Spalding公式在脊状表面湍壁摩擦力测量中的应用

在低速风洞中来流速度一定的情况下使用IFA300恒温热线风速仪测量了光滑表面和两种不 同尺寸的脊状表面湍流边界层平均速度分布剖面，并验证了试验段湍流发展的充分性；通过 应用Spalding壁面公式使用最小二乘法精准拟合了实验测量的边界层内层速度分布曲线，得 到了湍流边界层壁面摩擦速度并进一步求得湍流壁面摩擦应力，较准确地计算出脊状表面的 虚拟原点位置，并通过与对数律公式拟合结果比较分析，证实了该方法更加准确有效. 最后 分别计算了3种实验模型的湍流边界层动量损失厚度. 通过对比脊状表面与光滑表面动量损 失厚度和壁面摩擦应力，反映了动量损失厚度的大小与壁面摩擦应力的大小具有一致性，充 分证实了脊状表面在湍流中具有一定的减阻效果.     

脊状表面航行器模型减阻特性的风洞实验研究

针对航行器提高航程和航速的需要,开展脊状表面湍流边界层减阻的实验和数值仿真研究。在航行器模型的外表面加工具有特定形状、尺寸的脊状结构,导致湍流边界层的流动稳定性增强,壁面摩擦阻力降低。在风洞中对具有光滑表面和脊状表面的航行器模型在不同风速和攻角下进行阻力测试,得到其减阻特性曲线。实验结果表明,具有横向脊状表面的航行器模型在一定来流速度范围内具有很好的减阻效果,实验获得的最大减阻量为23.5%。数值仿真结果则发现,在脊状结构内形成了稳定的"二次涡",边界层内湍动能和湍流猝发强度降低,很好地揭示了减阻机理。     

脊状表面减阻特性的风洞试验研究

利用热线风速仪，对光滑表面和多个脊状表面在低速风洞中进行了表面流场测试。基于测得的边界层速度分布数据，利用对数律区速度分布公式，编程分别计算出光滑表面和脊状表面的壁面摩擦速度和虚拟原点。研究发现，脊状表面最大减阻量达13．5％；有减阻效果的脊状表面使边界层速度曲线上移、湍流强度下降；与光滑表面相比，脊状表面的位移厚度和动量损失厚度明显减小，也表明脊状表面具有减阻效果；位移厚度和动量损失厚度减少量随槽间距s^＋的增加呈现先变大后变小的趋势，在S^＋=12时达到最大。     

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.     

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.     

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。     

倾斜吹吸控制下湍流边界层减阻的直接数值模拟

雷诺切应力是壁湍流高摩擦阻力的重要来源, 有理论认为可以通过壁面生成负雷诺应力(数值上为正)的方式来削弱湍流流场中雷诺应力的分布, 以此获得流动减阻. 而通过对雷诺平均运动方程的法向二次积分, 可以发现壁面生成正雷诺应力(数值上为负)对壁面摩擦阻力系数才有负贡献. 文中在湍流边界层流动的控制区域下边界设置一系列倾斜狭缝, 利用该装置通过周期性吹吸的方法产生壁面生成正(负)雷诺应力, 并采用直接数值模拟方法考察和验证上文提到的减阻理论. 文中采用的湍流边界层流动模型, 其流动雷诺数(基于外流速度及动量损失厚度)从300 发展到860. 文中通过多组数值模拟算例, 考察了射流强度和频率对壁面摩擦阻力系数的影响, 并对比了壁面生成正或负雷诺应力对流动的影响. 研究表明, 壁面生成正雷诺应力控制的减阻率能达到3.26, 而壁面生成负雷诺应力控制的减阻效果较壁面生成正雷诺应力控制的要差; 壁面生成的正雷诺应力对壁面摩擦阻力有负贡献, 而壁面生成的负雷诺应力对壁面摩擦阻力有正贡献; 通过考察控制的收支比, 发现控制方案不能获得能量净收益.      

