The mapping of vortex fields around single and dual bluff bodies

The development and use of a system to measure important parameters of the vortex field in a coordinate grid around several single bluff bodies and dual bluff body combinations are described. Two hot wire sensors were used: a reference sensor at a fixed position and a moving sensor with position coordinates adjusted by using computer control. The strength, frequency and regularity of the vortex shedding are found from the auto-power spectral density of the moving sensor signal. The phase difference between moving and reference sensor signals is found from the cross-power spectral density function between the signals. The results are presented as maps of vortex parameters plotted as a function of sensor position coordinates. The main features of these maps, including the enhancement of vortex shedding from certain dual bluff body combinations, is then discussed.     

双钝体涡街流量计流体振动特性的试验研究

研究了双钝体旋涡脱离的流体动力学特性以及利用双钝体增强流体振动的最佳组合结构。给出了多种钝体组合结构的斯特劳哈尔数的试验结果,表明双钝体结构具有常定的斯特劳哈尔数。用50 mm口径涡街流量计进行的试验证明,双钝体组合在一定的条件下能获得理想的旋涡重叠,从而增强流体的振动。在钝体的轴对称点上,还观察到强度和频率相同,相位相差180°的流体振动点,利用优化的双钝体结构和差动传感技术,研制出小流量灵敏度和抗干扰性能很好的新型旋涡流量计。     

Response of a dual triangulate bluff body vortex flowmeter to oscillatory flow

In this paper, on an experimental facility, the measurement characteristics of a diameter 50 mm dual triangulate bluff body vortex flowmeter in steady flow and oscillatory flow were investigated. Then, the Hilbert Huang Transformation (HHT) method was used to assess the anti-interference performances and the vortex street stability in oscillatory flow for the dual triangulate bluff body vortex flowmeter and a single bluff body vortex flowmeter. Offline simulation was carried out on the anti-interference performances of the dual triangulate bluff body vortex flowmeter signal noise in oscillatory flow by the method of the EMD-scales filter. The major findings are: (a) in most case, the EMD-scales filter may be as good at de-noising effect for the dual bluff body vortex flowmeter in oscillatory flow than that for the single bluff body vortex flowmeter in oscillatory flow. The vortex street stability in oscillatory flow for the dual bluff body is similar to that for the single bluff body. (b) In some special case, the EMD-scales filter is unable to play a better de-nosing role for the dual bluff body vortex flowmeter in oscillatory flow. The invalid condition of the EMD-scales filter for the dual bluff body vortex flowmeter in oscillatory flow is different to that of the single bluff body vortex flowmeter and it was advanced in this paper. (C) The vortex street stability for the dual bluff body vortex flowmeter is better than that for the single bluff body vortex flowmeter.     

Influence of blockage and shape of a bluff body on the performance of vortex flowmeter with wall pressure measurement

Experimental and numerical investigations are carried out with various bluff body shapes to identify an appropriate shape which can be used for vortex flowmeter application. In both the cases vortex shedding frequency is inferred from the fluctuation of wall pressure. The numerical simulations are carried out with cylindrical and triangular bluff bodies to understand the vortex shedding phenomenon and to identify an appropriate turbulence model for this class of flows with wall pressure measurement. The simulations reveal that the k-ε RNG model predicts the Strouhal number closer to the experimental results than other models. The experimental investigations are carried out with several bluff body shapes, such as triangular, trapezoidal, conical, cylindrical and ring shapes, with water as the working medium. In this context, the effects of sampling rate, tap location and blockage effects are explored. The results suggest that the axisymmetric tapping is better than differential pressure tapping in terms of signal amplitude. The non-dimensional location of the static pressure tap is found to be 0.714 times diameter of pipe times blockage. The trapezoidal bluff body is found to be the best among all the bluff bodies investigated in terms of signal amplitude and constancy of Strouhal number. The vortex flowmeter performance is also measured under disturbed flow conditions created by using gate valve and bends. These results are significant because they provide an optimum bluff body shape and blockage, and also present the performance of vortex flow meter under disturbed flow conditions which is rather seldom reported in the literature.     

