基于滑速比的气水两相流气相流量计算方法研究

目前对气水两相流的分相流量的研究中,多是针对两相流总流量和液相分相流量进行,对气相分相流量的研究很少.本文利用文丘里管和含气率传感器对空气水两相流气相流量计算方法进行了研究,在均相流模型基础上考虑了滑速比因素造成的影响,探讨了两相流气相流量计算方法.结果表明,该方法相对于传统的均相流模型在计算精度上得到了显著提高.     

全息光盘存储器的数据存取与像素匹配

全息光盘存储(HDS)技术作为一种非常具有潜力的新型信息存储技术, 是下一代光盘发展的目标。在全息光盘存储系统中, 为了实现数据精确快速地写入和读出, 降低误码率, 需要在整个高分辨率页面上实现空间光调制器(SLM)与光电阵列耦合器(CCD)之间1∶1像素匹配。在体全息存储理论和光学设计理论基础上, 研究实现SLM和CCD像素1∶1匹配的方法, 提出了实现像素匹配的要求和条件, 并且按照此要求完成该全息光盘存储器的光学系统。实验中, 分别使用随机数据掩膜版和SLM实现了对CCD的512×512精确像素匹配, 在光学系统中引入存储介质条件下, 图像质量良好, 掩膜版和SLM原始误码率分别为2.5×10-4和1.5×10-4。     

Gamma-Gamma湍流信道中大气光通信系统误码特性分析

大气信道中的大气湍流是影响无线激光通信系统性能的主要因素之一, 其引起的强度闪烁效应会对接收信号的提取和还原造成很大干扰。基于Gamma-Gamma概率分布的大气湍流信道统计模型, 研究了利用副载波相移键控(PSK)强度调制技术的大气光通信系统的误码特性; 推导了副载波二进制相移键控(BPSK)及开关键控(OOK)两种调制模式下的系统误码率表达式; 对在一定条件下的大气光通信系统, 比较了副载波BPSK和OOK两种调制模式的误码特性; 分析了链路特征、接收口径尺寸、通信波长和天顶角等因素对系统误码率的影响。结果表明, 增大接收孔径和通信波长都能有效地降低系统误码率, 而天顶角的增大则会使系统误码率增加, 副载波BPSK调制模式的误码特性要优于OOK调制模式的误码特性。     

Optical signal quality metric based on statistical moments and Laguerre expansion

A new and straightforward optical signal quality metric, which is based on the approximation of the analytical probability density function by its Laguerre expansion, is proposed. This expansion uses statistical moments to approximate the true statistics, but, unlike the well-known Gaussian approximation, higher order moments can be used as well. The quality of the proposed metric is discussed and the metric is compared with the Gaussian approximation. It is shown that the new metric provides not only a good estimation of the bit error rate in the range of practical interest but, contrary to the Gaussian approximation, correctly estimates optimum decision threshold and accurately pictures the shape of the noise distribution.     

Lattice gas simulation and experiment study of evacuation dynamics

In this paper, evacuation dynamics in an office building is studied by experiment and simulation. A lattice gas (LG) model is developed. A parameter called ‘exit bias’ is introduced into the model to describe the occupants’ familiarity with different exits in a building. The evacuation experiment, which consists of seven scenarios under various conditions, is conducted to verify the model and calibrate the model’s input parameters such as pedestrian speed and exit bias. The effect of exit width on flow rate, and the effect of occupants’ familiarity with the building on their route selections, are studied. It is found that the accuracy of simulation depends a lot on the model’s pedestrian speed. The optimal pedestrian speed is decided by not only occupant characteristics, but also flow features determined by people distribution, building structure, environment pressure, etc. LG models with proper pedestrian speed are capable of simulating the dynamic process of orderly emergency evacuations.     

Pressure-driven transient flows of Newtonian fluids through microtubes with slip boundary

Recent advances in microscale experiments and molecular simulations confirm that slip of fluid on solid surface occurs at small scale, and thus the traditional no-slip boundary condition in fluid mechanics cannot be applied to flow in micrometer and nanometer scale tubes and channels. On the other hand, there is an urgent need to understand fluid flow in micrometer scale due to the emergence of biochemical lab-on-the-chip system and micro-electromechanical system fabrication technologies. In this paper, we study the pressure driven transient flow of an incompressible Newtonian fluid in microtubes with a Navier slip boundary condition. An exact solution is derived and is shown to include some existing known results as special cases. Through analysis of the derived solution, it is found that the influences of boundary slip on the flow behaviour are qualitatively different for different types of pressure fields driving the flow. For pressure fields with a constant pressure gradient, the boundary slip does not alter the interior material deformation and stress field; while, for pressure fields with a wave form pressure gradient, the boundary slip causes the change of interior material deformation and consequently the velocity profile and stress field. We also derive asymptotic expressions for the exact solution through which a parameter is identified to dominate the behaviour of the flow driven by the wave form pressure gradient, and an explicit formulae for the critical slip parameter leading to the maximum transient flow rate is established.     

