超声波流速测量的多普勒频移提取方法研究

为了提高超声波多普勒法测量复杂流体流量的精度,针对流体的超声回波频率的复杂性,本文研究多普勒流速测量中的频偏提取方法。以傅里叶分析为理论基础,设计了硬件电路并获得代表回波平均频率的信号,然后以该信号作为输入,以数字鉴频法获得回波多普勒频移。基于该方法设计了一种超声多普勒流量测量系统,实验结果显示:油水混合流体流量的测量误差在3%以内,从而证实了此频偏提取方法的有效性。     

A single-scan method for measuring flow along an arbitrary direction

In this article, we demonstrate a single-scan method to measure an average flow velocity vector along an arbitrary direction. This method is based on the MMME sequence and utilizes static and pulsed magnetic field gradients along multiple directions for the optimal determination of flow velocity components in three-dimensional space. Experimentally measured average flow velocities from the flow induced phase shift with a single-scan MMME sequence show excellent agreements with the known flow rate, and the signal decay of each echo due to a velocity distribution is also quantitatively verified with known laminar flow patterns.     

Modeling slender bodies with the method of regularized Stokeslets

The motion and flow generated by immersed structures in a fluid in the Stokes regime can be modeled with a variety of different numerical methods. The mathematical structure of the Stokes equations allows one to describe the flow around a three-dimensional object using only information regarding its geometry. This leads to computational techniques such as boundary integral methods or the method of regularized Stokeslets that discretize the surface of the immersed object in the flow. However, when the body in question is slender, a more computationally efficient alternative is to represent the flow by a one-dimensional discretization along the centerline of the object rather than a discretization of the boundary. Using an exact and an asymptotic solution describing the nontrivial three-dimensional fluid flow generated by a slender precessing spheroid, we present a careful analysis of the approximation of the flow using the method of regularized Stokeslets, where the regularized Stokeslets are placed along the centerline of the spheroid. Guidance is presented on how best to choose the numerical parameters within the method of regularized Stokeslets to minimize the error for a given application.     

摇摆条件下自然循环系统流量混沌脉动的检验与预测

利用替代数据法检验了摇摆条件下自然循环系统不规则复合型脉动的混沌特性, 并在此基础上进行混沌预测. 关联维数、最大Lyapunov指数等几何不变量计算结果表明不规则复合型脉动具有混沌特性, 但是由于计算结果受实验时间序列长度的限制和噪声的影响, 可能会出现错误的判断结果. 为了避免出现误判, 在提取流量脉动的非线性特征的同时, 需要用替代数据法进一步检验混沌特性是否来自于确定性的非线性系统. 本文用迭代的幅度调节Fourier 算法进行混沌检验, 在此基础上用加权一阶局域法进行混沌脉动的预测. 计算结果表明: 不规则复合型脉动是来自于确定性系统的混沌脉动, 加权一阶局域法对流量脉动进行混沌预测效果较好, 并提出动态预测方法. 关键词： 混沌时间序列 替代数据法 实时预测 两相流动不稳定性     

磁绝缘电压叠加器功率流的电路计算方法

以已建成的10级直线变压器驱动源系统为依托,以传输线计算方法为基础,通过引入空间电荷限制流和磁绝缘流阻抗模型,对感应电压叠加(IVA)真空功率流的电路计算方法进行了探索,开发了包含磁绝缘过程的全电路计算程序,提供了一种快速评估该IVA系统真空功率流的方法。计算结果表明：对磁绝缘状态下流阻抗的描述是该方法准确计算的关键;解决好算法的数值稳定性,避免数值振荡的发生,是将这一方法推广应用的重要前提。     

磁绝缘电压叠加器功率流的电路计算方法

以已建成的10级直线变压器驱动源系统为依托,以传输线计算方法为基础,通过引入空间电荷限制流和磁绝缘流阻抗模型,对感应电压叠加(IVA)真空功率流的电路计算方法进行了探索,开发了包含磁绝缘过程的全电路计算程序,提供了一种快速评估该IVA系统真空功率流的方法。计算结果表明:对磁绝缘状态下流阻抗的描述是该方法准确计算的关键;解决好算法的数值稳定性,避免数值振荡的发生,是将这一方法推广应用的重要前提。     

The flow velocity distribution from the doppler information on a plane in three-dimensional flow

In order to observe and estimate the flow of fluid in three-dimensional space, the pulsed Doppler method has been used widely. However, the velocity information acquired is only the velocity component in the beam direction of the wave even if an observation plane is formed by beam scanning. Accordingly, it is difficult to know the velocity distribution in the observation plane in tree-dimensional flow. In this paper, the new idea for processing the velocity distribution in the beam direction on an observation plane for transposing to flux distribution (flow function method) has been introduced. Further, the flow in an observation domain is divided into two kinds of flows, viz., the base flow which indicates the directivity of the flow in the observation domain and the vortex which is considered a two-dimensional flow. By applying the theory of a stream function to the two-dimensional flow, and by using the physical feature of a streamline to the base flow, the velocity component v which intersects perpendicularly to the beam direction is estimated. The flow velocity distribution in a scanning plane (observation plane) can be known from these two components of velocity, viz., beam direction componentu and perpendicular component to the beam directionv. The principle was explained by an example of the blood flow measurement in normal and abnormal heart chamber, by the ultrasonic Doppler method.     

