The understanding of the performance of a propeller in realistic operative conditions is nowadays a key issue for improving design techniques, guaranteeing safety and continuity of operation at sea, and reducing maintenance costs. In this paper, a summary of the recent research carried out at CNR-INSEAN devoted to the analysis of propeller loads in realistic operative scenarios, with particular emphasis on the in-plane loads, is presented. In particular, the experimental results carried out on a free running maneuvering model equipped with a novel force transducer are discussed and supported by CFD (Computational Fluid Dynamics) analysis and the use of a simplified propeller model, based on Blade Element Momentum Theory, with the aim of achieving a deeper understanding of the mechanisms that govern the functioning of the propeller in off-design. Moreover, the analysis includes the scaling factors that can be used to obtain a prediction from model measurements, the propeller radial force being the primary cause of failures of the shaft bearings. In particular, the analysis highlighted that cavitation at full scale can cause the increment of in-plane loads by about 20% with respect to a non-cavitating case, that that in-plane loads could be more sensitive to cavitation than thrust and torque, and that Reynolds number effect is negligible. For the analysis of cavitation, an alternative version of the BEMT solver, improved with cavitation linear theory, was developed.

The interference between the hull, propeller and rudder remarkably affects the control and maneuvering capabilities of marine vehicles. In case of twin screw/twin rudder ships, the asymmetric evolution of the wake past the hull causes the asymmetric functioning of the propeller-rudder system. Systematic investigations on this aspect for twin screw ships are limited. Available experimental data carried out on simplified hull-propeller-rudder system and captive model tests do not allow to completely understand the fluid mechanism at the basis of the hydrodynamic interaction that should be taken into account in simplified maneuvering mathematical models for preliminary predictions. In this paper the hull-propeller-rudder interactions phenomena for a twin screw/twin rudder model are investigated by URANS simulations, with a particular focus on the asymmetry of the propeller-rudder system. To this aim, captive model tests consisting of pure rudder and coupled drift-yaw motions corresponding to the steady phases of turning circle maneuvers at different rudder angles (?=15°÷35°) are performed at the speed correspondent to Fr=0.265. Moreover, a free running maneuvering simulation is also performed to gain more insight on the transient phase of the maneuver. An identity rudder lift methodology is applied to synthesize the hull-propeller-rudder interactions by means of a flow straightening coefficient; the analysis highlights that these effects are weak and invariant with respect to the rudder angle on the windward shaft, whereas on the leeward side these effects are extremely sensitive to the evolution of the hull and propeller wake.

The vortex-body interaction problem, that characterizes the flow field of a rudder placed downstream of a single-blade marine rotor, is investigated by numerical simulations. The particular topology of the propeller wake, consisting of a helicoidal vortex detached from the blade tips (tip vortex) and a longitudinal, streamwise oriented vortex originating at the hub (hub vortex), embraces two representative mechanisms of vortex-body collisions: the tip vortices impact almost orthogonally to the mean plane, whereas the hub vortex travels in the mean plane of the wing (rudder), perpendicularly to its leading edge. The two vortices evolve independently only during the approaching and collision phases. The passage along the body is instead characterized by strong interaction with the boundary layer on the rudder and is followed by reconnection and merging in the middle and far wake. The features of the wake were investigated by the l2-criterion and typical flow variables (pressure, velocity and vorticity) of the instantaneous flow field; wall pressure spectra were analysed and related to the tip and hub vortices evolution, revealing a non-obvious behaviour of the loading on the rudder, that can be related to undesired unsteady loads.

Marine propellers in behind-hull conditions develop, in addition to thrust and torque, in-plane loads that are strictly related to fatigue stress of the propulsive shaft bearings, hull-induced vibrations and the dynamic response of the ship while maneuvering or experiencing wave induced motions. An in-depth understanding of their nature as well as their quantification in typical design and off-design operative scenario is fundamental for improving ship design criteria. This issue is tackled in the present work by means of URANS simulations and simplified propeller theories to assess the correlation between inflow conditions and propeller loads. In particular, the analysis is carried out for the same twin screw model recently considered in free running maneuvering model tests (Ortolani et al., 2015a, 2015b) and further aims to provide a complementary and deeper insight to the outcome of these experiments. The first part of the study is focused on the straight ahead motion and the steady turning maneuvers with rudder deflections of 15°, 25° and 35° and Froude number equal to 0.26.

