ABSTRACT Simplified mathematical models based upon semi-empirical regression formulae describing ... more ABSTRACT Simplified mathematical models based upon semi-empirical regression formulae describing forces and moments acting on the hull during manoeuvres are commonly used for the preliminary evaluation of ship steering capabilities. Many semi-empirical regressions have been developed, and are broadly adopted, for the manoeuvrability prediction of single screw slow/medium speed ships. Their application for the study of twin screw vessels (cruise ships, RoRo ferries, megayachts, naval vessels, for which dedicated regression formulae are scarce) manoeuvring capabilities could lead to misleading results. 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 a novel procedure to properly account for the particular geometric and stern appendage characteristics of these kind of vessels is described. A thorough analysis has been performed on a ship model equipped with 13 different stern appendage configurations and new formulae have been developed in order to describe accurately their influence on ship manoeuvring behaviour.
The problem of ship maneuverability has currently reached a significant consideration, both for m... more 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 twinscrew naval vessels maneuverability behavior. From this analysis, stern appendages influence (including also nonlinear effects resulting from hull/appendage interactions) resulted one of the peculiar characteristics of this type of ship, clearly affecting their maneuvering capabilities.
ABSTRACT Propeller modelling in CFD simulations is a key issue for the correct prediction of hull... more ABSTRACT 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 χnavis 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 quantification of the radial force exerted by the propeller is of paramount importance for th... more The quantification of the radial force exerted by the propeller is of paramount importance for the improvement of the design procedures as well as for enhancing the prediction of the controllability and maneuvering qualities of oceangoing vessels. In this work the radial load experienced by the bearing strut of a twin screw configuration is experimentally investigated by free running model tests carried out at the CNR-INSEAN outdoor maneuvering basin. To this purpose the model has been equipped with a novel, in-house developed, two-component transducer. Results obtained for the straight ahead motion for a large range of speeds (0:05 o F N o 0:425) and turning circle maneuvers at different rudder angles (151-351) and three different speeds (F N ¼ 0.218, 0.32 and 0.35) are presented and discussed in this paper. The main task is also to identify, from a phenomenological perspective, the nature of this load, in particular during tight maneuvers, characterized by critical overloading of the propellers. The use of the free running, self-propelled model, is the best way to gain a deeper insight on the loads generated by the propeller during quasi-steady and, especially, transient motions that are difficult to reproduce in a towing tank via captive model tests.
ABSTRACT The onset and the nature of dynamic instabilities experienced by the wake of a marine pr... more ABSTRACT 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�, 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 �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.
9th IFAC Conference on Control Applications in Marine Systems (2013), 2013
Propulsion system can experience large power absorption fluctuations during tight maneuvers. In t... more Propulsion system can experience large power absorption fluctuations during tight maneuvers. In the case of a turning circle maneuver for a twin-screw ship, the power required by the two shaft lines can be completely different; in case of non conventional propulsion system, like cross-connect configurations, a compromise must be met in order to design a safe control system without affecting dramatically the vessel's maneuvering performance. In this work, a series of free running model tests have been carried out in order to investigate the influence of different propulsion system operation settings on the vessel's maneuvering characteristics.
The turning circle manoeuvre of a self-propelled tanker like ship model is numerically simulated ... more 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 considered is a twin screw single rudder vessel. 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 operations should be considered. The effects of the stern appendages (shaft lines and brackets) on the vessel's manoeuvring capabilities is analysed. Comparison with experimental data from free running tests will demonstrate the feasibility of the CFD computations and in particular of the proposed model for the propeller side force estimation.
The turbulent flow behind a rotating marine propeller is analysed by integration of the Reynolds-... more The turbulent flow behind a rotating marine propeller is analysed by integration of the Reynolds-Averaged Navier-Stokes Equations with both the Spalart & Allmaras eddy viscosity model (1) and by a Detached Eddy Simulation approach (2) in order to assess advantages and limits of the two different turbulence models. As far as global quantities (like thrust and torque) are concerned, it is shown that the two methods perform equally well. On the contrary, local flow features (like the evolution of the wake or the onset of tip vortices instability) are capured by DES, whereas the eddy viscosity modelling proves to be overly dissipative.