双钝体涡

涡街流量计测量应用广泛,但小流量信噪比低、抗干扰性能差.本论文提出以双钝体组合强化流体振动,从而降低涡街流量计的计量下限,提高其抗干扰性能.通过实验研究获得了优化的双钝体组合,表明双钝体组合能达到强化流体振动和提高流量计信噪比的目的.     

Numerical study of vortex shedding from different shaped bluff bodies

A computer code based on the stream function-vorticity formulation of the unsteady incompressible Navier Stokes equations has been adapted to treat turbulent flows using the Baldwin-Lomax turbulence model. The code was then used to analyse the flows inside vortex flowmeters with the aim of optimizing performance. The computed Strouhal numbers compared well with those acquired experimentally and the computed flow fields indicated how the bluff body shape can influence the size and strength of the shed vortices, with the T-shaped cylinder producing the largest vortices. In addition to the primary vortices shed from the bluff body, the computations gave a clear indication of the formation of secondary vortices near the meter wall and the front face of the bluff body.     

Vortex shedding mechanisms in single and dual bluff bodies

This paper presents studies of vortex shedding mechanisms in a single bluff body and two dual bluff body combination. These studies are based on video imaging of vortex shedding in open channel flow. Three thousand frames were analysed for each geometry under specified flow conditions and used to derive statistical distributions for the periods of individual cycle. For each of the combinations, the vortex mechanism for a typical half cycle is presented in six stages. The differences in mechanisms between the three geometries are highlighted and used to explain the differences in the statistical distribution of cycle periods.     

Control of laminar vortex shedding behind a circular cylinder using tabs

Small, thin flat plates (called tabs hereafter) are attached to the upper and lower surfaces of a circular cylinder to control vortex shedding and reduce the mean drag and lift fluctuations at the Reynolds number of 100. We vary the location and size of the tabs and the distance between the adjacent tabs. The maximum amount of drag reduction by the tabs is 17%. It is found that the tabs perturb twodimensional vortex shedding and introduce spanwise mismatch of vortex shedding, which weakens the strength of vortex shedding or even annihilates vortex shedding. The present result suggests that these tabs are an effective passive device for the control of vortex shedding behind two-dimensional bluff bodies.     

Comparisons of different bluff bodies in vortex wake transit time measurements

This paper investigates the effects of different types of bluff body on the ultrasonic vortex wake transit time measurement technique for flow metering. Comparisons are made between results obtained from three different bluff bodies of low blockage ratio.     

基于EEMD-Hilbert谱的气液两相流钝体绕流流型识别

为了有效识别气液两相流的流型,以水和空气为实验介质,以涡街流量计为元件诱发钝体绕流,通过管壁差压法获取气液两相流钝体绕流的尾迹波动信号,采用集总经验模态分解对信号进行分解,通过Hilbert变换得到Hilbert边际谱,利用最大互相关系数法对固有模态函数进行筛选,选取特征固有模态函数能量比分别与体积含气率、两相雷诺数构建流型图。结果表明,构建的两类新流型图对单相水、泡状流、塞状流、弹状流等典型流型的识别率分别可达91.67%和88.89%,能较好地满足工程实际应用的需求。     

三棱柱阻流体无阀压电泵的设计与试验

以三棱柱阻流体为无移动部件阀,结合3D打印技术的快速一体成型特点,设计并制作了以压电振子为动力源的三棱柱阻流体无阀压电泵。分析了该无阀压电泵的工作原理、理论流量和振子振动特性,推导出了它的的流量表达式。利用有限元法对三棱柱阻流体的流阻特性进行了仿真模拟,由其内部压强分布及进出口流速情况,定性分析了三棱柱阻流体的正反向流阻大小。最后,使用3D打印机制作了该无阀泵的试验样机,并进行了流阻和流量测量试验。试验结果表明:三棱柱阻流体具有正反向绕流流阻不等的特性,当驱动电压为550V,驱动频率为8 Hz时,该压电泵的输出流量达到最大,为29.8mL/min。结果证明了该三棱柱阻流体无阀压电泵具有良好的输送流体的能力。     