Measurement of gas flow parameters by the laser produced breakdown method

A method of measuring gas flow parameters, based on testing the gas flow by shock waves, is described. The shock waves in the examined flow were produced by a pulsed laser beam focused on to the surface of an object. To illustrate the method, experimental data are presented dealing with gas parameters on the surface of a free-flying object.     

吸气式高超声速飞行器机体/推进一体化内外流非定常耦合方法

基于吸气式高超声速飞行器机体/推进一体化的气动布局设计方式,文章提出了一种内外流一体化流场的耦合求解方法,其中燃烧室内流场采用考虑有限速率化学反应动力学模型的一维非稳态方法求解,进气道和尾喷管外流场采用二维CFD软件计算,进气道与燃烧室在耦合界面处通过一维平均方法实现静温、静压和Mach数等参数传递.并分别以日本国家航空与航天实验室（NAL）的氢燃料燃烧室模型作为内流场验证算例,以某典型高超声速飞行器一体化模型作为内外耦合流场验证算例.研究结果表明:有限速率化学反应准一维方法能较为准确地模拟燃烧室内燃烧流场,提出的内外流场耦合方法能够有效地计算出内外流耦合效应,计算后体压力分布与理论值较接近.该方法可为超燃冲压发动机的性能快速分析和吸气式高超声速飞行器机体/推进一体化的初步分析设计提供重要参考.      

基于混沌吸引子形态特性的两相流流型分类方法研究

利用高灵敏度差压传感器，在垂直上升管中采集到了80组气液两相流差压波动信号.利用放置参考截面的方法，建立了描述混沌吸引子形态的一般方法，在此基础上，提出了不同维数下的吸引子形态特征量进行组合的气液两相流流型分类新方法.研究结果表明：该方法对包括复杂过渡流型在内的气液两相流流型有很好分类效果，预示着混沌吸引子形态描述是研究非线性时间序列的实用有效途径. 关键词： 气液两相流 流型分类 吸引子形态 混合维     

A non-linear observer for unsteady three-dimensional flows

A method is proposed to estimate the velocity field of an unsteady flow using a limited number of flow measurements. The method is based on a non-linear low-dimensional model of the flow and on an expansion of the velocity field in terms of empirical basis functions. The main idea is to impose that the coefficients of the modal expansion of the velocity field gives the best approximation of the available measurements, while at the same time satisfying the non-linear low-order model as closely as possible. Practical applications may range from feedback flow control to the monitoring of the flow in non-accessible regions. The proposed technique is applied to the flow around a confined square cylinder, both in two- and three-dimensional flow regimes. Comparisons are provided with existing linear and non-linear estimation techniques.     

Visualisation of dynamic flow birefringence of cardiovascular models

In this paper, the method of dynamic flow birefringence (DFB) have been studied extensively under the consideration of an application to cardiovascular models. The method utilises the optical interference patterns observed in the birefringent flow for determination of the fluid shear stress and velocity distribution. In order to measure a flow in a cardiovascular model, an assumption of a simplified stress–optical relation in a pulsatile flow is suggested and special experimental techniques such as birefringent fluid for simulating blood and new experimental system have been developed. Application studies focus on pulsatile flows in typical models, namely arterial bifurcation and mechanical heart valves. Experimental results are discussed and compared with those of other researchers.     

Scaling analysis of phase fluctuations in experimental three-phase flows

The characterization of complex patterns arising from three-phase (e.g., oil-gas-water) flows is an important problem with significant engineering and industrial applications. Based solely on measured conductance fluctuation signals from experimental three-phase flows, we propose a method to characterize and distinguish three commonly observed flow patterns. Using the phase characterization method, we first calculate the instantaneous phase from the signals. Then, through performing a scaling analysis, detrended fluctuation analysis (DFA), we extract scaling behaviors associated with the phase fluctuations and find that the DFA scaling exponent is sensitive to the transition among different flow patterns, which can be used to characterize nonlinear dynamics of the three-phase flow. From a novel perspective, we investigate the three-phase flow in terms of phase characterization and scaling analysis. The results indicate that our method can provide new insights into the exploration of complex mechanism in flow pattern transition. The effectiveness of the method is demonstrated and its broader applicability is articulated.     

