Hydrodynamic Performance Analysis of a Collective and Cyclic Pitch Propeller Under Bollard Pull Condition Through Numerical Evaluation of Two-Dimensional Pitching Hydrofoils

Abstract

Propulsion and maneuvering of autonomous underwater vehicles require a combination of effective and efficient operation at both high and low speeds. The collective and cyclic pitch propeller (CCPP) is a novel system designed to provide the required operational flexibility through control of the propeller's blade pitch. Collective pitch control governs the forward generated thrust, whereas cyclic pitch control governs the generated maneuvering force(s)/side-force(s). In this article, a numerical analysis into the CCPP's hydrodynamic performance at bollard pull is set-up, reducing the complex three-dimensional flow problem to a two-dimensional problem. Through a force break-down model, the CCPP's hydrodynamic performance is related and matched to the operation of a pitching hydrofoil. Analysis of the two-dimensional numerical results can thereby provide insights into the performance of the three-dimensional CCPP. First, the performance of the pitching hydrofoils is investigated as such, relating the generated lift, drag, and moment to the occurrence of dynamic stall. Next, the methodology's applicability and limitations are discussed by comparing the numerical results with recent experiment CCPP work to allow the model to be used for a numerical evaluation of the CCPP's performance. Under the evaluated conditions, testing a range of collective and cyclic pitch angles under bollard pull, the side-force generation by the CCPP is shown to be highly dependent on the generated drag force at higher collective pitch angles. At low pitch angles, the side-force generation is controlled by the lift produced over the pitching blades, and efficient but not highly effective. As the collective pitch is increased, the generated drag affects both the effectiveness of the side-force and the side-force efficiency, defined by the large resulting side-force orientation. At larger collective pitch angles, the lift forces are overtaken by the drag generation, resulting in effective but inefficient side-force generation. Autonomous underwater vehicles (AUVs) have become a widely used and researched tool for underwater exploration and reconnaissance (Alam et al. 2014). AUVs distinguish themselves from other unmanned underwater vehicles in their ability to complete a pre-determined mission autonomously over large distances and long time periods, i.e., without the need for regular human interaction. The diversity in industry applications for AUVs has resulted in a wide range of AUV shapes and designs (Button et al. 2009). Applications include different areas such as underwater pipe-line inspection in the oil and gas industry, sample collection for marine biology research, and military surveillance missions (Chyba 2009). One key requirement of any AUV design, as a result of their specific mission profile and inherent functionality, is the combination of efficient long-endurance travelling capabilities with effective maneuverability at low speeds (Wernli 2000). Traditional maneuvering systems using control surfaces lose their efficiency at low speeds and lowspeed maneuvering aids such as side-or podded-thrusters reduce the long-endurance travelling efficiency. A novel propulsion and maneuvering system, aimed at providing both efficient long-endurance propulsion and effective maneuvering at all speeds, is the collective and cyclic pitch propeller (CCPP).