Strategies for the applications of flow control downstream of a bluff body

Flow over a bluff body is a common and significant case for engineering sciences. The well-known problem is the highly unsteady and vortical flow structure developing downstream of the body. The wake of the body has a complex flow dynamics. The chaotic flow structure in the wake causes serious structural deformation on the body; hence the flow is intended to be suppressed downstream of the body. There are numerous research papers for this purpose starting from 1960's. The literature survey points out that the investigations may be mainly classified as theoretical or experimental. Furthermore, the control of flow around bodies arranged as side-by-side or tandem was also studied in the past; however the flow control around a single bluff-body is reviewed in this paper. The investigations are currently continuing due to physical importance of the problem and there are increasing number of papers on flow control downstream of a bluff body which is mostly a circular cylinder. In this study, the recent development on the topic is considered such that the main results presented in papers for the last 15 years are reviewed and recalled in detail. The potential future investigation subjects are also discussed by referring to the major contributing solutions in previous related studies reviewed herein.     

Vortex flowmeter with enhanced accuracy and reliability by means of sensor fusion and self-validation

Due to the complementary qualities of the principles of vortex frequency estimation and vortex time of flight estimation they are ideally suited for an effective sensor fusion within flow measurement. A flow meter which combines the results of the two individual systems in an intelligent manner was built and beside theoretical considerations the performance of the fused system is demonstrated by a variety of measurements. In comparison with conventional vortex meters which only use the vortex frequency to estimate the flow rate, the measuring range can be extended by a factor of 8–10 and the accuracy of the system as well as the robustness to disturbances like a second fluid phase or depositions on the bluff body are strongly increased.     

低雷诺数下涡街流量计线性度的试验

在DN25水流量标准装置上对装有梯形漩涡发生体的涡街流量计在低雷诺数下进行试验研究,利用数字信号处理方法(频谱分析方法)得到涡街脱落频率,较传统模拟信号处理方法在保证较好线性度的基础上,有效地扩展了测量下限。绘制雷诺数—仪表系数曲线,发现梯形发生体在低雷诺数下仪表系数K呈递减趋势,当Re>2 000时K趋于常数,这与圆柱发生体在低雷诺数时曲线递增的趋势截然相反。通过分析得知,管道内流速分布的影响、发生体形状的影响和流速与频率关系的影响是导致上述变化趋势的3个主要影响因素。     

Vortex wake transit time measurements for flow metering

A method of flow measurement is described based on using a vortex wake as a flow tracer shed from a low-blockage-ratio bluff body, the velocity of which is measured by cross-correlation. Preliminary comparisons are made between measurement iof the vortex shedding frequency and the vortex wake transit time between the two ultrasonic beams to determine the flow rates. Whilst preliminary results are confined to a 50 mm diameter sensing head, there is no upper limit to pipe size using the same suitably extended bluff body.     

Development of the T-ring vortex meter

A new type of vortex-shedding meter has been developed, in which the bluff body is in the form of an annular ring with a T-shaped cross-section. Because this is axi-symmetric it disturbs the flow much less than a transverse bluff body. This leads to a meter which is considerably more repeatable, and hence potentially more accurate, than a conventional vortex meter. It also has excellent linearity, and an exceptionally low pressure drop. The paper describes the design and performance as well as the current problems with development of a vortex detector.     

湍流边界层内钝体扰流的流动与传热特性

应用大涡模拟方法对小尺度开缝圆柱涡流发生器强化传热和流动减阻的机理进行研究。水平开缝圆柱置于充分发展湍流边界层内,分析不同间隙比对开缝圆柱尾流、湍流边界层拟序结构以及槽道底面流动与换热特性的影响。为验证所采用数值方法的准确性与可靠性,将矩形空槽道的计算结果与前人直接数值模拟结果及与采用相关准则关系式所得结果进行对比。计算结果表明：湍流边界层内钝体扰流的尾迹流与壁面边界层的相互作用能够显著提高槽道的换热性能。与未开缝的基准圆柱相比,间隙比小于2.0时,开缝圆柱通道的整体热性能较好;间隙比为2.0时,其综合性能系数最大;间隙比大于2.0时,整体热性能较差。与矩形空槽道相比,最大努塞尔数可提高17.45%,最小摩擦因数可减小4.94%。     

