单边狭窄血管中超声多普勒信号的仿真研究

超声多普勒技术作为一种无损检测手段被广泛应用于血管狭窄的检测。以往的血管狭窄仿真信号的研究仅限于双边狭窄的对称情况,文章提出了一种单边狭窄血管中超声多普勒信号的仿真方法。首先用有限元分析方法(FEM)计算出狭窄血管中血流流速场分布情况,然后用总体分布非参数估计法计算出超声多普勒信号的功率谱密度(PSD),再用余弦叠加法获取仿真的超声多普勒时域信号。用快速傅里叶变换(FFT)计算仿真超声多普勒信号的频谱,从中计算最大频率、平均频率和频谱宽度等参数,分析它们在不同流速和狭窄程度下的特征,为血管疾病的诊断提供敏感的参数。     

超声多普勒信号的频谱分析

超声多普勒信号具有非平稳性，利用传统的短时傅里叶变换分析该信号不能同时得到较高的时间频率精度，为克服这一缺点，本文简要介绍了多普勒信号频谱分析的几种新方法，并将它们和传统的方法进行性能的比较。     

面向医学内窥诊断的数字超声多普勒成像系统

研究基于现场可编程门阵列（FPGA）的超声多普勒内窥成像系统,针对内窥系统超声探头体积小、回波信号微弱的特点,设计了具有较高增益和较低噪声的超声信号前端接收电路.在FPGA中对微弱信号进行全数字化处理,实现了正交解调与频谱分析等功能,系统具有电路匹配性好、信噪比高、处理速度快及体积小等优势;搭建基于多普勒物理模型的实验平台进行实验验证,分析对比不同实验条件下的声谱,验证了系统及信号处理方法的合理性和正确性.     

实时频谱分析仪中CCDF测量功能的实现

实时频谱分析仪可以对通信信号进行实时测试,符合现代通信分析仪的发展趋势。CCDF测量是实时频谱分析仪的关键技术之一,对于测量多载波信号非常有用。它普遍应用在2G和3G无线信号测量上。本文介绍了实时频谱分析仪的总体框架,重点阐述了CCDF测量功能的原理和处理流程。并以OFDM系统(正交频分复用)为应用背景验证了这种算法的有效性。     

全相位频谱校正技术在水声通信中的应用研究

多普勒效应是影响水下移动通信性能的主要因素,准确估计多普勒频移对提高通信系统可靠性具有重要意义。在进行离散频谱分析时,时域非整周期截断会造成频域能量的泄漏,导致频谱估计精度降低。全相位频谱分析具有良好的抑制频谱泄漏特性及相位不变性。仿真验证了全相位频谱校正技术相对于传统频谱校正技术在估计性能上的优势,并在此基础上探讨了全相位频谱校正技术在水声通信中的应用。采用全相位频谱校正技术进行多普勒频移估计,进而进行多普勒补偿以降低通信系统误码率。仿真结果表明,全相位频谱校正技术能够实现高精度多普勒频移估计,从而提高水下移动通信系统的可靠性。     

多普勒血流信号最大频率曲线的特征分析及应用

文章主要研究从超声多普勒血流信号的最大频率曲线中提取特征值,并用于临床血管疾病诊断的方法。先利用短时傅里叶变换分析采样所得的音频多普勒血流信号,接着用百分比法得到信号的最大频率曲线,然后提取最大频率曲线频谱各峰值的比作为特征值,并将它们用于血管疾病的诊断。通过对颈动脉多普勒血流信号的临床分析表明：此方法比传统的声谱参数法有更高的诊断准确性。     

远程水声扩频通信时变多普勒估计与码片同步联合处理算法

在低信噪比的远程浅海水声移动通信中,传统多普勒估计算法难以有效跟踪时变多普勒因子,针对该问题提出了一种基于时频联合搜索的新的时变多普勒跟踪算法。利用正交支路扩频码的扩频增益和互相关特性,结合迭代处理技术,搜索扩频码符号在不同码片相位下的多普勒频谱峰值,选择具有最大峰值的频谱对应的多普勒因子和码片相位,对信号进行多普勒补偿和码片同步。蒙特卡洛仿真表明,该算法能够在信噪比为-18 dB的情况下有效地对10 m.s~(-1)以内产生的时变多普勒进行逐符号跟踪补偿。算法经过海试测试,在远程浅海移动通信中成功完成了多普勒跟踪估计,通信误码率达到10~(-3)。     

