Effect of Upstream Obstacles on Air Layer Regimes in a Turbulent Boundary Layer

Abstract

Hydrodynamic drag significantly influences the fuel efficiency and operational performance of marine vessels. Gas Injection Drag Reduction (GIDR) techniques, such as Bubble Drag Reduction (BDR), Transitional Layer Drag Reduction (TLDR), and Air Layer Drag Reduction (ALDR), have shown promise in reducing skin friction drag by introducing gas into the boundary layer. However, the influence of hull protrusions, such as sacrificial anodes and fouling, on these techniques remains poorly understood. This research addresses the question: How do the placement and dimensions of upstream cylindrical obstacles affect the formation and stability of gas phases (bubbly, transitional, and air layer) in a turbulent boundary layer (TBL)? To investigate this, protrusions are represented as cylindrical objects placed upstream of the gas injector, exploring their effects on the gas injection regimes in a TBL.

The study examines three gas injection regimes: bubbly, transitional, and air layer. In the bubbly regime, the focus is on bubble morphology and velocity distributions. The transitional regime investigates non-wetted areas and air layer morphology. For the air layer regime, the study examines the length and stability of the air layer near the injector. Image processing techniques are used to distinguish the air layer from the surrounding fluid, providing accurate assessments of the properties of the air layer.

The results demonstrate that cylindrical obstacles have a significant effect on the gas phases. In the bubbly regime, they alter bubble morphology and velocity, producing localized velocity peaks. In the transitional regime, obstacles lead to the formation of non-wetted areas and funnel-shaped regions. For the air layer regime, although the length of the air layer remains mostly unaffected, its stability near the injector is disrupted by upstream obstacles due to wake-induced instabilities. These findings highlight the critical role of obstacle placement and dimensions in shaping gas injection regimes.

Future research is needed to enhance understanding in areas such as the integration of velocity measurements (e.g., Particle Image Velocimetry), widening the experimental field of view to capture more comprehensive flow dynamics, and exploring the impact of various obstacle sizes and geometries. These investigations will be crucial for refining gas injection drag reduction strategies and advancing practical marine applications.