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991.
Grain boundary migration between strained, substructured grains and newly appearing, strain free grains has been observed during static in-situ annealing of pre-deformed rocksalt in the SEM. With increasing temperature (T) the migration velocity increases and the character of grain boundary migration changes. As temperature increases there is an increase in the length of individual migrating boundary segments that move at similar rates. In addition, the frequency of migrating boundaries that form traces of a {100} boundary plane of at least one of the crystals involved increases, and moving grain boundaries between new and old grains change from highly irregular to smooth, straight boundaries. At the same time there is a decrease in the influence of the substructure of pre-existing strained grains on the grain boundary movement. Resultant microstructures reflect these changes. At 325–350 °C, the deformed-then-annealed microstructure is characterized by very irregular grain boundaries, a high abundance of 5–50 m scale remnants of old, substructured grains within new grains, giving a poikilitic microstructure. At 350–400 °C, grain boundaries often exhibit elongate embayments into the strained grains and most remnants of old, strained grains are located at former grain boundaries. At > 400 °C, grain boundaries between new and old, strained grains are straight to smoothly curved.The grain boundary velocity observations are explained by the effect of temperature on mobility coupled with local driving force variations. Additionally, at low annealing temperature, impurity (solute) drag and driving-force variations are influential, while at high temperature the anisotropy in grain boundary energy with crystallographic orientation becomes more important. Transferring the knowledge from our experiments to geological samples enables us to recognize and interpret similar microstructures in rocks, thereby making it is possible to relate microstructural characteristics to the pre-annealing and post-deformational annealing history. 相似文献
992.
RTK技术以其精度和自动化程度都比较高的优势,成为地籍测量中的重要技术手段之一。针对地籍测量的要求,从经济、精度、效率各方面探讨RTK技术在地籍测量中的应用问题,提出切实可行的技术方案。探讨了RTK技术用于地籍碎部测量的可行性,分析了各种误差的来源及对定位精度的影响,提出有效的减弱或消除措施。 相似文献
993.
994.
川滇黔接壤地区城镇化空间特征及驱动力分析 总被引:1,自引:0,他引:1
川滇黔接壤地区经济落后,整体城镇化水平较低,采用川滇黔接壤地区2008年各区县(市)的城镇化率作为测度指标,运用探索性空间数据分析(ESDA)方法,分析了本区域的城镇化水平的空间分布特征。通过对城镇化率的空间自相关系数Moran’s I的计算,发现各区县城镇化率的空间分布并非表现出完全随机性,而是具有明显的空间自相关性,具体表现为低值空间聚集。利用空间计量模型研究了城镇化水平的驱动机制。结果表明:空间误差模型优于空间滞后模型和OLS模型;相邻地区各因素的空间扩散效应对城镇化水平存在显著的影响,其弹性系数为0.416 1;第三产业增加值是城镇化率的最主要驱动因子,其弹性系数为0.777 7。因此,未来川滇黔接壤地区在推进城镇化时,应考虑城镇化水平的空间相关性,建立区域一体化开发政策,并积极发展第三产业,提高第三产业在国民经济中的比重。 相似文献
995.
996.
The Role of Shear in the Morning Transition Boundary Layer 总被引:1,自引:1,他引:0
Robert J. Beare 《Boundary-Layer Meteorology》2008,129(3):395-410
We use large-eddy simulation (LES) to better define the early stages of the morning transition boundary layer. Previous LES
studies relating to the morning transition boundary layer focus on the role of the entraining convective boundary layer (CBL).
By using a combination of different domain sizes and grid lengths, the full evolution from the stable boundary layer (SBL)
to the CBL is modelled here. In the early stages of the morning transition the boundary layer is shown to be a combination
of a shallow mixed layer capped by a significant shear driven stable boundary layer (the so-called mixed CBL–SBL state). The
mixed CBL–SBL state is the key to understanding the sensitivity to shear. Turbulent kinetic energy budgets also indicate that
it is shear driven. The negative flux from the mixed CBL–SBL state extends much further above the minimum than is typically
found for the CBL later in the day, and the depth of penetration scales as w
m
/N
i
, where w
m
is the combined friction and convective velocity scale and N
i
the static stability at the inversion top. 相似文献
997.
The microstructure of a stably stratified boundary layer, with a significant low-level nocturnal jet, is investigated based
on observations from the CASES-99 campaign in Kansas, U.S.A. The reported, high-resolution vertical profiles of the temperature,
wind speed, wind direction, pressure, and the turbulent dissipation rate, were collected under nocturnal conditions on October
14, 1999, using the CIRES Tethered Lifting System. Two methods for evaluating instantaneous (1-sec) background profiles are
applied to the raw data. The background potential temperature is calculated using the “bubble sort” algorithm to produce a
monotonically increasing potential temperature with increasing height. Other scalar quantities are smoothed using a running
vertical average. The behaviour of background flow, buoyant overturns, turbulent fluctuations, and their respective histograms
are presented. Ratios of the considered length scales and the Ozmidov scale are nearly constant with height, a fact that can
be applied in practice for estimating instantaneous profiles of the dissipation rate. 相似文献
998.
