Experimental Study on the Friction Drag Reduction of Superhydrophobic Surfaces in Closed Channel Flow

Abstract

Due to the importance of copper and its alloys in marine applications, the main objective of this research is to provide a simple, effective and low cost manufacturing approach to fabricate a superhydrophobic riblet copper surface with high drag reduction capability in laminar and turbulent flow regimes. Therefore, the riblets are produced by wire cut technique on the copper substrate and then by using a wet chemical method, a superhydrophobic coating is produced on the riblet surface. A pressure drop measurement system consists; pump, closed channel flow with a fabricated surfaces on the lower wall, connections and pressure drop transmitter is employed to measure the pressure drop in the close channel flow, for Reynolds number from 300 to 2769, in order to evaluate the ability of the fabricated surface to reduce the friction drag. The experimental results revealed that combining the abilities of the riblet and superhydrophobic surfaces increases the surface's ability to reduce friction drag. In addition, the riblet surface and superhydrophobic riblet surface on average decreased the friction drag by 10.33% and 42.65% correspondingly in water flow ranging from laminar to turbulent flow regime. Finally, according to the experimental results, the drag reduction performance of riblet surface is improved from 18.9% to 56.9% after superhydrophobic coating.

Investigation on the Mechanism of Drag Modification over Triangular Riblets

“…In terms of the 2D case, the riblets were primarily regarded as the typical 2D longitudinal grooved structures. So far, four kinds of explanations about the drag reduction mechanism of riblets have been publically re-ported: (1) spanwise inhibition, i.e., the spanwise movements of streamwise vortices were inhibited by riblets, which impaired the turbulent momentum transport near the wall surface and thereby reduced the skin-friction (Choi (1989), Chu and Karniadakis (1993), Monfared et al (2019)); (2) existence of protrusion height, i.e., the existence of the distance between virtual origin and riblet tip suppressed the spanwise shift of the viscous sublayer where the riblets were submerged, resulting in more stable low-speed streaks and lower bursting frequencies (Bechert and Bartenwerfer (1989), Luchini et al (1991), Gr u neberger and Hage (2011));(3) creation of a secondary vortex, i.e., a couple of secondary vortices generating near the riblet tip weakened the main streamwise vortical flow in the turbulent boundary layer, which retarded the strong interactions among neighboring streaks and subsequently kept a quasistatic flow in rib valley (Chu and Karniadakis (1993), Bacher and R. Smith (1986), Boomsma and Sotiropoulos (2016)); (4) uplifting of streamwise vortex, i.e., the ribs might uplift the streamwise vortices to decrease the ejection and sweeping events which were related with the high shear stress force in turbulent flows. Thus, the viscous drag force was reduced (Lee and Lee (2001), Martin andBhushan (2014), Huang et al (2016)).…”

Numerical Study on Laminar Drag Reduction for Superhydrophobic Surfaces with Continuous V-Shaped Microstructures

“…A Cassie state (Cassie and Baxter 1944) is maintained and the water does not completely contact the solid surface over flow, but forms an air-water interface with the gas between adjacent micropatterns, causing a concomitant reduction in wall shear stress and flow resistance. So far, the drag reduction technology of superhydrophobic surfaces has shown promising potential in applications such as underwater vehicles (Lee et al 2016;Monfared et al 2019), bionic sharkskin swimsuits (Monfared and Alidoostan 2020), water injection in oil reservoirs (Ijaola et al 2020), etc. Therefore, it is of great significance to study the drag reduction of superhydrophobic surfaces with flow field.…”