管道内均匀与非均匀PEO溶液湍流减阻特性研究

在自制重力式循环水槽中(最高试验Re数可达105),测试分析了管道内均匀与非均匀聚氧化乙烯(PEO)溶液的湍流减阻规律。结果表明,均匀PEO溶液减阻率(DR)随Re数和浓度增加而提高(最大减阻率70%),但浓度大于临界浓度(100p.p.m)后DR不再增加;而相同浓度均匀PEO溶液的减阻效果随时间逐渐减弱,且Re数越大,剪切破坏越严重,DR下降越明显。与均匀PEO溶液不同,非均匀PEO溶液(从管壁狭缝向内喷射PEO溶液所得)不受剪切破坏影响,减阻效果更稳定,且药品使用量小;同时通过增大喷射速率、Re数和喷射液浓度,可提高减阻率(最大减阻率50%)。对比可见,喷射减阻方式仅需消耗少量药剂,就能实现显著、稳定减阻,有望用于水下航行体湍流减阻。     

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.     

HPAM稀溶液在微圆管中流动特性的实验研究

实验研究了部分水解聚丙烯酰胺(HPAM)稀溶液(水溶液)在内径为10.1 ~325 \mu m石英微管中的高剪切速率(1 215 ~23 120 s ^{ -1} )流动. 结果表明, 聚合物溶液在管径小于100 \mu m微管中的流动具有明显的微尺度效应, 实测流速高于由同种溶液在常规管径圆管中的流动规律所预测的流速. 实测流速与预测流速之间的偏离程度与管径和剪切速率有关: 相同剪切速率下, 管径越小, 偏离越明显; 在小管径微管中, 偏离随剪切速率的增加而减小, 而当管径超过30.7 \mu m后, 偏离不再随剪切速率的变化而改变.      

高分子溶液壁湍流减阻机理的TRPIV实验研究

利用高时间分辨率粒子图像测速技术(TRPIV)对回流式水槽中低浓度高分子溶液壁湍流的减阻机理进行实验研究。通过对比分析高分子溶液和纯水平板湍流边界层在相同来流速度下的平均速度剖面、湍流强度和雷诺应力,发现高分子溶液的壁面摩擦阻力减小了21.77%,并且其缓冲层增厚,按对数律外移,雷诺应力减小;高分子聚合物主要在近壁区起到抑制湍流脉动的作用,而在主流区的作用不太明显。用流向局部平均多尺度速度结构函数和相干结构条件采样方法,检测并对比了高分子溶液和水的壁湍流相干结构“喷射”和“扫掠”事件中的脉动速度、展向涡量、雷诺应力等物理量的二维拓扑形态,发现高分子溶液近壁区相干结构在猝发时的脉动速度减小,涡量受到抑制,雷诺应力明显减小,说明高分子溶液湍流近壁区相干结构“喷射”和“扫掠”的强度变弱,猝发频率降低,动量和能量的输运减弱,揭示出高分子溶液减阻的重要机理。     

基于单个压电振子的湍流边界层主动控制

利用安装在壁面上的单个压电振子周期振荡,采用开环主动控制方案,实现了对平板湍流边界层相干结构猝发的主动控制和壁湍流减阻.根据不同的输入电压幅值和频率,完成了10种工况的实验.在压电振子下游2mm处,用热线风速仪和迷你热线单丝探针,精细测量湍流边界层不同法向位置瞬时流向速度信号的时间序列,分析了在Re_?=2183压电振子振动对湍流边界层平均速度剖面、减阻率和相干结构猝发过程的影响.实验结果表明,施加控制的工况使平均速度剖面对数律层上移,产生减阻效果;压电振子振幅越大,减阻率越高,减阻效果越明显;通过对施加控制前后流向瞬时速度的多尺度湍涡结构脉动动能的尺度分析,当压电振子振动频率与壁湍流能量最大尺度的猝发频率相近时,减阻率达到最大,为25%,说明控制壁湍流能量最大尺度相干结构的猝发是实现壁湍流减阻的关键;通过对比相干结构猝发的流向速度分量条件相位平均波形,发现施加控制的工况中相干结构猝发流向速度分量的波形幅值明显降低,且流向速度在扫掠后期高速阶段迅速衰减,缩短了高速流体的下扫过程,说明压电振子的振动能抑制相干结构的高速流体下扫过程,减弱高速流体与壁面的强烈剪切过程,并使近壁区域相干结构的振幅显著减弱,迁移速度加快,从而减小壁面摩擦阻力.      

