Experimental Investigation Of Particle Velocity Distributions In Windblown Sand Movement

With the PDPA (Phase Doppler Particle Analyzer) measurement technology, the probability distributions of particle impact and lift-off velocities on bed surface and the particle velocity distributions at different heights are detected in a wind tunnel. The results show that the probability distribution of impact and lift-off velocities of sand grains can be expressed by a log-normal function, and that of impact and lift-off angles complies with an exponential function. The mean impact angle is between 28 degrees and 39 degrees, and the mean lift-off angle ranges from 30 degrees to 44 degrees. The mean lift-off velocity is 0.81-0.9 times the mean impact velocity. The proportion of backward-impacting particles is 0.05-0.11, and that of backward-entrained particles ranges from 0.04 to 0.13. The probability distribution of particle horizontal velocity at 4 mm height is positive skew, the horizontal velocity of particles at 20 mm height varies widely, and the variation of the particle horizontal velocity at 80 mm height is less than that at 20 mm height. The probability distribution of particle vertical velocity at different heights can be described as a normal function.

Reconstructing The Vertical Distribution Of The Aeolian Saltation Mass Flux Based On The Probability Distribution Of Lift-Off Velocity

The probability distribution of lift-off velocity of the saltating grains is a bridge to linking microscopic and macroscopic research of aeolian sand transport. The lift-off parameters of saltating grains (i.e., the horizontal and vertical lift-off velocities, resultant lift-off velocity, and lift-off angle) in a wind tunnel are measured by using a Phase Doppler Particle Analyzer (PDPA). The experimental results show that the probability distribution of horizontal lift-off velocity of saltating particles on a bed surface is a normal function, and that of vertical lift-off velocity is an exponential function. The probability distribution of resultant lift-off velocity of saltating grains can be expressed as a log-normal function, and that of lift-off angle complies with an exponential function. A numerical model for the vertical distribution of aeolian mass flux based on the probability distribution of lift-off velocity is established. The simulation gives a sand mass flux distribution which is consistent with the field data of Namikas (Namikas, S.L., 2003. Field measurement and numerical modelling of acolian mass flux distributions on a sandy beach, Sedimentology 50, 303-326). Therefore, these findings are helpful to further understand the probability characteristics of lift-off grains in aeolian sand transport. (c) 2007 Elsevier B.V. All rights reserved.

强散射体产生的像面散斑对比度与随机表面及成像系统关系的研究

We study the statistical properties of the image speckles produced by strong-scattering objects in the 4f optical imaging system. Using the generic expression of the complex amplitude of speckles and the approximation of the double-exponential function, we first obtain the ensemble average of the speckle intensity. Then we derive the variance of the speckle intensity based on the rotational transformation of the real and imaginary parts of the complex amplitude of speckles. We finally obtain the expression for the contrast of the. speckles, which is explicitly related to the statistical parameters of random surface and to the parameters of the imaging system. Our results are an obvious improvement compared with those reported in the literature, where the relations including such implicit quantities as the average size of the scattering grains of the random surface and the number of scattering grains are usually used. The results of this paper would be helpful for the characterization of random surface by speckle contrast.

激光多普勒振动计用于水下声光通信

介绍了激光多普勒振动计(LDV)用于水下声光通信的应用背景,阐述了激光多普勒振动计的工作原理和两种相干检测方式。采用零差的相干探测方式,设计并实现了一套光纤结构的激光多普勒振动计。为了证明系统能够应用于水下声光通信,进行了对水下声源发出的声波频率和强度的探测实验。通过对实验数据的分析得出：第一,系统能够检测出水下声源发出的声波频率,对7 kHz附近的10个声波频率的测量标准偏差小于8 Hz; 第二,系统探测信号强度与水下声源发声的声压级成指数关系,对于水下目标通信所用的3.5 kHz和7 kHz声波频段的

Estimating material elasticity by spherical indentation load-relaxation tests on viscoelastic samples of finite thickness

