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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.     

回转体表面条纹沟槽减阻水洞实验研究

本文在条纹沟槽表面减阻理论分析的基础上,对条纹沟槽表面回转体进行了大量的水洞实验研究。实验模型表面条纹沟槽采用直接加工方法,头部线型为双参数平方根圆头曲线,尾部线型为双参数尖尾曲线。实验结果分析表明,与相同形状、尺寸的光滑表面回转体相比,条纹沟槽表面回转体在一定速度范围内存在很好的减阻效果,最大减阻量超过6%,且在小攻角范围内减阻量基本稳定,对回转体升力特性也没有影响。对比不同尺寸条纹沟槽的减阻效果发现,降阻量不但随条纹沟槽宽度S变化,而且随来流速度U∞变化,即与无因次沟槽宽度S (文中近似取S =S.LRe2Ckx,其中k为总阻力Cx的修正因子)存在着一定的关系。对于V型条纹沟槽具有减阻效果的S 的范围在10到60之间。本文研究证明,条纹沟槽表面减阻技术在水下航行器设计领域有着很好的应用前景,同时文中所取得的研究成果对该技术的应用具有一定的促进作用。     

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.     

湍流流动中鲨鱼皮表面流体减阻研究进展

快速游动的鲨鱼, 其皮肤表面沿流动方向有序地排列着沟槽状结构, 人们认为这种结构能在湍流流动 中减小表面摩擦阻力. 人们仿真这种生物结构来进行实验研究和应用, 通过复制和改善鲨鱼皮肤表面沟槽状 结构, 使得摩擦阻力最大减小了近10%. 在实验和模拟仿真中, 人们不断讨论和研究湍流流动阻力的形成机制 和沟槽减阻的理论特性. 本文综述了沟槽减阻理论特性的一些研究方法, 并且归纳定义了沟槽减阻最优几何 形状及其尺寸; 详细考虑流体流动的特点, 给出了一种用来选择最优沟槽形状及其尺寸的方法; 综述了目前的 沟槽加工制造技术. 由于鲨鱼皮肤表面存在少量黏液, 从仿生学的角度, 文章最后综述并展望了通过局部应用 疏水性材料来改变沟槽附近流场属性, 从而达到更大程度上减小阻力的目标.     

脊状表面减阻机理研究

针对脊状表面流场的特点,通过实验测量和数值模拟的方法对脊状表面微观流场进行了深入研究,获得了脊状表面湍流边界层的时均速度分布曲线、湍流度分布曲线和微观流场结构.为了得到脊状结构对壁面物性的影响,对脊状表面进行了疏水性测试,获得了液滴在脊状表面上的表观接触角,并通过水洞试验验证了脊状表面的减阻效果.研究表明,与光滑表面相比,脊状表面微观流场结构中存在"二次涡",近壁区的黏性底层厚度比平板的要厚得多,湍流度显著降低,且脊状表面表现出明显的疏水性.由此提出了基于壁面隔离效应、增大湍流阻尼效应和改变壁面物性效应的减阻机理.     

添加剂湍流减阻流动与换热研究综述

添加剂湍流减阻是指在液体的管道湍流中添加少量的高分子聚合物或某种表面活性剂从而使湍流阻力大大降低的现象.从其被发现至今,经过近半个世纪的研究(实验研究、理论分析、数值模拟和实际系统的应用研究),尽管对这一现象及其实际应用价值已有了较为深入的认识,但仍有许多方面尚有欠缺,例如对湍流减阻的机理仍然在探索中. 本文归纳评述了高分子聚合物或表面活性剂添加剂湍流减阻流动与换热现象的研究现状,从湍流减阻剂的特性、减阻剂的湍流减阻机理、湍流减阻发生时的换热机理、减阻流动速度场分布和换热控制等几个方面综述了添加剂湍流减阻流动与换热特性,并综述了湍流减阻剂在实际工业系统中的应用情况,在对添加剂湍流减阻机理、有湍流减阻发生时的对流换热机理等的理解方面进行了新的总结.     

翼型表面脊状结构减阻特性的数值仿真研究

对翼型表面脊状结构进行了数值仿真研究,初步发现了翼型表面脊状结构的减阻特性,并进一步研究了间隔对于翼型表面脊状结构减阻效果的影响规律。针对翼型表面脊状结构的特点,在数值计算过程中对其计算域、计算网格及流动参数进行了合理的处理。结果表明:脊状结构在翼型表面仍然具有很好的减阻效果,其表面边界层内湍流强度、湍动能明显低于光滑表面;有间隔的脊状结构减阻效果优于无间隔的脊状结构,间隔大小与脊状结构尺寸相当时,减阻效果最佳。     

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, $S^{+}\approx 76$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.      

