共查询到20条相似文献,搜索用时 0 毫秒
1.
Schooley AH 《Science (New York, N.Y.)》1968,160(3829):763-764
Passage of a submerged self-propelled body or other mixing device produces a region of more or less homogeneous fluid, in a fluid having a stable vertical density gradient (stratified), which initially expands vertically and then falls back (collapses). Maximum expansion Z(2) at time t(2) after the start of mixing are dependent variables related to the diameter Z(1) of propeller or mixer and to the V?is?l?-Brunt period T by T/t(2) = 2.5 and Z(2)t(2)/Z(1)T = 1.3. These scaling relations are first-order approximations. 相似文献
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Baker DJ 《Science (New York, N.Y.)》1971,172(3987):1029-1031
Velocity gradients induced in a rapidly rotaiting, density-stratified saltwater solution by a slowly rotating disk produce sharp vertical gradients of density, which appear as regularly spaced, curved horizontal sheets when the ratio of angular velocities exceeds a critical value. The existence of the sheets is apparently a finite amplitude manifestationl of a recently proposed viscous instability. 相似文献
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匀浆辊流浆箱以转动的孔辊作为匀整元件 ,匀整浆料的流速 ,具有整流和分散纤维的作用 .该文主要讨论波迹效应对匀浆辊设计的影响 ;匀浆辊流浆箱的匀浆辊 (闸辊 )的波迹效应产生的纤维定向条纹 ,导致整流效果不良 ,最终引起涂布、压光中的纸病 ,影响印刷质量 .经分析 ,这种波迹效应引起的纤维定向条纹 ,不同于简单的定量条纹 ,改变匀浆辊的运行参数、几何参数并不能消除 .匀浆辊的转动 ,必然产生哪怕纵向上轻微的涡流 ,就不可避免产生定向问题 .因此 ,匀浆辊流浆箱只适用于中低速纸机 . 相似文献
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The conformation of linear polymer chains trapped in a matrix of cross-linked polymer has been measured by neutron scattering. Three regimes were found depending on the length of the linear chain, Nl, with respect to the mesh size of the network, N(c). When N(c) > Nl, the radius of gyration, R(g), of the linear chain is the same as that observed in the uncrosslinked melt. When N(c) < Nl, R(g) shrinks according to the scaling relation R(g)(-1) approximately N(c)(-1) that has been predicted for isolated polymer chains trapped in a field of random obstacles. When N(c) < Nl the linear chains are observed to segregate. 相似文献
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A meromictic (chemically stratified) lake occupies a thawed depression in a pingo in interior Alaska, near Circle City. Increased salt concentration and anaerobic conditions characterize the zone extending from a maximum depth of 3 to 8.8 meters. The concentration of strontium and lithium is unusually high for lake water. 相似文献
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Holden C 《Science (New York, N.Y.)》1975,188(4190):815-817
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Ecosystem collapse in Pleistocene Australia and a human role in megafaunal extinction 总被引:1,自引:0,他引:1
Miller GH Fogel ML Magee JW Gagan MK Clarke SJ Johnson BJ 《Science (New York, N.Y.)》2005,309(5732):287-290
Most of Australia's largest mammals became extinct 50,000 to 45,000 years ago, shortly after humans colonized the continent. Without exceptional climate change at that time, a human cause is inferred, but a mechanism remains elusive. A 140,000-year record of dietary delta(13)C documents a permanent reduction in food sources available to the Australian emu, beginning about the time of human colonization; a change replicated at three widely separated sites and in the marsupial wombat. We speculate that human firing of landscapes rapidly converted a drought-adapted mosaic of trees, shrubs, and nutritious grasslands to the modern fire-adapted desert scrub. Animals that could adapt survived; those that could not, became extinct. 相似文献
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Schooley AH 《Science (New York, N.Y.)》1969,164(3886):1393-1394
Short bursts of submerged turbulent mixing in stratified water (every-where denser below than above) is shown to cause waves of surface convergence, divergence, and strain. Quantitative data are given for four experiments. 相似文献
