Turbulent boundary layers over gas-liquid interfaces

Skin friction drag reduction methods via air injection have become an area of significant innovation for the shipping industry in recent years. Apart from specific technical considerations for optimal performance and net energy savings, the associated flow geometry provides a very challenging fundamental problem in multiphase turbulent flows. In the present work, we analyse this problem in a laboratory setting using snapshot and time-resolved Particle Image Velocimetry and high-speed photography measurements. In particular, the effects of variable air injection under a flat plate on the incoming liquid turbulent boundary layer (TBL) upstream of the injection location (Re_τ = 3300) are statistically evaluated at a wall-parallel plane in the log region (y = 0.1δ). This includes steady or unsteady blockage effects on the incoming liquid flow due to the formation of air bubbles, wall-attached air patches and air layers downstream of the injection location. In addition, the correlation of spanwise-meandering velocity structures of the upstream liquid TBL (at the same wall-parallel plane) with the temporal evolution and coalescence of the injected air patches, is also analysed through simultaneous time-resolved measurements of both the upstream liquid and the downstream gas phases.