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Wind-Assisted Ship Propulsion: SAIL ASSIST GROUP
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This article presents an integrated analysis of the Flettner rotor + Flap concept. The Flettner rotor is a rotating cylinder that, due to the Magnus effect, can generate large aerodynamic forces relative to its area. Unfortunately, other than lift, this includes a substantial drag force, which results in sub-optimal performance when considering lift-to-drag efficiency. The lift-to-drag ratio for a wind propulsor is a key determinant for upwind performance. A wind-assisted ship will generally motor sail, mostly operating in upwind apparent wind conditions in which the performance for the lift-to-drag ratio of the Flettner rotor could be a short-coming.
The Flettner Rotor+Flap concept is a modification to the standard Flettner rotor design. The flap, which can be set at any angular position around the rotor, introduces the possibility to fix the separation point. This improves the aerodynamic properties of the Flettner Rotor, in particular its lift/drag ratio.
The results obtained using Blue Wasp’s Pelican performance prediction software, show that the flap significantly increases the aerodynamic thrust generated by the Flettner rotor for upwind sailing conditions. The improved lift/drag efficiency of the Flettner Rotor+Flap also results in a smaller tacking angle, meaning that the ship is able to sail closer to the wind thus increasing its operational profile. Differences in tacking angles around 15 degrees are reported. The evaluation of a shipping route on the North Sea shows that the difference in fuel savings between a ship deploying the Flettner Rotor+Flap and the Standard Flettner rotor can be as large as 35%.
The maturity of Reynolds-averaged Navier Stokes computational fluid dynamics (RANS-CFD) packages offers the ready assessment of the hydro-mechanic performance of a wind-assisted commercial ship. However, these simulations require intensive computational resources and complex software to generate results. To expedite results under different hull designs, or in the context of scenario analysis of the global shipping fleet, machine learning models are trained on a database of RANS-CFD for a systematic hull series. The model successfully trained on the RANS-CFD results with over 94% accuracy for 60 different hull variations. Using the trained model to evaluate new runs on individual hulls showed over 86% accuracy-commensurate with RANS-CFD uncertainty-allowing for rapid, accurate reproduction of wind-assist vessel response.
ARTICLE INFO Keywords Numerical error RANS Computational fluid dynamics Sailing vessels Emission control areas Green shipping ABSTRACT A Reynolds-averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for wind-assisted commercial ships. This paper describes the simulation verification exercise, performed in support of the experimental validation presented in Part 1 of this two-part series describing the RANS-CFD method employed in this research. The predominance of large-scale separated flow structures in the wake of the sailing ship, an artefact of sideforce production necessary for sailing, points to a careful verification exercise and estimate for the numerical uncertainty to support the systematic investigation of wind-assisted ship hydromechanics and meshing guidelines within the available computer resources. Methods for CFD uncertainty quantification are defined and implemented for verification cases at leeway angles equal to 0 ᵒ , 6 ᵒ , and 9 ᵒ. Analysis for four sets of grids with different meshing strategies and for varying time steps results in a grid definition and time step for simulation validation. Numerical uncertainty as adopted in Part 1 for validation is defined. Finally, the meshing strategy for full-scale simulation is described, as used for the production runs of the Delft Wind Assist Series.
In this paper, a vessel model for the performance of wind-assisted ships is combined with a routing tool to assess the fuel savings available from the installation of both one and two Flettner rotors when travelling along a Great Circle Route path. This is combined with an economic analysis to assess commercial viability for these hybrid concepts. The case study is performed in collaboration with DAMEN shipyards, who have provided a design for a wind-assist concept to sail in the Baltic Sea, that, since January 2015, is an Emission Control Area where a sulphur limit content of 0.1 % is enforced on the ship fuels. Results for this case study are presented in terms of fuel savings and payback period analysis, where the reference case is an identical ship sailing without wind propulsors. For the 5,150 dwt general cargo vessel travelling at a speed of 10 knots, average fuel savings of 2.99% were obtained in the Baltic Sea for the single Flettner scenario, and 6.11% for the double Flettner scenario. A discussion of key engineering and design constraints for these ships is included.
In this paper the state-of-the-art on wind-assisted propulsion for commercial ships is presented. The review shows that, albeit a considerable amount of research has been carried out over the years, there is still a substantial lack of knowledge on the actual performance of wind-assisted ships. Especially the aerodynamic interaction effects of wind propulsion systems as well as the hydrodynamic phenomena heel, leeway, sideforce and yaw balance are often simplified or neglected. A performance prediction program is presented and it aims to be a versatile design tool to better evaluate the use of wind energy as an auxiliary form of propulsion for commercial ships.
This paper uses a vessel model that assesses the performance of wind-assisted ships and uses a route optimisation tool to investigate the fuel savings available from one and two Flettner rotors on a Great Circle Route (GCR) and fuel-optimised route. The case study is performed in collaboration with DAMEN shipyards, who have provided a design for a wind-assist concept to sail in the Baltic Sea, that, since January 2015, is an Emission Control Area (ECA) where a sulphur limit content of 0.1 % is enforced on the ship fuels. Results for this case study are presented in terms of fuel savings and payback period analysis, where the reference case is an identical ship sailing without wind propulsors. For the 5,150 dwt general cargo vessel travelling at a speed of 10 knots, average fuel savings of 6.0% on the GCR (6.7% with fuel-optimised routing) were obtained in the Baltic Sea for the single Flettner scenario, and 13.4% on the GCR (14.8% with fuel-optimised routing) for the double Flettner scenario. At the current bunker price (550$/t), and not considering current available green incentive schemes or future possible carbon pricing measures, the payback period of the wind assist installation varies between 16.4 (two Flettner rotors, fuel-optimised routing) and 25.2 (one Flettner rotor, GCR) years. This work is a correction of an earlier publication by the same authors on the same case study (van der Kolk et al., 2019a), for which incorrect wind conditions were used.
