水下减阻技术研究综述

在简要回顾早期减阻研究的基础上,对现有典型水下湍流减阻技术进行了较深入地分析.重点介绍了脊状表面减阻、微气泡减阻和疏水/超疏水表面减阻的研究现状.分别从实验研究和理论研究两方面对其进行了阐述,并着重强调了各自的减阻机理. 此外,还简要介绍了柔顺壁面减阻、壁面振动减阻等其它减阻技术.展望了水下减阻技术今后的研究重点及其应用前景.      

超疏水沟槽表面通气减阻实验研究

减阻是解决航行体提速和增程的主要技术途径之一, 对缓解日益严峻的能源危机极为重要. 在重力式管道实验系统中, 测试给出了湍流状态下不同通气速率时减阻率随雷诺数及沟槽无量纲间距的变化规律和气膜铺展状态, 对比分析了单纯超疏水表面与超疏水沟槽表面上通气时减阻效果的差异.实验板材质为无色亚克力, 沟槽结构采用机械方法加工, 并在表面喷涂超疏水涂层. 结果表明, 持续通气能解决超疏水沟槽表面气膜层流失问题, 实现气膜层长时间稳定维持; 恒定雷诺数下, 随通气速率增大, 超疏水沟槽表面气膜铺展更趋均匀, 减阻率上升; 由于通气速率影响气膜横向扩展能力, 致使恒定通气速率下, 减阻率随雷诺数的变化呈现两种模式; 在固定雷诺数及通气速率时, 减阻率随沟槽尺寸的扩大先增后减, $S^{+}\approx 76$时减阻率最大. 分析其原因在于, 沟槽结构增大沾湿面积的同时, 显著提升了通气状态下超疏水表面气膜层的稳定性, 因而展示出与超疏水表面和沟槽表面均不相同的减阻规律, 且效果更佳.      

超疏水沟槽表面通气减阻实验研究

减阻是解决航行体提速和增程的主要技术途径之一,对缓解日益严峻的能源危机极为重要.在重力式管道实验系统中,测试给出了湍流状态下不同通气速率时减阻率随雷诺数及沟槽无量纲间距的变化规律和气膜铺展状态,对比分析了单纯超疏水表面与超疏水沟槽表面上通气时减阻效果的差异.实验板材质为无色亚克力,沟槽结构采用机械方法加工,并在表面喷涂超疏水涂层.结果表明,持续通气能解决超疏水沟槽表面气膜层流失问题,实现气膜层长时间稳定维持;恒定雷诺数下,随通气速率增大,超疏水沟槽表面气膜铺展更趋均匀,减阻率上升;由于通气速率影响气膜横向扩展能力,致使恒定通气速率下,减阻率随雷诺数的变化呈现两种模式;在固定雷诺数及通气速率时,减阻率随沟槽尺寸的扩大先增后减, S~+≈76时减阻率最大.分析其原因在于,沟槽结构增大沾湿面积的同时,显著提升了通气状态下超疏水表面气膜层的稳定性,因而展示出与超疏水表面和沟槽表面均不相同的减阻规律,且效果更佳.     

超疏水表面滑移理论及其减阻应用研究进展

减阻技术对提高原油采收率、降低液体流动阻力具有十分重要的意义.通过论述超疏水表面结构的基本理论、超疏水表面形成的主要影响因素和近年来仿生超疏水表面的制备方法,综合分析了超疏水表面滑移理论和基于这一理论的减阻技术的研究进展,并简单介绍了其在石油储层微孔道纳米降压减阻方面的应用,展望了超疏水表面滑移理论及其减阻技术的研究重点及应用前景.      

逆向涡对超疏水壁面减阻影响的TRPIV实验研究

超疏水壁面由于具有减阻和自清洁功能而成为国内外减阻和海洋防污等研究领域的热点之一,而20世纪湍流中相干结构的发现为湍流的控制指出新的方向,尤其近壁区涡结构对摩擦阻力贡献很大.利用高时间分辨率粒子图像测速技术,研究了超疏水壁面(SH)以及亲水壁面(PH)湍流边界层中正负展向涡的空间分布特征,研究逆向涡对超疏水壁面近壁区流动结构的影响和超疏水壁面的减阻机理.首先利用空间多尺度局部平均涡量的概念提取壁湍流发卡涡展向涡头(顺向涡)和逆向涡,实现了准确识别涡心并排除小尺度涡的干扰;然后根据检测到的顺向涡和逆向涡流线分布图,发现逆向涡始终处于正向涡的上游和下方,并且对正向涡的进一步发展起抑制作用;最后对两种壁面边界层中逆向涡数量以及出现概率进行对比,发现具有减阻效果的超疏水壁面边界层中出现更多逆向涡.说明逆向涡可抑制上方顺向涡与壁面的强烈剪切,并使靠近壁面的流体加速,从而产生减阻效果;超疏水壁面中涡结构具有更大的β角,使其更好地阻碍了发卡涡头附近强烈的喷射和扫略;超疏水壁面逆向涡出现概率明显大于亲水壁面.这些结果表明:超疏水壁面表现出的减阻特性(Reδ≈13 500,减阻5.8%)与两板产生逆向涡的差异有关.     

