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Analysis of a marine propeller operating in oblique flow. Part 2: Very high incidence angles
No full textThe 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
On the wake dynamics of a propeller operating in drift
No full textThe 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 λ2 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
Vortex structures in the wake of a marine propeller operating close to a free surface
Full text linkThe present paper analyses the vortical structures in the wake of a naval propeller operating underneath a free surface using detached-eddy simulation. We investigate the flow topology for several loading conditions and compare it with analogous observations behind a propeller operating in open water. We show that the wake topology is similar to that observed in open water only for low-loading conditions. For mild blade loading, the free surface's presence seems to stabilize the flow. On the contrary, for high blade loading, the mutual interaction between the vortex system and the free surface leads to vortex breakdown that overshadows the multiple pairing mechanisms observed in open-water conditions
Analysis of the performances of a marine propeller operating in oblique flow
No full textThe 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. © 2013 Elsevier Ltd
Analysis of propeller bearing loads by CFD. Part II: Transient maneuvers
No full textThe numerical study presented in Part I (Dubbioso et al., 2017) on the bearing loads developed by the propellers of a twin screw model during quasi–steady conditions is extended to transient maneuvers. In the previous study, numerical simulations highlighted that the hydrodynamic loads might experience significant peak at moderate turning rates due to complex interaction of the propeller with the wake. In the present paper, the complete turning circle maneuver at δ=35∘ at Fr=0.265 is numerically simulated in order to analyze the character of the blade loads during the transient phases after the actuation of the rudder (start and pull–out). The analysis shows that the overall degradation of the propeller performance may occur also at kinematic conditions weaker than those usually considered as the most critical ones (in general, tight maneuvers); therefore, these conditions should be accounted for also in the early design phases
Turning ability analysis of a fully appended twin screw vessel by CFD. Part I: Single rudder configuration
No full textThe turning circle manoeuvre of a naval supply vessel (characterized by a block coefficientCB~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
Numerical analysis of marine propellers low frequency noise during maneuvering. Part II: Passive and active noise control strategies
No full textManeuvering motion inevitably modifies the acoustic signature of marine propellers due to alteration of the wake field past the hull. This aspect is crucial to fulfill the stringent requirements for the noise abatement in the environment. Therefore, design procedures, mainly targeted to rectilinear motion in calm water, should encompass these conditions for achieving a low emission profile. In the present work, the analysis presented in Dubbioso et al., (2020) on the noise generated by a marine propeller in behind-hull is contextualized for the development of holistic tools for design and mission planning, with attention to both passive and active noise control strategies. Passive solutions consist of modification of stern appendages and inversion of propeller rotational direction, while active control is implemented by limiting of the absorbed power, load fluctuation and their combination. The noise sources are first computed by a non-interacting RANS–BEMT procedure and given as input to the acoustic analogy based on the Formulation 1A by Farassat. The test case is a modern twin screw ship undergoing rectilinear advance and turning maneuvers at two different rudder angles for a moderate speed at FN=0.265
Analysis of propeller bearing loads by CFD. Part I: Straight ahead and steady turning maneuvers
No full textMarine 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
Method for estimating parameters of practical ship manoeuvring models based on the combination of RANSE computations and System Identification
No full textIn this work a method for estimating parameters of practical ship manoeuvring models based on the combination of RANSE computations and System Identification procedure is investigated, considering as test case a rather slender twin screw and two rudders ship. The approach consists in the estimation of the hydrodynamic coefficients applying System Identification to a set of free running manoeuvres obtained from an in-house unsteady RANS equations solver, which substitute the usually adopted experimental tests at model or full scale. In this alternative procedure the numerical quasi-trials (in terms of kinematic parameters time histories and, if needed, forces time histories) are used as input for the System Identification procedure; the aim of this approach is to reduce external disturbances that, if not properly considered in the mathematical model, may compromise the identification results, or at least amplify the well-known “cancellation effects”. Furthermore, the CFD results provide information both in terms of flow field variables and hydrodynamic forces on the manoeuvring ship. These data may be adopted for a better understanding of the complex flow during manoeuvres, especially at stern, providing also additional information about the interaction between the various appendages (including rudders) and the hull. The identification procedure is based on an off-line genetic algorithm used for minimizing the discrepancy between the reference manoeuvres from CFD and those simulated with the system based modular model. The discrepancy was measured considering different metric functions and simplified formulations which consider only the main macroscopic parameters of the manoeuvre; the metrics have been analysed in terms of their capability in reproducing the time histories and in limiting the cancellation effect of the hydrodynamic derivatives
Experimental investigation of single blade loads by captive model tests in pure oblique flow. Part II: Propeller in-plane loads and preliminary comparison of single blade loads during transient phases
No full textOff–design and realistic operative conditions of the propulsion system have been progressively considered a frontier topic for the development of novel and successful design procedures. The availability of high accurate data on propeller performance, both experimental or numerical, is pivotal for the development of reliable tools and the enhancement of traditional ones. In this context, this paper continues the investigation described in (Ortolani et al., 2020), that was dedicated to analyze the single blade loads obtained by oblique towing tests and assess their consistency with those measured during the steady turning phase, reproduced by free running model tests. In this paper, the same analysis is broadened to two aspects. At first, the in–plane forces and moments that are obtained by a conversion from the rotating frame of the measure to the fixed frame are discussed. These loads need quantification, because they are the primary cause of damages of the shafting structure, vibratory loads and also contribute to dynamic response of the ship. Then, the study is switched to the comparison of single blade loads during transient motions of the turning maneuvers, at weak and tight rudder angles, performed at the same reference speed as the captive model test. The analysis lays the basis for the enhancement of ship system (comprehensive) mathematical models, already used in ship design, to real–time analysis of vibratory loads and emitted noise
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