Twin screw vessels' propulsion system experiences strong off design conditions during tight manoe... more Twin screw vessels' propulsion system experiences strong off design conditions during tight manoeuvres due to the propellers inflow asymmetry arising from the coupled yaw-drift motion. Unfortunately, simplified mathematical models based upon statistical data or ad hoc executed captive model test (PMM or CMT) do not provide such a detailed information. Indeed, free running model tests are the best mean in order to get ship's trajectory and kinematics parameters data and propulsion behaviour by recording the loads (thrust and torque) on the shafts. More insight into this complex aspect is desired in order to improve and generalize the application of existing manoeuvring mathematical models for the preliminary design of unconventional propulsive configuration control system.
Twin screw ships may experience considerably asymmetrical propeller functioning during maneuvers.... more Twin screw ships may experience considerably asymmetrical propeller functioning during maneuvers. This phenomenon may result in large power fluctuations during tight maneuvers, with increases of shaft torque up to and over 100% of the steady values in straight course with considerable unbalances. A multi-year joint research project supported by Italian Navy has been set up in order to deeply investigate the phenomenon, potentially dangerous for ships propulsion systems with coupled shaftlines, by means of large scale model testing and related numerical simulations.
Propulsion systems experience large power absorption fluctuations during tight maneuvers. In the ... more Propulsion systems experience large power absorption fluctuations during tight maneuvers. In the case of a turning circle maneuver for a twin-screw ship, the power required by the two shaft lines can be completely different; in case of non conventional propulsion system, like cross-connect configurations, a compromise must be met in order to design a safe control system, without dramatically affect the vessel's maneuvering performance. In order to investigate the influence of different propulsion system operation settings on the vessel's maneuvering characteristics, a series of had-hoc free running model tests have been carried out at the CNR-INSEAN outdoor maneuvering basin. In the present work experimental results will be presented and discussed, focusing on the ship maneuvering performance under different propulsion system control settings. Moreover, CFD have been used to provide a deeper insight on the propellers overloading and unbalancing: first, numerical computations have been carried out to capture the nominal wake in correspondence of the propeller disks; then, propeller loads were evaluated (off-line the RANSE simulation) by means of two simplified model based on Blade Element Momentum theory (BEMT) theory.
ABSTRACT The present work is aimed to assess the capability of a numerical code based on the solu... more ABSTRACT 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 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 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 and downstream wake will be presented as well. Verification of the numerical computations have been assessed by grid convergence analysis.
ABSTRACT This paper presents a comparison among different hydrodynamic models for the analysis of... more ABSTRACT This paper presents a comparison among different hydrodynamic models for the analysis of the unsteady loads delivered by a marine propeller working in an axial, non-uniform inflow. Specifically, for a propeller subjected to a wake-field dominated by local high-frequency changes in space, the unsteady hydroloads predicted by the Nakatake formulation are compared with those given by the Theodorsen and Sears theories, respectively. Drawbacks and potentialities of these approaches are highlighted to assess a computationally efficient hydrodynamic solver for the analysis of operating conditions where propeller blades are significantly perturbed by a multi-harmonic onset-flow. Guidelines coming from this investigation may drive the choice of a fast and reliable unsteady propeller modeling that represents a good trade-off between accuracy of simulation and cost of computation within implementation in Computational Fluid Dynamics (CFD) solvers. The hydrodynamic formulations herein proposed are validated through numerical comparisons with the (accurate but computationally expensive) propeller loads predicted by a fully 3-D panel-method Boundary Element Method (BEM) solver, suited for the analysis of propellers operating in a complex hydrodynamic environment.
ABSTRACT Simplified mathematical models based upon semi-empirical regression formulae describing ... more ABSTRACT Simplified mathematical models based upon semi-empirical regression formulae describing forces and moments acting on the hull during manoeuvres are commonly used for the preliminary evaluation of ship steering capabilities. Many semi-empirical regressions have been developed, and are broadly adopted, for the manoeuvrability prediction of single screw slow/medium speed ships. Their application for the study of twin screw vessels (cruise ships, RoRo ferries, megayachts, naval vessels, for which dedicated regression formulae are scarce) manoeuvring capabilities could lead to misleading results. 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 a novel procedure to properly account for the particular geometric and stern appendage characteristics of these kind of vessels is described. A thorough analysis has been performed on a ship model equipped with 13 different stern appendage configurations and new formulae have been developed in order to describe accurately their influence on ship manoeuvring behaviour.