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木材皱缩现象中的力学问题探讨 总被引:2,自引:0,他引:2
长期以来,木材皱缩现象一直被认为主要是因为木材中液体毛细张力产生的“负压强”压溃细胞壁造成的,该文对此提出了质疑.作者根据YoungLaplace定律设计了一组毛细负压强直接测量试验,发现凹液面上的毛细张力并不能使液体的压强降低.根据水在玻璃表面形成二维冰新结构这一最新发现,对毛细负压强及产生皱缩的主要动力问题提出了一些新的假设. 相似文献
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Hydrophobic collapse in multidomain protein folding 总被引:1,自引:0,他引:1
We performed molecular dynamics simulations of the collapse of a two-domain protein, the BphC enzyme, into a globular structure to examine how water molecules mediate hydrophobic collapse of proteins. In the interdomain region, liquid water persists with a density 10 to 15% lower than in the bulk, even at small domain separations. Water depletion and hydrophobic collapse occur on a nanosecond time scale, which is two orders of magnitude slower than that found in the collapse of idealized paraffin-like plates. When the electrostatic protein-water forces are turned off, a dewetting transition occurs in the interdomain region and the collapse speeds up by more than an order of magnitude. When attractive van der Waals forces are turned off as well, the dewetting in the interdomain region is more profound, and the collapse is even faster. 相似文献
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Mitchell JL 《Science (New York, N.Y.)》1976,193(4247):30-32
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对 3个典型黄土地区的原状黄土样在不同湿度状态下的湿陷特性进行了研究 ,探讨了含水率、干密度、压力对黄土湿陷性的影响规律 ,着重从结构性的角度解释了不同地区间黄土湿陷特性的差异。基于不同地区间黄土湿陷特性的差异这一事实 ,提出了黄土潜在湿陷率的概念。结果表明 ,这一概念可用于对黄土湿陷性的分析 ,并能较好地反映不同黄土种类的结构特性。 相似文献
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Reagan MJ Rowlett RM Garrison EG Dort W Bryant VM Johannsen CJ 《Science (New York, N.Y.)》1978,200(4347):1272-1275
In northwest Missouri, Lithic stage flake tools struck from prepared cores have been excavated underlying a Paleo-Indian fluted point assemblage. These assemblages were in two different loesses of the last glaciation. Thermoluminescent analysis of stone tools dates the Paleo-Indian occupations at 8690 +/- 1000 B.C. and 12,855 +/- 1500 B.C.; the Lithic stage occupations must be older than 13,000 B.C. on the basis of geologic correlation, lithic analysis, and cultural stratigraphy. 相似文献
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Phinney RA 《Science (New York, N.Y.)》1984,225(4667):1142-1143
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When a terawatt-peak-power laser beam is focused into a gas jet, an electron plasma wave, driven by forward Raman scattering, is observed to accelerate a naturally collimated beam of electrons to relativistic energies (up to 10(9) total electrons, with an energy distribution maximizing at 2 megaelectron volts, a transverse emittance as low as 1 millimeter-milliradian, and a field gradient of up to 2 gigaelectron volts per centimeter). Electron acceleration and the appearance of high-frequency modulations in the transmitted light spectrum were both found to have sharp thresholds in laser power and plasma density. A hole in the center of the electron beam may indicate that plasma electrons were expelled radially. 相似文献
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Xu G Broholm C Soh YA Aeppli G Ditusa JF Chen Y Kenzelmann M Frost CD Ito T Oka K Takagi H 《Science (New York, N.Y.)》2007,317(5841):1049-1052
Mesoscopic quantum phase coherence is important because it improves the prospects for handling quantum degrees of freedom in technology. Here we show that the development of such coherence can be monitored using magnetic neutron scattering from a one-dimensional spin chain of an oxide of nickel (Y2BaNiO5), a quantum spin fluid in which no classical static magnetic order is present. In the cleanest samples, the quantum coherence length is 20 nanometers, which is almost an order of magnitude larger than the classical antiferromagnetic correlation length of 3 nanometers. We also demonstrate that the coherence length can be modified by static and thermally activated defects in a quantitatively predictable manner. 相似文献