Influences of the Helical Strake Cross-Section Shape on Vortex-Induced Vibrations Suppression for A Long Flexible Cylinder

An experimental study on a bare flexible cylinder as well as cylinders fitted with two types of cross-sectioned helical strakes was carried out in a towing tank. The main purpose of this paper is to investigate the effects of strakes’ cross-section on the vortex-induced vibrations （VIV） suppression of a flexible cylinder. The square-sectioned and round-sectioned helical strakes were selected in the experimental tests. The uniform current was generated by towing the cylinder models along the tank using a towing carriage. The Reynolds number was in the range of 800–16000. The strain responses were measured by the strain gages in cross-flow （CF） and in-line （IL） directions. A modal analysis method was adopted to obtain the displacement responses using the strain signals in different measurement positions. The comparison of the experimental results among the bare cylinder, square-sectioned straked cylinder and round-sectioned straked cylinder was performed. The helical strakes can effectively reduce the strain amplitude, displacement amplitude, response frequencies and dominant modes of a flexible cylinder excited by VIV. And the mean drag coefficients of straked cylinders were approximately consistent with each other. In addition, the square-sectioned and round-sectioned strakes nearly share the similar VIV reduction behaviors. Sometimes, the strakes with round-section represent more excellent effects on the VIV suppression of response frequency than those with square-section.

Fabrication of High-Density Out-of-Plane Microneedle Arrays with Various Heights and Diverse Cross-Sectional Shapes

Out-of-plane microneedle structures are widely used in various applications such as transcutaneous drug delivery and neural signal recording for brain machine interface.This work presents a novel but simple method to fabricate high-density silicon(Si)microneedle arrays with various heights and diverse cross-sectional shapes depending on photomask pattern designs.The proposed fabrication method is composed of a single photolithography and two subsequent deep reactive ion etching(DRIE)steps.First,a photoresist layer was patterned on a Si substrate to define areas to be etched,which will eventually determine the final location and shape of each individual microneedle.Then,the 1st DRIE step created deep trenches with a highly anisotropic etching of the Si substrate.Subsequently,the photoresist was removed for more isotropic etching;the 2nd DRIE isolated and sharpened microneedles from the predefined trench structures.Depending on diverse photomask designs,the 2nd DRIE formed arrays of microneedles that have various height distributions,as well as diverse cross-sectional shapes across the substrate.With these simple steps,high-aspect ratio microneedles were created in the high density of up to 625 microneedles mm^(-2)on a Si wafer.Insertion tests showed a small force as low as~172μN/microneedle is required for microneedle arrays to penetrate the dura mater of a mouse brain.To demonstrate a feasibility of drug delivery application,we also implemented silk microneedle arrays using molding processes.The fabrication method of the present study is expected to be broadly applicable to create microneedle structures for drug delivery,neuroprosthetic devices,and so on.

Optimization of Cross-sectional Shapes of the Bi-2223/Ag Wires before Flat Rolling

Rolling process plays an important role in the manufacture of Bi-based high temperature superconductor tapes, and the plastic flow regularities of the superconducting wires during deformation will directly affect the ultimate quality of the tapes. In order to investigate the effect of cross-sectional shapes before fiat rolling on the performance and homogeneity of the tapes, some numerical models of Bi-2223/Ag wires with different cross-sectional shapes including circular, square, elliptical and racetrack cross-sections are constructed during the rolling process. By comparing the relative density, logarithmic strain ratio and length-width ratio on the filaments, it is revealed that Bi-2223/Ag wire with special-shaped cross-section can achieve better conductivity than the round wire, in particular, the racetrack cross-sectional wire has the second best performance among four wires. Based on material processability and experimental condition, tri-pass racetrack drawing technique is employed to optimize the process and obtain racetrack cross-sectional wire. The rolling process of Bi-2223/Ag wire with racetrack cross-section causes more intensive deformation of filaments in the center of the tape and achieves the filaments with larger length-width ratio. Also, the deformation distribution of filaments verifies the numerical results. Consequently, the racetrack drawing technique can be utilized for a reference during the mechanical processing and to increase the current transmission capacities of Bi-2223/Ag tapes.

