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).

Uncertainty estimation of a CFD-methodology for the performance analysis of a collective and cyclic pitch propeller

“…First of all, the results show that a negative thrust force is produced under both bollard pull and captive conditions at zero collective pitch, where it is expected for the CCPP to not produce a resulting thrust force. In earlier numerical work [19], using a two-dimensional methodology, the negative force was not observed and the current model confirms the assumption of the force generation being a threedimensional phenomenon. Second, the generation of a side-force can be seen in all cases, with the force increasing as the cyclic pitch angle is increased.…”

“…forces in the yz-plane. The CCPP is intended to be fitted on a torpedo-shaped AUV and was designed with specific behavioural characteristics and geometry specifications, as discussed in previous work [19,18] and an overview of the main characteristics presented in Table 2. One of the most important characteristics of the CCPP is the blade rake angle, i.e.…”

“…Currently, a one-to-one relation is assumed between the pitch angle and the resulting force / motion, as can be derived from the directional arrows included in the cyclic pitch parameter indices (referring to the intended AUV motion, as expected from the applied pitch profile). Unfortunately, both experimental and numerical CCPP research has shown that the relation is not as straightforward as outlined [14,19,18]. Depending on the applied operational conditions, a shift will occur between the orientation of the intended / expected side-force, and the resulting side-force because of the generation of a side-force component perpendicular to the intended side-force direction.…”

“…The project uses Computational Fluid Dynamics (CFD) to investigate the complex and highly unsteady flow and force behaviour involved in the constant pitching of the propeller's blades. Numerical investigations provide some clear advantages and benefits when compared to more traditional experimental research, as discussed in the related earlier published work by the current author [17,18,19]. In the initial stage, a two-dimensional numerical model, relating the CCPP'S performance to that of pitching hydrofoils, was developed [17] and used as a preliminary performance analysis tool [19].…”

“…Numerical investigations provide some clear advantages and benefits when compared to more traditional experimental research, as discussed in the related earlier published work by the current author [17,18,19]. In the initial stage, a two-dimensional numerical model, relating the CCPP'S performance to that of pitching hydrofoils, was developed [17] and used as a preliminary performance analysis tool [19]. The twodimensional study investigated the CCPP under bollard pull condition (no forward velocity) and showed the importance of the applied collective pitch angle, and the resulting drag generation, on the effectiveness and efficiency of the manoeuvring force.…”