The present investigation focuses on the effects of the stern appendages and the propulsion system on the hydro-loads generated by the propeller during off-design conditions, with particular emphasis on the in-plane components. Recent experimental investigations carried out by free running model tests [7,8] and CFD analysis [5] for a modern twin screw model, highlighted that maneuvers at small drift angles and yaw rates might be as critical as the tighter ones due to complex propeller-wake interactions. Therefore, design criteria should take into account also these operative conditions, in order to reduce the effects of propeller-wake interaction phenomena that degrade the overall propulsive efficiency, induce shaft/hull structural vibration and increase noise emission. In the present study we analyze the effects of geometric and propulsive modifications with respect to the twin screw configuration studied in [5]. In particular, the effect of the centreline skeg, propeller direction of rotation and control strategies of the propulsion plant on the propeller bearing loads have been investigated from the analysis of the nominal wake in maneuvring conditions, computed by unsteady RANSE simulations coupled with a propeller model based on Blade Element Theory. The considered test cases were turning circle maneuvers with different rudder angles at FN = 0.265.

The turning circle manoeuvre of a naval supply vessel (characterized by a block coefficient ^CB~0.60) is simulated by the integration of the unsteady Reynolds-Averaged Navier Stokes equations coupled with the equations of rigid body motion with six degrees of freedom. The model is equipped with all the appendages, and it is characterised by an unusual single rudder/twin screws configuration. This arrangement causes poor directional stability qualities, which makes the prediction of the trajectory a challenging problem. As already shown in previous works, the treatment of the in-plane loads exerted by the propellers is of paramount importance; to this aim each propeller is simulated by an actuator disk model, properly modified to account for oblique flow effects. The main goal of the present paper is to assess the capability of the CFD tool to accurately predict the trajectory of the ship and to analyse the complex flow field around a vessel performing a turning manoeuvre. Distribution of forces and moments on the main hull, stern appendages and rudder are analysed in order to gain a deeper insight into the dynamic behaviour of the vessel. Validation is provided by the comparison with experimental data from free running tests.

Interaction of the vortex systems detached from a propeller with a rudder installed in its wake is investigated by CFD. The correct prediction of this phenomenon is of great interest in naval hydrodynamics research, it being the source of irradiated noise and vibratory loads. The phenomenology is addressed by simulating a single bladed propeller (INSEAN E779A) and a rudder characterized by a rectangular plane area and symmetric sectional shape (NACA0020 profiles). The main focus is on the hydro-loads developed by the rudder and their correlation with the different phases of the interaction of the tip vortex with the rudder. The phenomenon is also investigated, through a preliminary computation on a coarser mesh, on the actual propeller geometry (4-bladed).

The analysis of a propeller operating in off-design conditions is one of the most attractive and challenging topics in naval hydrodynamics, because of its close connections with different aspects of ship design and performances. For these reasons, wake dynamics and propeller loads are analyzed in the present paper by means of a numerical code based on the solution of the Reynolds averaged Navier-Stokes equations, whose capability to capture propeller hydrodynamics in these extreme conditions are also investigated. The test case considered is the CNR-INSEAN E779A propeller model, for which a detailed experimental database exists for axial flow conditions; propeller geometry and computational domain are discretized by means of an overlapping grid approach.A wide range of incidence angles (10-50°) at two different loading conditions are considered, in order to analyze the propeller performance during severe off-design conditions, similar to those experienced during very complicated maneuvering scenarios. Details of average and instantaneous loads are reported, for both the complete propeller and for a single blade.The present paper is an extension of the analysis of propeller performance in oblique flow, recently proposed in [1]; here, the focus is on propeller performance at very high angle of incidence. The k - {small element of} and a DES turbulence models have been exploited also, in order to provide a reliable verification of the numerical results in the absence of experimental data in these extreme operating conditions. © 2013 Elsevier Ltd.