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.      

Herschel-Bulkley模型下磁流变减振器旁路泊肃叶流动分析

制备了羰基铁粉(CIP)的质量分数为70%的硅树脂基磁流变胶。用安东帕MCR-301型流变仪对其流变特性进行了测量,并对测试结果进行了Herschel-Bulkley本构模型参数识别。最后对MRG-70在该模型下磁流变减振器旁路中的泊肃叶(Poiseuille)流动展开了分析。试验及分析结果表明,在外加磁场下,Herschel-Bulkley本构模型中非牛顿指数n1。剪切应力随半径的增大而线性增大;当τ_rτ_y形成剪切流,反之则为柱塞流动。在剪切流动区域内,流速随半径增大而非线性降低,剪切速率随半径增大而非线性增大。磁感应强度越大,柱塞流区域越大,柱塞流区域内剪切速率恒为0。体积流量随磁感应强度增强而非线性减小。在0mT～500mT之间体积流量随磁感应强度变化剧烈,进一步增加磁感应强度则变化缓慢。     

TRPIV MEASUREMENT OF DRAG-REDUCTION IN THE TURBULENT BOUNDARY LAYER OVER RIBLETS PLATE

采用高时间分辨率粒子图像测速技术对沟槽壁面平板湍流边界层速度矢量场的时间序列及其统计量进行了实验测量,讨论了在同一来流速度下沟槽壁面对平均速度剖面﹑雷诺切应力及湍流强度的影响. 用流向速度分量的多尺度空间局部平均结构函数辨识壁湍流多尺度相干结构,用条件采样和相位平均技术提取壁湍流多尺度相干结构喷射和扫掠事件的脉动速度、展向涡量的二维空间拓扑形态. 结果表明,与同材料光滑壁面对比,沟槽壁面实现了10.73%的摩阻减小量;沟槽壁面湍流边界层湍流强度及雷诺切应力皆比光滑平板湍流边界层对应统计量小,说明沟槽壁面有效降低了湍流边界层内流体的脉动. 通过比较壁湍流相干结构猝发事件各脉动速度分量与展向涡量的空间分布特征,肯定了沟槽壁面的减阻效果,发现沟槽壁面通过抑制相干结构猝发事件实现减阻.     

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.      

Pathline analysis of traveling wavy blowing and suction control in turbulent pipe flow for drag reduction

Skin friction drag is much greater in turbulent flows as compared with that in laminar flows. It is well known that traveling wave control can be used to achieve a large drag reduction. In the present study, a direct numerical simulation of a turbulent pipe flow was performed to clarify the mechanism of the drag reduction caused by the traveling wave control. The flow induced by the control was evaluated using pathline analysis. Near the wall, a “closed flow” was formed, wherein the injected particles return to the wall owing to the suction flow. The random component of Reynolds shear stress was perfectly suppressed in the closed flow, which suggests that there was no turbulence. The controlled flow was categorized into four patterns, and each flow characteristic and drag reduction effect was discussed. When the closing rate is high, the drag decreases, while when the closing rate is low, i.e., when the injected particles are released into the main flow, the turbulence is maintained. If the thickness of the layer suppressing turbulence is insufficient, a significant effect in terms of the drag reduction cannot be expected. The large drag reduction owing to the traveling wave control can be attributed to the elimination of turbulence in the region near the wall.     

Characteristic turbulent structure of a modified drag-reduced surfactant solution flow via dosing water from channel wall

