Spectrum of Doppler ultrasound signals from nonstationary blood flow

A new formulation for the Doppler signal generation process in pulsatile flow has been developed enabling easier identification and quantification of the mechanisms involved in spectral broadening and the development of a simple estimation formula for the measured rms spectral width. The accuracy of the estimation formula was tested by comparing it with the spectral widths found by using conventional spectral estimation on simulated Doppler signals from pulsatile flow. The influence of acceleration, sample volume size, and time window duration on the Doppler spectral width was investigated for flow with blunt and parabolic velocity profiles passing through Gaussian-shaped sample volumes. Our results show that, for short duration windows, the spectral width is dominated by window broadening and that acceleration has a small effect on the spectral width. For long duration windows, the effect of acceleration must be taken into account. The size of the sample volume affects the spectral width of the Doppler signal in two ways: by intrinsic broadening and by the range of velocities passing through it. These effects act in opposite directions. The simple spectral width estimation formula was shown to have excellent agreement with widths calculated using the model and indicates the potential for correcting not only for window and nonstationarity broadening but also for intrinsic broadening.     

Experimental study of nonstationary liquid flow in capillary-porous bodies

This paper describes the method and results of an experimental study of liquid flow in models of capillary-porous bodies and in capillary-porous bodies under isothermal conditions.     

Noninvasive Young's modulus evaluation of tissues surrounding pulsatile vessels using ultrasound Doppler measurement

This paper presents an indirect approach to estimating the mechanical properties of tissues surrounding the arterial vessels using ultrasound (US) Doppler measurements combined with an inverse problem-solving method. The geometry of the structure and the dynamic behavior of the inner fluid are first evaluated using a novel dual-beam US system. A numerical phantom associated with a parametric finite element simulator that calculates the hydrodynamic pressure and the displacement on the walls' boundaries is then built. The simulation results are iteratively compared to the US measurement results to deduce the value of the unknown parameters, i.e., the Young's modulus and the pressure resulting from the downstream load. The feasibility of the proposed approach was experimentally tested in vitro using a phantom composed of a latex tube surrounded by a cryogel tissue-mimicking material.     

超短激光脉冲腔外光谱展宽与压缩技术

超短激光脉冲是超快光学和强场激光物理研究领域的重要驱动光源,通常可以通过脉冲后压缩方法获得。本文详细介绍了超短脉冲后压缩的发展现状、原理和相关技术,包括块状固体材料压缩、薄片组压缩、多通腔压缩、中空波导压缩以及光子晶体光纤压缩等。并通过对现有的脉冲后压缩技术进行总结,为未来的研究与发展方向提供了理论指导。     

Experimental evaluation of the plastic zones for steady mode-III crack growth

Summary A steady state quasi-static growth of a crack in antiplane shear in a strain hardening material was studied under small-scale yielding conditions. The extent of the plastic zone from the tip of the crack was determined as function of the size and geometry of the corresponding caustic developed around the crack tip. The method can be readily extended without considerable changes for large scale yielding.The topography of the plastically deformed zone was numerically and experimentally defined, its shape resembling a spiral ramp with its ground level coinciding with the crack axis and the two flanks of the ramp on either side of the crack axis going up or down until a complete half tour is covered. The position and characteristics of these ramps are changed, depending on the strain hardening coefficient from the elastic to purely plastic materials. As a consequence, the main crack extends by developing multiple radial microcracks following the steps of the ramps. These helical clasters of cracks straddle the main crack and prepare a new front for subsequent steps of propagation.Finally, a criterion is established relating the dimensions of the optical set-up and the mechanical and optical properties of the material ascertaining the creation of the initial curve of the caustic inside the elastic-plastic boundary of the deformed plate.With 11 Figures     

机械三叶心瓣模型的定常流实验研究

对机械人工三叶心脏瓣膜的模型进行定常流测试,并与QT型双叶瓣膜进行比较。通过使用Soildworks2005中的插件COSMOSFloWorks2005作为定常流测试的模拟软件,分别在定常流5L/min、10L/min、15L/min不同流量,不同开启角度79°、83°、87°下,测试出三叶瓣膜的跨瓣压差。测试实验结果表明三叶瓣膜跨瓣压差的确远小于双叶瓣膜,而且血流动力表现优于双叶瓣,符合瓣膜的设计要求。     

