Development of a propeller Lifting-Line tool for analysis and design.

Abstract

The development of marine propellers is fundamental in ship design, as the appropriate hull-propeller combination may provide thrust-torque ratio optimization, greenhouse gas emissions reductions, and vibration minimization. Propeller design is already a complex task that requires the knowledge of many areas in addition to the use of diverse engineering tools. Particularly in hydrodynamics, these tools can be classified according to a trade-off between cost and fidelity: while it is sometimes more interesting to have a not-as-accurate, but a fast tool -as during early-stage design -, other times it is more appropriate a more precise, however costly, tool -as during refinement stage. This work presents a modern formulation of the Lifting-Line Theory for preliminary hydrodynamic propeller analysis, i.e., given a propeller geometry, the formulation is capable of obtaining the thrust and torque for a given advance velocity. Its importance lies in the fast configuration assessment and on the possibility of rapidly changing geometry characteristics. The development of the formulation starts from the wing lifting-line, in which adaptations are made to incorporate the influence of viscosity on lift and drag and to make it suitable for wings with dihedral and sweep at flow regimes until stall onset. Once established, the wing lifting-line is then adapted to the moderately loaded propeller problem, i.e., those whose operation sheds almost constant-pitch helical wakes; therefore, given the hypothesis, the adaptation conveys the properties of the influence of viscosity on thrust and torque, and the suitability to simulate propellers with rake and skew distributions. Both formulations are verified and validated according to procedures recommended by credited societies; verification is limited to mesh convergence analysis, while validation is done through comparison between numerical and experimental data, obtained from tests available on the literature. For both wings and propellers, verification presented satisfactory convergence rates for at least one of the proposed discretization schemes, indicating the possibility of simulating wings with dihedral and sweep and propellers with rake and skew; validation presents adequate adherence between numerical and experimental results for regions in which the thin-foil-theory is valid (wings) and near design advance ratio (propellers), results that indicate the capability of the methods for simulating wings and propellers with the proposed geometries, the limitations associated with the flow regimes, and the influence of the viscosity on forces and moments. In summary, the proposed formulations meet the expectations and special attention is given to the propeller one as it is oriented towards hydrodynamic analysis, typically not possible for such methods. Additionally, the inclusion of viscosity and the possibility of simulating raked and skewed propellers without further corrections broadens the range of application, thus creating a relatively low-cost-good-fidelity tool.