The problem of ship maneuverability has currently reached a significant consideration, both for m... more 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 twinscrew naval vessels maneuverability behavior. From this analysis, stern appendages influence (including also nonlinear effects resulting from hull/appendage interactions) resulted one of the peculiar characteristics of this type of ship, clearly affecting their maneuvering capabilities.
ABSTRACT Propeller modelling in CFD simulations is a key issue for the correct prediction of hull... more ABSTRACT 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 χnavis 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 quantification of the radial force exerted by the propeller is of paramount importance for th... more The quantification of the radial force exerted by the propeller is of paramount importance for the improvement of the design procedures as well as for enhancing the prediction of the controllability and maneuvering qualities of oceangoing vessels. In this work the radial load experienced by the bearing strut of a twin screw configuration is experimentally investigated by free running model tests carried out at the CNR-INSEAN outdoor maneuvering basin. To this purpose the model has been equipped with a novel, in-house developed, two-component transducer. Results obtained for the straight ahead motion for a large range of speeds (0:05 o F N o 0:425) and turning circle maneuvers at different rudder angles (151-351) and three different speeds (F N ¼ 0.218, 0.32 and 0.35) are presented and discussed in this paper. The main task is also to identify, from a phenomenological perspective, the nature of this load, in particular during tight maneuvers, characterized by critical overloading of the propellers. The use of the free running, self-propelled model, is the best way to gain a deeper insight on the loads generated by the propeller during quasi-steady and, especially, transient motions that are difficult to reproduce in a towing tank via captive model tests.
ABSTRACT The onset and the nature of dynamic instabilities experienced by the wake of a marine pr... more ABSTRACT 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�, 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 �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.
9th IFAC Conference on Control Applications in Marine Systems (2013), 2013
Propulsion system can experience large power absorption fluctuations during tight maneuvers. In t... more Propulsion system can experience large power absorption fluctuations during tight maneuvers. In the case of a turning circle maneuver for a twin-screw ship, the power required by the two shaft lines can be completely different; in case of non conventional propulsion system, like cross-connect configurations, a compromise must be met in order to design a safe control system without affecting dramatically the vessel's maneuvering performance. In this work, a series of free running model tests have been carried out in order to investigate the influence of different propulsion system operation settings on the vessel's maneuvering characteristics.
The turning circle manoeuvre of a self-propelled tanker like ship model is numerically simulated ... more 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 considered is a twin screw single rudder vessel. 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 operations should be considered. The effects of the stern appendages (shaft lines and brackets) on the vessel's manoeuvring capabilities is analysed. Comparison with experimental data from free running tests will demonstrate the feasibility of the CFD computations and in particular of the proposed model for the propeller side force estimation.
The turbulent flow behind a rotating marine propeller is analysed by integration of the Reynolds-... more The turbulent flow behind a rotating marine propeller is analysed by integration of the Reynolds-Averaged Navier-Stokes Equations with both the Spalart & Allmaras eddy viscosity model (1) and by a Detached Eddy Simulation approach (2) in order to assess advantages and limits of the two different turbulence models. As far as global quantities (like thrust and torque) are concerned, it is shown that the two methods perform equally well. On the contrary, local flow features (like the evolution of the wake or the onset of tip vortices instability) are capured by DES, whereas the eddy viscosity modelling proves to be overly dissipative.
Twin screw vessels' propulsion system experiences strong off design conditions during tight manoe... more Twin screw vessels' propulsion system experiences strong off design conditions during tight manoeuvres due to the propellers inflow asymmetry arising from the coupled yaw-drift motion. Unfortunately, simplified mathematical models based upon statistical data or ad hoc executed captive model test (PMM or CMT) do not provide such a detailed information. Indeed, free running model tests are the best mean in order to get ship's trajectory and kinematics parameters data and propulsion behaviour by recording the loads (thrust and torque) on the shafts. More insight into this complex aspect is desired in order to improve and generalize the application of existing manoeuvring mathematical models for the preliminary design of unconventional propulsive configuration control system.