基于小波变换的高分辨率信号频谱分析方法

本文介绍了一种基于小波变换的高分辨率频谱分析方法，该方法对信号频谱分辨率有明显的改善效果，尤其是在短数据采样点上，仍然具有较高的频谱分析效果，适合于快速变化信号的频谱分析。     

双天线雷达测速仪的研发

为解决现有机车测速仪的不足,研发一种基于非对称发射角度的双天线雷达测速仪。该测速仪由双雷达微波模块、信号调理模块、数据处理模块和电源模块组成。双雷达微波模块与地面分别构成不同的发射角,以不同的频率发射雷达波,根据能量和频谱特征接收各自发射的雷达波产生的反射波,将其传入信号调理模块分别处理后,再输入数据处理模块利用多普勒效应相关算法分别进行分析解算。模拟测试结果表明,该测速仪具有测速精度高、系统稳定、数据可靠等优点。     

下肢动脉闭塞性病变的多普勒血流频谱形态观察

目的：观察下肢动脉闭塞性病变的多普勒血流频谱。方法：选择32例下肢动脉栓塞患者作为研究对象并行彩色多普勒超声检查,观察血流频谱及血流频谱指标,并与健侧做比较。结果：32例下肢动脉闭塞患者,闭塞长度最长28cm,最短4.8cm,平均长度17.4±4.9cm;闭塞前段血流频谱形态失常,频窗消失,频带增宽,舒张期反向血流消失,出现静脉样血流频谱或者舒张期正向血流频谱,闭塞后段频谱多普勒呈单向且血流速度减低或无血流频谱。下肢动脉血流频谱各项指标显示,患侧PSV明显低于健侧,AT、RI、PI明显高于患侧（p＜0.05）。结论：多普勒超声血流频谱异常改变是下肢动脉闭塞性疾病一项重要的检查方式,有较高的临床价值。     

双光束激光多普勒测速实验系统的不确定度分析

激光多普勒测速技术被广泛应用于流体速度的测量。为减小双光束激光多普勒测速实验系统的测量误差,依据计量技术规范JJF 1059—1999 中规定测量不确定度的分析步骤和方法,针对系统的测量结果,从数学模型和测量方法2个方面,分析了流速测量中不确定度的主要来源。采用不确定度的A类评定和B类评定,对测量结果的各不确定度分量进行评定,得到了测量结果的合成标准不确定度和扩展不确定度。     

选带细化频谱分析在超声波流量测量中的应用

通过中频调制为基础提取流体速度方向信息,引入选带细化频谱分析技术对解调后多普勒信号进行高精度的频谱估计,该方法主要有三个方面的优点:能判断流速的方向;降低流速测量下限;提高了流速测量的动态响应速度、实时性以及稳定性.在此基础上,以数字信号处理器(DSP)为平台设计了超声流量计,实验分析表明1 024点ZOOM-FFT算法在选定频带范围内与16 384点FFT有相近的分辨率.     

高精度的超声波在线流量测量

针对超声波检测流量过程中的流场分布问题，提出一种高精度在线检测的实用方法及其实现装置。基于混合长度理论对流速进行补偿，建立了测量的数学模型，给出了测量系统的结构框图，并且分析了测量误差的来源以及消除误差的方法。     

组合式光纤流量传感器

提出了一种新型的组合式光纤流量传感器，介绍了它的工作原理，详细讨论了光脉冲转换器的设计要点，据此设计了传感头，并给出了它的实验结果。     

旋进射流喷嘴的频率及流场特性研究

在圆筒套管内设置一块孔板构成旋进射流喷嘴，由此获得持续稳定的旋进射流，研究了喷嘴最优的结构参数。用热线风速仪测量出口射流的速度并进行功率谱分析，得到旋进射流的频率及流场特性。研究表明，旋进射流具有较高的紊流强度，速度场沿径向的分布趋向平缓，其进动频率随流量增大而加快，喷嘴的尺寸和结构对射流进动有影响。     