Density matrix of a quantum field in a particle-creating background

We examine the time evolution of a quantized field in external backgrounds that violate the stability of vacuum (particle-creating backgrounds). Our purpose is to study the exact form of the final quantum state (the density operator at the final instant of time) that has emerged from a given arbitrary initial state (from a given arbitrary density operator at the initial time instant) in the course of evolution. We find a generating functional that allows one to obtain density operators for an arbitrary initial state. Averaging over states of the subsystem of antiparticles (particles), we obtain explicit forms of reduced density operators for the subsystem of particles (antiparticles). Analyzing one-particle correlation functions, we establish a one-to-one correspondence between these functions and the reduced density operators. It is shown that in the general case a presence of bosons (e.g., gluons) in the initial state increases the creation rate of the same type of bosons. We discuss the question (and its relation to the initial stage of quark–gluon plasma formation) whether a thermal form of one-particle distribution can appear even if the final state of the complete system is not in thermal equilibrium. In this respect, we discuss some cases when pair-creation by an electric-like field can mimic the one-particle thermal distribution. We apply our technics to some QFT problems in slowly varying electric-like backgrounds: electric, SU(3) chromoelectric, and metric. In particular, we analyze the time and temperature behavior of the mean numbers of created particles, provided that the effects of switching the external field on and off are negligible. It is demonstrated that at high temperatures and in slowly varying electric fields the rate of particle-creation is essentially time-dependent.     

Comparison of tumor histology to dynamic contrast enhanced magnetic resonance imaging-based physiological estimates

Purpose

The purpose of this study was to compare histologically determined cellularity and extracellular space to dynamic contrast-enhanced magnetic resonance imaging (DCE MRI)-based maps of a two-compartment model's parameters describing tumor contrast agent extravasation, specifically tumor extravascular extracellular space (EES) volume fraction (v_e), tumor plasma volume fraction (v_p) and volume-normalized contrast agent transfer rate between tumor plasma and interstitium (K_TRANS/V_T).

Materials and Methods

Obtained v_e, v_p and K_TRANS/V_T maps were estimated from gadolinium diethylenetriamine penta-acetic acid DCE T₁-weighted gradient-echo images at resolutions of 469, 938 and 2500 μm. These parameter maps were compared at each resolution to histologically determined tumor type, and the high-resolution 469-μm maps were compared with automated cell counting using Otsu's method and a color-thresholding method for estimated intracellular (V_{intracellular}) and extracellular (V_{extracellular}) space fractions.

Results

The top five K_TRANS/V_T values obtained from each tumor at 469 and 938 μm resolutions are significantly different from those obtained at 2500 μm (P<.0001) and from one another (P=.0014). Using these top five K_TRANS/V_T values and the corresponding tumor EES volume fractions v_e, we can statistically differentiate invasive ductal carcinomas from noninvasive papillary carcinomas for the 469- and 938-μm resolutions (P=.0017 and P=.0047, respectively), but not for the 2500-μm resolution (P=.9008). The color-thresholding method demonstrated that v_e measured by DCE MRI is statistically similar to histologically determined EES. The V_{extracellular} obtained from the color-thresholding method was statistically similar to the v_e measured with DCE MRI for the top 10 K_TRANS/V_T values (P>.05). DCE MRI-based K_TRANS/V_T estimates are not statistically correlated with histologically determined cellularity.

Conclusion

DCE MRI estimates of tumor physiology are a limited representation of tumor histological features. Extracellular spaces measured by both DCE MRI and microscopic analysis are statistically similar. Tumor typing by DCE MRI is spatial resolution dependent, as lower resolutions average out contributions to voxel-based estimates of K_TRANS/V_T. Thus, an appropriate resolution window is essential for DCE MRI tumor diagnosis. Within this resolution window, the top K_TRANS/V_T values with corresponding v_e are diagnostic for the tumor types analyzed in this study.     

Strain rate sensitivity of ultra-fine and nanocrystaline metals and alloys

The mechanical behavior of metals and alloys is strongly related to grain size. In particular, the grain refining leads to the increase in yield strength in the ultra-fine grain (<1 μm) and nanocrystalline (<100 nm) regimes.Instrumented nanoindentation measurements allow a rapid evaluation of mechanical properties of materials, and the possibility to perform tests in a very wide range of loads. The strain rate sensitivity of ultra-fine and nanocrystalline metals can be derived by changing loading rates. The present paper presents the results on the strain rate sensitivity of ultra-fine grain metals produced by equa-channel angular pressing and nanocrystalline materials produced via electrodeposition. The results were obtained by systematic experiments performed at different loading rates (3, 30 and 300 mN/s) showing broad ranges of variations for the investigated metals. Also, the strain rate sensitivity of the studied materials was derived from the load vs. depth curves.     

温度、应变率和空位缺陷对氮化硼纳米管压缩性能的影响

采用分子动力学方法,分别模拟了完好的和含有缺陷的氮化硼纳米管的轴向压缩过程。原子间的相互作用采用Tersoff多体势函数来描述。结果表明,同尺寸的锯齿型氮化硼纳米管的临界轴向压缩强度高于扶手型氮化硼纳米管,这与碳纳米管的研究结果一致。发现纳米管的压缩强度,如临界轴向内力在低温下受温度影响明显,并且和应变率的大小有关。然而,应变率对纳米管的弹性变形没有影响。另外,还发现空位缺陷降低了纳米管的力学性能。与完好的纳米管相比,含有缺陷的纳米管轴向压缩强度对于温度的影响并不敏感。