Experimental investigation into the active ventilation drag reduction on surfaces with different wettabilities

Underwater vehicles, ships, and pipelines face significant energy consumption challenges, with fluid–solid friction being a primary cause. Active ventilation, which injects gas to create interfacial layers, is a promising drag reduction technique. This study experimentally investigates its performance on surfaces with contrasting wettability in a rectangular channel flow. A custom-built floating-unit force sensor was developed to measure local wall friction with high spatial resolution. The effects of ventilation rate Q and flow velocity v on drag reduction rate (DR) and the corresponding interfacial gas morphology were systematically examined. For hydrophilic surfaces, ventilation generally increases drag due to bubble-induced flow disturbance. A competing mechanism between localized density reduction and flow disturbance is identified, with the latter dominating at higher Q, leading to a systematic increase in the drag increase rate. In contrast, on superhydrophobic surfaces, DR improves significantly with increasing Q, which is attributable to the formation and stabilization of a continuous gas layer that enhances interfacial slip. Under optimal conditions (Q = 3.2 ml/s and v = 1.00 m/s), the mid-section of the superhydrophobic surface achieved a peak DR of 66.58%. Furthermore, the influence of v exhibits strong spatial heterogeneity. On hydrophilic surfaces, higher v reduces bubble size and near-wall residence time, thereby reducing downstream wall resistance. On superhydrophobic surfaces, increased v can either stabilize and reorganize the gas layer or cause shear-induced thinning and fragmentation, depending on the streamwise location and gas supply. This study provides crucial insights for optimizing active ventilation strategies in practical engineering applications.