Optimal Cross-Sectional Shape of Gas/Air Ducts

In industrial plants, ships, and buildings, a large amount of gas and air ducts are applied for equipment connection, HVAC, medium transport, and exhaust, etc. These ducts can be designed in varied cross-sectional shapes, such as round or rectangle. The author reveals through geometric calculation of the duct cross-sectional shapes and engineering experiences that the round cross-section is an optimal shape in the duct system. The round duct has the shorter perimeter than the other cross-sectional shape ducts and the stronger structure in the same working condition. The material saving of the round duct due to the shorter perimeter is quantitatively determined. In the pater, it is shown that the round duct is economically attractive. The economic analysis for the material cost saving is illustrated by an example. For a long duct system, the material and material cost savings are significant. It is suggested that the round duct in the gas and air duct system should have priority as long as the field conditions are allowed. In the paper, the material cost saving is also converted to PW, AW, and FW used for LCC economic analysis.

The recirculation flow after different cross-section shaped high-rise buildings with applications to ventilation assessment and drag parameterization

The building cross-section shape significantly affects the flow characteristics around buildings,especially the recirculation region behind the high-rise building.Eight generic building shapes including square,triangle,octagon,T-shaped,cross-shaped,#-shaped,H-shaped and L-shaped are examined to elucidate their effects on the flow patterns,recirculation length L and areas A using computational fluid dynamics(CFD)simulations with Reynolds-averaged Navier-Stokes(RANS)approach.The sizes and positions of the vortexes behind the buildings are found to be substantially affected by the building shapes and subsequently changing the recirculation flows.The recirculation length L is in the range of 1.6b-2.6b with an average of 2b.The maximum L is found for L-shaped building(2.6b)while the shortest behind octagon building(1.6b).The vertical recirculation area Av is in the range of 1.5b^(2)-3.2b^(2)and horizontal area Ah in 0.9b^(2)-2.2b^(2).The L,Av and Ah generally increase with increasing approaching frontal area when the wind direction changes but subject to the dent structures of the#-shaped and cross-shaped buildings.The area-averaged wind velocity ratio(AVR),which is proposed to assess the ventilation performance,is in the range of 0.05 and 0.14,which is around a three-fold difference among the different building shapes.The drag coefficient parameterized by Ah varies significantly,suggesting that previous models without accounting for building shape effect could result in large uncertainty in the drag predictions.These findings provide important reference for improving pedestrian wind environment and shed some light on refining the urban canopy parameterization by considering the building shape effect.

Analysis of the mass of behind-armor debris generated by RHA subjected to normal penetration of variable cross-section EFP

Analyzing the mass of behind-armor debris (BAD) generated by Rolled Homogeneous Armor (RHA) subjected to normal penetration of variable cross-section Explosively Formed Projectile (EFP) is the purpose of this paper. So theoretical analysis, numerical simulation and experimental data are combined to analyze the influence of variable cross-section characteristic on the time history of crater radius. Moreover the relationships between time history of crater radius (as well as mass of BAD) and the thickness of RHA (from 30mm to 70 mm) and the impact velocity of EFP (1650 m/s to 1860 m/s) are also investigated. The results indicate that: 1) being compared to the variable cross-section characteristic is ignored, the theoretical time history of crater radius is in better agreement with the simulation results when the variable cross-section characteristic is considered;2) being compared to the other three conditions of plug, the theoretical mass of BAD is in the best agreement with the simulation results when the shape of plug is frustum of a cone and the angle between generatrix and bottom is 45- and the axial length of mushroom is considered.

蜗壳断面形状对大型双吸泵性能影响的数值模拟

为研究蜗壳不同断面形状对大型双吸离心泵性能的影响,在确保其他因素不变的条件下,设计3种不同的蜗壳方案进行瞬态数值模拟。结果表明:3种蜗壳断面方案在设计工况下,圆形蜗壳的效率最高为90.7%,其次是梨形蜗壳,效率为90.2%,矩形蜗壳的效率最低为88.9%,且圆形蜗壳的高效区有所扩大;在各流量工况下圆形蜗壳能量损失率均为最少,在设计工况时圆形蜗壳的能量损失率为0.3%,矩形蜗壳的能力损失率为1.1%;在大流量和设计流量下,圆形蜗壳的叶轮所受径向力均最小,梨形蜗壳次之,矩形蜗壳最大;在设计工况下,3种蜗壳方案对蜗壳上的压力脉动均呈现周期性变化,圆形蜗壳在各检测点处压力脉动的振幅幅度均为最小。

