Ducted Propellers: Behaviour in Waves and Scale Effects
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- Institutt for marin teknikk 
Ducted propellers find their application in a wide range of marine vessels, such as cargo ships, tugboats, submarines, trawlers etc. Preference over the open propeller depends on a variety of factors, the most common being increased thrust at high propeller loadings. Different duct designs have been developed based on the operational requirements, the most popular type being the accelerating duct. With the entire marine industry pushing for energy efficiency and optimized design each passing day, it has been imperative to evaluate certain aspects which had not typically been classified as ‘design criteria’, but, bear the prospect of influencing the actual performance of the propulsion unit, and hence, the operation of the vessel for which they are employed. One important aspect is related to the nature of the sea itself. For most of its life span, a vessel has to operate in waves, and hence the hydrodynamics of the propulsion unit might be strongly affected by sea conditions, which is not calm water, as used as design condition. The first part of this work focusses on the behaviour of a cargo vessel in waves, when propelled by a ducted propeller. The main point of interest is the hydrodynamic behaviour, which includes the added resistance, motions, and accelerations of the vessel in presence of waves. The study is based on results from model seakeeping tests, as well as a linear strip theory based potential flow solver ShipX Veres. The influence of the duct has been evaluated by comparison of the behaviour of the same vessel propeller by an open propeller of similar design. It was found that- the duct has very little influence on the seakeeping properties for the large cargo vessel. However, interesting differences have been observed between the propulsion factors in waves for the open and ducted propulsion cases, in spite of much similar calm water characteristics. A very striking feature for the ducted propeller is the significant increase in the propulsion point effective wake fraction in waves compared to calm water conditions. The other aspect of investigation in this work is the scale effect, which affects the propeller characteristics when model scale open water data are used to evaluate the performance in full scale. Scale effect in itself is a widely researched topic in ship hydrodynamics, considering applications in both ship resistance and propulsion domains, but there is a scarcity of knowledge and no generally recognized scaling procedure for ducted propellers. Common experiences report a ducted propeller in full scale to be operating in a lighter loading than that it is designed for, when calculations are performed using model values. Due to a large difference of Reynolds number between the model and full scale conditions, the corresponding flow characteristics are different, and often a change of flow regime is observed between them. For a ducted propeller, these changes affect the propeller and duct forces, as well as the interaction between them, which strongly influence the integral characteristics and efficiency. Based on extensive research, many approaches have been developed for the scaling of open propeller characteristics, one of them being the ITTC method. However, due to load-dependent strong duct propeller interaction, use of a simplified scaling approach is not possible for a ducted propeller. A systematic study has been carried out in this work to calculate the scale effects of a controllable-pitch propeller working within different duct designs. CFD calculations in both model and full scales using the RANSE solver of Star-CCM+ has been the backbone of this study. Model scale open water tests and paint tests at different propeller speeds have been carried out to estimate the propeller and duct forces, as well as the streamline patterns over the propeller blade and duct. It is observed that the duct thrust and therefore also the duct induced velocities increase in the full scale, causing the propeller to operate at a higher effective advance number compared to model scale conditions. An important objective of the scale effect study is to develop a suitable scaling procedure for general use. The proposed scaling approach is derived from CFD calculations at different Reynolds numbers and loading conditions. It is a component-based procedure, where the scale effects on the pressure and friction components of the propeller and duct forces are calculated separately based on dependencies on factors like Reynolds number, thrust loading, blade pitch ratio, and duct geometry. The aim of such a scaling procedure is to obtain the full scale open water diagram from the model scale diagram, without full scale calculations for each individual case. The procedure shows promising results, but more work is needed before it can be adopted for general use.