Propeller

The paper offers an analytical formulation of the two errors embodied in the momentum theory. The first one originates from to the use of the differential form of the axial momentum equation and the second one from the linearisation of the tangential velocity terms. Both errors are evaluated comparing the axial velocity at the disk as predicted by the momentum theories with that one obtained thorough a semi-analytical actuator disk method based on the exact solution of the flow. Several cases characterised by different values of the thrust and advance coefficient are analysed, and the range of validity of the momentum theories is discussed in depth.

This paper completes the work presented in the companion paper [Bontempo et al., Appl. Ocean Res., 58 (2016) 322 - 330] by presenting the investigation of the flow around a propeller ducted with a so-called accelerating duct.
To this aim, both the axial momentum theory and a nonlinear actuator disk method are used.
The straightforward application of the first approach reveals that if the duct and rotor thrusts are concordant, then a beneficial effect on the propulsive efficiency can be readily obtained by enclosing a propeller in an accelerating duct.
When the more advanced nonliner actuator disk method is applied to verify the outcomes of the axial momentum theory additional information on the performance of the device are obtained.
Moreover, the nonlinear actuator disk method is also employed to investigate, through experimental design techniques, the effect of the key geometrical parameters of the duct onto the efficiency and robustness of this kind of propulsive system.
In particular, it has been found that a propulsive efficiency gain can be achieved through a duct thickness, camber and chord increase, and through an incidence decrease.

In this paper, we propose a Simulation-Based Design Optimization (SBDO) approach for the design of a Pre-Duct type Energy Saving Device (ESD). Pre-Ducts reduce the wake losses, contribute to a better interactions between the propeller and the hull and generate, in the end, generate an additional thrust. An integrated design approach, based on a parametric description of the duct geometry and on RANSE calculations together managed by an optimization algorithm, is developed to this aim, i.e. to maximize the thrust delivered by a Wake Equalizing Duct (WED). The Japan Bulk Carrier test case, for which experimental data regarding the effectiveness of WED is available in the literature , is considered as a reference. Results from the design activity show sensible improvements of the overall propul-sive efficiency when the Pre-Duct design is tailored to the actual hull wake shape thanks to the flexibility provided by the fully automated design approach proposed in this paper.

The paper describes the assessment of two different actuator disc models as applied to the flow around open propellers. The first method is based on a semi-analytical approach returning the solution for the nonlinear differential equation governing the axisymmetric, steady, inviscid and incompressible flow around an actuator disc. Despite its low computational cost, the method does not require simplifying assumptions regarding the shape of the slipstream, e.g. the wake contraction is not disregarded or prescribed in advance. Moreover, the presence of a tangential velocity in the wake as well as the spanwise variation of the load are taken into account. The second one is a commonly used procedure based on CFD techniques in which the effects of the propeller are synthetically described through a set of body forces distributed over the disc surface. Both methods avoid the difficulties and the computational costs associated with the resolution of the propeller blades geometrical details. The comparison is based on an in-depth error analysis of the two procedures which results in a set of reference data with controlled accuracy. An excellent agreement has been documented between the two methods while the computational complexity is obviously very different. Among other things the comparison is also aimed at verifying the accuracy of the semi-analytical approach at each point of the computational domain and at quantifying the effect of the errors embodied in the two methods on the quality of the solution, both in terms of global and local performance parameters. Furthermore, the paper provides a set of reference solutions with controlled accuracy that could be used for the verification of new and existing computational methods. Finally, the computational cost of the semi-analytical model is quantified, thus providing a valuable information to designers who need to select a cost effective and reliable analysis tool.

The propeller performance has been investigated using a benchmark Duisburg Test Case ship with RANSE. First, the hydrodynamic characteristics of propeller in case of open water have been analyzed by a commercial CFD program and the results are compared with those of experimental data. Later, the flow around the bare hull has been solved and the frictional resistance value and form factor of the ship have been obtained and compared with those of ITTC57 formulation and experimental results for validation. The free surface effect has been ignored. A good agreement has been obtained between the results of RANSE and experiments at both stages. Then the ship - propeller interaction problem was solved by RANSE and the differences in thrust, torque and efficiency of propeller as compared with the open–water numerical results have been discussed.