Melting phenomenon in magneto hydro-dynamics steady flow and heat transfer over a moving surfacein the presence of thermal radiation

The Lie group method is applied to present an analysis of the magneto hydro-dynamics(MHD) steady laminar flow and the heat transfer from a warm laminar liquid flow to a melting moving surface in the presence of thermal radiation.By using the Lie group method,we have presented the transformation groups for the problem apart from the scaling group.The application of this method reduces the partial differential equations(PDEs) with their boundary conditions governing the flow and heat transfer to a system of nonlinear ordinary differential equations(ODEs) with appropriate boundary conditions.The resulting nonlinear system of ODEs is solved numerically using the implicit finite difference method(FDM).The local skin-friction coefficients and the local Nusselt numbers for different physical parameters are presented in a table.     

Time-delayed phase-control for suppression of the flow-induced noise from an open cavity

A flow over an open cavity causes a cavity resonance, which is a feedback mechanism between the acoustic waves and the pressure fluctuation of the cavity flow. Previous research on the reduction of the cavity resonance has focused on suppressing the flow disturbance. This paper presents a time-delayed phase-control method to reduce the global noise of the cavity. Acoustic feedback of the cavity noise, which amplifies the flow disturbance, can be generally reduced by this control method, regardless of the flow physics. The positions of the sensors and the actuator are determined to increase the control efficiency. Experiments show that this control method reduces the peak of the flow oscillation by suppressing the acoustic feedback.     

A clustering-based approach to vortex extraction

Since vortex is an important flow structure and has significant influence on numerous industrial applications, vortex extraction is always a research hotspot in flow visualization. This paper presents a novel vortex extraction method by employing a machine learning clustering algorithm to identify and locate vortical structures in complex flow fields. Specifically, the proposed approach firstly chooses an objective, physically based metric that describes the vortex-like behavior of intricate flow and then normalizes the metric for applying on different flow fields. After that, it performs the clustering on normalized metric to automatically determine vortex regions. Our method requires relatively few flow variables as inputs, making it suitable for vortex extraction in large-scale datasets. Moreover, this approach detects all vortices in the flow simultaneously, thereby showing great potential for automated vortex tracking. Extensive experimental results demonstrate the efficiency and accuracy of our proposed method in comparison with existing approaches.     

Force-coupling method for flows with ellipsoidal particles

The force-coupling method, previously developed for spherical particles suspended in a liquid flow, is extended to ellipsoidal particles. In the limit of Stokes flow, there is an exact correspondence with known analytical results for isolated particles. More generally, the method is shown to provide good approximate results for the particle motion and the flow field both in viscous Stokes flow and at finite Reynolds number. This is demonstrated through comparison between fully resolved direct numerical simulations and results from the numerical implementation of the force-coupling method with a spectral/hp element scheme. The motion of settling ellipsoidal particles and neutrally buoyant particles in a Poiseuille flow are discussed.     

Interactive method for computation of viscous flow with recirculation

An interactive method is proposed for the solution of two-dimensional, laminar flow fields with identifiable regions of recirculation, such as the shear layer driven cavity flow. The method treats the flow field as composed of two regions, with an appropriate mathematical model adopted for each region. The shear layer is computed by the compressible boundary layer equations, and the slowly recirculating flow by the incompressible Navier-Stokes equations. The flow field is solved iteratively by matching the local solutions in the two regions. For this purpose a new matching method utilizing an overlap between the two computational regions is developed, and shown to be most satisfactory. Matching of u, v, as well as u/y is amply accomplished using the present approach, and the stagnation points corresponding to separation and reattachment of the dividing streamline are computed as part of the interactive solution. The interactive method is applied to the test problem of shear layer driven cavity. The computational results are used to show the validity and applicability of the present approach.     

耗散粒子动力学处理复杂固体壁面的一种有效方法

耗散粒子动力学(dissipative particle dynamics,DPD)作为一种介观尺度拉格朗日型粒子方法,已经成功地应用于微纳米流动和生化科技的研究中.复杂固体壁面的处理和壁面边界条件的实施一直是DPD方法发展及应用的一个障碍.提出了处理复杂固体壁面的一种新的方法.复杂固体区域通过冻结随机分布并且达到平衡状态的DPD粒子代表;所冻结的DPD粒子位于临近流动区域的一个截距内;在靠近固体壁面的流动区域中设置流动反弹层,当流动DPD粒子进入此流动层后反弹回流动区域.应用这种固体壁面处理方法对简单流动区域的Poiseuille流动和复杂多孔介质内的流动进行了分析.研究表明,这种新的固体壁面处理方法能够有效模拟复杂固体区域,准确实施壁面边界条件.