The onset and the nature of dynamic instabilities experienced by the wake of a marine propeller set in oblique flow are investigated by means of detached eddy simulations. In particular, the destabilization process is inspected by a systematic comparison of the wake morphology of a propeller operating in pure axisymmetric flow and in drift with angle of 20 degrees, under different loading conditions. The wake behaviour in oblique flow shows a markedly different character with respect to the axisymmetric condition: in the latter, the destabilization is triggered by an increasing interaction of the main vorticity confined in the tip vortex; whereas, in the former, the role of the secondary vorticity (oriented in the streamwise direction) as well as the hub vortex seems to be crucial. The features of the wake have been investigated by the lambda(2) criterion (Jeong & Hussain, J. Fluid Mech., vol. 285, 1995, pp. 69-94) and typical flow variables (pressure, velocity and vorticity), for both the averaged and instantaneous flow fields. Moreover, in order to further inspect the evolution of the vortical structures, as well as their interaction and destabilization, the spectra of the kinetic energy have been considered. This investigation aims to broaden the knowledge from previous works on the subject of rotor wake instabilities, focusing on the differences between an ideal (axisymmetric) and actual operating conditions occurring in typical engineering applications.

The present work is aimed to assess the capability of a numerical code based on the solution of the Reynolds averaged Navier--Stokes Equations for the study of propeller functioning in off design conditions; this aspect is becoming of central interest in naval hydrodynamics research because of its crucial implications on design aspects and performance analysis of the vessel during its operational life. A marine propeller working in oblique flow conditions is numerically simulated by the unsteady Reynolds averaged Navier-Stokes equations (uRaNSe) and a dynamically overlapping grid approach. The test case considered is the CNR-INSEAN E779A propeller model. Two different loading conditions have been considered at different incidence angles (10° to 30°) in order to analyze the propeller performance during idealized off-design conditions, similar to those experienced during a tight manoeuvre. The main focus is on hydrodynamic loads (forces and moments) that act on a single blade, on the hub and on the complete propeller; peculiar characteristics of pressure distribution on the blade will be presented as well. Verification of the numerical computations have been asses sed by grid convergence analysis.

The present work is aimed to assess the capability of a numerical code based on the solution of the Rey- nolds averaged Navier-Stokes equations for the study of propeller functioning in off design conditions; this aspect is becoming of central interest in naval hydrodynamics research because of its crucial impli- cations on design aspects and performance analysis of the vessel during its operational life. A marine pro- peller working in oblique flow conditions is numerically simulated by the unsteady Reynolds averaged Navier-Stokes equations (uRaNSe) and a dynamically overlapping grid approach. The test case consid- ered is the CNR-INSEAN E779A propeller model. Two different loading conditions have been analyzed at different incidence angles (10-30°) in order to characterize the propeller performance during idealized off-design conditions, similar to those experienced during a tight manoeuvre. The main focus is on hydro- dynamic loads (forces and moments) that act on a single blade, on the hub and on the complete propeller; peculiar characteristics of pressure distribution on the blade and downstream wake will be presented as well. Verification of the numerical computations have been assessed by grid convergence analysis

Propeller modelling in CFD simulations is a key issue for the correct prediction of hull-propeller interactions, manoeuvring characteristics and the flow field in the stern region of a marine vehicle. From this point of view, actuator disk approaches have proved their reliability and computational efficiency; for these reasons, they are commonly used for the analysis of propulsive performance of a ship. Nevertheless, these models often neglect peculiar physical phenomena which characterise the operating propeller in off-design condition, namely the in-plane loads that are of paramount importance when considering non-standard or unusual propeller/rudder arrangements. In order to emphasize the importance of these components (in particular the propeller lateral force) and the need of a detailed propeller model for the correct prediction of the manoeuvring qualities of a ship, the turning circle manoeuvre of a self-propelled fully appended twin screw tanker-like ship model with a single rudder is simulated by the unsteady RANS solver ?navis developed at CNR-INSEAN; several propeller models able to include the effect of the strong oblique flow component encountered during a manoeuvre have been considered and compared. It is emphasized that, despite these models account for very complex and fundamental physical effects, which would be lost by a traditional actuator disk approach, the increase in computational resources is almost negligible. The accuracy of these models is assessed by comparison with experimental data from free running tests. The main features of the flow field, with particular attention to the vortical structures detached from the hull are presented as well.

