Abstract
This paper presents an experimental study of horizontal channel flow with air-filled superhydrophobic grooves. Air–water interfaces in the grooves are visualized in a range of the channel Reynolds number, Re, (2000 ≤ Re ≤ 5000) while flow characteristics are evaluated using particle tracking velocimetry measurements at Re = 3000 and 4000. Near the air–water interface in the superhydrophobic groove, turbulent eddies and hence the Reynolds shear stress appreciably attenuate owing to a lack of energy supply through the interface, and it takes a notable distance for the Reynolds shear stress to recover downstream of the groove. Additionally, a secondary cross-flow from the solid surface region between two grooves towards the air–water interface appears and sweeps eddies between the grooves towards the interface. The decay of turbulent eddies and the sweeping of eddies from the solid surface to the air–water interface reduce friction drag both in and immediately downstream of the grooved region.
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Acknowledgements
The authors gratefully acknowledge support from a Grant-in-Aid for Scientific Research (A) (Grant no. 17H01245). In addition, the authors thank Dr. M. Tanaka of Kyoto Institute of Technology for useful comments.
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Appendix: Comparison of normalized streamwise mean velocity u/ub between Re = 3000 and 4000
Appendix: Comparison of normalized streamwise mean velocity u/ub between Re = 3000 and 4000
Figure
15 shows the comparison of normalized streamwise mean velocity u/ub between Re = 3000 and 4000.
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Kitagawa, A., Shiomi, Y., Murai, Y. et al. Transient velocity profiles and drag reduction due to air-filled superhydrophobic grooves. Exp Fluids 61, 237 (2020). https://doi.org/10.1007/s00348-020-03070-x
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DOI: https://doi.org/10.1007/s00348-020-03070-x