A two-step viscoelastic spherical indentation method is proposed to compensate for 1) material relaxation and 2) sample thickness. In the first step, the indenter is moved at a constant speed and the reaction force is measured. In the second step, the indenter is held at a constant position and the relaxation response of the material is measured. Then the relaxation response is fit with a multi-exponential function which corresponds to a three-branch general Maxwell model. The relaxation modulus is derived by correcting the finite ramp time introduced in the first step. The proposed model takes into account the sample thickness, which is important for applications in which the sample thickness is less than ten times the indenter radius. The model is validated numerically by finite element simulations. Experiments are carried out on a 10% gelatin phantom and a chicken breast sample with the proposed method. The results for both the gelatin phantom and the chicken breast sample agree with the results obtained from a surface wave method. Both the finite element simulations and experimental results show improved elasticity estimations by incorporating the sample thickness into the model. The measured shear elasticities of the 10% gelatin sample are 6.79 and 6.93 kPa by the proposed finite indentation method at sample thickness of 40 and 20 mm, respectively. The elasticity of the same sample is estimated to be 6.53 kPa by the surface wave method. For the chicken breast sample, the shear elasticity is measured to be 4.51 and 5.17 kPa by the proposed indentation method at sample thickness of 40 and 20 mm, respectively. Its elasticity is measured by the surface wave method to be 4.14 kPa. © 2011 IEEE.

The effect of organic matter accumulation on phosphorus release in sediment of Chinese shallow lakes

The effects of organic matter in sediment on phosphorus release were studied by field investigations in eight Chinese shallow freshwater lakes with different trophic status and a laboratory experiment. The sediment organic matter content paralleled the trophic status, ranging from 6.1 to 173.0 g kg(-1) (dry weight), with the mean value of 63.1 g kg(-1) (dry weight). It was positively proportional to Soluble reactive phosphorus concentration in the interstitial water in a form of exponential function, but inversely related to the sediment Fe/P ratio. The sediment alkaline phosphatase activity was significantly related not only to the organic matter content (r = 0.829, P < 0.01, n = 120), but also to the soluble reactive phosphorus concentration in interstitial water (r = 0.454, P < 0.01, n = 42). In the laboratory experiment, the addition of organic matter (dry materials of an aquatic macrophyte) into the sediment significantly enhanced alkaline phosphatase activity and soluble reactive phosphorus release. However, in the treatment with organic matter added and aeration, this release was generally prevented in spite of an increase in APA. Hence, sediment organic matter can effectively accelerate phosphorus release by enzymatic hydrolysis and anaerobic desorption. The latter mechanism seems to be more important.

Antibunching and blinking from a single colloidal CdSe quantum dot

We have investigated the optical properties of single CdSe/ZnS nanocrystals by conducting combinations of experiments on antibunching and photoluminescence intermittence under different experimental conditions. Based on photoluminescence in an antibunching experiment, we analyzed the emission lifetime of QDs by using stretched exponentials. The difference between the parameters obtained from average lifetimes and stretched exponents were analyzed by considering the effect of nonradiative emission. An Auger-assisted tunneling model was used to explain the power law exponents of off time distribution. The power law exponent under high excitation power was correlated with a higher Auger ionization rate. Using the parameters obtained from stretched exponential function and power law, the antibunching phenomena at different time and under different excitation intensity were analyzed.

Localized exciton dynamics in AlInGaN alloy

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved PL (TRPL) at various temperatures. The fast red-shift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/,tau)13], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed QDs or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent beta on temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered QDs. Furthermore, the localized states are found to have 0D density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Enhanced hydrogen desorption from Si sites during low-temperature Si1-xGex growth by disilane and solid-Ge molecular beam epitaxy

A phenomenological model is proposed to explain quantitatively the interesting compositional dependence on the Ge incorporation rate during low-temperature growth of Si1-xGex by disilane and solid-Ge molecular beam epitaxy, based on enhanced hydrogen desorption from Si sites due to the presence of Ge atoms. The hydrogen desorption rate constant for disilane on Si sites is fitted to an exponential function of Ge incorporation rate and a possible physical explanation is discussed. Simulated results are in excellent agreement with experimental data. (C) 1999 American Institute of Physics. [S0021-8979(99)02109-X].

Optical properties of AIInGaN quaternary alloys

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved photoluminescence (TRPL). The fast redshift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/tau)(beta)], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed quantum dots or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent 8 on the temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered quantum dots. Furthermore, the localized states are found to have OD density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature.

