Drag reduction mechanism of the biomimetic superhydrophobic surface on the boundary layer of underwater gliders

The buoyancy-driven underwater glider serves as a highly efficient tool for achieving ocean-wide, long-term, and continuous fine-scale observations. However, the performance of underwater gliders in the gliding range and speed is significantly influenced by biological attachments. To investigate potential solutions for this issue, this study explores the drag reduction mechanism of the Petrel-L underwater glider's main body based on the biomimetic superhydrophobic surface (BSHS). The flow field surrounding the underwater glider is analyzed through particle image velocimetry, and mechanical measurements are obtained with force balance techniques. The drag reduction effect of the BSHS with/without biological attachments is accurately determined through force balance analysis. Additionally, the impact mechanism of the boundary layer on Petrel-L with/without biological attachments is investigated with a range of analytical techniques, including proper orthogonal decomposition, conditional phase averaging, finite-time Lyapunov exponent, and quadrant analysis. At an angle of attack (AOA) of 3°, the BSHS realizes a drag reduction of approximately 15.2% when there are no biological attachments. When biological attachments are introduced, the flow drag of the underwater glider increases significantly, but BSHS achieves a drag reduction of about 16.8%. The drag reduction ability of BSHS is mainly reflected in its reduction of the streamwise fluctuation velocity within the boundary layer and achieves relaminarization of the boundary layer under the influence of AOA. These findings suggest that the BSHS retains its remarkable drag reduction capability for the underwater glider, even in the presence of adverse AOA and biological attachments. The current study demonstrates the immense potential of BSHS in the realm of underwater vehicles and offers theoretical and empirical support for future investigations into hydrodynamic performance optimization and passive drag reduction technologies for underwater vehicles.