Experiments on a large-scale Flettner rotor were carried out in the boundary-layer test section of Politecnico di Milano wind tunnel. The rotating cylinder used in the experimental campaign (referred to as Delft Rotor) had a diameter of 1.0 m and span of 3.73 m. The Delft Rotor was equipped with two purpose-built force balances and two different systems to measure the pressure on the rotor’s outer skin. The goal of the experiments was to study the influence of different Reynolds numbers on the aerodynamic forces generated by the spinning cylinder. The highest Reynolds number achieved during the experiments was \( {\text{Re}} = 1.0 \cdot 10^{6} \).
Wind-assisted ship propulsion is an effective short-to mid-term mitigation option for the maritime shipping industry's essential course for rapid decarbonisation. Wind propulsion devices such as the Flettner rotor develop an aerodynamic thrust that can replace main engine thrust, promising large reductions for the fuel consumption of ships. This study assesses the installation of three Flettner rotor devices on a 19,500 DWT tanker operating in the North Sea with fuel-optimised weather routing, using a comprehensive vessel model including operational constraints. Yearly averaged fuel savings of 29.5% are possible, corresponding to an annual CO 2 reduction of 3,330 tonnes and a payback period of 9.7 years given the present-day fuel price, excluding a carbon price. Accurate assessment of the potential savings requires collaboration amongst detailed modelling efforts spanning multiple disciplines. Significant interaction effects in the vessel modelling with implications for vessel operation with wind assist are discussed. For example, aerodynamic interaction effects and depowering required to maintain vessel operability limits the available wind-assist thrust, a level of vessel modelling not present in previous studies. Increased voyage time for minimum-fuel route optimisation is presented terms of equivalent transport work, and the implications for the economic assessment and environmental impact are discussed. The economic case for wind-assist, presented as a net present value analysis, is placed within the present-day regulatory climate, including Green Port Fee incentives and prospective carbon pricing. The study shows that wind propulsion can contribute to the mitigation agenda of international shipping in regions with strong wind speeds such as those in the North Sea. A compelling business case exists based on fuel savings and existing regulatory measures, a case that is further bolstered by protection against environmental obsolescence.
A Reynolds-averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for wind-assisted commercial ships. This paper is Part 1 of a two-part series describing the RANS-CFD method adopted for this study. The modelling challenge presented by large separated flow structures in the wake of a sailing ship points to a conscientious validation study. A validation data set, consisting of hydrodynamic forces acting on the ships sailing with a leeway angle, was collected at the Delft University of Technology towing tank facility, for bare-hull and appended cases. Appended cases were designed to represent a broad range of appendage typologies: Rudder, Bilge-keels, Skeg, and Barkeel. A validation statement is made for simulations for the entire bare-hull series and for appended geometries, excepting the Bilge-keel case. The simulation method is described in Part 2, including the assessment of the numerical uncertainty.
A Reynolds-Averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for Wind-Assisted commercial ships. The modelling challenge presented by large separated flow structures in the wake of the sailing ship points to a conscientious validation study. A validation data set, consisting of hydrodynamic forces acting on the ship sailing with a leeway angle, was collected at the Delft University of Technology towing tank facility, for bare-hull and appended cases. Four hull geometries were selected to represent of the Delft Wind-Assist Systematic Series. Appended cases were designed to represent a broad range of appendage topologies: Rudder, Bilge-keels, Skeg, and Barkeel. The direct validation exercise for the bare-hull case was successful, with the validation level for the sideforce equal to 9.5% (fine mesh: 9M cells). An extended validation statement is made for simulations for the entire series. This exercise was successful for leeway angles equal to í µí»½ = [3 í µí± , 6 í µí± ]. The validation level (base mesh, 3M cells) for each force component is: í µí± Val X ′ =12%, í µí± Val Y ′ =17%, í µí± Val N ′ =10%. Appended simulations for bilge keels were not considered successful. Other appended geometries were validated for select operating conditions and force components. The numerical uncertainty is the dominant contribution for the validation level, motivating a proportionate refinement of the grid for further simulations.
This paper deals with the hydrodynamic sideforce production of a wind-assisted ship. The subject is introduced in physical terms, and the importance of the hydrodynamic sideforce is established, before classical models are reviewed. Finally, the complications arising from diverse appendage topologies are discussed, and a theory for zero-aspect ratio lifting surfaces is discussed. The paper concludes with a short summary of ongoing work.
The TU Delft is developing a performance prediction tool that will allow for ready assessment of wind-assist concepts using regression based force models. Reynolds-Averaged Navier Stokes (RANS) simulations will be a primary tool during this study. The advent of the numerical towing tank brings possibilities but also new challenges. The predominance of large, separated flow structures in the wake of the sailing ship, and the particular interest in the lateral force generation of the hull, points to a conscientious grid verification study. Here, it is sufficient to achieve parity among uncertainty contributions within the larger context of the project. Diverse procedures are available for evaluating the numerical uncertainty of a RANS simulation. Principal methods were defined and implemented for verification cases at leeway angles of 0, 9, and 20 degrees. The uncertainty for lateral force at 9 degrees leeway for the base grid (2E6 cells) was estimated to be 8.3%.
