Achieving underwater stable drag reduction on superhydrophobic porous steel via active injection of small amounts of air

Over the years, numerous researchers have proposed air lubrication methods to reduce drag in underwater vehicles. This method is challenged by buoyancy and water flow, which facilitate air escape from solid surfaces, necessitating substantial air to achieve significant drag reduction. The adherence of hydrophobic interfaces to submerged air bubbles reduces air escape, stabilizing it on solid surfaces for effective drag reduction. However, high flow velocities or pressure conditions can destabilize the drag reduction effect by disrupting the air-water interface. To address this issue, the paper proposes a method that integrates superhydrophobic surfaces with active air injection. This approach involves preparing stable superhydrophobic surfaces by combining laser ablation, heat treatment, and spraying hydrophobic coatings on porous steel surfaces, and using active air injection to maintain the stability of the air layer. Flow field analysis revealed that active air injection into superhydrophobic porous steel sustains a stable air layer, even at high flow velocities, reaching a drag reduction exceeding 25%. Moreover, at high Reynolds numbers, high drag reduction can be achieved at significantly low flow rates compared to smooth surface. This approach shows promise for maintaining a stable air-water interface on superhydrophobic surfaces and sustaining drag reduction in engineering applications.