【作者】 刘朝阳；

【导师】 姜楠；

【作者基本信息】 天津大学 ， 力学， 2022， 硕士

【摘要】 本文对超疏水微沟槽和微凸柱结构壁面湍流边界层的减阻机理进行了实验研究。使用高时间分辨率粒子图像测速仪（TRPIV,Time-Resolved Particle Image Velocimetry）测量了亲水壁面、超疏水微沟槽及微凸柱结构壁面湍流边界层内的瞬时速度场,对比分析了3种壁面的壁面摩擦切应力,发现超疏水结构壁面都产生了减阻效果,但超疏水微沟槽结构壁面的减阻率要大于超疏水微凸柱结构壁面,其中超疏水微沟槽结构壁面的减阻率达到了13.8%,而超疏水微凸柱结构壁面的减阻率为10.2%。通过对比分析湍流边界层内3种壁面对应的平均速度剖面、湍流脉动强度和雷诺切应力剖面,证实流体在超疏水壁面具有滑移速度,而且在15<y⁺<100区域的同一法向高度上,亲水壁面、超疏水微沟槽及微凸柱结构壁面对应的流向湍流脉动强度依次减弱,同时在30<y⁺<80区域的同一法向高度上,超疏水微凸柱壁面、亲水壁面及超疏水微沟槽结构壁面对应的法向湍流脉动强度依次减弱。在整个法向高度上,亲水壁面、超疏水微凸柱及微沟槽壁面的雷诺切应力的最大值依次减小。通过以∧_ci准则识别出的顺向涡为条件进行条件采样和相位平均,分别与亲水表面对比,发现在y⁺≈63附近位置处超疏水微沟槽壁面展向涡诱导的第四象限事件幅值减弱,其构成的扫掠事件强度减小,进而实现减阻。为进一步分析湍流脉动能量,使用本征正交分解,将湍流边界层内全场的瞬时脉动速度在时间上和流-法向空间进行了求和并进行了无量纲化,用来表征流场的脉动程度。结果表明,超疏水微凸柱结构壁面的展向滑移特性增阻,削弱了其流向滑移带来的减阻效果。超疏水微沟槽结构壁面的流向滑移特性能有效地抑制湍流脉动,从而实现更好的减阻效果。除此之外,验证了泰勒冻结假说在超疏水壁面湍流边界层的适用性以及通过空间两点相关分析得出滑移与壁面粗糙度对湍流结构的作用相反,其中,滑移抑制湍流结构,而壁面粗糙度诱导湍流结构。更多还原

【Abstract】 The drag reduction mechanism of the wall turbulent boundary layer with superhydrophobic micro-riblets and micro-convex posts is studied experimentally.The instantaneous velocity field in the turbulent boundary layer of the hydrophilic wall,superhydrophobic micro-riblets wall and micro-convex posts wall are measured by Time-Resolved Particle Image Velocimetry（TRPIV）.The frictional shear stress of the three kinds of walls are compared and analyzed.It is found that the superhydrophobic walls achieve drag reduction effect.However,the drag reduction rate of the superhydrophobic micro-riblets wall is higher than that of the superhydrophobic micro-convex posts wall,and the drag reduction rate of the superhydrophobic micro-riblets wall is 13.8%,while the drag reduction rate of the superhydrophobic micro-convex posts wall is 10.2%.Through comparison and analysis on three kinds of wall corresponding average velocity profile in the turbulent boundary layer,turbulence intensity and Reynolds shear stress profile,it is found that the fluid indeed has sliding speed in the superhydrophobic wall,and in the area of the same normal height at15<y⁺<100,streamwise turbulence intensity corresponding hydrophilic wall,superhydrophobic micro-riblets and micro-convex posts wall decreases.At the same time,in the area of the same normal height at 30<y⁺<80,the wall-normal turbulence intensity of superhydrophobic micro-convex posts,hydrophilic wall and superhydrophobic micro-riblets wall decrease successively.Over the whole wall-normal height,the maximum values of Reynolds shear stress on hydrophilic wall,superhydrophobic micro-convex posts and micro-riblets wall decrease successively.Based on the conditional sampling and phase averaging of the spanwise vortex identified by the ∧_ci criterion,it is found that the amplitude of the fourth quadrant event induced by the spanwise vortex of superhydrophobic micro-riblets wall weakens nearby at y⁺≈63,which leads to the intensity of its sweep events decreasing,and then the drag reduction is realized.In order to further analyze the turbulent fluctuation energy,the instantaneous fluctuation velocity of the whole turbulent boundary layer is summed in time and streamwise-normal space by using the Proper Orthogonal Decomposition and is dimensionless to characterize the fluctuation degree of the flow field.The results show that the spanwise slip characteristics of superhydrophobic micro-convex posts increase the drag and weaken the drag reduction effect caused by the streamwise slip.The streamwise slip characteristics of the superhydrophobic micro-riblets wall can effectively suppress turbulence fluctuation and achieve better drag reduction effect.In addition,the applicability of Taylor’s freezing hypothesis in the superhydrophobic wall turbulent boundary layer is verified,and the effect of slip and wall roughness on turbulent structure is opposite through spatial two-point correlation analysis.The turbulence structure is inhibited by slip and induced by wall roughness.更多还原