Multi-scale analysis on particle dynamic of vertical curved 90° bend in a horizontal-vertical pneumatic conveying system

To reveal the particle dynamic characteristic in the bend, high-speed particle image velocimetry (PIV) and wavelet transform were used to measure and analyze the particle velocity in a horizontal-vertical pneumatic conveying system. The pressure drop and particle velocity are analyzed to elucidate the macroscopic motion properties of particles in the different radius ratio bend firstly. Then the methods of continuous wavelet transform and one-dimensional discrete orthogonal wavelet transform are used to analyze the particle dynamic characteristic in the different regions of the bend pipe in terms of time–frequency characteristics of particle fluctuation velocity, fluctuation energy distributions of wavelet components, and auto-correlation of various frequencies. The results show that the particles are mainly small-scale motion in the rapidly decreasing region, while the large-scale motion increases in the accelerating region near the inlet and the stable region near the outlet. And the results of the wavelet component show that the acceleration and deceleration of particles in the bend will decrease the proportion of high-frequency fluctuation energy. The auto-correlation coefficient of the high-frequency component decays slower and has a longer period at the critical position of the three regions.     

Volume fraction and velocity fields of nearly uniform granular flows in a narrow channel geometry with smooth bed

The combined knowledge of the velocity and volume fraction fields is crucial for investigating the dynamics of granular flows, especially in the dense-collisional regime where both frictional and collisional dissipation mechanisms are significant. A laboratory investigation on steady dry granular flows in a straight channel is reported, where slip conditions are allowed at the basal surface and side walls. The stochastic-optical method (SOM), proposed by Sarno et al. (2016) for estimating the volume fraction in granular mixtures, is applied for the first time to granular flows. The velocity at the free surface and at the flume sidewall is measured by using a multi-pass particle image velocimetry (PIV) approach. The measurements of the velocity and volume fraction reveal a superimposition of different dynamic structures, which can be distinguished by means of a volume fraction threshold. Additionally, the profiles of measured volume fraction are exploited to estimate the pressure distribution, so as to numerically describe the velocity profiles by using the μ(I) rheology. It is found that the employment of the experimental volume fraction is superior in describing the flow dynamics, especially near the free surface.     

Some aspects concerning modelling the flow and heat transfer in horizontal mantle heat exchangers in solar water heaters

An experimental and numerical investigation has been undertaken to study the heat transfer process in horizontal mantle heat exchangers used in solar water heaters. A rectangular cavity has been used as a simplified geometry. With the aid of particle image velocimetry (PIV) the flow field in the centre‐plane of the rectangular cavity has been visualized. Three‐dimensional flow simulations were performed using a commercial CFD package. The impinging jet formed by the inlet flow directed towards the opposite wall was found to produce localised turbulence in the cavity, with an inlet Reynolds number as low as 360. This turbulence was found to effect the flow field and heat transfer in the cavity when the inlet Reynolds number was above 1200. It is shown that, with the boundary conditions used in this study, most of the heat transferred was in the bottom half of the cavity. This is not the ideal situation for optimization of solar water heating systems. Copyright © 1999 John Wiley & Sons, Ltd.     

A modified optical flow algorithm based on bilateral-filter and multi-resolution analysis for PIV image processing

Particle Image Velocimetry (PIV) technology is an efficient and powerful testing method to investigate the characteristics of flow field. The topic of PIV post-processing techniques has roused researchers׳ wide concern for its great influence on the success of flow field measurement. The traditional correlation algorithms have their innate defects. In the present study, a modified optical flow algorithm is proposed to overcome these deficiencies based on bilateral-filter and multi-resolution analysis of PIV image processing. The algorithm is designed based on the principle of multilayer segments, in which the isotropic diffusion method is employed to calculate the low-resolution layer of the image and the nonlinear filtering method is used to process the high-resolution layer. This new algorithm can reduce image noise effectively and maintain the details of the image boundary. In addition, the design of nonlinear filter makes the optical flow equation simpler, and the optimal velocity mapping factor method needs less iteration and reduces the computational load. The algorithm is first tested on synthetic time-resolved channel flow images, and the computational results from the simulated particle images are found to be in reasonable agreement with the given simulated data. The algorithm is then applied to images of actual up-channel flow, and the results also confirmed that the algorithm proposed in the present study has good performance and reliability for post-processing PIV images.     