柔性壁面湍流边界层相干结构控制的实验研究

本文利用热膜测速技术对刚性壁面和柔性壁面湍流边界层的流向速度分量进行了实验测量,首先研究了柔性壁面对平均速度分布和湍流度分布的影响,结果表明:柔性壁面的边界层速度分布在对数律层向上有所平移,缓冲层加厚,具有一般的壁面减阻特征;而柔性壁的湍流度比刚性壁的湍流度要低,分布也更为平坦。然后综合运用自相关法和条件采样技术研究了湍流近壁区的相干结构,结果表明:刚性壁自相关曲线的第二峰值出现的时间比柔性壁的短,柔性壁的猝发频率比刚性壁的低许多。实验结果表明柔性壁面具有一定的减阻作用。     

基于表面设计减阻的水下航行体模型实验与数值分析

针对水下航行体高速航行时提高航速的需要,开展近壁面湍流边界层减阻的研究。进行表面设计使旋涡流动加强导致局部空化从而产生壁面附着的微气泡,以降低流体粘性系数,伴随近壁面湍流结构的改变来实现一定幅度的减阻。设计并进行轴对称体模型在高压循环水洞中的空化现象观测和阻力测量并做了相应条件下的数值计算。。实验和数值结果分析的基本结论比较一致:某种特征表面形貌在湍流中有一定幅度的减阻,局部空化效应在中低流速下牺牲压差阻力而导致航行总阻力升高,在高流速条件下则降低总阻力。     

柔性壁面湍流减阻机理与多尺度相干结构控制的实验研究

用热膜测速仪以高于对应最小湍流时间尺度的分辨率,精细测量了水槽中刚性壁面和柔性壁面平板湍流边界层不同法向位置流向速度分量的时间序列信号,利用湍流边界层近壁区域的对数律平均速度剖面与壁面摩擦速度、流体粘性系数、壁面摩擦切应力等内尺度物理量的关系,在准确测量湍流边界层近壁区域对数律平均速度剖面的基础上,通过非线性迭代求解壁面摩擦速度以及湍流边界层壁面摩擦切应力.结果表明柔性壁的湍流边界层速度分布在对数律层有所上移,缓冲层增厚,说明柔性壁面具有一定的减阻作用. 利用湍流多尺度局部平均结构函数的概念和多尺度局部平均结构函数的瞬时强度因子、平坦因子检测多尺度相干结构及其间歇性的方法,提取了湍流边界层多尺度相干结构的条件相位平均波形.对比研究了刚性壁面和柔性壁面平板湍流边界层近壁区域多尺度相干结构的条件相位平均波形及其间歇性的统计特征,分析了柔性壁面具有减阻作用的物理机理.     

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.      

Riblet drag reduction in mild adverse pressure gradients: A numerical investigation

Riblet films are a passive method of turbulent boundary layer control that can reduce viscous drag. They have been studied with great detail for over 30 years. Although common riblet applications include flows with Adverse Pressure Gradients (APG), nearly all research thus far has been performed in channel flows. Recent research has provided motivation to study riblets in more complicated turbulent flows with claims that riblet drag reduction can double in mild APG common to airfoils at moderate angles of attack. Therefore, in this study, we compare drag reduction by scalloped riblet films between riblets in a zero pressure gradient and those in a mild APG using high-resolution large eddy simulations. In order to gain a fundamental understanding of the relationship between drag reduction and pressure gradient, we simulated several different riblet sizes that encompassed a broad range of s⁺ (riblet width in wall units), similarly to many previously published experimental studies. We found that there was only a slight improvement in drag reduction for riblets in the mild APG. We also observed that peak values of streamwise turbulence intensity, turbulent kinetic energy, and streamwise vorticity scale with riblet width. Primary Reynolds shear stresses and turbulence kinetic energy production however scale with the ability of the riblet to reduce skin-friction.     