减阻用表面活性剂溶液分子动力学模拟研究进展

减阻用表面活性剂在能源动力及化工领域有着广泛应用,在管道流体中加入少量表面活性剂可以使流动阻力大大降低从而节约能源,对于表面活性剂减阻机理的讨论也是近些年学者关注的热点之一.本文不仅对课题组前些年在表面活性剂溶液流变性、湍流减阻、减阻与传热的相关性、布朗动力学模拟方面的工作进行了概述,而且详细介绍了近三年来在表面活性剂粗粒化分子动力学模拟方面的研究成果.粗粒化模拟是近年来发展起来的方法,目前已广泛应用于化学、生物等诸多领域.在粗粒化分子动力学模拟方面的工作包括:表面活性剂溶液的流变性能与微观结构、表面活性剂溶液湍流减阻机理研究、湍流减阻失效分析三个部分.通过对表面活性剂溶液分子动力学模拟研究进展的回顾,作者认为,利用粗粒化分子动力学模拟方法可以合理揭示表面活性剂胶束的结构与流变性的对应关系,对胶束的断裂与再连接能力进行多维度的评价,如胶束的拉伸能、断裂能、最大拉伸长度、结合能、$\zeta$电势、疏水基驱动作用等方面.并对"黏弹说"减阻机理进行分子模拟层面的验证,对实际应用中的湍流减阻失效原理进行初步分析.最后,根据对近几年分子动力学模拟工作的总结,展望了未来粗粒化分子动力学模拟在表面活性剂方面的研究方向.      

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.     

Drag characteristics of longitudinal and transverse riblets at low dimensionless spacings

A surface grooved with microscopic riblets aligned parallel to the flow is an effective means to reduce the turbulent skin friction up to 10% compared to a smooth surface. The maximum drag reduction is found for a dimensionless rib spacing s ⁺ in the range of 15–17. For s ⁺ < 10, a linear behaviour of the drag reduction curve is predicted by viscous theory. This linear slope of the drag reduction curve is in contradiction to Schlichting’s postulation of a hydraulically smooth behaviour of small-scale roughness in a turbulent flow. This regime of evanescent dimensionless rib spacings is investigated experimentally by direct wall shear stress measurements in a fully developed channel flow. Additionally, a numerical calculation of the viscous flow over riblets was carried out to predict the drag reducing behaviour. The experimental results show a linear drag reducing behaviour down to s ⁺ = 0.3, which is in good agreement with the numerical results of the viscous simulation. The postulation of Schlichting’s hydraulically smooth regime of a rough surface was not confirmed, neither for a riblet surface nor for a surface geometry with grooves oriented perpendicular to the flow. In the latter case, the drag increases as a quadratic function of the roughness height.     

Unsteady viscous flow over a grooved wall: A comparison of two numerical methods

A numerical study is made of the unsteady two-dimensional laminar flow of an incompressible fluid over a periodically grooved wall. Two independent finite difference techniques are employed. One is based on the vorticity-stream function and the other on the vorticity-velocity (i.e. induction law) formulation. The fluid motion is initiated impulsively from rest and is assumed to be spatially periodic in the streamwise direction. The numerical formulations are derived in detail. The generation of vorticity at the solid surface is modelled differently in the two approaches, and this is found to play an important role in determining the surface pressure distribution and the drag coefficient. The flow field is examined during the early transient phase of development, during which the greatest changes occur. Results are presented for a moderate Reynolds number (based on groove depth) equal to 100. It is found that the vorticity-stream function approach does not produce a spatially periodic wall pressure distribution, and therefore global conservation of total vorticity is not achieved. This results in substantial errors in the predictions for the drag coefficient. These deficiencies are not found in the results obtained by the vorticity-velocity formulation.     

Cylinder in viscous incompressible fluid flow

We present a technique for calculating the temperature field in the vicinity of a cylinder in a viscous incompressible fluid flow under given conditions for the heat flux or the cylinder surface temperature. The Navier-Stokes equations and the energy equation for the steady heat transfer regime form the basis of the calculations. The numerical calculations are made for three flow regimes about the cylinder, corresponding to Reynolds numbers of 20, 40, and 80. The pressure distribution, voracity, and temperature distributions along the cylinder surface are found.It is known that for a Reynolds number R>1 the calculation of cylinder drag within the framework of the solution of the Oseen and Stokes equations yields a significant deviation from the experimental data. In 1933 Thom first solved this problem [1] on the basis of the Navier-Stokes equations. Subsequently several investigators [2, 3] studied the problem of viscous incompressible fluid flow past a cylinder.It has been established that a stable solution of the Navier-Stokes equations exists for R40 and that in this case the calculation results are in good agreement with the experimental data. According to [2], a stable solution also exists for R=44. The possibility of obtaining a steady solution for R>44 is suggested.Analysis of the results of [2] permits suggesting that the questions of constructing a difference scheme with a given order of approximation of the basic differential relations which will permit obtaining the sought solution over the entire range of variation of the problem parameters of interest are still worthy of attention.Calculation of the velocity field in the vicinity of a cylinder also makes possible the calculation of the cylinder temperature regime for given conditions for the heat flux or the temperature on its surface. However, we are familiar only with experience in the analytic solution of several questions of cylinder heat transfer with the surrounding fluid for large R within the framework of boundary layer theory [4].     