轴流泵站蜗壳形进水池的优化设计及研究

文章以轴流排涝泵站为例,采用数值模拟方法,对蜗壳形进水池进行优化设计及研究。针对蜗壳形进水池的安放角、偏心距及螺旋线螺旋角3个主要参数,分析了各参数对泵站水力性能及水泵入口流速分布均匀性的影响。研究结果表明:安放角对泵站性能影响较大,需对电机功率进行复核,防止过载;偏心距为0时,水力性能最优;螺旋线形状对泵站性能影响较小,但较小的螺旋角有利于提高水泵入口流速均匀性。

离心泵侧壁式压水室的模态计算分析

基于有限元分析软件ANSYS对一种具有特殊结构形式的离心泵压水室进行了模态计算分析,获得了无预应力和有预应力两种状态下压水室的前6阶固有频率和模态振型。分析结果表明:侧壁式压水室的各阶固有频率均远离主要激励频率,可有效避免共振的发生;压水室的变形主要发生在进口法兰端和扩散段,可相应增加两处的刚度以优化压水室的振动特性;有预应力状态下压水室的各阶固有频率较无预应力状态下出现约1%的增幅,2种状态下压水室的变形方式基本一致。分析结果为带有侧壁式压水室离心泵的安全运行和结构优化提供了一定的理论指导。

基于CFD的离心通风机蜗壳型线结构性能分析

选取某一大比转速离心通风机为研究对象,分别以等边基元法、不等边基元法和阿基米德螺旋线方程法绘出蜗壳外型线。在三维建模软件CATIA中完成风机实体造型,用F luent进行三维数值模拟,得到详细的风机内部流场。对比以上3种不同的蜗壳绘制方法对通风机流动性能的影响,对计算结果进行了分析并得出了结论。

离心式通风机蜗壳型线设计方法的理论分析与试验研究

详细阐述了蜗壳理论计算方法与工程作图法之间的对应关系 。

蜗壳形状对高速部分流泵性能的影响

利用FLUENT6.3.21软件,选择了S-A湍流模型,对同一叶轮配两种蜗壳的高速部分流泵进行了内部流场数值模拟和性能预测,通过比较分析,提出采用矩形螺旋蜗壳能提高关死点扬程,得到较为平坦的扬程曲线,且能提高泵的效率。并通过内部流场分析,解释了引起外特性差异的原因。

离心通风机蜗壳型线绘制方法的改进

针对离心式通风机的蜗壳内壁型线设计常用两种近似作图方法存在的问题,提出了一种更为合理的新近似作图法,新作图法很好地解决了4段圆弧间相接和相切的问题,并可分别按阿基米德螺旋线和对数螺旋线近似作图。

大型泵站蜗壳出水流道型线的优化与试验

对大型泵站蜗壳出水流道的型线进行了多方案的优化设计和试验研究:将导水锥型线进行了优化,在流道宽度不变的条件下,仅将出水室蜗壳段弧线曲率半径适当加大,同时延长扩散管的长度,减小扩散管出口及蜗壳断面流速.最终得到了水力性能优良的蜗壳出水流道型线方案.在模型比为10.27,叶片角度(-2°)和转速(1400 r/min)都保持不变的情况下,该流道形式的模型泵装置效率达到78%以上,比优化前提高了6个百分点,达到或已超过常用流道型式立式泵装置效率,为蜗壳进出水流道在大型低扬程泵站中推广和应用奠定了基础.