The paper presents an extension to ducted rotors of the nonlinear actuator disk theory of Conway (J. Fluid Mech., vol. 365, 1998, pp. 235–267) and it is exact for incompressible, axisymmetric and inviscid flows. The solution for the velocities and the Stokes stream function results from the superposition of ring vortices properly arranged along the duct surface and the wake region. Using a general analytical procedure the flow fields are given as a combination of one-dimensional integrals of expressions involving complete as well as incomplete elliptic integrals. The solution being exact, the proper shape of the slipstream whether converging or diverging is naturally accounted for, even for heavy loads. A semi-analytical method has been developed that enables the flow induced by an actuator disk housed in a contoured duct to be solved duly accounting for the nonlinear mutual interaction between the duct and the rotor. Non-uniform load distributions, rotor wake rotation and ducts of general shapes and thickness distribution can be dealt with. Thanks to its reduced numerical cost, the method is well suited for the design and/or analysis of ducted rotors for marine, wind and aeronautical applications.

Propeller optimisation is always the focus of the propeller design process, as such process is aimed at finding the best compromise between often conflicting objectives accounting for many design constraints. The use of optimization algorithms combined with blade shape modification techniques has been proposed by a number of research groups in the past few years and has proved to have a potential for practical applications, but integration in the everyday propeller design process is still beyond to be so consolidated. In the past one and a half year, CETENA, University of Genoa and the Fincantieri's Naval Vessels Business unit have teamed to set up a propeller optimization software environment to be used by Fincantieri's propeller designers in their everyday design work. A specification of the optimisation environment was worked out based on an analysis of the current design process, in order for the new procedures and tools to allow continuity of current practices while offering new possibilities in the comparative investigation of design variants and the identification of the very best design solution. The resulting optimisation environment integrates different software tools and consists of three main components linked by JAVA script architecture: a software tool for blade shape modification and definition (1), a BEM code for the evaluation of the propeller performance (2) and the open-source software DAKOTA to guide the optimisation (3). The software suite has been setup such as to be compatible with both Windows and Linux operating systems, in order to take advantage of all available computational resources, from single Windows workstations to the company's cluster, which operates under Linux. An ad-hoc '2D modeler' has been developed by the University of Genoa to model and modify the propeller design table, while a 3D modeller has been developed by CETENA to generate the 3D propeller description suitable for CFD RANS computations, starting from the 2D 296 propeller design curves. The propeller performance is evaluated using CRS BEM code PROCAL and the CRS Empirical Tip Vortex model. Measures of merit usually include: the propeller efficiency, pressure coefficients at specific locations on the blade back and face sides and, for an hydro-acoustic optimization, also the tip vortex induced pressure. The paper will provide details of the setup of the optimization environment focusing on an industrial application to design by optimization a naval propeller. The specific needs of the final users of the optimization suite will be highlighted and the expected benefits will be discussed, related to the time frame of the design process and the possibility of performing thorough investigations of design variants.

The 9 bladed propeller attached to the rear of the flying vehicles with a co-axial rotor assembly provides extra thrust to enhance the speed of the vehicle. The CFD analysis and further results and discussions using FLUENT provide the numerical computations for the improved velocity near wall effects of the propeller.

The paper presents the validation of a generalised semi-analytical actuator disk model as applied to the study of the flow around ducted propellers. The method, which returns the exact solution as a superposition of ring vortex, duly accounts for the rotation of the wake, the convergence of the slipstream, and the nonlinear mutual interaction between the duct and the propeller. Furthermore, it can deal with an arbitrary radial distribution of the load and ducts of general shape. In order to validate the previously mentioned actuator disk model, results obtained through it are compared with those provided by the so-called " CFD actuator disk method ". The latter is a widely diffused tool for the analysis of the flow around open and ducted propellers which models the rotor by means of radial profiles of blade forces distributed over a disk surface. In this paper, evidence has been given of the excellent agreement between the results of the two methods. Thanks to its extremely reduced computational cost the semi-analytical method is well suited to be integrated into design systems based on the repeated analysis scheme of hierarchical type.

The paper analyses the flow around a marine propeller ducted with a so-called decelerating nozzle both through the axial momentum theory and the nonlinear semi-analytical actuator disk model. While the well-known and widely diffused axial momentum theory can be successfully employed only to qualitatively investigate the characteristics of the flow around a ducted propeller, the nonlinear and semi-analytical method can instead evaluate the thrust exerted by the duct for different values of the overall thrust and advance coefficients. There are several advantages characterising the more advanced actuator disk method. Specifically, the wake convergence and rotation may be fully taken into account, the shape of the duct and of the radial distribution of the load can be of general type, and, finally, the mutual interaction between the duct and the propeller may be readily dealt with. The methods are employed to investigate the effects of the decelerating nozzle on the efficiency and on the cavitation condition of the propeller. In particular, the influence of some duct geometrical parameters on the device performance is thoroughly analysed providing useful insights on the operating principles of this kind of propulsive systems.