caused by the presence of gas in space between Earth and the source. The distinctive shape that the dispersion imposes on the initial pulse is an indicator of the amount of material between us and the source, and hence an indicator of the distance to the host galaxy. Analysis by Gajjar and the Breakthrough Listen team revealed 15 new pulses from FRB 121102. The observations show for the first time that fast radio bursts emit at higher frequencies than previously observed, with the brightest emission occurring at around 7 GHz. The extraordinary capabilities of the backend receiver, which is able to record several gigahertz of bandwidth at a time, split into billions of individual channels, enable a new view of the frequency spectrum of FRBs, and should shed additional light on the processes giving rise to FRB emission. Gajjar said. Whether or not fast radio bursts turn out to be signatures of extraterrestrial technology, Breakthrough Listen is helping to push the frontiers of a new and rapidly growing area of our understanding of the universe around us, Siemion said. Explore further: Mysterious bursts of energy do come from outer space 天文学研究新成果(3) 天体物理学家将土星的卫星和土星环转变为音乐 Astrophysicists convert moons and rings of Saturn into music August 30, 2017 The orbital periods, scaled frequencies, and musical notes of Saturn's major moons. The frequencies have been increased by 27 octaves from their true values by astrophysicists at the University of Toronto so they can be heard by human ears. Credit: SYSTEM Sounds/NASA/JPL-Caltech/Elisabetta Bonora/Marco Faccin After centuries of looking with awe and wonder at the beauty of Saturn and its rings, we can now listen to them, thanks to the efforts of astrophysicists at the University of Toronto (U of T). To celebrate the Grand Finale of NASA's Cassini mission next month, we converted Saturn's moons and rings into two pieces of music, says astrophysicist Matt Russo, a postdoctoral researcher at the Canadian Institute for Theoretical Astrophysics (CITA) in the Faculty of Arts Science at U of T. The conversion to music is made possible by orbital resonances, which occur when two objects execute different numbers of complete orbits in the same time, so that they keep returning to their initial configuration. The rhythmic gravitational tugs between them keep them locked in a tight repeating pattern which can also be converted directly into musical harmony. Wherever there is resonance there is music, and no other place in the solar system is more packed with resonances than Saturn, says Russo. The Cassini spacecraft has been collecting data while orbiting Saturn since its arrival in 2004 and is now in the throes of a final death spiral. It will plunge into the planet itself on September 15 to avoid contaminating any of its moons. The orbital periods of the six 1st order resonances of Janus that affect the ring system. The 1:1 resonance is with Janus' co-orbital moon Epimetheus. The corresponding frequencies of these resonances were scaled up by 23 octaves by astrophysicists at the University of Toronto, producing a musical scale. Credit: SYSTEM Sounds/NASA/JPL/Space Science Institute Russo was joined by astrophysicist Dan Tamayo, a postdoctoral researcher at CITA and the Centre for Planetary Sciences at U of T Scarborough, and together they were able to play music with an instrument measuring over a million kilometers long. The musical notes and rhythms both come from the orbital motion of Saturn's moons along with the orbits of the trillions of small particles that make up the ring system. Saturn's magnificent rings act like a sounding board that launches waves at locations that harmonize with the planet's many moons, and some pairs of moons are themselves locked in resonances, says Tamayo. Music of the moons and rings For the first piece which follows Cassini's final plunge, the researchers increased the natural orbital frequencies of Saturn's six large inner moons by 27 octaves to arrive at musical notes. What you hear are the actual frequencies of the moons, shifted into the human hearing range says Russo. The team then used a state of the art numerical simulation of the moon system developed by Tamayo to play the resulting notes every time a moon completes an orbit. The moon system has two orbital resonances which give rhythmic and harmonic structure to the otherwise unsteady lullaby-style melody. The first and third moons Mimas and Tethys are locked in a 2:1 resonance so that Mimas orbits twice for every orbit of Tethys. The same relationship links the orbits of the second and fourth moons Enceledus and Dione, and the combination of the two simple rhythms creates interesting musical patterns as they fall in and out of synchronicity. A wood carving of Saturn's main ring system designed for the visually impaired, commissioned by astrophysicists at the University of Toronto. One will be able to feel many complex structures within the rings while also listening to their audio form. Credit: SYSTEM Sounds Since doubling the frequency of a note produces the same note an octave higher, the four inner moons produce only two different notes close to a perfect fifth apart, says Russo, who is also a graduate of U of T's Jazz performance program. The fifth moon Rhea completes a major chord that is disturbed by the ominous entrance of Saturn's largest moon , Titan. Russo and Tamayo are joined in the project by Toronto musician, and Matt's long-time bandmate, Andrew Santaguida. Dan understands orbital resonances as deeply as anyone and Andrew is a music production wizard. My job is to connect these two worlds. Titan actually gives the Cassini probe the final push which sends it hurtling towards its death in the heart of Saturn. The music follows Cassini's final flight over the ring system by converting the constantly increasing orbital frequencies of the rings into a dramatic rising pitch; the volume of the tone increases and decreases along with the observed bright and dark bands of the rings. The death of Cassini as it crashes into Saturn is heard as a final crash of a final piano chord, which was inspired by The Beatles' A Day in the Life, in which a rich major chord follows a similarly tense crescendo. In addition to the soundtrack, Russo has had a large wood carving made of Saturn's rings so people can follow along with their fingertips while listening. The carving will be part of a tactile-audio astronomy exhibit at the Canadian National Institute for the Blind's Night Steps fundraising