Velocity Fluctuations in a Particle-Laden Turbulent Flow over a Backward-Facing Step

Dilute gas-particle turbulent flow over a backward-facing step is numerically simulated. Large Eddy Simulation (LES) is used for the continuous phase and a Lagrangian trajectory method is adopted for the particle phase. Four typical locations in the flow field are chosen to investigate the two-phase velocity fluctuations. Time-series velocities of the gas phase with particles of different sizes are obtained. Velocity of the small particles is found to be similar to that of the gas phase, while high frequency noise exists in the velocity of the large particles. While the mean and rms velocities of the gas phase and small particles are correlated, the rms velocities of large particles have no correlation with the gas phase. The frequency spectrum of the velocity of the gas phase and the small particle phase show the -5/3 decay for higher wave number, as expected in a turbulent flow. However, there is a "rising tail' in the high frequency end of the spectrum for larger particles. It is shown that large particles behave differently in the flow field, while small particles behave similarly and dominated by the local gas phase flow.     

Experimental proof of Doppler bandwidth invariance

A line flow of scatterers crossing the sound field produced by a focused circular transducer at uniform velocity originates a quasi-triangular Doppler spectrum. It is known that the spectrum shape and width depend on the line flow to beam axis angle, as well as on the transducer geometry. It has recently been theoretically predicted that this spectrum width is independent of the flow line location in the sound field. Experimental verification of the new theorem, based on the use of a thread phantom operated at various orientations, ranges, and offsets, with respect to the ultrasound transducer, is presented. The tests were made with a computerized pulsed Doppler system designed to perform optimal real-time spectral analysis of data obtained in this application. The prototype system and the experimental procedure adopted for demonstrating in vitro the invariance of the Doppler spectral bandwidth are described.     

Doppler angle estimation using correlation

Conventional Doppler techniques can only detect the axial component of blood flow. To obtain the transverse flow component, an approach based on the dependence of Doppler bandwidth on Doppler angle has been widely investigated. To compute the bandwidth, a full Doppler spectrum is often required. Therefore, this approach has not been applied to real-time, two-dimensional Doppler imaging because of the long data acquisition time. To overcome this problem, a correlation-based method is proposed. Specifically, variance of the Doppler spectrum is used to approximate the square of the Doppler bandwidth. Because variance is computed efficiently and routinely in correlation-based color Doppler imaging systems, implementation of this method is straightforward. In addition, the two-dimensional velocity vector can be calculated and mapped to different colors using the color mapping function of current systems. Simulations were performed, and experimental data were also collected using a string phantom with the Doppler angle varying from 23 degrees to 82 degrees . Results indicate that the correlation-based method may produce significant errors if only a limited number of flow samples are available. With averaging, however, the Doppler angles estimated by the correlation-based method can achieve good agreement with the true angles by using only four flow samples with proper variance averaging.     

Numerical Modelling of Flow Boiling Heat Transfer in Horizontal Metal‐Foam Tubes

The flow boiling heat transfer performance in horizontal metal‐foam tubes is numerically investigated based on the flow pattern map retrieved from experimental investigations. The flow pattern and velocity profile are generally governed by vapour quality and mass flow rate of the fluid. The porous media non‐equilibrium heat transfer model is employed for modelling both vapour and liquid phase zones. The modelling predictions have been compared with experimental results. The effects of metal‐foam morphological parameters, heat flux and mass flux on heat transfer have been examined. The numerical predictions show that the overall heat transfer coefficient of the metal‐foam filled tube increases with the relative density (1‐porosity), pore density (ppi), mass and heat flux.     

Flow Instability of Molten GaAs in the Czochralski Configuration

The flow and heat transfer of molten GaAs under the interaction of buoyancy, Marangoni and crystal rotation in the Czochralski configuration are numerically studied by using a time-dependent and three-dimensional turbulent flow model for the first time. The transition from axisymmetric flow to non-axisymmetric flow and then returning to axisymmetric flow again with increasing centrifugal and coriolis forces by increasing the crystal rotation rate was numerically observed. The origin of the transition to non-axisymmetric flow has been proved to be baroclinic instability. Several important characteristics of baroclinic instability in the CZ GaAs melt have been predicted. These characteristics are found to be in agreement with experimental observations.