Numerical investigation of particle velocity distributions in aeolian sand transport

Particle velocity distribution in a blowing sand cloud is a reflection of saltation movement of many particles. Numerical analysis is performed for particle velocity distribution with a discrete particle model. The probability distributions of resultant particle velocity in the impact-entrainment process, particle horizontal and vertical velocities at different heights and the vertical velocity of ascending particles are analyzed. The probability distributions of resultant impact and lift-off velocities of saltating particles can be expressed by a log-normal function, and that of impact angle comply with an exponential function. The probability distribution of particle horizontal and vertical velocities at different heights shows a typical single-peak pattern. In the lower part of saltation layer, the particle horizontal velocity distribution is positively skewed. Further analysis shows that the probability density function of the vertical velocity of ascending particles is similar to the right-hand part of a normal distribution function, and a general equation is acquired for the probability density function of non-dimensional vertical velocity of ascending particles which is independent of diameter of saltating particles, wind strength and height. These distributions in the present numerical analysis are consistent with reported experimental results. The present investigation is important for understanding the saltation state in wind-blown sand movement. (C) 2009 Elsevier B.V. All rights reserved.

CFD-DEM simulation of three-dimensional aeolian sand movement

A three-dimensional CFD-DEM model is proposed to investigate the aeolian sand movement. The results show that the mean particle horizontal velocity can be expressed by a power function of heights. The probability distribution of the impact and lift-off velocities of particles can be described by a log-normal function, and that of the impact and lift-off angles can be expressed by an exponential function. The probability distribution of particle horizontal velocity at different heights can be described as a lognormal function, while the probability distribution of longitudinal and vertical velocity can be described as a normal function. The comparison with previous two-dimensional calculations shows that the variations of mean particle horizontal velocity along the heights in two-dimensional and three-dimensional models are similar. However, the mean particle density of the two-dimensional model is larger than that in reality, which will result in the overestimation of sand transportation rate in the two-dimensional calculation. The study also shows that the predicted probability distributions of particle velocities are in good agreement with the experimental results.