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.     

Experimental and numerical investigation of flow control on bluff bodies by passive ventilation

In this work, the so-called natural or passive ventilation drag reduction method is investigated experimentally and numerically. Passive ventilation is performed by directly connecting the high pressure region at the front of a body to the lower pressure in the near wake using a venting duct; in this manner, a net mass flux is established within the wake. In particular, in aerodynamic applications it appears suitable to attain a global reduction in the drag of a body moving in a fluid and a reduction in turbulence levels by means of a global modification of the body wake. Velocity field investigations using particle image velocimetry measurements and a Reynolds averaged numerical code are employed at moderately high Reynolds numbers to clarify the effectiveness of drag reduction on a vented bluff body. The numerical and experimental results agree qualitatively, but the amount of reduction for the vented body (about 10%) is underestimated numerically. The effectiveness of drag reduction has been proved both for smooth and rough (single strip) models. Direct balance measurements are used for comparisons.     

Wave drag of an ellipsoid of revolution rapidly moving in a horizontal direction in a uniformly stratified fluid

The problem of the wave drag of ellipsoids moving in a uniformly stratified ideal fluid is considered by means of modeling the bodies by surface distributions of mass sources. Analytical results are obtained using the distributions known from the theory of a uniform fluid, which make it possible to describe the emission of internal waves by rapidly moving ellipsoids of revolution (spheroids) in the limit of large Froude numbers. An asymptotically simplified form of the dependence of the wave drag on the Froude number and the spheroid axis ratio is found. In the particular case of a sphere, the result confirmed earlier by Greenslade by making comparisons with a numerical calculation and experimental data is obtained.     

Friction Drag Variation via Spanwise Transversal Surface Waves

The introduction of spanwise velocity is a promising technique to effect the near-wall turbulent flow field to influence friction drag. However, the essential physical mechanism which significantly reduces friction drag has not been understood, yet. It is the objective of this numerical study to improve the fundamental knowledge on the drag reduction mechanism. The investigation is based on spanwise traveling transversal surface waves which are applied to modify the near-wall flow field and to influence friction drag. Two actuation configurations are analyzed in detail. Compared with an unactuated flat plate boundary layer simulation the first wave setup, which represents a low frequency wave at an amplitude larger than the viscous sublayer, leads to a reduced wall-shear stress resulting in friction drag reduction of up to 9%. The second wave setup, which possesses a higher frequency and an amplitude in the range of the viscous sublayer, yields an increase of friction drag of about 8%. Unlike previous investigations which focus on excitation setups to lower friction drag, the comparison of the two wave setups in this study allows to identify the effects which on the one hand, lead to drag reduction and on the other hand, result in drag increase. That is, due to the pronounced differences the major effects determining the friction distribution are more evident. The two key features for drag reduction are the damping of the wall-normal vorticity fluctuations above the entire surface and the decrease of turbulence production. Furthermore, the effect of rearranging streamwise vorticity, which has been stated to be responsible for drag reduction, is found to occur at increasing and decreasing drag, i.e., it is not the effect that lowers the friction drag.     

动力刚化问题的实验研究

应用频散可控耗散格式对环形激波在圆柱形激波管内绕射、反射和聚焦 的问题进行了数值模拟研究. 研究结果表明环形激波形成强烈聚焦的关键因素是环形激波在 圆柱形管道中向对称轴运动时，绕射激波就不断加速而不作通常情况下的衰减；不同马赫数 的环形激波绕射也产生不同马赫数及形状的准柱形激波，导致聚焦效果和位置的差异；另外， 环形激波聚焦于一个点而圆柱形激波聚焦于一条线,两者有本质不同.     

幂律流体圆柱绕流的格子波尔兹曼模拟简

基于插值补充格子波尔兹曼方法和幂律流体的本构方程,建立了贴体坐标系下适用于幂律流体的格子波尔兹曼模型,模拟了幂律流体的圆柱绕流问题,采用非平衡外推格式处理圆柱表面的速度无滑移边界,利用应力积分法确定曳力系数和升力系数,并与基于标准的格子波尔兹曼方法和有限容积法获得的数值数据进行对比,吻合良好. 进行了网格无关性验证之后,分析了稳态流动时,不同雷诺数下幂律指数对于尾迹长度、分离角、圆柱表面黏度分布、表面压力系数及曳力系数的影响,以及非定常流动中,幂律指数对于流场、曳力系数、升力系数和斯特劳哈尔数的影响. 获得的变化规律与基于其他数值模拟方法得到的结果相一致,充分验证了模型的有效性和正确性. 结果表明：插值补充格子波尔兹曼方法可以用来模拟幂律流体在具有复杂边界流场内的流动问题,通过引入不同的非牛顿流体本构方程,该方法还可以进一步应用于其他类型的非牛顿流体研究中.