基于滑移理论的超疏水表面减阻性能分析

采用滑移理论计算和数值模拟方法研究超疏水表面的减阻性能.层流流动状态的数值模拟和理论计算结果的一致性较好,并可证明滑移速度与通道内流体速度之比与无量纲压降比的数量级相当;湍流状态的数值仿真结果表明,在目前可实现的滑移速度范围内,超疏水表面对水下航行器的流体阻力影响不显著.     

微纳结构超疏水表面的湍流减阻机理研究

超疏水表面的优异性质使其在现代生活和工业生产中具有十分广泛的潜在应用价值. 本文采用了碳纳米管缠绕技术和聚氟硅氧烷疏水化处理方法制备了具有二级微纳米结构的超疏水表面. 测量了由该超疏水表面构建的槽道中的流动压降,将其与普通表面构建的槽道内的流动压降进行比较,发现在层流情况下,流动阻力减小最多达到了22.8%. 在湍流的情况下,超疏水表面的减阻比例约为53.3%,减阻效果比层流更加明显.利用PIV (particle image velocimetry) 技术测量了具有超疏水表面的槽道内的速度场,通过超疏水表面速度滑移和湍动脉动场信息,分析了湍流减阻效果比层流更加明显的物理机制.     

超空泡技术现状、问题与应用

超空泡技术可以使运动体在水中的阻力降低90{\%}左右, 辅以先进的推进技术, 运动体在水中将可以实现超高速的``飞行'. 超空泡减阻技术对海战武器的研制产生了巨大的影响,目前, 俄罗斯已经研制成功了速度达100\,m/s的``暴风'超空泡鱼雷; 美、德、法等国都正在进行超空泡减阻技术的基础与应用研究; 我国也于近年开展了超空泡技术的基础研究. 本文综述了超空泡技术研究的现状、问题与应用. 介绍了空化的基本原理、超空泡概念及其减阻机理; 总结了空化器设计、通气超空泡的生成与控制技术、空泡稳定性及超空泡航行体稳定性技术等若干关键技术; 回顾了国内外超空泡技术试验与数值模拟研究进展等. 最后分析了大型超空泡武器研究的技术难点, 并对超空泡技术领域的研究方向进行了展望.      

材料表面润湿性调控及减阻性能研究

设计合成不同结构的自组装分子,使其可以在不改变表面粗糙度的情况下改变表面的润湿性能;利用低表面能涂层修饰粗糙表面得到超疏水表面.采用流变仪和水洞试验分别在层流和湍流流动状态下测试了具有不同润湿行为的亲、疏水材料的减阻性能.结果表明:在层流流动状态,随着不同表面的接触角从13°增加到45°、113°和161°,减阻率随之从1.8%增大到7.2%、7.9%和14.9%;在湍流流动状态下,自组装涂层接触角为13°、45°和113°的三组模型的平均减阻率为0.8%、1.9%和6.8%,最大减阻率分别可达3.6%、9.2%和18.0%.两种流体流动中均存在材料表面水接触角增加减阻效率增大的行为.     

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 在条纹沟槽表面减阻的研究发展及其优点的基础上 对条纹沟槽表面减阻进行水洞实验,研究在航行器外表面加工条纹沟槽所具有的阻 力特性. 通过对光体、0.1mmV型条纹沟槽表面和0.2mmV型条纹沟槽表面3 种不同模型在零攻角、不同水流速度下进行阻力测量, 测得条纹沟槽表面减阻特 性曲线. 从特性曲线中可以看出减阻效果约为8.3%. 实验的结果表明, 条纹沟槽 表面能明显降低水下航行器的阻力.     

水下湍流减阻途径分析

综述了壁湍流边界层分层结构及近壁区湍流猝发过程,对现有典型水下湍流减阻技术进行了较深入地总结.根据各类减阻方法的特点对水下湍流减阻方法进行了分类,并分析了各类减阻方法对壁湍流流场的影响特征,总结了维持或延长层流、干扰湍流"猝发"、壁面隔离、增大湍流阻尼和改变壁面物性等5种水下湍流减阻可行途径,为开展水下湍流减阻研究及发现新的减阻方法提供参考.     