模拟增温对川西亚高山人工云杉林土壤碳含量和土壤呼吸的影响

全球气候变化已经成为不争的事实，其中全球变暖是近年来国内外的研究热点之一。土壤碳库作为陆地生态系统最大的碳库，气温升高必然会导致一系列的土壤碳储量和碳通量的变化，这些微小的变化又可能导致大气CO2浓度的变化并强化这种变暖的趋势。目前，土壤碳循环对温度升高的响应仍然是陆地碳循环研究最缺乏的部分，对土壤有机碳动态变化的研究仍存在着很大的不确定性与争议。四川西部的亚高山人工针叶林是青藏高原东部高寒林区的重要组成部分，是研究全球变化对森林生态系统影响的关键地区和重要森林类型。本研究通过采用原位人工模拟增温装置（Open-top chambers，OTCs）对川西米亚罗60年人工云杉林土壤实施增温，研究高海拔地区森林，尤其是人工森林系统下的土壤有机碳 含量、土壤呼吸及土壤酶活性对温度升高的响应。结果表明： 1. 增温处理的660天（2005年11月至2007年9月）期间，增温条件下的平均气温和土壤平均温度分别比对照提高0.43 ℃和0.27 ℃；0~10 cm土壤含水量在增温的不同时期均有不同程度的降低。 2. 土壤蔗糖酶、蛋白酶和脲酶活性在温度升高的不同阶段均有不同程度的提高。在增温处理300天（2006.09）、540天（2007.05）、600天（2007.07）和660天（2007.05）后，0~10 cm层的蔗糖酶活性分别比对照提高了36.36%（P<0.05）、24.31%、14.54%（P<0.05）和7.22%，脲酶活性分别提高了12.90%、24.19%（P<0.01）、34.48%（P<0.05）和14.64%(P<0.05)，蛋白酶活性分别提高了31.37%、1.99%、3.70%和17.80%。10~20 cm层的土壤酶活性也均有不同程度的提高，但均没有显著差异。蔗糖酶、脲酶和蛋白酶活性均呈现出随土层加深而减弱的趋势。 3. 土壤过氧化氢酶和多酚氧化酶活性在增温的第1年内均有不同程度的提高，但在增温的第2年内比对照有所降低。增温300天后（2006.09），过氧化氢酶和多酚氧化酶在0~10 cm层分别比对照增加3.76%和49.25%（P<0.05），10~20 cm层分别增加了5.54%和29.67%。在增温的第2年内，增温540天（2007.05）、600天（2007.07）和660天（2007.09）后，0~10 cm层的过氧化氢酶活性分别比对照降低了27.70%（P<0.05）、4.34%和1.47%，多酚氧化酶活性分别降低了5.86%、11.76%（P<0.05）和7.47%。增温的第2年内，10~20 cm层的过氧化氢酶和多酚氧化酶活性也均有不同程度的降低，但差异均未达到显著水平。不同土层之间相比较，过氧化氢酶活性随土层加深而降低，多酚氧化酶活性随土层加深而增加。 4. 土壤有机碳和有机质在增温的不同阶段，含量比对照均有所降低；且随增温时间的延长，降低的幅度下降。0~10 cm层的土壤有机碳和土壤有机质在增温300天（2006.09）、540天（2007.05）、600天（2007.07）和660天（2007.09）后分别降低了8.69%、4.35%、3.80%和2.44%，差异均未达到显著水平。土壤全氮含量在增温后与对照相比无明显的增加或者降低趋势。增温条件下的土壤C/N比与对照相比有所降低，但在增温各阶段的差异均不显著。10~20 cm层的有机碳、有机质和C/N比也有不同程度的降低趋势，但差异均不显著。不同土层之间相比，0~10 cm层的有机碳、有机质、全氮含量和C/N比均高于10~20 cm层，呈现出随土层加深而降低的趋势。 5. 土壤呼吸速率在增温第1年内，与对照相比明显提高，但在增温处理2年后，与对照相比无显著变化。增温300天（2006.09）和360天（2006.11）后分别提高了13.32%和21.17%，差异显著。增温处理540天（2007.05）到660天（2007.09）期间，与对照相比，不仅没有明显的提升，反而有些月份比对照有所降低，对温度升高的敏感性降低，呈现出对温度升高的适应性。土壤呼吸的日呼吸速率呈现单峰曲线形式，在14:00~20:00期间达到最大值，在4:00~10:00期间具有最低值。土壤呼吸的季节变化，呈现出与外界环境温度相一致的趋势，在7月份（夏季） 最高，11月份（冬季）最低。土壤呼吸与2 cm土壤温度、5 cm土壤温度和空气温度均呈极显著指数相关，与0~10 cm土壤含水量呈线性相关，相关性达到显著水平，但低于土壤呼吸与温度的相关性。 The past century has seen a marked increase in atmospheric carbon dioxide concentrations and a concomitant warming that has drawn scientific attention to the link between global carbon stocks and climate change. In particular, the decomposition and turnover of soil organic matter is recognised as an important determinant of carbon driven climate change. The slightly variation in soil organic carbon will result in the increase of atmospheric carbon dioxide concentrations and reinforce the tendency of warming. The experiment was conducted in Subalpine coniferous forest in western Sichuan province. Subalpine coniferous forest in western Sichuan was a important part of eastern Qinghai-Tibetan Plateau, which play a important role in reseaching the sensitivity of forest ecosystem to climate change. To investigate the effects of elevated temperature on soil organic carbon content, soil respiration rates, and soil enzyme activities in subalpine Picea asperata plantations, a esimulated warming measure was applied with Open-top chambers. The results were as followed: 1) During the period from Nov. 2005 to Sep. 2007, mean air temperature and soil temperature were respectively 0.43℃ and 0.27℃ the ambient higher. Soil water content decreased to different exent in different months in warmed plots than in unwarned plots at depth of 0-10 cm. 2) In general, elevated temperature enhanced the soil enzyme activities of invertase, protease, and urease. In the first year of warming—after 300 days’ treatment (in Sep,2006), the activities of invertase, protease, and urease increased by 36.36%, 12.90% and 31.37% respectively at the depths of 0-10 cm，among which the activity of invertase reached statistic significance. In the second year of warming, invertase activity increased by 24.31% after 540 days’ treament (in May, 2007), 14.54% after 600 days’ treament (in Jul, 2007) and 7.22% after 660 days’ treatment (in Sep, 2007) at the depths of 0-10 cm, and the differences in July and Septemmber were statistically significant. Elveated temperature also increased the activity of urease in the second year of warming and had significant effects in May and July. The activity of protease in warmed plots was also higher than in unwarmed plots at depths of 0-10 cm, but there was no significant difference. Elevated temperature had no significant effects on all soil enzyme acitivities at the depths of 10-20 cm in the first and sencond year. The values of above-mentioned soil enzyme all decreased with soil layers. 