Turbulent channel flow over riblets with superhydrophobic coating

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 ReH = 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 ∼1λ (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 〈u2〉 over the s+ = 8.6 and 17.3 surfaces while 〈u2〉 reduced over the s+ = 34.6 surface in the near wall region of y+ < 40. A smaller 〈v2〉 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 〈v2〉 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 the superhydrophobic coated counterparts 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.