直管中流体流速对超声波防垢效果的影响

在实验室中采用循环水模拟装置,在相同流体、相同管道、相同温度的条件下,研究了流体流速对超声波防垢效果的影响。结果表明：流体流速变大,造成流体中的实际声强减小,使得超声波防垢率下降,防垢效果变差;当流体流速变大时增大超声波输入功率,防垢率就会上升;超声波频率越高,流体流速对超声防垢效果影响越大。     

Evaluation of coherent Doppler lidar velocity estimators in nonstationary regimes

We evaluate the mean velocity estimator performance for coherent Doppler lidar measurements of wind fields with wind shear and nonuniform system response as a function of target range. Performance of the velocity estimates is characterized by the bias and standard deviation that are determined by computer simulations. Results are for solid-state lasers with a Gaussian transmitted pulse. We consider data with high signal energy that produces negligible random outliers.     

Rheological behavior of a homogeneous suspension in nonstationary flow

Forces and moments are calculated for the interaction between phases in the nonstationary flow of a moderately concentrated suspension of small spheres; the equations of motion are derived in closed form.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 6, pp. 1007–1013, June, 1978.     

Tunneling-induced spectral broadening of a single atom in a three-dimensional optical lattice

We have investigated the spectral broadening in the near-resonance fluorescence spectrum of a single rubidium atom trapped in a three-dimensional (3D) optical lattice in a strong Lamb-Dicke regime. Besides the strong Rayleigh peak, the spectrum exhibited weak Stokes and anti-Stokes Raman sidebands. The line width of the Rayleigh peak for low potential depths was well explained by matter-wave tunneling between the first-two lowest vibrational states of 3D anisotropic harmonic potentials of adjacent local minima of the optical lattice.     

基于编码激励超声流速剖面测量的实验研究

相比常规的声学流速剖面仪,用于人体血流速度剖面测量的超声系统需要更高的信噪比增益和空间分辨率.为此,首先采用编码激励技术来改进传统的测量硬件系统,构造出一种具有"零相关窗"的互补码序列激励换能器发射声波,其优良的距离选通特性能提取出每个分辨单元的速度信息,同时大幅度提高回波信噪比.然后分析提取的分辨单元回波信号的特征,提出基于二维频谱图的邻帧差分法来消除相邻单元的干扰,对预处理后的信号进行高分辨率的速度参数估计,从而得到整个血管区域的速度剖面.通过搭建模拟人体血流测量实验装置进行仿血流的流速剖面测量实验.实验结果表明,采用此方法,在发射换能器的中心频率为5 MHz时,能达到0.3 mm的纵向分辨率,取得良好的测量效果.     

Identification of vacancies in electron irradiated GaSb by coincidence Doppler broadening spectroscopy

In this paper we present the results of coincidence Doppler broadening (CDB) measurements and positron lifetime spectroscopy (PLS) on the semiconductor material GaSb. Gallium vacancy with positron lifetime of about 283 ps (V_{Ga, 283 ps}) was identified in as-grown sample by CDB technique and PAS technique. For electron irradiated samples with dosages of 10¹⁷ cm^− 2 and 10¹⁸ cm^− 2, the PAS showed almost the same defect-related positron lifetime of about 285 ps. CDB experiments indicated that defects in irradiated samples were related to Ga vacancies.     

Observation of spectral broadening caused by self-phase modulation in highly multimode optical fiber

We observed spectral broadening caused by self-phase modulation in 400- and 600-mum core diameter fibers using amplified, Q-switched, Nd:YAG laser pulses with peak powers to 150 kW. The degree of spectral broadening was not dependent linearly on the fiber length as in single-mode fibers because of the more complicated modal evolution in highly multimode fiber. Furthermore, even slight stress near the input end of the fiber reduced the observed broadening. The results have significant implications for the delivery of high-peak-power laser beams through optical fiber with high-output beam quality for industrial applications.     

