Friction drag reduction of Taylor-Couette flow over air-filled microgrooves

Reducing drag under high turbulence is a critical but challenging issue that has engendered great concern. This study utilizes hydrophilic tips in super hydrophobic(SHP) grooves to enhance the stability of plastron, which results in a considerable drag reduction (DR)upto 62%,at Reynolds number(Re) reaching 2.79x104. Theeffect of the spacing width w of the microgrooves on both DR and flow structures isinvestigated. Experimental results demonstrate that DR increases as either microgroove spacingworReincreases. The velocity fields obtained usingparticleimagevelocimetryindicate that the air-filled SHP grooves induce a considerable wall slip. This slipsignificantly weakens the intensity of Taylor rolls, reduces local momentum transport, and consequently lowers drag. This phenomenon becomes more pronounced with increasingw. Furthermore, to quantify the multiscale relationship between global response and geometrical as well as driving parameters,DR similar to(w,phi s,Re), a theoretical model isestablished based on angular momentum defect theory and magnitude estimate. It is demonstrated that a decrease in the surface solid fraction can reduce wall shear, and anincrease in the groove width can weaken turbulence kinetic energy production, rendering enhanced slip and drag reduction. This research has implications for designing and optimizing turbulent-drag-reducing surfaces in various engineering applications, such as transportation and marine engineering