Doppler Lidar Measurements of Vertical Velocity Spectra in the Convective Planetary Boundary Layer 总被引:1,自引:1,他引:0
We utilized a Doppler lidar to measure spectra of vertical velocity w from 390m above the surface to the top of the daytime convective boundary layer (CBL). The high resolution 2μm wavelength
Doppler lidar developed by the NOAA Environmental Technology Laboratory was used to detect the mean radial velocity of aerosol
particles. It operated continuously during the daytime in the zenith-pointing mode for several days in summer 1996 during
the Lidars-in-Flat-Terrain experiment over level farmland in central Illinois, U.S.A. The temporal resolution of the lidar
was about 1 s, and the range-gate resolution was about 30m. The vertical cross-sections were used to calculate spectra as
a function of height with unprecedented vertical resolution throughout much of the CBL, and, in general, we find continuity
of the spectral peaks throughout the depth of the CBL. We compare the observed spectra with previous formulations based on
both measurements and numerical simulations, and discuss the considerable differences, both on an averaged and a case-by-case
basis. We fit the observed spectra to a model that takes into account the wavelength of the spectral peak and the curvature
of the spectra across the transition from low wavenumbers to the inertial subrange. The curvature generally is as large or
larger than the von Kármán spectra. There is large case-to-case variability, some of which can be linked to the mean structure
of the CBL, especially the mean wind and the convective instability. We also find a large case-to-case variability in our
estimates of normalized turbulent kinetic energy dissipation deduced from the spectra, likely due for the most part to a varying
ratio of entrainment flux to surface flux. Finally, we find a relatively larger contribution to the low wavenumber region
of the spectra in cases with smaller shear across the capping inversion, and suggest that this may be due partly to gravity
waves in the inversion and overlying free atmosphere. 相似文献
999.
A parametrization method used to account for the effects of flow separation and wall roughness on the lower boundary condition
for turbulent boundary layers is investigated against direct numerical simulation and laser Doppler anemometry data. The numerical
simulation represents flow over a smooth, flat surface with a prescribed external adverse pressure gradient. The water-channel
experiments cover flow over smooth and rough hills for two specified Reynolds numbers. Global optimization algorithms based
on four different direct search methods are used to assess the parametrization function, C, in terms of local mean velocity profiles and the parametrization parameters u
* (friction velocity), ∂
x
p (local pressure gradient), z
0 (effective roughness) and d (zero-plane displacement). The study investigates regions of attached and reversed flows, and forty-two velocity profiles
are compared with the proposed expression for the function C, including two profiles that satisfy the solution of Stratford. 相似文献
1000.
Exploring the Effects of Microscale Structural Heterogeneity of Forest Canopies Using Large-Eddy Simulations 总被引:4,自引:4,他引:0
Gil Bohrer Gabriel G. Katul Robert L. Walko Roni Avissar 《Boundary-Layer Meteorology》2009,132(3):351-382
The Regional Atmospheric Modeling System (RAMS)-based Forest Large-Eddy Simulation (RAFLES), developed and evaluated here,
is used to explore the effects of three-dimensional canopy heterogeneity, at the individual tree scale, on the statistical
properties of turbulence most pertinent to mass and momentum transfer. In RAFLES, the canopy interacts with air by exerting
a drag force, by restricting the open volume and apertures available for flow (i.e. finite porosity), and by acting as a heterogeneous
source of heat and moisture. The first and second statistical moments of the velocity and flux profiles computed by RAFLES
are compared with turbulent velocity and scalar flux measurements collected during spring and winter days. The observations
were made at a meteorological tower situated within a southern hardwood canopy at the Duke Forest site, near Durham, North
Carolina, U.S.A. Each of the days analyzed is characterized by distinct regimes of atmospheric stability and canopy foliage
distribution conditions. RAFLES results agreed with the 30-min averaged flow statistics profiles measured at this single tower.
Following this intercomparison, two case studies are numerically considered representing end-members of foliage and midday
atmospheric stability conditions: one representing the winter season with strong winds above a sparse canopy and a slightly
unstable boundary layer; the other representing the spring season with a dense canopy, calm conditions, and a strongly convective
boundary layer. In each case, results from the control canopy, simulating the observed heterogeneous canopy structure at the
Duke Forest hardwood stand, are compared with a test case that also includes heterogeneity commensurate in scale to tree-fall
gaps. The effects of such tree-scale canopy heterogeneity on the flow are explored at three levels pertinent to biosphere-atmosphere
exchange. The first level (zero-dimensional) considers the effects of such heterogeneity on the common representation of the
canopy via length scales such as the zero-plane displacement, the aerodynamic roughness length, the surface-layer depth, and
the eddy-penetration depth. The second level (one-dimensional) considers the normalized horizontally-averaged profiles of
the first and second moments of the flow to assess how tree-scale heterogeneities disturb the entire planar-averaged profiles
from their canonical (and well-studied planar-homogeneous) values inside the canopy and in the surface layer. The third level
(three-dimensional) considers the effects of such tree-scale heterogeneities on the spatial variability of the ejection-sweep
cycle and its propagation to momentum and mass fluxes. From these comparisons, it is shown that such microscale heterogeneity
leads to increased spatial correlations between attributes of the ejection-sweep cycle and measures of canopy heterogeneity,
resulting in correlated spatial heterogeneity in fluxes. This heterogeneity persisted up to four times the mean height of
the canopy (h
c
) for some variables. Interestingly, this estimate is in agreement with the working definition of the thickness of the canopy
roughness sublayer (2h
c
–5h
c
). 相似文献