Low-order AR models for mean and maximum frequency estimation in the context of Doppler color flow mapping

Autoregressive (AR) techniques are investigated by developing mean and maximum frequency estimators suitable for use in Doppler color flow mapping systems, where they are most needed. The estimators are based on low-order (for computational efficiency) AR models applied to complex signals whose real and imaginary parts are the in-phase and quadrature components of the analytical Doppler signal, respectively. A large number of simulated data sequences generated by a sinusoidal computer model and having different number of samples, spectral shapes, bandwidths, and signal-to-noise ratios are used to examine the performance (bias and variance) of the estimators in a systematic manner. Comparisons are made with the established autocorrelation technique, whose output is shown to be identical to one of the AR mean frequency estimators described.     

Statistics of ultrasonic spectral parameters for prostate and liverexaminations

A theoretical analysis was performed to describe statistical characteristics of calibrated spectral parameters used for ultrasonic tissue evaluation in the prostate and liver. The analysis assumes that radiofrequency (rf) echo signals exhibit Gaussian statistics. It derives the probability density function (pdf) of spectral parameters that are computed using sliding-window analysis techniques. The analysis relates the standard deviations of linear-regression spectral-parameter estimates to system and analysis parameters including bandwidth, center frequency, and the length of the sliding analysis window. The analysis also derives the pdf for mid-band fit parameter images. Theoretical results are found to agree well with clinical data from homogeneous segments in liver and prostate. The results offer a basis for evaluating spectral-estimator precision and for conducting future studies of lesion detectability based on spectral features     

A mathematical modeling of pulsatile flow in carotid artery bifurcation

Real carotid arteries are elastic and perfused with a non-Newtonian fluid. The blood flow is pulsatile characterized by the reverse flow regions at the non-divider wall of internal carotid artery and external carotid artery. A coupling effect between fluid flow and elastic deformation is introduced. In addition we have calculated volumetric flow rate, pressure gradient and Impedance due to pulsatile flow in elastic carotid artery. We analyzed the flow at various locations in common carotid artery and internal carotid artery for different frequencies 60, 90, 120 pulse/min and Reynolds number 500 using a compliance mismatch model of carotid artery. Reverse flow of blood increases the impedance and should not be discarded. The problem has been analyzed mathematically and the agreement between present results and the available literature was found to be quite satisfactory.     

A priori evaluation of dynamic subgrid models of turbulence in square duct flow

A priori tests of two dynamic subgrid-scale (SGS) turbulence models have been performed using a highly resolved direct numerical simulation database for the case of turbulent incompressible flow in a straight duct of square cross-section. The model testing is applied only to the homogeneous flow direction where grid filtering can be applied without the introduction of commutation errors. The first SGS model is the dynamic (Smagorinsky/eddy viscosity) SGS model (DSM) developed by Germano et al. [1] while the second is the dynamic two parameter (mixed) model (DTM) developed by Salvetti and Banerjee [2]. For the Smagorinsky model we have used both the Fourier cut-off filter and a modified Gaussian filter which has the property that it removes aliasing errors in consistent a priori model-testing for spectral-based datasets. Results largely consistent with those found for plane channel flow are observed but with some slight differences in the corner regions. As found in prior studies of this sort, there is a very poor correlation of the modelled and exact subgrid-scale dissipation in the case of the DSM. The DSM over-predicts subgrid-scale dissipation on average. Instantaneously, the model provides an inaccurate representation of subgrid-scale dissipation, in general underestimating the magnitude by approximately one order of magnitude. On the other hand, the DTM shows excellent agreement with the exact SGS dissipation over most of the duct cross-section with a correlation coefficient of approximately 09.     

Influence of spectral broadening on femtosecond wavelength conversion based on four-wave mixing in silicon waveguides

Femtosecond wavelength conversion in the telecommunication bands via four-wave mixing in a 1.5 mm long silicon rib waveguide is theoretically investigated. Compared with picosecond pulses, the spectra are greatly broadened for the femtosecond pulses due to self-phase modulation and cross-phase modulation in the four-wave mixing process, and it is difficult to achieve a wavelength converter when the pump and signal pulse widths are close to or less than 100 fs in the telecommunication bands because of the spectral overlap. The influence of the spectral broadening on the conversion efficiency is also investigated. The conversion bandwidth of 220 nm and peak conversion efficiency of -8 dB are demonstrated by using 500 fs pulses with higher efficiency than the picosecond pulse-pumped efficiency when the repetition rate is 100 GHz.