The modification of the near-wall structure is very important for the control of wall turbulence. To ascertain the effect of near-wall modulation on the viscoelastic drag-reduced flow, the modified characteristics of a surfactant solution channel flow were investigated experimentally. The modulation was conducted on the boundary of the channel flow by injecting water from the whole surface of one side of the channel wall. The diffusion process of the injected water was observed by using the planar laser-induced fluorescence technique. The velocity statistics and characteristic structure including the spatial distributions of instantaneous streamwise velocity, swirling strength, and Reynolds shear stress were analyzed based on the velocity vectors acquired in the streamwise wall-normal plane by using the particle imaging velocimetry technique. The results indicated that the disturbance of the injected water was constricted within a finite range very near the dosing wall, and the Reynolds shear stress was increased in this region. However, the eventual drag reduction rate was found to be increased due to a relatively large decrement of viscoelastic shear stress in this near-wall region. Moreover, the flow structure under this modulation presented obvious regional characteristics. In the unstable disturbed region, the mixing of high-speed and low-speed fluids and the motions of ejection and sweep occurred actively. Many clockwise vortex cores were also found to be generated. This characteristic structure was similar to that in the ordinary turbulence of Newtonian fluid. Nevertheless, outside this disturbed region, the structure still maintained the characteristics of the drag-reduced flow with non-Newtonian viscoelastic additives. These results proved that the injected Newtonian fluid associated with the modified stress distribution creates a diverse characteristic structure and subsequent enhanced drag reduction. This investigation can provide the experimental basis for further study of turbulence control.     

Numerical and experimental investigation of turbulent characteristics in a drag-reducing flow with surfactant additives

Cetyltrimethyl ammonium chloride (CTAC) surfactant additives, because of their long-life characteristics, can be used as promising drag-reducers in district heating and cooling systems. In the present study we performed both numerical and experimental tests for a 75 ppm CTAC surfactant drag-reducing channel flow. A two-component PIV system was used to measure the instantaneous streamwise and wall-normal velocity components. A Giesekus constitutive equation was adopted to model the extra stress due to the surfactant additives, with the constitutive parameters being determined by well-fitting apparent shear viscosities, as measured by an Advanced Rheometric Expansion System (ARES) rheometer. In the numerical study, we connected the realistic rheological properties with the drag-reduction rate. This is different from previous numerical studies in which the model parameters were set artificially. By performing consistent comparisons between numerical and experimental results, we have obtained an insight into the mechanism of the additive-induced drag-reduction phenomena.

Our simulation showed that the addition of surfactant additives introduces several changes in turbulent flow characteristics: (1) In the viscous sublayer, the mean velocity gradient becomes gentler due to the viscoelastic forces introduced by the additives. The buffer layer becomes expanded and the slope of the velocity profile in the logarithmic layer increases. (2) The locations where the streamwise velocity fluctuation and Reynolds shear stress attain their maximum value shifted from the wall region to the bulk flow region. (3) The root-mean-square velocity fluctuations in the wall-normal direction decrease for the drag-reducing flow. (4) The Reynolds shear stress decreases dramatically and the deficit of the Reynolds shear stress is mainly compensated by the viscoelastic shear stress. (5) The turbulent production becomes much smaller and its peak-value position moves toward the bulk flow region. All of these findings agree qualitatively with experimental measurements.

Regarding flow visualization, the violent streamwise vortices in the near wall region become dramatically suppressed, indicating that the additives weaken the ejection and sweeping motion, and thereby inhibit the generation of turbulence. The reduction in turbulence is accomplished by additive-introduced viscoelastic stress. Surfactant additives have dual effects on frictional drag: (1) introduce viscoelastic shear stress, which increases frictional drag; and (2) dampen the turbulent vortical structures, decrease the turbulent shear stress, and then decrease the frictional drag. Since the second effect is greater than the first one, drag-reduction occurs.     

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.     

The influence of flow-induced non-Newtonian fluid properties on turbulent drag reduction

Friction factors and velocity profiles in turbulent drag reduction can be compared to Newtonian fluid turbulence when the shear viscosity at the wall shear rate is used for the Reynolds number and the local shear viscosity is used for the non-dimensional wall distance. On this basis, an apparent maximum drag reduction asymptote is found which is independent of Reynolds number and type of drag reducing additive. However, no shear viscosity is able to account for the difference between the measured Reynolds stress and the Reynolds stress calculated from the mean velocity profile (the Reynolds stress deficit). If the appropriate local viscosity to use with the velocity fluctuation correlations includes an elongational component, the problem can be resolved. Taking the maximum drag reduction asymptote as a non-Newtonian flow, with this effective viscosity, leads to agreement with the concept of an asymptote only when the solvent viscosity is used in the non-dimensional wall distance.     