降低超高层建筑横风向响应气动措施研究进展

通过风洞测压试验,对比不同风向下、不同倒角半径的矩形高层建筑表面风压分布、整体风力及斯托罗哈数St;采用PIV试验,给出建筑的近尾流流动特性,并从流场作用角度,揭示倒角化处理对于矩形高层建筑风荷载特性的影响机理。研究表明：临界风向下,在建筑一侧分离的剪切层发生流动再附,形成分离泡;此时,建筑的阻力达谷值,升力和St达最大值。相比而言,倒角化矩形高层建筑的临界风向小于无气动措施的工况。St主要受到横风向投影宽度和尾流涡对间距的影响,在一定的风向范围内,当倒角半径达一定数值,St将有所增大。在建筑的整体阻力方面,倒角化处理将使得建筑尾流涡对尺寸减小;涡对横向流速增大,涡量掺混运动加剧,旋涡强度减弱。在此作用下,建筑整体阻力降低。在建筑的整体升力方面,采用倒角化处理后,旋涡脱落的不规则性和随机性增大,脱落强度减弱,这促使建筑整体升力减小;但倒角化处理对于升力的减小效应并非见于所有风向。     

PIV系统测量误差的评价方法研究

通过解决高速高精度的转速控制、安全性控制等技术问题研制了PIV基准速度场系统,最高速度达200m/s,且达到了可用于对PIV系统标定的误差要求;介绍了利用基准速度系统对PIV测量误差评价的实验方法,解决了示踪粒子模拟、体激光投射的PIV测量方法等关键技术问题,并完整的介绍了实验的流程;通过PIV系统误差来源分析、速度基准系统不确定度分析以及PIV测量结果分析等完整的介绍了数据分析的方法与流程,得到了1套PIV设备的误差标定结果。     

PIV测速技术分析异型坯连铸结晶器流场

通过建立1[∶]1异型坯结晶器物理模型，采用PIV粒子测速技术，研究断面尺寸为767.3 mm×383.1 mm×103.2 mm异型坯结晶器不同工艺参数条件和不同水口结构对结晶器内流场的影响。PIV实验结果表明，减小拉速和增大水口底部内径可以有效地减小冲击深度，结晶器深度860 mm处水口中心最大流股速度分别下降了27.96%和41.46%；增加拉速和减小水口浸入深度可以提高流场下旋涡上顶点位置。通过减小拉速和浸入深度，增大水口底部内径可以改善结晶器内流场。     

基于PIV测量的涡轮流量计响应分析

应用粒子成像测速技术获得了涡轮流量计叶片入口流场的速度分布信息,并基于该测量结果,运用T-G模型理论得出流量计的响应.通过与以往所采用的几种典型的入口速度分布计算得到的结果比较分析表明,基于PIV测量的结果更接近于涡轮流量计的真实响应.还比较分析了涡轮入口速度分布对涡轮流量计响应的影响机理,相关结果可望为改进涡轮流量计响应的计算分析方法以及优化设计提供有价值的参考.     

Experimental study on the confined flow over a circular cylinder with a splitter plate

An experimental study was carried out to investigate the effect of a splitter plate on wake flows downstream of a circular cylinder symmetrically placed in a confined channel. A particle image velocimetry (PIV) measurement was applied to visualize the flow structure and analyze changes in the vortex shedding process. The control elements of the splitter plate length, L/D (D is the cylinder diameter) was varied from 0 to 1.5 and Reynolds number, Re_D was considered at 2400 and 3000. The experimental results showed that the splitter plate had an influence on stabilization of wake turbulences in a confined channel. For shorter splitter plate length of L/D=0.5 and 0.75 cases, flow structures were significantly modified and the vortex shedding frequency decreased as compared with bare cylinder cases. For longer splitter plate length of L/D=1, 1.25 and 1.5 cases, the generation of a secondary vortex was observed based on the snapshot proper orthogonal decomposition (snapshot POD) analysis. In addition, turbulent characteristics corresponding to turbulent kinetic energy (TKE) and Reynolds shear stress correlations took the lowest values and the dominant vortex shedding frequency disappeared. There was an optimal value of the splitter plate length at L/D=1 on suppression of velocity fluctuations. Moreover, the stabilizing effect of a splitter plate was more obvious at Reynolds number of Re_D=3000 than that at Re_D=2400.     

Flow characteristic of the horizontal cylinder placed on the plane boundary

In this study, the flow characteristic of the circular cylinder laid on the free surface is studied quantitatively using the particle image velocimetry technique. The flow phenomena in the upstream and downstream regions of the horizontal cylinder and free surface junctions increase the instability of the vortical flow structure significantly. Since the scour and burial process of the pipelines laid on the seabed depends mostly on the hydrodynamic behavior of the flow, the present investigation concentrates on the characterization of the flow structures using the time-averaged and instantaneous flow data. Distributions of velocity vectors, patterns of streamlines, vorticity contours and corresponding Reynolds stress correlations characterize the flow structure in detail.