Stefano Gaggero

RANS and PANEL method for unsteady flow propeller analysis

A spatial non uniform inflow is the usual inflow to which a marine propeller is subjected to. Inside a ship wake, in inclined shaft condition, in tandem/contra rotating configurations, the spatial non uniformity of the incoming flow on the propeller plane leads to the unsteadiness responsible of thrust and torque fluctuations, induced pressures, cavitation and the associated noise.Any design method and, moreover, each analysis tool should be able to treat these phenomena, to carry out a more consistent ship oriented propeller design and to better understand the flow features and the performances of an already designed propeller. In the present work two different numerical approaches, a commercial RANS solver and a in house developed Panel Method, are addressed and applied for the analysis of marine propellers subjected to an oblique inflow, showing, through the comparison with the experimental measures, their capabilities and their limitations.

EFD and CFD Characterization of a CLT Propeller

In the present paper an experimental and numerical analysis of an unconventional CLT propeller is carried out. Two different numerical approaches, a potential panel method and an RANSE solver, are employed. Cavitation tunnel experiments are carried out in order to measure, as usual, thrust, torque, and cavity extension for different propeller working points. Moreover, LDV measurements are performed to have a deep insight into the complex wake behind the propeller and to analyze the dynamics of generated tip vortexes. The numerical/experimental analysis and comparison of results highlight the peculiarities of this kind of propellers, the possibility to increase efficiency and reduce cavitation risk, in order to exploit the design approaches already well proven for conventional propellers also in the case of these unconventional geometries.

EFD and CFD Design and Analysis of a Propeller in Decelerating Duct

Ducted propellers, in decelerating duct configuration, may represent a possible solution for the designer to reduce cavitation and its side effects, that is, induced pressures and radiated noise; however, their design still presents challenges, due to the complex evaluation of the decelerating duct effects and to the limited amount of available experimental information. In the present paper, a hybrid design approach, adopting a coupled lifting line/panel method solver and a successive refinement with panel solver and optimization techniques, is presented. In order to validate this procedure and provide information about these propulsors, experimental results at towing tank and cavitation tunnel are compared with numerical predictions. Moreover, additional results obtained by means of a commercial RANS solver, not directly adopted in the design loop, are also presented, allowing to stress the relative merits and shortcomings of the different numerical approaches.

An Investigation on the Discrepancies Between RANSE and BEM Approaches for the Prediction of Marine Propeller Unsteady Performances in Strongly Non-Homogeneous Wakes

In the present work an analysis of the reliability of different numerical approaches, namely BEM and RANSE solver, for the prediction of unsteady performances of marine propellers is presented. To this aim, the well-known Seiun-Maru Highly Skewed Propeller operating in different wakes is considered. The results of this analysis show that, in correspondence to particularly challenging conditions, i.e. when very pronounced ship wakes are present, the two approaches may provide considerably different results in terms of propeller mechanical characteristics and pressure distributions on the blades. This problem, which is implicitly eliminated when the thrust identity approach is applied (as almost always performed, as an example, for propeller analyses aimed to the evaluation of cavitation extension and / or induced pressures), may become critical in case the codes are used for numerical self-propulsion tests, where a high accuracy in the prediction of the unsteady propeller performances in correspondence to a prescribed value of the advance coefficient, instead of the thrust coefficient, is mandatory. The analyses carried out allow to underline which are the potentially more problematic cases, in terms of wakes characteristics, and to suggest some possible reasons of the encountered discrepancies, which will need further analyses to enhance the prediction capability of numerical codes.Copyright

Fatigue Strength Assessment of Propellers by Means of Weakly Coupled CFD and FEM Analyses

Propeller design considerably evolved during years, continuously imposing new challenges to the designer, from usual high efficiency and avoidance of erosive cavitation up to reduction of radiated noise and pressure pulses for “high added value” ships. New more stringent limits, together with a general requirement for performances optimization, leads to the need of a continuous review of design procedures which, in some cases, still rely on rather “old-fashioned” approaches.The aim of this paper is to analyse in detail the importance of considering fatigue phenomena in the propeller design, proposing a procedure for the evaluation in time of the stress field acting on a blade during one revolution, which is then considered for fatigue assessment.Copyright

Ship propeller side effects: pressure pulses and radiated noise

Abstract The present paper deals with the side effects of propellers cavitation, i.e. pressure pulses and radiated noise. These effects are gaining more and more importance for commercial ships for different reasons. Pressure pulses significantly affect comfort onboard, thus their reduction is of utmost importance for all ships carrying passengers. As regards the underwater radiated noise, in the last decade interest has shifted from navy applications to commercial ships, due to the concern for the rising background noise in the oceans. The propellers, generating noise directly in water, represent one of the main contributions to the overall underwater noise emitted from ships. Due to the complexity of the mechanisms of propeller noise generation, different complementary strategies have to be followed to properly analyze the problem, ranging from induced pressure pulses to broadband noise and cavitation. In the present work, part of the activities carried out in the framework of the collaborative EU FP7 project AQUO (Achieve QUieter Oceans by shipping noise footprint reduction, www.aquo.eu) are reported. The paper presents the investigations carried out on a specific test case represented by a single screw research vessel, which is analyzed with three different strategies: numerical calculations, model scale investigations and fullscale measurements.