event for the visually impaired in Toronto on September 15, the same day the Cassini mission is scheduled to end. Resonances of Janus translated into music The second piece demonstrates the scales played by Janus and Epimetheus, two small irregular moons that share an orbit just outside Saturn's main ring system. Together they are an example of 1:1 resonance, the only one in the solar system. The pair orbit at slightly different distances from Saturn but with a difference that is so negligible they swap places every four years. The composition simulates the final few months of Cassini's mission, while Janus is inching closer to Epimetheus before stealing its place in 2018. Together, the two moons play a unison drone but with a constantly shifting rhythm that repeats every eight years. Russo played a C# note on his guitar once for every orbit while a cello sustains a note for each resonance within the rings. Each ring is like a circular string, being continuously bowed by Janus and Epimetheus as they chase each other around their shared orbit, says Russo. Cassini recently captured an image of one of the ripples this creates within the rings. To turn this into music, Russo and Santaguida used the brightness variations in this image to control the intensity of the cello. Saturn's dancing moons now have a soundtrack, says Russo. Russo, Tamayo and Santaguida are the same group who converted the recently discovered TRAPPIST-1 planetary system into music a few months ago. They've dubbed their astro-sonic side-project SYSTEM Sounds ) and hope to continue exploring the universe for other evidence of naturally occurring harmonic resonance. Explore further: Image: Saturn and rings, 7 June 2017
我以前就没给人家写过推荐语,也没觉得自己有这个资格,尤其步几位大师大牛之后,虽然我认为平民也高贵,但其实还是很知道做人要有自知之明,要知道谦虚。所以这篇博文就是想为这篇: 铅字背后的惶恐:收到《天文观测完全手册》之后 做个解释。 虽然此举冒着把一个无人注意的黑点越描越黑越大的危险,但鉴于科学网人的牙尖嘴利,我还是决定赶紧做一下自嘲。(再有些人来嘲笑我时,我可以说我先前已经羞愧过了。) 轻舟兄弟是 2014 年新进科学网的青年才俊,他的文字见识思想我都很佩服。 借着《我是歌手》刚刚火热的当口,做个不太贴切的类比:当我在微信中表达出对李健的膜拜时,立刻有科学网众位好友对我的非理性热粉表示劝解,其实说实话李健的歌中非常打动我的并不很多,包括他的《传奇》、《贝加尔湖》,并不在我《单曲循环》之列,虽然我也觉得好听。他的才华是很纯净、很雅致、很内敛,是水满自溢的才华,但不像很多欧美流行经典那么丰富那么充满感染力,总还是显得单薄了,(单薄我以为是中国流行音乐的通病,无论民谣还是摇滚……),还缺少那种驾驭各种美好元素的能力。 文字也一样,我更偏爱丰厚的文字,轻舟的文字总是能引经据典如同己出,将各种美好的元素熔铸一起却不显造作,中西合璧,文理贯通……虽然多是长篇大论,需要正儿八经的时间来读,但一读之下必然多方面受益匪浅。 我于是曾经对儿子说,现在的书籍如此之多,所以只能有所选择。这样的文字是可以学着天文、物理辅助语文修辞,还顺带哲学思考、历史地理、美术摄影、排版印刷……所以是好的。 If you can see it, you can be it. 我总觉得小孩子们还是美学修养最重要,知道什么样的工作是漂亮的,知道去欣赏到更多的美:科学的美、文学的美、艺术的美。 可惜我提供那些美好能力不足,更多的家庭也是这样,而学校和社会在这方面也做得更是不够,所以我觉得中国的孩子们很可惜并不能浸润在足够丰富的美好中,所以也许我们创造奇迹也还需要时日。 好在好的作品总是越来越多起来了。 我向来不喜习练世故,却偏爱新潮,所以年龄积累,反倒更多了以年轻人为师的时候。对于轻舟的作品,我一直就是打算佩服佩服,学习学习,借鉴借鉴,参考参考……我是不敢在如今的年轻人面前指手画脚的,以前偶尔出于关爱对有些异才不走寻常路的年轻人进行些鼓励或建议,后来每每想来总是羞愧,觉得人家比你懂得多多了,还给你这么大的面子听你唠叨这许多。 后来交流得多了,也会在是称呼我阿姨还是老师……之类的问题上开下玩笑,但没想到有一天,轻舟兄弟忽然请我为他的新书写个推荐语。 我当然是很谦虚的坚辞不受,我说我可以很认真写个读后感,但写推荐语是不敢的。但可能轻舟觉得需要给我个面子,还是很热情的邀请…… 后来我忽然想起这个“女”字来。 我觉得也许我可以从女性和母亲的角度来说一下,我总以为虽然女性已经很有些伟大人物出现,但是社会对女性的教育引导还很不如人意,而女性作为主力的家庭教育也因此存在各种问题。而且作为母亲,我对社会的科普未来有更多的期望,也是可以写写。 前段时间辅导一个高二的女孩儿,同学的女儿。是个很漂亮有思想很爱科普也很能言善辩热爱艺术但自觉和看韩剧的同学没有共同语言的女孩。她倒是愿意听我的,所以学习热情比以前高涨了许多。跟她聊起看星星,她也是满眼憧憬,我开玩笑说那是要找个认识星星的男朋友才行。 我并不在意有多少热爱科学的女孩子能够成名成家,但我很在意科学的美好和精神能给她的一生和她的家庭后代带来更愉悦的体验。我总希望更多的男生爱上星空,并与他也爱着星空的女孩儿一起在夜空里指星谈天,就像木心和那个湖州的女孩谈论圣经的美好一样,比起世俗的送花送戒指我觉得要浪漫得多了。 我总觉得爱天空的男孩女孩要比爱钻戒的男孩女孩更在意我们世界的未来,因而更可靠些。 当爱的内涵和形式也可以更丰富美好起来的时候,也许我们就离诺贝尔奖不远了。 扯远了,扯回来,于是我就非常艰难的写了那么一段推荐语(我发现我的确脑子发散的太厉害,所以适合交流辩论,让我正儿八经写点东西总是自觉很艰难): 充满了古老谜思和高新科技的天文学世界壁垒重重,路径稀缺,无人引领,难入其门,虽然人人心向往之,可几人有幸有夜观天象的爷爷?或通晓星学的男朋友呢?好在阅读是生活的起点。无论你是偏好哲思,还是喜爱人文,或者擅长技术,都能从这本书中找到兴趣的切入点,并由此出发,领略宇宙的无限可能。 虽然轻舟觉得“爷爷和男朋友那里很诡异啊。”但我觉得也许这样可以为那些喜欢天文学的女孩子勾画一幅美丽图景。如果她们没有懂天文的爷爷,她们还可以找一个懂星星的男朋友。《美丽心灵》中约翰·纳什为女友指着星空进行爱的模式识别的时候,我想这种情迷比起《色戒》里的一个大钻戒的迷失要有品位得多。我更希望看到中国有如前者的影视作品。 但后来又遇到问题,是关于我的简介的问题,我这个潍坊学院副教授太不响亮,也没啥社会兼职,开过的重要政治会议就是潍坊学院的无党派人士年会啥的,实在是没啥名头。我说我写读后感博文吧,非要我写啥推荐语,实在和别人不配套啊,也就是科学网知名博主响亮点。这确实是我说的,但我那不是开玩笑吗?我真不是那么自以为是自以为傲滴。话说我也不是没开过玩笑,怎么您这会儿就对玩笑没有了识别能力呢? 反正等我看到的时候,已然不能再改了,于是我在一拨大牛大师科学网大大的知名博主后面,被冠以科学网知名博主亮相了。让我不但每每看到这本书的时候就羞愧啊羞愧,而且拿书送人的时候还要解释半天,那我也不能不送是吧?! 我真的很羞愧。 不说了。 以后谁也不许说。o(╯□╰)o
A possible solution of Friedmann equations Xin-an Zhang (Xi’an Polytechnic university, 710048,China) zxafafa@sohu.com Abstract : In this paper, from the analysisof the acoustic quantum energy characteristics, it was found that the acousticwaves show negative energy characteristics in the energy transfer process.Thereby, the result of negative pressure can be deduced. This result is justwhat the cosmic dark energy model needs. Combining the latest cosmic acousticdetection results, the author made a point, that it is the energy transfer ofthe acoustic wave producing the negative energy and the negative pressure.After analysis, the state equations obtaining from two modals inferred from thispoint of view can not only meet the requirements of the dark energy model butalso solve the problems of singularity.Theother conclusion of this paper is that the universe has the boundary. The twomodels established by the author can not only explain the dark energy and alsoprove that the energy density equals matter density. In addition, the Friedmann equations have found solutions accordingly. Keywords : sound wave, negative (dark)energy, state equation, universe boundary, singularity Main content Considering the Energy Density and Matter Density is equal and assuming the proportion of the dark energy be x, we have (1) Then, (2) where, P is the maximal pressure of sound, is the energy density as well as the matter density, c is sound speed. Taking Eq(2) into Firedmenn eqaution (1), we have (3) where, a is the sacled factor, , C 1 ,C 2 are constant of integration REFERENCE Xu,R.X., Introduction to astrophysics,Beijing:Beijinguniversity press,2006,239-241 See full paper in the link Friedmann equations-two models.doc
