【作者】 豆照良；

【导师】 陈大融；

【作者基本信息】 清华大学 ， 机械工程， 2010， 博士

【摘要】 减小水面舰艇及水下航行体行进时所受摩擦阻力,对于提高航行体航速及航程、降低能量消耗等,具有重要意义。为此,多年来人们就湍流边界层减阻问题进行了大量的理论研究和试验验证,并形成了诸如柔顺壁、高分子聚合物、疏水材料以及表面形貌等多种减阻技术。然而,由于湍流边界层减阻理论的核心是通过改变湍流边界层结构来降低摩擦阻力,因此无法降低占据主要份额的层流状态下的摩擦阻力,工程应用时的实际减阻效果受到限制。本文通过对荷叶及鱼鳞表面微观结构的研究,提出了通过一种具有二元复合结构表面形貌的聚合物涂层来实现界面效应减阻的构想,即以微米级凹坑内驻留的微气泡为气核,在形貌效应作用下使近壁面流场空化以促使微气泡生长,进而在固/液界面间构建出气相结构,用气/液剪切代替固/液剪切,这样在降低层流条件下的摩擦阻力的同时,还可以有效抑制湍流的发生和发展,从而大幅度降低壁面摩擦阻力。利用自行研制的喷涂实验台,采用高压空气喷涂工艺制备出了具有亚毫米级、微米级二元复合结构的聚合物涂层。对涂料成膜过程进行实验研究和数值模拟的结果表明,溶剂挥发导致涂膜体系中出现温度梯度和表面张力梯度,由此引发的界面流和界面变形是涂层表面形貌形成的根本原因。通过调整涂料组分配比和喷涂工艺参数,以控制涂膜体系的温度梯度和表面张力梯度,可有效地控制涂料成膜过程中的界面流和界面变形,最终实现对涂层表面形貌的控制。利用小型平板水洞对聚合物涂层的减阻性能与其表面形貌特征参数之间的关系进行了研究,并提出了形貌优化准则。利用小型高速水洞对形貌优化后的聚合物涂层进行了减阻性能测试,结果表明该聚合物涂层可有效降低摩擦阻力,在14~20m/s的试验速度范围内,涂层减阻率稳定在13%~18%之间,且涂层减阻率随来流速度增加呈现增长趋势。采用计算流体力学方法对表面形貌作用下流场空化并构建出气相结构的过程进行了数值模拟,初步分析了二元复合结构聚合物涂层的减阻机理。将聚合物涂层减阻技术用于正式赛艇比赛中,取得了良好的比赛成绩,验证了该减阻技术用于工程实际的可行性和有效性。更多还原

【Abstract】 The researches on skin friction drag reduction have attracted significant attentions due to the practical values in engineering applications such as increasing vehicle speed, decreasing energy consumption and so on. So far, enormous theoretical and experimental investigations on turbulent drag reduction have been carried out and many drag reduction technologies have been developed, for example compliant surface, polymer, hydrophobic surface, and micro-structued surface like riblets. However, the technologies based on turbulent drag reduction theory can only reduce skin friction drag of turbulent flow, and can not reduce that of laminar flow dominating in many flow conditions, which has hampered the implementation of turbulent drag reduction technologies in the industry.In the paper, a novel drag reduction technology was proposed by designing a lotus and fish scale like surface with submilli-microscale binary structure. On the designed surface, gas phase is supposed to be generated due to vortex cavitation and developed in low pressure condition provided by surface topography. With the generated gas phase at solid-liquid interface, the shear force at the solid-liquid interface will be replaced by gas-liquid interfacial shear, not only reducing skin friction of laminar flow, but also restraining the growth of turbulence flow. As a result, the skin friction will be reduced effectively.The designed surface was fabricated with a simple spray-painting technique. By using a self-developed spray-painting apparatus, a mixture including resins, solvents, and micro particles was coated on a substrate. The polymer coating with designed binary structure surface formed spontaneously after the solidification of wet coating film.Experiments were performed and computational fluid dynamic methods were conducted to investigate the formation mechanism of binary structure surface. And according to the results, Benard convection and interfacial deformation driven by the gradients of temperature and surface tension due to solvents evaporation played a determinant role in the formation process of surface topography of polymer coating. By controlling the gradients of temperature and surface tension in the wet coating film, the interfacial deformation and surface topography of polymer coating were controlled.Experimental reseaches on the optimization of surface characteristic parameters were carried out so as to maximize the drag reduction property of polymer coating. Coating samples with optimized surface were tested in a minisized high-speed water tunnel, and a relative drag reduction rate was calculated. The experimental results showed that, the fabricated coating with designed binary structured surface had a steady drag reduction rate of 13%~18% at the flow speed of 14~20m/s, and the drag reduction efficiency became more remarkable with the incrment of flow speed.To further investigate the drag reduction property of designed surface, comparison experiments between coated rowing shell and smooth rowing shell were performed in a deep towing tank, and the test results re-proved the validity of fabricated polymer coating in reducing skin friction drag.Finally, by means of computational fluid dynamic methods, the processes of vortex cavitation caused by surface topography were numerically simulated. Based on the simulation, the effect mechanism of binary structured surface on drag reduction was analyzed.更多还原