Application of Passive Drag Reduction Techniques on a Scaled-Down Underwater Vehicle

• Author / Creator

  Reholon Inojosa, Desiree

• Natural Resources Canada (NRCan) has been using an autonomous underwater vehicle (AUV) to map the vast Canadian Artic seabed. The logistics of under-ice survey is currently complicated due to limited lifetime of lithium-ion batteries of AUVs. An AUV with longer endurance would allow exploration of a larger seafloor area from a single Arctic camp, reducing costs and advancing the rate of exploration. A 1:15 scaled-down model of NRCan AUV was designed and manufactured to test different passive drag reduction techniques with the goal of improving its endurance in future. The developed scaled-down underwater vehicle has a modular design consisting of the front edge, central body, and aft body sections. The main component of the aft body is a replaceable cylinder of 0.190 m length (l), where the different drag reduction techniques are applied. The aft body module is mounted on a submersible s-beam load cell for direct measurement of the axial force (i.e., drag) on this module. The vertical forces of weight and buoyancy in the aft body have been balanced in order to avoid off-axis forces and momentums on the load cell. Three passives drag reduction methods were studied, including: superhydrophobic surfaces (SHS), air injection, riblets, and also their combinations. The experiments were carried out in the high-speed water loop of the University of Alberta, over a Reynolds number (Re) range of 5.0×10^5 to 1.5×10^6 (based on the free-stream velocity of 0.9 to 3.3 m/s and model length of 0.508 m. The test section of this facility is equipped with transparent walls for visualization and is 0.25 m wide, 0.45 m high and 2.1 meters long. For the study of an SHS with random texture, direct measurement of the drag force is complemented with the simultaneous use of shadow-based long-range microscopic particle tracking velocimetry (micro-PTV). The use of the shadowgraph technique over the body-of-revolution allowed the visualization of the air plastron and its characteristic over time and with increasing Re number. Consistent with previous investigations, the water movement over the surface enhanced the air depletion. The largest air thickness was observed at the lowest Re = 5.0×10^5, which also showed a larger value of drag reduction. Conversely, a faster air depletion rate was observed at higher velocities. The depletion of air exposed the SHS microstructures and reduced the drag reduction. The direct measurement of the drag force resulted in a maximum drag reduction (DR) of 36% at the lowest Re = 5.0×10^5, and gradually decreased with higher velocities to a 5.6% at Re = 1.5×10^6. The magnitude of slip velocity was estimated at the top of peaks of the SHS texture, showing a larger slip velocity with increasing Re number. The use of the porous surface increased drag when compared with the smooth surface, and no improvement was observed with air injection. The efficiency of this technique was strongly influenced by the non-uniform distribution of the bubbles over the surface. The use of a superhydrophobic coating over the porous cylinder (porous SHS) reduced drag in comparison with the non-coated porous cylinder. Although the injection of air did not improve the porous SHS efficiency, it was possible to replenish the air layer that gradually dissolved into the flow stream. The replenished porous SHS could keep its superhydrophobicity as long as the air was supplied. The use of rectangular riblets with normalized spanwise tip spacing of s+ = 15 – 40 increased drag in comparison with the smooth surface. Yet, the use of the superhydrophobic coating over the riblets reduced drag in comparison with the riblets surface without the coating. The combined effect of rectangular riblets (s+ = 30 – 80) and a low air injection rates, did not result in the formation of grooves filled with air. This configuration also increased drag when compared with the smooth surface. Overall, the performance of riblets might have been adversely affected by the existence of a crossflow component around the AUV body, and manufacturing imperfections of the grooves.

• Subjects / Keywords

  • Superhydrophobic Surfaces
  • Drag Reduction
  • Passive Drag Reduction Techniques
  • Axisymmetric Body
  • Underwater Vehicle

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3V698T1N

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.