你找不到工作,能怨爹妈没本事吗? 唉,肤浅的我终于还是没有忍住...... 这几天大家讨论的都是通贯天人、会通中西的“大问题”,本来俺们是不想插嘴的。谁知一不留神没管住,到别人博客下评论的频率飙升了。 这个争论由来已久,今天又被翻出来,大家活动活动思维也是极好的嘛。讨论问题要鼓励,“帽子”横飞就不必了,一会儿“爱国复古”,一会儿“崇洋媚外”,一点都不好玩儿! 要说写这篇博文不带点儿“情绪”,那是骗人的。“情绪”从哪里来呢?—— 注意,俺们对事不对人 !!! 现代科学崛起于西方,发轫于古希腊,这俺们都不否认——我一向为苏格拉底、柏拉图、亚里士多德诸公大唱赞歌(不信去查前面的博文)。我向来服膺薛定谔“老帅哥”在《自然与希腊》( N a ture and the Greeks )中的论断: There was need to explain(though I was myself not so thoroughly convinced of it)that in passing the time with narratives about ancient Greek thinkers and with comments on their views I was not just following a recently acquired hobby of mine; that it did not mean, from the professional point of view, a waste of time, which ought to be relegated to the hours of leisure; that it was justified by the hope of some gain in understanding modern science and thus inter alia also modern physics. 这有必要做出解释(尽管我自己还不太确认下面这些说法):讲述古希腊哲人的故事并对其观点发表评论不仅仅是向大家展示我的个人兴趣;从专业的角度讲,我也不认为这是把本可用于休息的时间浪费了;它的真正意义在于我们有可能对现代科学尤其是现代物理有新的理解。 “言必称希腊”在我这儿也是历来作风。但是,我给苏格拉底、柏拉图、亚里士多德雕个像,并不妨碍我给“孔夫子”(实际上是我们的传统)上柱香啊!——美国立国不过两百多年,尚且重视自己的清教徒传统。怎么我们一谈论这个“大问题”,动不动就把孔孟朱王、老庄孙子一系拿出来“陪绑”呢? 俺们家李中堂诗云“三百年来伤国步”,近世以来我们落后了,这没什么好争辩的。 在过去百年、现在以及将来可预见的未来,我们还要不断吸收学习掌握现代科学,建立科学文明,这也是极好的嘛 。但别人的东西好,我们的就真的一无是处了吗?——苏联人民建设“新世界”够暴风骤雨了吧,德国纳粹来了,斯大林同志不还得把东正教莫斯科大牧首抬出来,请神甫们上前线,鼓舞士气吗(作为宣传工具的苏联电影里都还得表现表现)? 再说了, 这个万恶的“科学”不应该是大家的吗?如果数千年来,中国人都没有为科学事业“添过砖,加过瓦”,而这个文明还能延续到今天,那这个所谓的“科学”真的有所宣称的价值和普遍意义吗 ?—— 一个狭隘得只容得下一种传统“科学”还是科学吗?美国人搞的是“民主”,中国人搞的就不叫“民主”,这不是文化上的“霸权主义”吗?—— 我写下这句后,自己都觉得好难得 。 俺在朋友们的博文下的评论基本都是一件事情: 其他学科我不清楚,中国古代至少在算学和天文学上都有系统的理论体系(基本模型、演算法则、现象预测和数据积累),并且其中的演算法则和数据积累在今天某些领域(比如星表和历法编订与修正)还在发挥一定作用 ,比如《九章算术》(特别是刘徽的《九章算术注》)区别于欧式几何公理体系的“计算”体系(这个体系在宋元时代达到高峰),元代郭守敬主持的天文与大地测量(天文学最先发展起来的分支就是天体测量学)。 有朋友问,那古人的工作有没有现在还在使用的? 我孤陋寡闻,暂时只想到这么几个: “杨辉三角”、秦九韶的“大衍总数术”(数论同余理论的“中国剩余定理”)、朱世杰的“招差术”(高次内插法)、“西风东渐”后李善兰根据沈括“垛积术”建立的组合数学“李善兰恒等式”..... 本来我还想说祖冲之、祖暅父子的祖氏原理,但想到那就是卡瓦列里早期积分学,怕诸位瞧不上,就不列在上面了。 有朋友又说:“杨辉三角”又不是牛顿二项式定理? 那我就要问多一句嘴了: 你现在用的牛顿第二定律公式根本不是牛顿的,是欧拉修改的,而且这个修改放到狭义相对论中看还是画蛇添足的,把“牛哥”本来深邃的洞见给“庸俗”了;你现在用的麦克斯韦方程也不是麦克斯韦的,是赫兹大刀阔斧修改过的;现在用的热力学卡诺定理是卡诺根据“热质模型”推演出来的;如果没有牛顿定律和广义相对论的等效原理,“日心模型”和“地心模型”之争根本没有物理意义...... 那牛顿、麦克斯韦、卡诺.....那些东西又算什么?——“ 科学”本来就是有历史维度的 ,为什么要割裂历史孤立地讨论呢?——别人的历史是历史,我们的历史就不是历史了吗? 那些把动辄把中国传统归结为“阴阳”、“太极”、“五行”的朋友,您真的查阅过张衡、祖冲之、郭守敬......在编算星表和历法的工作吗?—— 如果您能用“阴阳”、“太极”、“五行”(且不说它们自有价值)这些东西推算星表和历法,且能达到上述几位工作的精度,请您务必教教我 ! 如果您要追问“中国传统”在今天的发展,就我知道的,建议诸位去查阅吴文俊先生、张景中先生在“计算数学”、“机械化证明”领域的工作。 好吧,好吧,就算祖宗们不如西方贤哲(注意,又是一个让步从句!),那: 你考不上大学,找不到工作,能怨你爸妈没本事吗? 能怨你祖父母没本事吗? 能怨你曾祖父母没本事吗? ...... 能怨你十八代祖宗没本事吗? 其实以前,俺也觉得我们的指南针只能拿来看风水(堪舆),火药只能拿来做炮仗......但现在,厚着脸皮打个比方:就像“戊戌”之前的梁启超变成了回归“清华”的梁启超,选择作一个“开明的保守主义者”——现代科学很重要,咱们科学网上这么多朋友还得靠他养活呢!祖宗的东西里面也有“传家宝”,你想丢都丢不掉。 林徽因先生(她当得起“先生”二字)说:你们今天拆了真古董,有一天就要造许多假古董!!! 我国以世界之古国,居东亚之天府,本应绍汉唐之遗烈,作并世之先进。将来建国完成,必于世界历史,居独特之地位。盖并世列强,虽新而不古;希腊、罗马,有古而无今。惟我国家,亘古亘今,亦新亦旧,斯所谓“周虽旧邦,其命维新”者也…… ——冯友兰《国立西南联大纪念碑碑文》 PS:本周《以镜窥天——天文观测手册》(暂定名)就截稿了。临时起意,加班加点,见缝插针,向里面追加中国天文学史的内容!等俺有空了,好好考证考证孔孟朱王这一系在“格物穷理”上的洞见,现在我看好周敦颐的《太极图说》......
《中国科学: 物理学 力学 天文学》英文版第6期在线发布 HIGHLIGHT Home | Current Issue | All Past Issues | Search SCIENCE CHINA Physics, Mechanics Astronomy 2014 , Volume 57 , Issue 6 have been published online on 22 April , 2014 , in this issue, some articles maybe interest: Shell model calculations for the allowed Gamow-Teller β-decays of light nuclei PDF , is contributed by Prof. REN ZhongZ hou (Associate Editor) from Nanjing University. Herein they use shell model calculations to calculate the allowed Gamow-Teller β-decay information of Li, Be, B, C, and N isotopes which are of interest and compare to determined experimental results; Plasmonic-enhanced two-photon fluorescence with single gold nanoshell PDF , is contributed by Prof. GONG QiHuang (Associate Editor in-Chief) from Peking University, a paper proposed a configuration consisting of single plasmonic nanoshell and fluorescent emitters to efficiently achieve high two-photon fluorescence; A high-efficiency aerothermoelastic analysis method PDF , is contributed by Prof. YANG Chao from Beihang University, by using two-way coupling form and unified hypersonic lifting surface theory, a new aerothermoelastic analysis method with high efficiency and accuracy is established; Unmanned air vehicle flow separation control using dielectric barrier discharge plasma at high wind speed PDF , is contributed by Prof. ZHANG Xin from Northwestern Polytechnical University, has developed a novel plasma actuator to improve the plasma authority at high wind speed and explored the flow separation control of an Unmanned Aerial Vehicle (UAV) at the wind speed of 100 m/s; Competition of the multiple Grtler modes in hypersonic boundary layer flows PDF , is contributed by Prof. FU Song (Associate Editor in-Chief) from Tsinghua University, by using the linear stability theory (LST) and parabolized stability equation (PSE), this paper propose a competition mechanism of the multiple Grtler modes in hypersonic boundary layer flows, and the study is a guidance for future studies of the secondary instability of Grtler vortices and flow transition in hypersonic boundary flows; Double-peaked decay of transient photovoltage in nanoporous ZnO/n-Si photodetector PDF , is contributed by Prof. ZHAO Kun from China University of Petroleum, by using a nanoporous ZnO/n-Si heterojunction structure, a new type of photo-energy detector of infrared pulsed laser has been proposed. Anyway, we think all of the articles in this issue are equally important and worth reading, we hope these articles are benifit to your research work. We really appreciate your kindly citing for the papers from SCIENCE CHINA Physics, Mechanics Astronomy w hen you submit your papers to other journals! Outside China please click http://link.springer.com/journal/11433 for the fulltext PDF. Best wishes, Editorial Office SCIENCE CHINA Physics, Mechanics Astronomy SCIENCE CHINA Physics, Mechanics Astronomy 2014, Volume 57, Issue 6 Home | Current Issue | All Past Issues | Search Article 1005 Shell model calculations for the allowed Gamow-Teller β -decays of light nuclei PDF (777KB) Cite | LI HanTao, REN ZhongZhou. Shell model calculations for the allowed Gamow-Teller β -decays of light nuclei. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1005-1012. Abstract: The allowed Gamow-Teller β -decay information of Li, Be, B, C, and N isotopes under the frame work of nuclear shell model is calculated herein. Theoretical results of Q values, half-lives, excitation energies, log ft values, branching fractions, and β -delayed proton/neutron emission probabilities are tabulated and compared with experimental data. The deviations from the observations are also analyzed. The 11 Be nucleus is well known for its anomaly ground state J π =1/2 + . Thus, we compared the theoretical energy levels with the experimental data and the agreements for low excitation states are consistent. The quenching factor is also evaluated and discussed. Articles 1013 The Brillouin zones and band gaps of a two-dimensional phononic crystal with parallelogram lattice structure PDF (1205KB) Cite | HU JiaGuang, XU Wen. The Brillouin zones and band gaps of a two-dimensional phononic crystal with parallelogram lattice structure. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1013-1019. Abstract: We present a detailed theoretical study on the acoustic band structure of two-dimensional (2D) phononic crystal. The 2D phononic crystal with parallelogram lattice structure is considered to be formed by rigid solid rods embedded in air. For the circular rods, some of the extrema of the acoustic bands appear in the usual high-symmetry points and, in contrast, we find that some of them are located in other specific lines. For the case of elliptic rods, our results indicate that it is necessary to study the whole first Brillouin zone to obtain rightly the band structure and corresponding band gaps. Furthermore, we evaluate the first and second band gaps using the plane wave expansion method and find that these gaps can be tuned by adjusting the side lengths ratio R , inclined angle θ and filling fraction F of the parallelogram lattice with circular rods. The results show that the largest value of the first band gap appears at θ =90° and F =0.7854. In contrast, the largest value of the second band gap is at θ =60° and F =0.9068. Our results indicate that the improvement of matching degree between scatterers and lattice pattern, rather than the reduction of structural symmetry, is mainly responsible for the enhancement of the band gaps in the 2D phononic crystal. 1020 Velocity field of wave-induced local fluid flow in double-porosity media PDF (1790KB) Cite | BA Jing, ZHANG Lin, SUN WeiTao, HAO ZhaoBing. Velocity field of wave-induced local fluid flow in double-porosity media. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1020-1030. Abstract: Under the excitation of elastic waves, local fluid flow in a complex porous medium is a major cause for wave dispersion and attenuation. When the local fluid flow process is simulated with wave propagation equations in the double-porosity medium, two porous skeletons are usually assumed, namely, host and inclusions. Of them, the volume ratio of inclusion skeletons is low. All previous studies have ignored the consideration of local fluid flow velocity field in inclusions, and therefore they can not completely describe the physical process of local flow oscillation and should not be applied to the situation where the fluid kinetic energy in inclusions cannot be neglected. In this paper, we analyze the local fluid flow velocity fields inside and outside the inclusion, rewrite the kinetic energy function and dissipation function based on the double-porosity medium model containing spherical inclusions, and derive the reformulated Biot-Rayleigh (BR) equations of elastic wave propagation based on Hamilton's principle. We present simulation examples with different rock and fluid types. Comparisons between BR equations and reformulated BR equations show that there are significant differences in wave response characteristics. Finally, we compare the reformulated BR equations with the previous theories and experimental data, and the results show that the theoretical results of this paper are correct and effective. 1031 Sound propagation from the shelfbreak to deep water PDF (1669KB) Cite | QIN JiXing, ZHANG RenHe, LUO WenYu, PENG ZhaoHui, LIU JianJun, WANG DeJun. Sound propagation from the shelfbreak to deep water. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1031-1037. Abstract: Motivated by a phenomenon in an experiment conducted in the Northwestern Pacific indicating that the energy of the received signal around the sound channel axis is much greater than that at shallower depths, we study sound propagation from the transitional area (shelfbreak) to deep water. Numerical simulations with different source depths are first performed, from which we reach the following conclusions. When the source is located near the sea surface, sound will be strongly attenuated by bottom losses in a range-independent oceanic environment, whereas it can propagate to a very long range because of the continental slope. When the source is mounted on the bottom in shallow water, acoustic energy will be trapped near the sound channel axis, and it converges more evidently than the case where the source is located near the sea surface. Then, numerical simulations with different source ranges are performed. By comparing the relative energy level in the vertical direction between the numerical simulations and the experimental data, the range of the air-gun source can be approximated. 1038 Plasmonic-enhanced two-photon fluorescence with single gold nanoshell PDF (1179KB) Cite | ZHANG TianYue, LU GuoWei, SHEN HongMing, Perriat P, Martini M, Tillement O, GONG QiHuang. Plasmonic-enhanced two-photon fluorescence with single gold nanoshell. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1038-1045. Abstract: Single gold nanoshell with mutilpolar plasmon resonances is proposed to enhance two-photon fluorescence efficiently. The single emitter single nanoshell configuration is studied systematically by employing the finite-difference time-domain method. The emitter located inside or outside the nanoshell at various positions leads to a significantly different enhancement effect. The fluorescent emitter placed outside the nanoshell can achieve large fluorescence intensity given that both the position and orientation of the emission dipole are optimally controlled. In contrast, for the case of the emitter placed inside the nanoshell, it can experience substantial two-photon fluorescence enhancement without strict requirements upon the position and dipole orientations. Metallic nanoshell encapsulating many fluorescent emitters should be a promising nanocomposite configuration for bright two-photon fluorescence label. The results provide a comprehensive understanding about the plasmonic-enhanced two-photon fluorescence behaviors, and the nanocomposite configuration has great potential for optical detecting, imaging and sensing in biological applications. 1046 Deformation study of bicrystalline and nano-polycrystalline structures using phase field crystal method PDF (21118KB) Cite | LONG Jian, ZHANG Shuai, ZHAO YuLong, LONG QingHua, YANG Tao, CHEN Zheng. Deformation study of bicrystalline and nano-polycrystalline structures using phase field crystal method. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1046-1056. Abstract: Deformation behaviors of bicrystalline and nano-polycrystalline structures of various tilt angles and inclination angles in two dimensions are investigated in detail using a two-mode phase field crystal model. The interaction between grain boundary (GB) and dislocation is also examined in bicrystals and nano-polycrystals that both contain asymmetric and symmetric tilt GBs, with energy analysis being carried out to analyze these processes. During deformation simulations, we assume the volume of each simulation cell at every time step is coincident with that of the initial state just before deformation. Our simulation results show that the behaviors of symmetric and asymmetric GBs in bicrystals and nano-polycrystals differ from each other depending on tilt angle and inclination angle. A new dislocation emission mechanism of interest is observed in bicrystals which contain low angle symmetric tilt GBs. Low angle GB has a higher mobility relative to high angle GB in both bicrystalline and nano-poly-crystalline structures, as does asymmetric GB to symmetric GB. The generation, motion, pileup and annihilation of dislocations, grain rotation and grain coalescence are observed, which is consistent with the simulation results obtained by molecular dynamics. These simulation results can provide strong guidelines for experimentation. 1057 Spin filtering magnetic modulation and spin-polarization switching in hybrid ferromagnet/semiconductor structures PDF (725KB) Cite | XU HuaiZhe, WANG LiYing, YAN QiQi, ZHANG YaPing, ZHANG ShiChao. Spin filtering magnetic modulation and spin-polarization switching in hybrid ferromagnet/semiconductor structures. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1057-1062. Abstract: Electron spin-polarization modulation with a ferromagnetic strip of in-plane magnetization is analyzed in a hybrid ferromagnet/semiconductor filter device. The dependencies of electron spin-polarization on the strip's magnetization strength, width and position have been systematically investigated. A novel magnetic control spin-polarization switch is proposed by inserting a ferromagnetic metal (FM) strip eccentric in relation to off the center of the spin filter, which produces the first energy level spin-polarization reversal. It is believed to be of significant importance for the realization of semiconductor spintronics multiple-value logic devices. 1063 Modulation of Fano resonances in symmetry-broken gold-SiO 2 -gold nanotube dimers PDF (758KB) Cite | WU DaJian, YU HaiQun, JIANG ShuMin, WU XueWei, LIU XiaoJun. Modulation of Fano resonances in symmetry-broken gold-SiO 2 -gold nanotube dimers. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1063-1067. Abstract: Fano resonances in the symmetry-broken gold-SiO 2 -gold (BGSG) nanotubes and the associated dimers have been investigated based on the finite element method. In the BGSG nanotube, the symmetry breaking induced the interactions of the inner gold core and outer gold nanoshell plasmons of all multipolar orders and hence the red-shifts of the plasmon resonance modes and the enhanced quadrupole mode peaks were observed. The interference of the quadrupole mode peak with the subradiant dipole mode caused a Fano-dip in the scattering spectrum. By increasing the core offset-value in the BGSG nanotube, the Fano dip with low energy showed a red-shift and became deeper. Unexpectedly the plasmon coupling between a GSG nanotube and a BGSG nanotube can lead to two strong Fano dips in the scattering spectra of the dimer. It was further noted that the thin side of the BGSG nanotube located at two sides of the dimer gap can lead to the strong near-field coupling between two BGSG nanotubes and hence a deeper and broader Fano dip. 1068 Wave dispersion and attenuation in viscoelastic isotropic media containing multiphase flow and its application PDF (926KB) Cite | YANG Lei, YANG DingHui, NIE JianXin. Wave dispersion and attenuation in viscoelastic isotropic media containing multiphase flow and its application. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1068-1077. Abstract: In this paper, we introduce the complex modulus to express the viscoelasticity of a medium. According to the correspondence principle, the Biot-Squirt (BISQ) equations in the steady-state case are presented for the space-frequency domain described by solid displacements and fluid pressure in a homogeneous viscoelastic medium. The effective bulk modulus of a multiphase flow is computed by the Voigt formula, and the characteristic squirt-flow length is revised for the gas-included case. We then build a viscoelastic BISQ model containing a multiphase flow. Through using this model, wave dispersion and attenuation are studied in a medium with low porosity and low permeability. Furthermore, this model is applied to observed interwell seismic data. Analysis of these data reveals that the viscoelastic parameter tan d is not a constant. Thus, we present a linear frequency-dependent function in the interwell seismic frequency range to express tan d . This improves the fit between the observed data and theoretical results. 1078 Elastic stress fields caused by a dislocation in Ge x Si 1- x /Si film-substrate system PDF (2619KB) Cite | WANG HuYi, YU Yong, YAN ShunPing. Elastic stress fields caused by a dislocation in Ge x Si 1- x /Si film-substrate system. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1078-1089. Abstract: The elastic stress fields caused by a dislocation in Ge x Si 1- x epitaxial layer on Si substrate are investigated in this work. Based on the previous results in an anisotropic bimaterial system, the image method is further developed to determine the stress field of a dislocation in the film-substrate system under coupled condition. The film-substrate system is firstly transformed into a bimaterial system by distributing image dislocation densities on the position of the free surface. Then, the unknown image dislocation densities are solved by using boundary conditions, i.e., traction free conditions on the free surface. Numerical simulation focuses on the Ge 0.1 Si 0.9 /Si film-substrate system. The effects of layer thickness, position of the dislocation and crystallographic orientation on the stress fields are discussed. Results reveal that both the stresses σ xx , σ xz at the free surface and the stress σ xy , σ yy , σ yz on the interface are influenced by the layer thickness, but the former is stronger. In contrast to the weak dependence of stress field on the crystallographic orientation the stress field was strongly affected by dislocation position. The stress fields both in the film-substrate system and bimaterial system are plotted. 1090 Fuel-optimal trajectory design using solar electric propulsion under power constraints and performance degradation PDF (1417KB) Cite | ZHANG Peng, LI JunFeng, GONG ShengPing. Fuel-optimal trajectory design using solar electric propulsion under power constraints and performance degradation. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1090-1097. Abstract: The fuel-optimal transfer trajectories using solar electric propulsion are designed considering the power constraints and solar array performance degradation. Three different performance degradation models including linear, positive and negative ex-ponential degradations are used in the analysis of three typical rendezvous missions including Apophis, Venus and Ceres, respectively. The optimal control problem is formulated using the calculus of variations and Pontryagin's maximum principle, which leads to a bang-bang control that is solved by indirect method combined with a homotopic technique. In demonstrating the effects of the power constraints and solar array performance degradation on the power budget and fuel consumption, the time histories of the power profile and the fuel consumptions are compared for the three missions. This study indicates that it is necessary to consider the power constraints and solar array performance degradation for the SEP-based low-thrust trajectory design, espacially for long-duration outbound flights. 1098 Multi-pulse homoclinic orbits and chaotic dynamics of a parametrically excited nonlinear nano-oscillator with coupled cubic nonlinearities PDF (3491KB) Cite | ZHANG Wei, HUANG YuTong, YAO MingHui. Multi-pulse homoclinic orbits and chaotic dynamics of a parametrically excited nonlinear nano-oscillator with coupled cubic nonlinearities. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1098-1110. Abstract: In this paper, the complicated dynamics and multi-pulse homoclinic orbits of a two-degree-of-freedom parametrically excited nonlinear nano-oscillator with coupled cubic nonlinearities are studied. The damping, parametrical excitation and the nonlinearities are regarded as weak. The averaged equation depicting the fast and slow dynamics is derived through the method of multiple scales. The dynamics near the resonance band is revealed by doing a singular perturbation analysis and combining the extended Melnikov method. We are able to determine the criterion for the existence of the multi-pulse homoclinic orbits which can form the Shilnikov orbits and give rise to chaos. At last, numerical results are also given to illustrate the nonlinear behaviors and chaotic motions in the nonlinear nano-oscillator. 1111 A high-efficiency aerothermoelastic analysis method PDF (2262KB) Cite | WAN ZhiQiang, WANG YaoKun, LIU YunZhen, YANG Chao. A high-efficiency aerothermoelastic analysis method. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1111-1118. Abstract: In this paper, a high-efficiency aerothermoelastic analysis method based on unified hypersonic lifting surface theory is established. The method adopts a two-way coupling form that couples the structure, aerodynamic force, and aerodynamic thermo and heat conduction. The aerodynamic force is first calculated based on unified hypersonic lifting surface theory, and then the Eckert reference temperature method is used to solve the temperature field, where the transient heat conduction is solved using Fourier's law, and the modal method is used for the aeroelastic correction. Finally, flutter is analyzed based on the p-k method. The aerothermoelastic behavior of a typical hypersonic low-aspect ratio wing is then analyzed, and the results indicate the following: (1) the combined effects of the aerodynamic load and thermal load both deform the wing, which would increase if the flexibility, size, and flight time of the hypersonic aircraft increase; (2) the effect of heat accumulation should be noted, and therefore, the trajectory parameters should be considered in the design of hypersonic flight vehicles to avoid hazardous conditions, such as flutter. 1119 Investigation of the unstable flow phenomenon in a pump turbine PDF (2024KB) Cite | YIN JunLian, WANG DeZhong, WALTERS D. Keith, WEI XianZhu. Investigation of the unstable flow phenomenon in a pump turbine. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1119-1127. Abstract: Instability of pump turbine with S-shaped curve is characterized by large fluctuations of rotational speed during the transient processes. For investigating this phenomenon, a numerical model based on the dynamic sliding mesh method (DSSM) is presented and used to numerically solve the 3D transient flow which is characterized by the variable rotation speed of runner. The method is validated by comparison with measured data for a load rejection process in a prototype pump turbine. The results show that the calculated rotation speed agrees well with the experimental data. Based on the validated model, simulations were performed for the runaway process using an artificially assumed operating condition under which the unstable rotation speed is expected to appear. The results confirm that the instability of runner rotational speed can be effectively captured with the proposed method. Presented results include the time history profiles of unit flow rate and unit rotating speed. The internal flow characteristics in a typical unstable period are discussed in detail and the mechanism of the unstable hydraulic phenomenon is explained. Overall, the results suggest that the method presented here can be a viable alternative to predict the dynamic characteristics of pump turbines during transient processes. 1128 Large-eddy simulation of the generation and propagation of internal solitary waves PDF (3005KB) Cite | ZHU Hai, WANG LingLing, TANG HongWu. Large-eddy simulation of the generation and propagation of internal solitary waves. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1128-1136. Abstract: A modified large-eddy simulation model, the dynamic coherent eddy model (DCEM) is employed to simulate the generation and propagation of internal solitary waves (ISWs) of both depression and elevation type, with wave amplitudes ranging from small, medium to large scales. The simulation results agree well with the existing experimental data. The generation process of ISWs is successfully captured by the DCEM method. Shear instabilities and diapycnal mixing in the initial wave generation phase are observed. The dissipation rate is not equal at different locations of an ISW. ISW-induced velocity field is analyzed in the present study. The structure of the bottom boundary layer (BBL) of internal wave packets is found to be different from that of a single ISW. A reverse boundary jet instead of a separation bubble exists behind the leading internal wave while separation bubbles appear in other parts of the wave-induced velocity field. The boundary jet flow resulting from the adverse pressure gradients has distinctive dynamics compared with free shear jets. 1137 Measuring the kinetic parameters of saltating sand grains using a high-speed digital camera PDF (1345KB) Cite | ZHANG Yang, WANG Yuan, JIA Pan. Measuring the kinetic parameters of saltating sand grains using a high-speed digital camera. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1137-1143. Abstract: A high-speed digital camera is used to record the saltation of three sand samples (diameter range: 300-500, 200-300 and 100-125 μm). This is followed by an overlapping particle tracking algorithm to reconstruct the saltating trajectory and the differential scheme to abstract the kinetic parameters of saltating grains. The velocity results confirm the propagating feature of saltation in maintaining near-face aeolian sand transport. Moreover, the acceleration of saltating sand grains was obtained directly from the reconstructed trajectory, and the results reveal that the climbing stage of the saltating trajectory represents an critical process of energy transfer while the sand grains travel through air. 1144 Study on the dynamic characteristics of flow over building cluster at high Reynolds number by large eddy simulation PDF (7735KB) Cite | WANG BoBin, WANG ZhiShi, CUI GuiXiang, ZHANG ZhaoShun. Study on the dynamic characteristics of flow over building cluster at high Reynolds number by large eddy simulation. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1144-1159. Abstract: In this paper, the dynamic characteristics of building clusters are simulated by large eddy simulation at high Reynolds number for both homogeneous and heterogeneous building clusters. To save the computational cost a channel-like flow model is applied to the urban canopy with free slip condition at the upper boundary. The results show that the domain height is an important parameter for correct evaluation of the dynamic characteristics. The domain height must be greater than 8 h ( h is the average building height) in order to obtain correct roughness height while displacement height and roughness sublayer are less sensitive to the domain height. The Reynolds number effects on the dynamic characteristics and flow patterns are investigated. The turbulence intensity is stronger inside building cluster at high Reynolds number while turbulence intensity is almost unchanged with Reynolds number above the building cluster. Roughness height increases monotonously with Reynolds number by 20% from Re * =10 3 to Re * =10 5 but displacement height is almost unchanged. Within the canopy layer of heterogeneous building clusters, flow structures vary between buildings and turbulence is more active at high Reynolds number. 1160 Unmanned air vehicle flow separation control using dielectric barrier discharge plasma at high wind speed PDF (2211KB) Cite | ZHANG Xin, HUANG Yong, WANG WanBo, WANG XunNian, LI HuaXing. Unmanned air vehicle flow separation control using dielectric barrier discharge plasma at high wind speed. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1160-1168. Abstract: The present paper described an experimental investigation of separation control of an Unmanned Aerial Vehicle (UAV) at high wind speeds. The plasma actuator was based on Dielectric Barrier Discharge (DBD) and operated in a steady manner. The flow over a wing of UAV was performed with smoke flow visualization in the Φ 0.75 m low speed wind tunnel to reveal the flow structure over the wing so that the locations of plasma actuators could be optimized. A full model of the UAV was experimentally investigated in the Φ 3.2 m low speed wind tunnel using a six-component internal strain gauge balance. The effects of the key parameters, including the locations of the plasma actuators, the applied voltage amplitude and the operating frequency, were obtained. The whole test model was made of aluminium and acted as a cathode of the actuator. The results showed that the plasma acting on the surface of UAV could obviously suppress the boundary layer separation and reduce the model vibration at the high wind speeds. It was found that the maximum lift coefficient of the UAV was increased by 2.5% and the lift/drag ratio was increased by about 80% at the wind speed of 100 m/s. The control mechanism of the plasma actuator at the test configuration was also analyzed. 1169 Lagrangian coherent structures analysis of gas-liquid flow in a bubble column PDF (6183KB) Cite | WU Qin, WANG GuoYu, HUANG Biao, Bai ZeYu. Lagrangian coherent structures analysis of gas-liquid flow in a bubble column. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1169-1177. Abstract: The objective of this paper is to apply a new identifying method to investigating the gas-liquid two-phase flow behaviors in a bubble column with air injected into water. In the numerical simulations, the standard k - ε turbulence model is employed to describe the turbulence phenomenon occurring in the continuous fluid. The Finite-Time Lyapunov Exponent (FTLE) and Lagrangian Coherent Structures (LCS) are applied to analyze the vortex structures in multiphase flow. Reasonable agreements are obtained between the numerical and experimental data. The numerical results show that the evolution of gas-liquid in the column includes initial and periodical developing stages. During the initial stage, the bubble hose is forming and extending along the vertical direction with the vortex structures formed symmetrically. During the periodical developing stage, the bubble hose starts to oscillate periodically, and the vortexes move along the bubble hose to the bottom of column alternately. Compared to the Euler-system-based identification criterion of a vortex, the FTLE field presents the boundary of a vortex without any threshold defined and the LCS represents the divergence extent of infinite neighboring particles. During the initial stage, the interfaces between the forward and backward flows are highlighted by the LCS. As for the periodical developing stage, the LCS curls near the vortex centers, providing a method of analyzing a flow field from a dynamical system perspective. 1178 Competition of the multiple Grtler modes in hypersonic boundary layer flows PDF (1167KB) Cite | REN Jie, FU Song. Competition of the multiple Grtler modes in hypersonic boundary layer flows. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1178-1193. Abstract: Competition of multiple Grtler modes in hypersonic boundary layer flows are investigated with the local and marching methods. The wall-layer mode (mode W) and the trapped-layer mode (mode T) both occur in the compressible boundary layer where there exists a temperature adjustment layer near the upper edge. The mode T has the largest growth rate at a lower Grtler number while the mode W dominates at larger Grtler numbers. These two modes are both responsible for the flow transition in the hypersonic flows especially when Grtler number is in the high value range in which the crossover of these two modes takes place. Such high Grtler numbers are virtually far beyond the neutral regime. The nonparallel base flows, therefore, cease to influence the stability behavior of the Grtler modes. The effects of the Mach number on the multiple Grtler modes are studied within a chosen Mach number of 0.95, 2, 4 and 6. When the flow Mach number is sufficiently large, e.g., Ma ≥q4, the growth rate crossover of the mode T and mode W occurs both in the conventional G - β map as well as on the route downstream for a fixed wavelength disturbance. Four particular regions (Region T, T-W, W-T and W) around the crossover point are highlighted with the marching analysis and the result matches that of the local analysis. The initial disturbance of a normal mode maintains the shape in its corresponding dominating region while a shape-transformation occurs outside this region. 1194 On the non-synchronous rotation of binary systems PDF (596KB) Cite | LI ZhiXiong, HUANG RunQian, WANG ShuMin. On the non-synchronous rotation of binary systems. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1194-1200. Abstract: During the evolution of the binary system, many physical processes occur, which can influence the orbital angular velocity and the spin angular velocities of the two components, and influence the non-synchronous or synchronous rotation of the system. These processes include the transfer of masses and angular momentums between the component stars, the loss of mass and angular momentum via stellar winds, and the deformation of the structure of component stars. A study of these processes indicates that they are closely related to the combined effects of tide and rotation. This means, to study the synchronous or non-synchronous rotation of binary systems, one has to consider the contributions of different physical processes simultaneously, instead of the tidal effect alone. A way to know whether the rotation of a binary system is synchronous or non-synchronous is to calculate the orbital angular velocity and the spin angular velocities of the component stars. If all of these angular velocities are equal, the rotation of the system is synchronous. If not, the rotation of the system is non-synchronous. For this aim, a series of equations are developed to calculate the orbital and spin angular velocities. The evolutionary calculation of a binary system with masses of 10 M ⊙ +6 M ⊙ shows that the transfer of masses and angular momentums between the two components, and the deformation of the components structure in the semidetached or in the contact phase can change the rotation of the system from synchronous into non-synchronous rotation. 1201 Investigating the abundance enrichment pattern of heavy elements in the only observed CEMP-r/s star J004441.04-732136.4 of the SMC PDF (108KB) Cite | CUI WenYuan, ZHANG Bo, ZHAO Gang. Investigating the abundance enrichment pattern of heavy elements in the only observed CEMP-r/s star J004441.04-732136.4 of the SMC. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1201-1205. Abstract: The post-AGB star J004441 is the first and the only one CEMP-r/s star found in SMC. Herein, we investigate the observed abundance pattern of the heavy elements using our parametric model. A consistent fitting results was obtained for the sample star. Based on the low r =0.08, the s-process nucleosynthesis occurred in the interior is supposed to belong to the single neutron-exposure event. The median value of τ 0 =0.44(T 9 /0.348) 1/2 mbarn -1 supports a higher efficiency of the s-process nucleosynthesis relative to J004441 than that of the solar system, however, the value is not sufficiently high to favor the formation of a lead star. Thus, J004441 does not belong to lead star group. The large C s value of J004441 supports the intrinsic characteristic of the s-enrichment. The C r value is similar with that found in halo CEMP-r/s stars, which indicates that the r-process contributions is critical during heavy element enrichment. This star has a metallicity of =-1.34, which is larger than that of Galaxy halo CEMP-r/s stars. The reason may be because of the different history of metallicity enrichment between the SMC and the Galaxy halo. Letter 1206 Double-peaked decay of transient photovoltage in nanoporous ZnO/n-Si photodetector PDF (787KB) Cite | LIU Hao, FU Cheng, ZHAO Kun. Double-peaked decay of transient photovoltage in nanoporous ZnO/n-Si photodetector. SCIENCE CHINA Physics, Mechanics Astronomy, 2014, 57 (6): 1206-1208. Abstract: In the present work, a nanoporous ZnO/n-Si structure has been proposed as a new type infrared photodetector. Triggered by one laser pulse with wavelength of 1064 nm, this structure exhibits a double-peak decay of transient photovoltage. Also, the time interval between these two peaks increases linearly with the increase of irradiated pulsed energy, indicating the promising application of this hetero-junction in photo-energy detection of infrared pulsed laser. A possible mechanism for this particular photoresponse has been discussed. 订阅《中国科学: 物理学 力学 天文学》微信公众号,手机同步关注最新热点文章、新闻、科技资讯, 请添加微信号 SCPMA2014 或扫描下方图片关注.
开启粒子天文学的新视窗 现代天文观察,实际上已经充分利用了电磁波谱的每一个通道,这包括:射电、微波、红外、可见光、紫外光、 X 射线和 γ 射线等。近年来,天文学家特别感兴趣的是来自太空的高能带电粒子,即粒子能量高于 10 19 eV 的宇宙射线。这是因为,在我们现在的高能物理加速器中,根本无法产生能量如此之高的粒子,甚至无法用现有的物理知识解释相关的物理过程。在一篇评述文章中 P. M. Bauleo 和 J. R. Martino 介绍了这一领域的新进展。 10 8 eV - 10 20 eV 的能谱范围内,能量低于 10 10 eV 的粒子主要来自太阳。这是因为太阳风磁场,使那些来自太阳系外能量较低粒子的路径偏折,结果阻止了它们到达地球。至于能量高于 10 18 eV 的粒子,无论是它的加速过程还是它的起源,都没有一个令人信服的解释。能量在 10 20 eV 以上的射线粒子极为罕见,每一千年每 km 2 大约只有几个。这个特别低的粒子流通量,要求巨大的观测台站。例如,位于阿根廷的 Pierre Auger 天文台是地球上最大的高能宇宙射线观测台,它占地 3000 km 2 ,其中大部分面积以高能粒子探测器覆盖。 能量在 4 × 10 19 eV 以上的宇宙射线,不可能无衰减地穿越太空到达地球。它的传播过程主要受到宇宙微波背景辐射的影响。在一个相对于超高速运动质子静止的参考系中,温度仅 2.7K 的背景辐射 , 看上去就像是能量约 10 8 eV 的 γ “光子海”。如果在静止参考系中,光子的能量大于 150MeV ,产生 π 介子的光散射反应将成为可能。这使得超高能质子无扰动通过太空的距离减少到大约 1.6 亿光年。 (戴闻 编译自 Nature 458 (2009): 847 - 851 )