等离子体减阻技术的研究进展

利用等离子体的特性减小飞行器的气动阻力是一种新概念的减阻思路.目前国外的一些风洞实验和计算结果已证明了等离子体减阻效果的显著性.本文介绍了国外等离子体减阻技术的研究进展,包括:逆向等离子体喷流减阻、基于等离子体边界层控制减阻和局部施加能量点源减阻.分析了等离子体减阻的基本原理及所涉及的一些关键技术点.最后针对我国的减阻研究情况,提出了等离子体减阻技术的研究方向.      

有气体溶解和没有气体溶解的水中石墨烯间的疏水引力

凝聚物理学界发现溶解在水中的气体会在疏水表面吸附,由此认为当疏水物体间距足够小的时候,吸附在疏水表面的气体会相互联通形成纳米气泡桥,纳米气泡桥连结疏水物质形成疏水引力,但是关于纳米气泡桥的形成过程和形态,力学界还没有一个清晰的描述.采用分子动力学方法,研究了在有气体溶解的水中和没有气体溶解的水中两片石墨烯间的引力作用,分析了两种情况下各相密度分布的变化过程、结构相图的变化过程和平均力势的变化过程,详细阐明了纳米气泡桥的形成和消失过程,并定量计算了纳米气泡桥的作用效果和作用距离.模拟结果表明:两片石墨烯在有气体溶解的水中和无气体溶解的水中的疏水引力都是由纳米气泡桥引起的.当石墨烯间距小于0.5nm时,无论水中是否有气体溶解,疏水引力由真空纳米气泡桥引起;当石墨烯间距大于0.5nm时,在没有气体溶解的水中,疏水引力由水蒸气纳米气泡桥引起;而在有气体溶解的水中,疏水引力由所溶气体形成的纳米气泡桥引起.      

水下条纹沟槽表面近壁区流场的数值分析

近年来对于水下条纹沟槽表面减阻的研究多集中在试验方面,理论分析和数值计算进行的却不多。而由于受实验条件的限制,条纹沟槽表面减阻的微观机理也一直没有搞清楚。本文正是通过理论方面的分析和计算来进一步揭示条纹沟槽表面的微观流场。首先基于“第二涡群”论对条纹沟槽表面流场的减阻特性进行了理论分析,阐述了其减阻机理;在此基础上采用B．L两层代数模型,建立了条纹沟槽表面水下流场的简化数学模型;并利用有限差分法对近壁区流场进行了数值计算．首次通过理论计算的方法获得了条纹沟槽表面水下微观流场的速度分布;为以后进一步从理论和实验上研究条纹沟槽减阻特性奠定了重要的基础。     

CHARACTERISTIC OF DROPLET OSCILLATION ON THE SURFACE OF RECTANGULAR HYDROPHOBIC GROOVES

液滴振荡行为是液滴运动中的重要伴随现象,具有重要科研价值.由于液滴撞击疏水沟槽板时运动行为与光滑表面明显不同,可以推测疏水沟槽表面液滴振荡特性也将会呈现与众不同的行为特点.采用高速摄像技术,研究了矩形疏水沟槽表面上水滴高度和接触线振荡行为随沟槽尺寸和撞击速度的变化规律.结果发现,矩形疏水沟槽造成的各向润湿异性使得振荡过程中水滴在平行沟槽方向上的接触线长度大于垂直方向,但并不影响水滴高度方向上衰减振荡的周期,即水滴振荡周期与沟槽间距无关;同时由于疏水沟槽表面上存在能垒束缚效应,致使水滴振荡过程中接触线的铺展和回缩运动不服从典型阻尼振荡规律,而呈现振荡数次后直接趋稳的特点.如水滴以0.61 m/s撞击时,接触线经历2次振荡后即维持稳定,但此时水滴仍在持续振荡中.另外,还初步分析了水滴振荡周期与沟槽间距无关的原因.     

Modeling of drag reduction in turbulent channel flow with hydrophobic walls by FVM method and weakly-compressible flow equations

In this paper the effects of hydrophobic wall on skin-friction drag in the channel flow are investigated through large eddy simulation on the basis of weaklycompressible flow equations with the MacCormack's scheme on collocated mesh in the FVM framework. The slip length model is adopted to describe the behavior of the slip velocities in the streamwise and spanwise directions at the interface between the hydrophobic wall and turbulent channel flow. Simulation results are presented by analyzing flow behaviors over hydrophobic wall with the Smagorinky subgrid-scale model and a dynamic model on computational meshes of different resolutions. Comparison and analysis are made on the distributions of timeaveraged velocity, velocity fluctuations, Reynolds stress as well as the skin-friction drag. Excellent agreement between the present study and previous results demonstrates the accuracy of the simple classical second-order scheme in representing turbulent vertox near hydrophobic wall. In addition, the relation of drag reduction efficiency versus time-averaged slip velocity is established. It is also foundthat the decrease of velocity gradient in the close wall region is responsible for the drag reduction. Considering its advantages of high calculation precision and efficiency, the present method has good prospect in its application to practical projects.     

