Drag Reduction Through Air-Trapping Discrete Grooves in Underwater Applications

Vehicles travelling underwater experience drag and the frictional drag costs 60% of the total energy consumption. Using trapped air serving as a lubricant is a promising way to reduce drag. The trapped air plays a significant role in drag reduction, and most failures in drag reduction are related to instability, escape, and dissolution of the trapped air. In this work, discrete grooves are employed to trap air and reduce drag. Through the analysis of the trapped air stability, the groove length and width are believed to be the main factors that influence the air escape and instability, and thus they are limited in this work to avoid these problems. The air dissolution is inevitable. The effective way to mitigate the air dissolution is to deepen the groove depth. The groove depth in this work varies from 0.5 mm to 4 mm. The numerical simulation is employed to analyze the flow field, reveal the drag reduction mechanism, and optimize the groove length. The experimental measurements are conducted to verify our design. The result confirms our design that the discrete grooves successfully avoid air escape and instability, mitigate air dissolution, and reduce drag. This work is meaningful for underwater vehicles to travel with low energy consumption and high speed.