An experimental study on drag reduction of superhydrophobic rotating disk with air plastron
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摘要: 在冯卡门旋流中，对均匀超疏水表面与网纹超疏水表面在雷诺数Re~O（105）量级上的减阻性能与表面气膜状态进行了实验观测。2种超疏水表面均使用物理喷涂法在有机玻璃板上喷涂纳米疏水颗粒制备。网纹超疏水表面制备时增加了丝网掩模的步骤，因此其表面增加了毫米级网格纹理。实验结果表明：对于冯卡门旋流中的超疏水表面减阻而言，存在一个临界雷诺数Rec，当Re < Rec时，超疏水表面具有稳定的减阻效果，减阻率高达30%；当Re>Rec时，减阻效果随Re的增加快速丧失。相较均匀超疏水表面，网纹超疏水表面可以有效提高其表面附着的气膜层的动态稳定性。此外，可通过主动补气的方式有效恢复网纹超疏水表面气膜层，进而恢复减阻效果，这将为超疏水表面实现可持续的减阻提供新的技术方案。Abstract: Drag reduction performance of superhydrophobic disks in a Von Kármán swirling flow with Re~O(105) was experimentally studied. Two superhydrophobic disks, which have different microstructures, i.e., one with micron-scale homogeneous roughness (abbreviated as SHS#1) and the other with additional millimeter-scale nonhomogeneous grid pattern (abbreviated as SHS#2), have been tested. Both SHS#1 and SHS#2 are prepared by the method of physically spraying nano-scale hydrophobic particles onto an acrylic-plate substrate. The grid pattern on SHS#2 is obtained by applying a mask of wire mesh during the spraying procedure. The mean skin-friction drag on the rotating disk was measured by a torquemeter. It is shown that for the superhydrophobic surface to reduce drag in Von Kármán swirling flow, there is a critical Reynolds number Rec. When Re < Rec, the superhydrophobic surface has a stable long-term drag reduction effect, with drag reduction ratio up to 30%; but when Re>Rec, the drag reduction effect is rapidly lost with the increase of Re. Compared to SHS#1, SHS#2 can effectively improve the dynamic stability of the air plastron attached on the surface. Additionally, the air plastron on the superhydrophobic surface can be effectively restored by pulse air injection, and so can the drag reduction effect. This observation indicates a promising strategy for reliable and sustainable drag reduction via superhydrophobic surface.
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Key words:
- superhydrophobic /
- drag reduction /
- Von Kármán swirling flow /
- air plastron /
- dynamic stability
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图 2 实验用各种平板及光学轮廓图
Figure 2. Superhydrophobic preparation diagram and optical profiles of different disks for experiment
图 4 不同实验表面的稳态摩擦扭矩和平均摩阻系数随雷诺数和最大切速度的变化
Figure 4. Variations of steady-state friction torque and average friction coefficient of different experimental surfaces with Reynolds number and maximum shear speeds
图 5 超疏水圆盘减阻失效过程与不同Re下的减阻有效时间
Figure 5. Superhydrophobic disk drag reduction failure process and effective time of drag reduction under different Re
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