进气口位置及蜗壳结构对微型发电涡轮输出扭矩的影响分析

增加涡轮输出扭矩是提高微型排气回收高效涡轮节能发电系统转换效率的关键。为分析进气口位置以及蜗壳结构对涡轮输出扭矩的影响,建立涡轮受力及输出扭矩的数学模型;揭示不同进气口位置、不同蜗壳结构对涡轮输出扭矩的影响规律,进而设计一种使涡轮产生较大扭矩且对气缸排气特性影响较小的蜗壳,为微型涡轮发电系统的有效集成奠定了基础。

蜗壳断面形状及叶轮位置对蜗壳式轴流泵性能的影响

根据速度系数法设计了1种对称的马蹄形断面蜗壳和2种非对称的圆形断面蜗壳与相同的轴流泵叶轮组合,并基于标准k-ε模型封闭的雷诺平均方程,应用ANSYS CFX 14.5软件,对设计的3个蜗壳式轴流泵内部的三维流动开展数值模拟.当采用马蹄形蜗壳时,设计流量点的扬程和效率最低,蜗壳内部压力分布不如非对称的圆形断面蜗壳均匀.选择水力效率相对较高的蜗壳,将4种轴向位置不同的叶轮与该蜗壳组合,并进行三维流动数值模拟,结果表明:叶轮出口与蜗壳进口中间平面距离40 mm时,轴流泵效率最高,叶轮出口与蜗壳进口中间平面距离80mm时,轴流泵效率最低.此时,过流段和蜗壳内有明显回流和旋涡.轴流式叶轮与蜗壳的相对位置对蜗壳轴流泵的扬程-流量曲线和效率-流量曲线都有明显的影响.

The lining responses for shallow mountain tunnels subjected to frost heaving

Mountain tunnels in cold regions are vulnerable to adverse effects of freezing action.Thus,it is necessary to identify the lining responses of shallow mountain tunnels subjected to freezing action.To quantify the influence of freezing action and key design parameters(such as cross-sectional shape;lining thickness;and waterproof measures)on the lining response,a thermal-hydro-mechanical coupled finite element(FE)model is established and verified.Then,specific consideration is given to the lining internal force and resulting axial stresses.And the influences of the cross-sectional shape,concrete parameters,and waterproof measures on the lining responses are investigated.Generally,the rectangular tunnel has the worst security;the circular tunnel is the safest.On the other hand,when the thermal conductivity is less than 2.2[W/(m·K)],a greater thermal conductivity will cause a greater risk of damage to the lining.Moreover,the drainage plate can reduce the value of minimum axial stresses,whether frozen or not,even eliminating the tensiondamaged area.Overall,this study helps to estimate the lining responses and prevent frost damages for shallow mountain tunnels during freezing period.

离心风机蜗壳型线设计方法的分析研究

蜗壳是离心式通风机的一个重要部件,其结构是复杂的空间曲面体。蜗壳设计较明显的影响参数是蜗壳的型线、蜗壳截面形状和布置方式,如何绘制蜗壳型线将直接影响蜗壳内的流动损失。本文对现有文献介绍的通风机蜗壳的工程设计方法进行了分析,总结了各设计方法的特点及其适用范围,并就对离心风机性能影响较明显的蜗壳型线和截面形状等因素进行总结分析,对离心风机蜗壳型线设计方法的改进提供了一定的工程设计借鉴价值和学术价值。

蜗壳截面形状对舰用离心泵性能特性的影响

为研究蜗壳截面形状对舰用离心泵性能的影响作为后续研究舰用离心泵噪声振动特性的理论依据,在蜗壳基圆、各断面面积、隔舌螺旋角、隔舌安放角和蜗壳出口面积相同的情况下,改变蜗壳截面形状,分别设计A型、B型和C型蜗壳。对3种蜗壳型式离心泵进行数值计算。结果表明,3种型式蜗壳离心泵的外特性变化趋势基本一致。小流量工况的A型蜗壳离心泵效率略小,大流量工况的B型和C型蜗壳离心泵效率小。随着流量的增大,B型和C型蜗壳离心泵效率下降幅度较大。3种型式蜗壳离心泵在相同工况条件下,压力场、速度场、湍动能和空化现象的变化趋势大致相同。3种型式蜗壳离心泵流场特性在隔舌流道和蜗壳流道处均不相同,表明改变蜗壳截面形状主要影响叶轮靠近隔舌流道和蜗壳流道内的流场。

离心风机蜗壳型线设计方法探讨

对传统和新理论蜗壳型线设计方法进行分析,总结出各设计方法的优缺点。提出考虑曲率对湍流结构的影响,引入流量修正系数为主线设计蜗壳型线,以提高风机性能。