Turbulent channel flow over riblets with superhydrophobic coating
Turbulent channel flow over riblets with superhydrophobic coating
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DOI：
10.1016/j.expthermflusci.2018.02.001
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发表时间：
2018-06-01
影响因子：
3.2
通讯作者：
Ghaemi, Sina
中科院分区：
文献类型：
--
作者：
Abu Rowin, Wagih;Hou, Jianfeng;Ghaemi, Sina
应用超疏水涂层（SHC）后的脊表面的性能通过平面粒子图像测速（PIV）测量在脊尖端间距s（+）= 8.6、17.3和34.6（使用壁单位归一化）下进行评估。这三个肋尺寸分别对应于尺寸过小（小的阻力减小）、最佳（最大阻力减小）和尺寸过大（阻力增加）的肋。所有的实验都是在恒定Re-H = 4360（基于通道高度H和平均速度）的湍流水槽流中进行的。超疏水层通过喷涂厚度类似于1 λ（壁单元）的微米/纳米颗粒形成。结果表明，在y（+）< 15的近壁区域，涂覆超疏水层后，s（+）= 8.6和s（+）= 17.3的脊上的平均速度较小，而在s（+）= 34.6的脊上，涂覆SHC后的平均速度相对于未涂覆的脊大。在y（+）< 40的近壁区，< u（2）>在s（+）= 8.6和17.3的表面上增加，而< u（2）>在s（+）= 34.6的表面上减少。在施加SHC时，在所有三个肋的近壁区域（y（+）< 50）中观察到较小的值，而对于较小的肋，减小较小。在涂覆s（+）=34.6的沟槽时，< v（2 >）峰也远离壁移动。在s（+）= 8.6和s（+）= 17.3的肋上的雷诺剪切应力相对于超疏水涂覆的对应物没有显著不同，而< uv >在涂覆s（+）= 34.6的肋后，在y（+）< 30处观察到大的减小。基于加权积分的减阻率（DR）估算结果< uv >表明，经过SHC处理后，s（+）= 8.6和s（+）= 17.3的沟槽的减阻率分别为6.0%和10.1%。在过大的s（+）= 34.6的肋上的SHC将性能从在未涂覆表面上的9.0%阻力增加（DI）提高到1.2%DR，相当于涂覆肋后阻力降低10.2%。过大肋（s（+）> 30）的较大改善与较大肋谷中的SHC的有效性以及因此喷射和扫掠运动的衰减相关联。SHC拓宽了较大肋的操作范围，这更容易制造。
The performance of riblet surfaces after applying a superhydrophobic coating (SHC) is evaluated by planar particle image velocimetry (PIV) measurement at riblet tip spacing of s(+) = 8.6, 17.3, and 34.6 (normalized using wall unit). The three riblet sizes correspond to an undersized (small drag reduction), an optimum (maximum drag reduction), and an oversized (drag increase) riblet, respectively. All the experiments are carried out in a turbulent water channel flow at constant Re-H = 4360 (based on channel height H and average velocity). The superhydrophobic layer is formed by spray coating of micro/nano particles with a thickness of similar to 1 lambda (wall unit). The results show smaller mean velocity over the s(+) = 8.6 and s(+) = 17.3 riblets when coated with the superhydrophobic layer at near-wall region of y(+) < 15 while the mean velocity over the s(+) = 34.6 riblet with SHC is larger relative to the non-coated counterpart. The SHC increased < u(2)> over the s(+) = 8.6 and 17.3 surfaces while < u(2)> reduced over the s(+) = 34.6 surface in the near wall region of y(+) < 40. A smaller value is observed in the near-wall region (y(+) < 50) of all three riblets upon applying the SHC while the reduction is smaller for smaller riblets. The < v(2 >) peak also shifts away from the wall upon coating the s(+) =34.6 riblet. The Reynolds shear stress over s(+) = 8.6 and s(+) = 17.3 riblets is not considerably different relative to superhydrophobic coated counterpart while a large reduction of < uv > is observed at y(+) < 30 after coating the s(+) = 34.6 riblet. The estimation of drag reduction (DR) based on weighted integral of < uv > shows 6.0% and 10.1% reduction of drag over the s(+) = 8.6 and s(+) = 17.3 riblets after the SHC process, respectively. SHC on the oversized s(+) = 34.6 riblet improves the performance from 9.0% drag increase (DI) over the non-coated surface to 1.2% DR, equivalent to 10.2% reduction of drag upon coating the riblet. The larger improvement of oversized riblets (s(+) > 30) is associated with the effectiveness of the SHC in the larger riblet valley and consequently attenuation of ejection and sweep motions. The SHC broadens the operation range of larger riblets, which are easier to manufacture.