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。     

Parametric Study on a Sinusoidal Riblet for Drag Reduction by Direct Numerical Simulation

The direct numerical simulation of fully developed turbulent channel flow with a sinusoidal riblet surface has been carried out at the friction Reynolds number of 110. Lateral spacing of adjacent walls in a sinusoidal riblet is varied sinusoidally in the streamwise direction. The average lateral spacing of a sinusoidal riblet is larger than the diameter of a quasi-streamwise vortex and its wetted area is smaller than that of ordinary straight-type riblets. We investigate the effect of sinusoidal riblet design parameters on the drag reduction rate and flow statistics in this paper. The parametric study shows that the maximum total drag reduction rate is approximately 9.8% at a friction Reynolds number of 110. The riblet induces downward and upward flows in the expanded and contracted regions, respectively, which contribute to periodic Reynolds shear stress. However, the random Reynolds shear stress decreases drastically as compared with the flat surface case, resulting in the reduction of total drag owing to the sinusoidal riblet. We also performed vortex tracking to discuss the motion of the vortical structure traveling over the sinusoidal riblet surface. Vortex tracking and probability analysis for the core of the vortical structure show that the vortical structure is attenuated owing to the sinusoidal riblet and follows the characteristic flow. These results show that the high skin-friction region on the channel wall is localized at the expanded region of the riblet walls. In consequence, the wetted area of the riblet decreases, resulting in the drag-reduction effect.     

Drag measurements on V-grooved surfaces on a body of revolution in axial flow

In water flows with velocities of up to 9 m/s the friction drag of a body of revolution in axial flow was investigated for dependence on the body surface structure. This was done for different types of riblet film fixed on the surface with the riblet direction aligned with the flow. The lateral spacing between the triangular shaped riblets varied between 0.033 mm and 0.152 mm. In all cases the riblet spacing was equal to the riblet height. For comparison a smooth reference film was used.Depending on the Reynolds number and the non-dimensional riblet spacings ⁺, a turbulent drag reduction of up to 9% could be achieved with riblets in comparison with the flow over a smooth surface.In the region of transition to turbulent flow and with non-dimensional riblet spacings ofs ⁺10–15 drag reductions of up to 13% were obtained. It is therefore conjectured, that in addition to hampering the near wall momentum exchange, the riblets can delay the development of initial turbulent structures in time and space.     

Flow field analysis of a turbulent boundary layer over a riblet surface

The near-wall flow structures of a turbulent boundary layer over a riblet surface with semi-circular grooves were investigated experimentally for the cases of drag decreasing (s ⁺=25.2) and drag increasing (s ⁺=40.6). One thousand instantaneous velocity fields over riblets were measured using the velocity field measurement technique and compared with those above a smooth flat plate. The field of view was 6.75 × 6.75 mm² in physical dimension, containing two grooves. Those instantaneous velocity fields were ensemble averaged to get turbulent statistics including turbulent intensities and turbulent kinetic energy. To see the global flow structure qualitatively, flow visualization was also carried out using the synchronized smoke-wire technique under the same experimental conditions. For the case of drag decreasing (s ⁺=25.2), most of the streamwise vortices stay above the riblets, interacting with the riblet tips frequently. The riblet tips impede the spanwise movement of the streamwise vortices and induce secondary vortices. The normalized rms velocity fluctuations and turbulent kinetic energy are small near the riblet surface, compared with those over a smooth flat plate. Inside the riblet valleys, these are sufficiently small that the increased wetted surface area of the riblets can be compensated. In addition, in the outer region (y ⁺ > 30), these values are almost equal to or slightly smaller than those for the smooth plate. For the case of drag increasing (s ⁺=40.6), however, most of the streamwise vortices stay inside the riblet valleys and contact directly with the riblet surface. The high-speed down-wash flow penetrating into the riblet valley interacts actively with the wetted riblet surface and increases the skin friction. The rms velocity fluctuations and turbulent kinetic energy have larger values compared with those over a smooth flat plate. Received: 24 March 1999/Accepted: 10 March 2000     

Flow over convergent and divergent wall riblets

Fast swimming sharks have small riblets on their skin, which are assumed to improve the swimming performance of the fish. Fluid dynamic experiments in water as well as in air confirm this assumption. With riblet surfaces as compared to smooth surfaces, drag reductions up to about 10% were measured. The overall riblet pattern on sharks shows parallel riblets directed from head to tail, but besides this overall pattern fast swimming sharks have also small areas with converging riblets and others with diverging riblets. In the present study the velocity field over convergent and divergent riblet patterns is investigated by hot-wire measurements in turbulent pipe flow. Significant changes in the near wall velocity field were found.