The turning circle maneuver of a self-propelled tanker like ship model is numerically simulated through the integration of the unsteady Reynolds averaged Navier-Stokes (uRaNS) equations coupled with the equations of the motion of a rigid body. The solution is achieved by means of the unsteady RANS solver ?navis developed at CNR-INSEAN. The focus here is on the analysis of the maneuvering behavior of the ship with two different stern appendages configurations; namely, a twin screw with a single rudder and a twin screw, twin rudder with a central skeg. Each propeller is taken into account by a model based on the actuator disk concept; anyhow, in order to correctly capture the turning maneuvering behavior of the model, a suitable model which takes into account for oblique flow effects has to be considered. Results from a preliminary verification assessment are discussed; validation of the predicted trajectory and the kinematical parameters is provided by comparison with experimental data from free running tests. Maneuvering abilities of the two configurations are discussed; in order to better understand the different behavior of the two configurations, an in depth analysis of the force and moments on the hull and on the individual appendages is provided.

The problem of ship maneuverability has currently reached a significant consideration, both for merchant ships, with the adoption of IMO standards, and naval ships, with the production of various documents by NATO Specialist Teams. In literature, many works regarding maneuverability of single-screw slow/medium speed ships are present, while a lack of information about twin-screw ships (cruise ships, Ro/Ro ferries, megayachts, naval vessels) exists. These ships are usually characterized by different hull forms and more complex stern configuration because of the presence of appendages such as skegs, shaft lines, and brackets, which can strongly affect maneuverability behavior. In this work various prediction methods, namely statistical regressions, system identification, and RANSE, are investigated to evaluate twin-screw naval vessels maneuverability behavior. Results of this analysis clearly evidence importance of stern appendages influence on maneuverability capabilities of this type of ship (including also nonlinear effects resulting from hull/appendage interactions).

The turning circle maneuver of a self-propelled tanker like ship model is numerically simulated through the integration of the unsteady Reynolds averaged Navier-Stokes (uRaNS) equations coupled with the equations of the motion of a rigid body. The solution is achieved by means of the unsteady RANS solver Xnavis developed at CNR-INSEAN. The focus here is on the analysis of the maneuvering behavior of the ship with two different stern appendages configurations; namely, a twin screw with a single rudder and a twin screw, twin rudder with a central skeg. Each propeller is taken into account by a model based on the actuator disk concept; anyhow, in order to correctly capture the turning maneuvering behavior of the model, a suitable model which takes into account for oblique flow effects has to be considered. Results from a preliminary verification assessment are discussed; validation of the predicted trajectory and the kinematical parameters is provided by comparison with experimental data from free running tests. Maneuvering abilities of the two configurations are discussed; in order to better understand the different behavior of the two configurations, an in depth analysis of the force and moments on the hull and on the individual appendages is provided.

The turning circle manoeuvre of a self-propelled tanker like ship model is numerically simulated through the integration of the unsteady Reynolds Averaged Navier-Stokes (URANS) equations coupled with the equations of the motion of a rigid body. The solution is achieved by means of the unsteady RANS solver developed at CNR-INSEAN. The model is considered with two different stern appendages configurations (each one providing a different dynamic behaviour): twin screw with a single rudder and twin screw, twin rudder with a central skeg. Each propeller is taken into account by a model based on the actuator disk concept; anyhow, in order to correctly capture the turning manoeuvring behaviour of the model, a suitable description of the propeller performance in oblique flow operation has be considered. Comparison with experimental data from free running tests will demonstrate the feasibility of the CFD computations. The main features of the flow field, with particular attention to the vortical structures detached from the hull is presented as well.

The problem of ship manoeuvrability has reached nowadays a significant consideration, both for merchant ships, with the adoption of IMO standards, and naval ships, with the production of various documents by NATO Specialist Teams. In literature many works regarding manoeuvrability of single screw slow/medium speed ships are present, while a lack of information about twin screw ships (cruise ships, RoRo ferries, megayachts, naval vessels) exists. These ships are usually characterised by different hull forms and more complex stern configuration due to the presence of appendages like skegs, shaft lines and brackets, which can strongly affect manoeuvrability behaviour. In this work various prediction methods, namely statistical regressions, system identification and RANSE, have been investigated in order to evaluate twin screw naval vessels manoeuvrability behaviour. From this analysis stern appendages influence (including also non linear effects resulting from hull-appendages interactions) resulted one of the peculiar characteristics of this type of ships, clearly affecting their manoeuvring capabilities