Drag reduction effectiveness and shear stability of polymer-polymer and polymer-fibre mixtures in recirculatory turbulent flow of water

The turbulent drag reduction caused by polymer-polymer and polymerfibre mixtures has been measured in recirculatory flow of water. Shear stability studies have also been made on a number of drag reducing polymers, asbestos fibres and their mixtures in recirculatory turbulent flow of water. Reynolds numbers ranged from 20,000 to 57,000. Both positive and negative deviations from linear additive behaviour have been observed in drag reduction caused by the polymer-polymer mixtures depending upon their compositions, flow rate and polymer species in the mixture. The drag reduction by the mixtures has been predicted by using simple mixture rule equations including an interaction parameter. This interaction parameter is believed to depend upon the polymer interaction in the polymer mixture. The random coil size and rigidity of the polymer molecules appear to be responsible for the synergism observed in the drag reduction caused by the mixture. In general, mixtures having larger solvation number seem to give positive synergism.Synergism in drag reduction by the polymer-fibre mixtures has also been observed. The simple mixture law equation with interaction parameter is also applicable in predicting the drag reduction by the mixtures as above. The random coil size of the polymer molecules and the rigidity of the polymer-fibre system appear to be responsible for the synergism observed in drag reduction. In the shearstability studies it has been observed that the decrement in drag reduction (DR) is higher than the the decrement in absolute viscosity in most cases. Carboxymethyl cellulose is found to be the most shear stable polymer followed by guar gum, xanthan gum and polyacrylamide. The mixtures exhibiting synergism in causing drag reduction are found to be more shear stable.     

Robust design and numerical simulation on drag reduction by a mixture film for liquid turbulence in a channel

An important way of increasing the speed and lowering the fuel consumption of ships is by decreasing the frictional drag. One of the most promising techniques for reducing drag is the use of air bubbles. The goal of this investigation is to establish a set of optimum robust parametric levels for drag reduction by a mixture (air–water) film in turbulent channel flow. Based on the conditions laid out by the Taguchi orthogonal array method, turbulent flows, with air bubbles injected into a channel, are simulated using commercial computational fluid dynamics software. The local shear stress on the upper wall is computed to evaluate the efficiency of drag reduction. Many factors can affect drag reduction. The factors investigated in this study are the rate of air injection, bubble size, area of air injection, flow speed, and measured position of the shear stress. These factors have been investigated through the analysis of variance, which has revealed that the rate of air injection and water flow speed dominate the efficiency of drag reduction by a mixture film. According to the results, the drag can be reduced by an average of 83.4%; and when the configuration of the parametric levels is optimum the maximum drag reduction of 88.5% is achieved. Copyright © 2008 John Wiley & Sons, Ltd.     

Influence of wavy structured surfaces and large scale polymer structures on drag reduction

Drag reduction was studied for turbulent flow over a structured wall that contained 600 sinusoidal waves with a wavelength of 5 mm and an amplitude of 0.25 mm. A concentrated solution of a co-polymer of polyacrylamide and sodium acrylate was injected into the flow through wall slots. Laser Doppler velocimetry was used to measure turbulence. A fluorescence technique was developed that enabled us to demonstrate the existence, under certain circumstances, of large gelatinous structures in the injected polymer solution and in the flow channel.At maximum drag reduction, the Reynolds shear stress was zero and the velocity field was the same as found for a smooth surface. Larger drag reductions could be realized for a wavy wall because the initial drag was larger. The influences of polymers on the turbulent fields are similar for smooth and wavy boundaries. These results are of interest since the interaction with the wall can be quite different for water flow over smooth and wavy boundaries (which are characterized as being completely rough). An important effect of polymers is a decreasing relative importance of high frequency fluctuations with increasing drag reduction that is characterized by a cut-off frequency. This cut-off is the same for smooth and wavy walls at maximum drag reduction. The sensitivity of drag reduction to the method of preparing and delivering the polymer solution suggests that aggregation of polymers could be playing an important role for the system that was studied. For example, drag reduction was enhanced when large polymer structures are present.