Twin screw ships may experience considerably asymmetrical propeller functioning during maneuvers.... more Twin screw ships may experience considerably asymmetrical propeller functioning during maneuvers. This phenomenon may result in large power fluctuations during tight maneuvers, with increases of shaft torque up to and over 100% of the steady values in straight course with considerable unbalances. A multi-year joint research project supported by Italian Navy has been set up in order to deeply investigate the phenomenon, potentially dangerous for ships propulsion systems with coupled shaftlines, by means of large scale model testing and related numerical simulations.
Propulsion systems experience large power absorption fluctuations during tight maneuvers. In the ... more Propulsion systems experience large power absorption fluctuations during tight maneuvers. In the case of a turning circle maneuver for a twin-screw ship, the power required by the two shaft lines can be completely different; in case of non conventional propulsion system, like cross-connect configurations, a compromise must be met in order to design a safe control system, without dramatically affect the vessel's maneuvering performance. In order to investigate the influence of different propulsion system operation settings on the vessel's maneuvering characteristics, a series of had-hoc free running model tests have been carried out at the CNR-INSEAN outdoor maneuvering basin. In the present work experimental results will be presented and discussed, focusing on the ship maneuvering performance under different propulsion system control settings. Moreover, CFD have been used to provide a deeper insight on the propellers overloading and unbalancing: first, numerical computations have been carried out to capture the nominal wake in correspondence of the propeller disks; then, propeller loads were evaluated (off-line the RANSE simulation) by means of two simplified model based on Blade Element Momentum theory (BEMT) theory.
ABSTRACT The present work is aimed to assess the capability of a numerical code based on the solu... more ABSTRACT 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 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 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 and downstream wake will be presented as well. Verification of the numerical computations have been assessed by grid convergence analysis.
ABSTRACT This paper presents a comparison among different hydrodynamic models for the analysis of... more ABSTRACT This paper presents a comparison among different hydrodynamic models for the analysis of the unsteady loads delivered by a marine propeller working in an axial, non-uniform inflow. Specifically, for a propeller subjected to a wake-field dominated by local high-frequency changes in space, the unsteady hydroloads predicted by the Nakatake formulation are compared with those given by the Theodorsen and Sears theories, respectively. Drawbacks and potentialities of these approaches are highlighted to assess a computationally efficient hydrodynamic solver for the analysis of operating conditions where propeller blades are significantly perturbed by a multi-harmonic onset-flow. Guidelines coming from this investigation may drive the choice of a fast and reliable unsteady propeller modeling that represents a good trade-off between accuracy of simulation and cost of computation within implementation in Computational Fluid Dynamics (CFD) solvers. The hydrodynamic formulations herein proposed are validated through numerical comparisons with the (accurate but computationally expensive) propeller loads predicted by a fully 3-D panel-method Boundary Element Method (BEM) solver, suited for the analysis of propellers operating in a complex hydrodynamic environment.
Propeller modeling approaches for off–design operative conditions, 2019
In adverse situations, such as maneuvering and motion in waves, severe variations of the propelle... more In adverse situations, such as maneuvering and motion in waves, severe variations of the propeller inflow may be experienced, resulting in an increase of propeller thrust and torque and in the generation of in-plane loads. This may cause undesired hull-vibratory loads, stress of the propulsive system and even affect somehow the ship dynamic response. Thus, a reliable prediction of these phenomena during design phases is necessary to comply with the increasingly stringent constraints on safety at sea, propulsive efficiency, vibration and noise pollution. In the present work, the capabilities of a propeller solver based on a potential, boundary element method, routinely used in the optimization process of the propulsive device, to analyze the propeller performance under different maneuvering conditions are considered. After a first validation against simulations considering a simple oblique flow, the analysis is broadened to a propeller operating in the wake field of a twin screw ship in different maneuvering conditions, for which experimental results from free running tests in model scale are available. The solver is compared also to a steady blade element approach in order to achieve an overview of the respective pros and cons in view of their inclusion in simulations.
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Papers by G. Dubbioso