TRPIV experimental investigation of drag reduction mechanism in turbulent boundary layer over superhydrophobic-riblet surface
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摘要: 利用TRPIV实验分别测量了湍流边界层在亲水壁面、超疏水壁面以及沟槽超疏水复合壁面上的瞬时速度场，对比分析了3种壁面的摩擦阻力，发现沟槽超疏水复合壁面的减阻率能够达到20.7%，而超疏水壁面只有14.6%。通过对比分析湍流边界层在3种壁面上的湍流脉动强度，发现法向湍流脉动强度在3种壁面上无明显变化，而在y+ < 150区域的同一法向高度上，流向湍流脉动强度在沟槽超疏水复合壁面上相对于亲水壁面的减小程度比超疏水壁面更高。为了进一步研究不同尺度的湍流脉动在不同壁面的变化情况，本文采用基于傅里叶变换的空间滤波法，将瞬时脉动速度场分解为空间流向波长大于δ的大尺度部分和波长小于δ的小尺度部分，发现超疏水壁面和沟槽超疏水复合壁面对大尺度流向湍流脉动强度的抑制作用可以到达y+=150的法向位置，而对小尺度流向湍流脉动强度的抑制作用只能到达y+=100的法向位置。采用以顺向涡为条件的大尺度脉动速度的条件相位平均方法，发现在yref=0.1δ处超疏水壁面和沟槽超疏水复合壁面相比于亲水壁面都存在正的大尺度流向脉动与负的法向脉动增强、负的大尺度流向脉动与正的法向脉动减弱以及脉动速度的0等值线偏离条件相位平均参考点的趋势，且沟槽超疏水复合壁面的移动趋势最弱。通过对比3种壁面不同法向高度的顺向涡强度值，同法向高度上亲水壁面、超疏水壁面以及沟槽超疏水复合壁面的涡强度值依次减弱，表明沟槽超疏水复合壁面比超疏水壁面能更有效地抑制近壁区涡结构的运动，从而实现更好的减阻效果。Abstract: The instantaneous velocity vector fields of turbulent boundary layers over the hydrophilic surface, the superhydrophobic (SH) surface and the superhydrophobic-riblet (SR) surface were measured using Time-Resolved Particle Image Velocimetry(TRPIV). Drag reduction rates of 14.6% and 20.7% for the SH surface and the SR surface respectively were acquired by comparing with the friction coefficient of the hydrophilic surface. By comparing the tendency of the turbulence intensity, it is found that the normal turbulence fluctuation intensity of the hydrophilic surface, the SH surface and the SR surface has no remarkable differences, but the streamwise turbulence fluctuation intensity shows a weakening trend in the region of y+ < 150 at the same wall-normal position. By using the spatial filtering method based on Fourier transform, the instantaneous fluctuating velocity field is divided into the large-scale part with the wavelength greater than δ and the small-scale part with the wavelength less than δ. It is found that the inhibitory effect of the SH surface and the SR surface on the streamwise turbulence fluctuation intensity of the large-scale part can reach the wall-normal position of y+=150, while the inhibitory effect on the streamwise turbulence fluctuation intensity of the small-scale part can only reach the normal position of y+=100. Through the conditional sampling and phase average methods, it is found that at the region of yref=0.1δ, compared with the hydrophilic surface, the positive large-scale streamwise fluctuating intensity and the negative wall-normal fluctuating intensity on the SH surface and the SR surface are increasing while the negative large-scale streamwise fluctuating and positive wall-normal fluctuating intensities on the SH surface and the SR surface are decreasing, and there is a gap between the contour with the value of zero and the reference position of the conditional sampling. Comparing the vortical strength of TBL on different wall, it is found that the vortex intensity value of the hydrophilic surface, the SH surface and the SR surface becomes weaker in turn, and hence we can conclude that the SR surface could acquire a higher drag reduction rate than the SH surface, via suppressing the motion of vortices at the near wall region.
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图 7 yref=0.1δ时大尺度流向脉动速度条件相位平均等值线图
Figure 7. Conditional phase average contour of large scale streamwise fluctuating velocity condition at yref=0.1δ
图 8 yref=0.1δ时大尺度法向脉动速度条件相位平均等值线图
Figure 8. Conditional phase average contour of large scale normal fluctuating velocity condition at yref=0.1δ
表 1 基本湍流减阻参数
Table 1. Basic turbulent drag reduction parameters
参数 亲水壁面 超疏水壁面 沟槽超疏水复合壁面 U∞/(m·s-1) 0.26 0.26 0.26 uτ/(m·s-1) 0.0119 0.0110 0.0105 Reτ 621 574 552 τw/(kg·m-1·s-2) 0.140 745 0.120 886 0.109 254 Cf 0.003 965 0.003 388 0.003 144 η 14.6% 20.7% 
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