壁面温度在多参数下对平板边界层的影响研究

为了得到壁面温度在不同来流速度、不同湍流强度条件下对边界层转捩与减阻的影响规律,本文采用Transitionk-kl-ω模型对低来流速度下无压力梯度的光滑平板进行了数值模拟。结果表明,随着来流速度的升高,壁温升高所起到的减阻效果更好,即高来流速度对壁面温度更为敏感。当来流处于中高湍流强度下时,壁温升高能起到推迟转捩的作用,且随着湍流强度的升高,转捩推迟的效果越好,但减阻效果正好相反;当来流处于低湍流强度下时,壁温升高会使得转捩提前发生。壁温升高抑制了边界层内流体的脉动程度,使得层流的稳态不易被破坏,流动更加稳定;同时,壁温升高使得边界层内流体的速度梯度减小,从而降低了壁面摩擦系数,故壁温升高能起到推迟边界层转捩与减阻的作用。     

Experimental study on the effects of shear induced structure in a drag-reducing surfactant solution flow

In this paper, The drag reduction characteristics of surfactant solutions have been experimentally studied, as well as, the shear viscosities of turbulent drag-reducing surfactant solution have been measured as a function of concentration, shear rate and temperature by using AG-G2 (TA Instruments, New Castle, USA) rheometer. In comparison the rheological property with the macroscopic behavior of the solutions in turbulent channel flow, a deeper insight into the mechanisms of drag-reducing surfactant solution has been obtained. For no shear induced structure of surfactant solutions they just show features shear thinning, but the drag reduction is very significant phenomenon. Surfactant solution of the shear induced structure is not a surfactant fluid drag reduction of the necessary elements.     

固体颗粒对沟槽湍流边界层影响的实验研究

本文采用粒子图像测速技术(particles image velocimetry, PIV)研究固体颗粒对放置在平板湍流边界层中的平壁和沟槽壁面减阻效果的影响. 实验对清水和加入粒径为155 μm聚苯乙烯颗粒的流法向二维速度场信息进行采集, 对不同工况下的平均速度剖面、雷诺应力和湍流度等统计量进行对比, 分析流体在边界层中的行为. 运用空间局部平均结构函数提取了不同工况湍流边界层喷射?扫掠行为的空间拓扑结构并进行比较. 结果发现, 在不同的壁面条件下, 粒子加入后的对数律区中无量纲速度均略大于清水组, 雷诺切应力有所降低, 湍流度有所减弱. 对于不同流场速度下的沟槽而言, 颗粒的加入均降低了壁面附近的阻力, 而颗粒单独作用于光滑壁面的减阻效果并不明显. 加入粒子后的相干结构数目有所增加, 法向脉动速度下降. 沟槽壁面附近的相干结构数目有所增加, 法向脉动速度在自由来流速度较大时有所上升, 在速度较小时有所下降. 这表明不同减阻状况下的沟槽均能将大涡破碎成更多的涡, 并且粒子的加入强化了这种破碎作用.      

Wall Versus Centerline Polymer Injection in Turbulent Channel Flows

This experimental study compares the mean and turbulence characteristics of turbulent channel flows with polymer injection at the wall and at the centerline to assess the impact of the injection location on drag reduction. It also contrasts the drag reduction performance of a hydrolyzed polymer versus a non-ionic polymer under the same conditions. Wall injection of non-ionic and hydrolized polymers resulted in 23% and 9% larger drag reduction than corresponding centerline injection, respectively. In all cases, the polymer was structured and the presence of macromolecular polymer structures, even when concentrated mostly away from the wall, seemed to be able to affect the turbulence structure in the flow.     

A qualitative assessment of the role of a viscosity depending on the third invariant of the rate-of-deformation tensor upon turbulent non-Newtonian flow

The numerical simulation of some non-Newtonian effects in wall and wall-free turbulent flows, such as drag reduction in pipe flows or the decrease in transverse normal Reynolds stresses, has been attempted in the past with a limited degree of success on the basis of modified wall functions applied to traditional turbulence models (k–ε), rather than through more realistic rheological constitutive equations. In this work, it is qualitatively shown that if the viscosity function of a generalised Newtonian fluid is assumed to depend on the third invariant of the rate of deformation tensor, there is an increase of the viscous diffusion terms, but especially, of the dissipation of turbulence kinetic energy by a factor equal to the Trouton ratio of the fluid, divided by the Trouton ratio of the solvent, thus indicating a possible way to improve rheological–turbulence modelling.