D21.5 - Applications of fully-RANS and cou- pled RANS/BEM methodologies to perform systematic design of unconventional propellers in given wake field

This report describes the results of marine propeller design and optimization studies performed by partners INSEAN and MARIN in the framework of WP21, Task 21.3, of the STREAMLINE Project. The present report is written in fulfilment of Deliverable D21.5. The 7000 DWT tanker selected as a case study for analysis and design activities in WP21 is taken here as baseline for optimization studies. Main objective is to increase the hydrodynamic efficiency of the original propeller by two approaches: (i) investigate the capability to achieve consistent efficiency improvements through unconventional blade shape deformation techniques combined with fully automated numerical optimization procedures, and (ii) evaluate a ducted screw layout specifically designed by CFD as an alternative to the original conventional propeller. The former approach is pursued by CNR-INSEAN. An innovative shape manipulation methodology developed under STREAMLINE WP35 and referred to as 'conformal free form deformation' is applied to investigate the possibility to achieve large improvements of propeller efficiency beyond results in the order of 1-2% that are expected by using standard design and optimization techniques, as already demonstrated by results documented in STREAMLINE Deliverable D21.4. The shape manipulation method is combined with propeller hydrodynamics modelling by an inviscid-flow solver by BEM and with a multidimensional unconstrained optimization model based on local as well as global search algorithms. Numerical applications address the optimization of the original propeller in open water as well as in behind conditions. Results of the optimization study demonstrate the capability of the proposed methodology to identify unconventional combinations of blade pitch and tip-raking that determine propeller efficiency gains of about 3% as compared to a modern propeller designed by using state-of-art industry best practice. The conflict between blade shape details enhancing open water efficiency and those generating a favourable pressure distribution throughout a propeller revolution in behind wake conditions is discussed as a factor limiting further efficiency improvements. The risk of cavitation is also identified as a limiting factor. The ducted propeller design study is presented by MARIN. First, the flow over the Ka4-70 propeller in a 19B duct in open-water conditions is analysed in order to make an assessment of the computational methodology based on the RANSE code ReFRESCO by MARIN. The full geometry of blade and duct is taken into account. The numerical simulations are done with a fixed rotation rate with varying incoming (uniform) flow velocity. Numerical results compared to experimental data for the same operating conditions in an open-water set-up show a good agreement for the complete open water range. Furthermore, viscous-flow calculations by RANSE allow for a detailed analysis enhancing the understanding of the flow field around the propeller and offers a good complement to experimental analysis. The viscous-flow methodology by MARIN is then applied to the design of a ducted propeller assembly where a streamlined optimised duct is introduced in order to improve the overall hydrodynamic efficiency. For this study, the duct flow is simulated as an axisymmetric 2D flow with propeller effects taken into account via an actuator disk simulating a screw with an infinite number of blades. Multi-objective optimisation of the duct geometry is performed, in which the two object functions are the force components in main stream direction for both a relatively low and high loading of the propeller. Results demonstrate that a combined improvement of both object functions can be obtained: compared to the duct 19A, both object functions are improved by more than 80%. The analysis of flowfield details reveals that larger thrust is obtained at a risk of flow separation on duct walls that should be carefully analysed during the design process.