3) Eleavted temperature enhanced the activities of catalase and polyphenol oxidase in the first year of warming while they turned out downtrend in the second year. The activity of catalase increased by 3.76% and 5.54% at depths of 0-10 cm and 10-20 cm respectively in the first year—after 300 days’ warming (in Sep, 2006), the differences of which had no statistical significance. The activity of polyphenol oxidase was significantly increased by 49.25% at depths of 0-10 cm and not significantly increased by 29.67% at depths of 10-20 cm after 300 days’ warming. In the second year of warming, the catalase activity was significantly decreased by 27.70% after 540 days’ treament (in May, 2007) and not significantly decreased by 4.34% and 1.47% after 600 days’ (in Jul, 2007) and 660 days’ treament (in Sep, 2007) respectively. The activities of catalase and polyphenol oxidase at depths of 10-20 cm were decreased to different extent, but there was no significant difference. Catalase activity stepped down with soil layers while polyphenol oxidase activity stepped up. 4) Increased temperature in both the first year and the second year resulted tendency of decrease in the contents of soil organic carbon and soil organic matter, and C/N ratios at soil depths of 0-10 cm and 10-20 cm. However, with the prolonged warming, the tendency of decrease gradually tapered off and the extent of decrease in the second year of experiment were lower than that in the first year. The contents of soil organic carbon and soil organic matter were all decreased 8.69% by warming in the first year and dcreased 4.35%, 3.80% and 2.44% in May, July and September of the second year, but no significant difference were found. The C/N ratios increased 8.52% in the first year of warming and had less increment in the second year, all of which were not statistical significant. Eleveated temperature had no obvious effect on the content of tatol N in two year consecutive warming experiment. The contents of soil organic carbon and soil organic matter, total N and C/N ratios all had the tendency of dcreasing with soil layers. 5) Soil respiration rates were significantly enhanced by 13.32% and 21.17% after 300 days’ (in Sep, 2006) and 360 days’ (in Nov, 2006) treament in the first year of warming, but the same showed no obvious difference in the second year of treatment, which was assumed the adaptability of soil respiration with a certain heightened temperature. Diurnal soil resspiration showed a daily variation with a minimum value between 4:00 and 10:00 h and a maximum value between 14:00 and 20:00 h, coinciding with the minimum and maximum values of soil temperature at 2 cm. Soil respiration rates exhibited a pronounced seasonal variation with minimum values in Novmber and a maximum value in July, approximately coinciding with the seasonal variation of air and soil temperature. An exponential function provided the best fit for soil respiration with temperature while a quadric equation was used to estimate the effect of soil moisture on soil respiration, which were all significantly correlated. Soil respiraion rate was more highly correlated with the soil temperature than soil moisture.

Structural and Bluish-White Luminescent Properties of Li+-Doped BPO4 as a Potential Environmentally Friendly Phosphor Material

Many efforts have been devoted to exploring novel luminescent materials that not contain expensive or toxic elements, or do not need a mercury vapor plasma source. In this paper, BPO4 and Li+-doped BPO4 powder samples were prepared by the Pechini-type sol-gel (PSG) process. The structure and optical properties of the resulting samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), photoluminescence (PL) excitation and emission spectra, kinetic decay, and X-ray photoelectron spectra (XPS), respectively. It was found that PSG -derived Li+-doped BPO4 annealed at 960 degrees C exhibited bright bluish-white emission centered at 416 nm. The luminescence decay curves analysis indicates that each sample has two kinds of lifetimes (5.9 ns and 0.529 ms) and two types of kinetic decay behaviors which can be fitted into a single-exponential function and a double-exponential function, respectively.