Experimental verification and numerical simulation of a vortex flowmeter at low Reynolds numbers

This study presents experimental verification and numerical simulations of a vortex flow meter in the Reynolds number range between 8300 and 50,000. A custom-designed bluff body with a wedge back shape was used in the flowmeter. A shedding frequency of the flowmeter was measured in an air duct using a hot-film probe. To evaluate the accuracy of the flowmeter, a measurement uncertainty analysis was performed. Numerical simulations of the vortex flowmeter were performed with the open source code OpenFOAM. Transient simulations of periodic vortex shedding behind the bluff body were performed using different simulation methods depending on the pipe Reynolds number, such as Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Unsteady Reynolds Averaged Navier Stokes (URANS) method. The simulated vortex shedding frequencies matched the experimental data very well. Experiments and simulations demonstrated a clear linear dependence of the shedding frequency on the volumetric flow rate over the entire range of Reynolds numbers. In addition, numerical simulations were used to study the main mechanisms of vortex formation and shedding behind the considered bluff body.     

Contact mechanics and the wear of metals

It is commonly observed that metallic wear debris takes the form of thin platelets, leading to the term ‘delamination wear’. Modelling this phenomenon has proved a stiff challenge in Contact Mechanics since the fractures which give rise to wear particles lie parallel, or nearly so, to the surface; i.e. on planes of maximum compressive stress. Sectioning the surface layer beneath a wear track has revealed it to have acquired severe plastic strains, which suggests that the cracks are ductile fractures, driven by plastic strain rather than elastic stress intensity. The paper reviews recent research into the progressive plastic deformation of surfaces in repeated sliding: the process known as ‘ratchetting’. Included is an analysis of ‘running-in’ of rough surfaces by repeated sliding and a discussion of the criterion of rupture under cyclic plastic strain.     

Numerical simulation of flow past rectangular cylinders with different aspect ratios using the incompressible lattice Boltzmann method

This paper presents a numerical study of a uniform flow past a rectangular cylinder using the incompressible lattice Boltzmann method (ILBM). Firstly, we use the ILBM to simulate the flow past a square cylinder symmetrically placed in a two-dimensional channel and results are validated against the well-resolved results obtained using finite-difference method and finite-volume method. Secondly, the effects of the aspect ratio defined as R = width/height on the fluid forces, vortex shedding frequency and the flow structures in the wake are investigated. Aspect ratios ranging from 0.15 to 4.00 and four Reynolds numbers Re = 100, 150, 200 and 250 are selected for the investigation. The results show that the effects of aspect ratio on physical quantities such as drag and lift coefficients, Strouhal number and the vortex shedding mechanism are very notable in the range between 0 and 2. In general, the drag coefficient decreases with the aspect ratio and the decreasing rate is more distinct in the range of 0.15 ≤ R ≤ 2.0. There is no local maximum found at around R = 0.6 in the drag coefficient as reported for higher Reynolds numbers in the literature. However the root-mean-square value of the lift coefficient shows a maximum value at R ≈ 0.5 for all Reynolds numbers selected. The variation of Strouhal number with R appears to be different for four selected Reynolds numbers. Especially for Re = 250, a discontinuity in St, as has been observed for higher Reynolds numbers, is observed at around R = 1.45 where multiple peaks are found in the result of Fourier spectrum analysis of the lift force and irregular vortex shedding behavior with no fixed shedding frequency is observed from the instantaneous vorticity contours. Such discontinuity is not observed for Re = 100, 150 and 200. The present results using the LBM are compared with some existing experimental data and numerical studies. The comparison shows that the LBM can capture the characteristics of the bluff body flow well and is a useful tool for bluff body flow studies.