Abstract
Underwater vehicles broadly fall into two categories: Autonomous Underwater Vehicles (AUVs) and Remotely Operated Vehicles (ROVs). AUVs are untethered, battery-powered platforms designed for pre-programmed survey missions — bathymetric mapping, pipeline inspection, environmental monitoring — operating autonomously over extended ranges. ROVs, by contrast, are tethered systems connected to a surface vessel via an umbilical cable, providing real-time operator control and typically higher thrust capacity for precision intervention tasks such as subsea valve operation, structural inspection, and repair.
Hybrid AUV/ROV systems combine both operational modes: autonomous transit and survey capability with the option to switch to tethered, operator-controlled mode for close-up inspection and intervention. This dual-purpose architecture offers significant operational flexibility for offshore energy, marine infrastructure, and defence applications.
The hydrodynamic challenge in designing such systems is substantial. The hull must balance low drag for efficient autonomous transit with the thrust-to-weight ratio needed for hovering and precise station-keeping in ROV mode. At operational speeds, drag forces scale with the square of velocity, making hull form optimisation critical for battery endurance. In hovering mode, the vehicle must generate sufficient vertical and lateral thrust to maintain position against ocean currents — requiring careful analysis of added mass coefficients and hydrodynamic stability derivatives.
Prof. Afzal's team conducted a pre-feasibility study for Aeroservices Ltd. The scope encompassed computation of drag profiles across a range of Reynolds numbers and operational speeds, estimation of thrust-to-weight ratio requirements for both transit and hovering modes, and determination of hydrodynamic coefficients — drag coefficient (Cd), lift coefficient (Cl), and added mass terms — using a combination of 3D CFD analysis, potential flow methods, and semi-empirical correlations.
The pre-feasibility data directly informs downstream engineering decisions: propulsion system sizing (thruster count, motor selection, propeller design), battery capacity and endurance calculations, and structural design of the drone hull. By establishing the hydrodynamic envelope early in the design process, the study reduces the risk of costly design iterations during detailed engineering.
Key Findings
- Drag profiles computed across a range of Reynolds numbers for both transit and hovering operational modes.
- Thrust-to-weight ratio requirements established for autonomous survey and tethered intervention configurations.
- Hydrodynamic coefficients (Cd, Cl, added mass) determined using CFD, potential flow, and semi-empirical methods.
Methodological Approach
3D CFD analysis, potential flow methods, and semi-empirical correlations for drag coefficient, lift coefficient, and added mass estimation across the operational speed envelope.
Implications for Hydraulic Practice
Pre-feasibility hydrodynamic data informs propulsion sizing, battery endurance calculations, and hull structural design, reducing risk of costly iterations during detailed engineering for Aeroservices Ltd.
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