28 research outputs found
Propeller modeling approaches for off–design operative conditions
No full textIn 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 uRANS 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 BEM 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 CFD simulations
Numerical analysis of marine propellers low frequency noise during maneuvering
No full textIn this paper the effects of maneuvering motion on the hydro–acoustic performance of a marine propeller in
behind–hull configuration is investigated. Velocities induced by ship motions markedly modify the inflow to the
propeller and hence noise sources and emission can be different with respect to the rectilinear motion, that
traditionally is exclusively considered during the design process. This problem is crucial from a practical
perspective, because the success of many types of operations at sea relies on the stealthy qualities of the vehicle
and, moreover, for it can provide an additional aid to preserve marine life. In this paper this topic is tackled by a
multidisciplinary approach that involves the use of hydrodynamic solvers and an acoustic analogy based on the
Formulation 1A by Farassat. In particular, the noise sources, input to the acoustic analogy, are computed by an
hybrid approach consisting of CFD simulation to obtain the inflow to the propellers and a blade element mo
mentum theory solver (BEMT), enhanced with linear, partially cavitating hydrofoil theory to account for cavi
tating conditions too. 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 F N = 0.265
Experimental investigation of single blade loads by captive model tests in pure oblique flow
No full textManeuvering motion is a critical off-design condition experienced by the propeller during realistic operations. Failures of the propulsive system, loss of efficiency and modification of the propeller side effects (propeller-hull induced pressure and noise) are the undesired consequences of these working conditions. Free running model tests, still representing the primary approach for a reliable performance assessment, requires facilities and devices that are not commonly affordable; alternatively, rectilinear towing tank can be used for maneuvering investigations by static or dynamic tests and can be a valid alternative to investigate propeller performance in offdesign. On these basis, in this paper the propeller performance in maneuvering conditions is investigated by
means of oblique towing tests in case of a twin screw model equipped with a novel set-up for single blade loads measurements.In the experiments, the drift angle and the advance speed of the model varied systematically, to focus on the relation between propeller operating conditions and loads. Moreover, the averaged and periodic blade loads are compared, in terms of the equivalent drift angle, to the measurements obtained by free running model tests, in order to demonstrate the reliability of pure oblique flow tests for the preliminary quantification of the off-design loads developed by the propeller
Analysis of the flow field around a rudder in the wake of a simplified marine propeller
No full textThe vortex–body interaction problem, which 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 \unicode[STIX]{x1D706}_{2}-criterion (Jeong & Hussain, J. Fluid Mech., vol. 285, 1995, pp. 69–94) 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.</jats:p
Free-Surface Effects on the Evolution of a Marine Propeller's Wake
No full textThe 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 over-shadows the multiple pairing mechanisms observed in open-water conditions.The structure of propeller wakes received great attention in the past (Kerwin, 1986) and in recent years (Felli et al., 2011; Muscari et al., 2013; Di Mascio et al., 2014a; Muscari et al., 2017a; Magionesi et al., 2018) because the evolution of the main vortical structures that emanate from the blade tips and the hub is tightly related to vibrations, noise and aerodynamic/hydrodynamic performances. The wake of a rotor has a very complex topology, with several vortical systems with various shapes and strengths. Three main structures are generally present: tip vortices, blade vortex sheets and hub vortex. For each blade, the tip vortex and the blade vortex sheet correspond to the trailing vortex system of a wing, whose local strength is proportional to the radial variation of blade circulation. Since the most significant circulation variation appears at the blade end, vorticity is markedly higher at the tip than in the rest of the blade vortex sheet, with a distinguishable concentrated tip vortex. The hub vortex system is a well-defined streamwise structure, where the vorticity is equal in strength and opposite in orientation to the sum of blade vortex systems (in a simplified portrait of inviscid flow plus horseshoe vortices, it consists of the sum of each horseshoe vortex for the blades). In a more realistic picture of a viscous fluid, the blade vortex sheet also carries the vorticity generated in the boundary layer that detaches at the trailing edge. Moreover, horseshoe vortices are also present at each blade root. All these structures interact during downstream convection in a peculiar way that depends on the blade load (i.e. local incidence). For some loading conditions, the regular vortex system becomes unstable. These basic mechanisms were largely studied for the four-bladed E779A marine propeller by means of flow visualizations (Felli et al., 2011) and further corroborated by numerical simulations (Muscari et al., 2013; Ahmed et al., 2020; Wang et al., 2021a,b). The tip vortex is the first to experience helical instabilities associated with self- and coil-to-coil interaction due to self-induction. These perturbations propagate along the vortex tube and cause transverse oscillations of the cores. Further downstream, due to the weakening of the blade vortex sheet, the tip and hub vortices are no longer linked and evolve independently as separate structures, giving rise to destabilization process (Okulov and Sørensen, 2007). In facts, in the absence of the blade sheet (that damps tip vortex oscillations), the interactions between consecutive tip vortices are strengthened, giving rise to long-wave instabilities and promoting the merging of two adjacent tip vortices. Reduced- order analysis using proper orthogonal decomposition and dynamic mode decomposition of detached-eddy simulations proved that the merging process consists of a sequence of modes with an asymmetric evolution of the coupled tip vortices at the periphery of the slipstream (Magionesi et al., 2018). Consequently, the hub vortex experiences transverse oscillations that amplify downstream, driving a low-frequency precession motion of the outer structures. These mechanisms amplify vortex stretching and tilting, with complete redistribution of the vorticity that leads to the complete loss of coherence in the wake and transition to almost homogeneous turbulence. Experimental observations and numerical simulations show that all vortex pairing mechanisms and instabilities move upstream when the blade load increases; this happens because the distance between tip vortices decreases, thus increasing the mutual interaction
Analysis of twin screw ships\u27 asymmetric propeller behaviour by means of free running model tests
No full textTwin screw ships may experience considerably asymmetric propeller functioning during manoeuvres. This phenomenon may result in large power fluctuations during tight manoeuvres, with increases of shaft torque up to and over 100% of the steady values in straight course and considerable unbalances; this, in its turn, may be potentially dangerous, especially in case of particularly complex propulsion plant configurations, such as those with coupled shaftlines. A joint research project supported by the Italian Navy has been set up in order to deeply investigate the phenomenon, by means of large scale model testing and related numerical simulations. In the present work, the extensive experimental campaign results on a free running model of a twin-screw ship are presented, allowing to obtain a deeper insight of the problem. In particular, tests have been carried out simulating different simplified control schemes, starting from the most common constant rate of revolution tests and including different control strategies (constant torque and power). Usual standard manoeuvres (turning circle, zigzag and spiral) have been carried out, providing results for asymmetric shaft functioning and ship manoeuvrability behaviour. Results from the present analysis allow to obtain the complete model for the time domain simulation of asymmetric shaft functioning
Inclined-Flow Propeller Hydroacoustics by the Permeable Ffowcs Williams Hawkings Equation
No full textIn this paper the noise hydrodynamically generated by noncavitating marine propellers in non-
axial flow condition is investigated. The permeable Ffowcs Williams and Hawkings Equation
(FWH-P), solved by a Boundary Element Method (BEM), is used to radiate sound outward a
suitable fictitious surface that encloses all the sources of sound related to the propeller and flow-
field around it. To this aim, the fluctuating velocity and pressure field distributions required upon
the permeable surface are obtained by a Detached Eddy Simulation (DES). For validation pur-
poses, the acoustic signatures evaluated by the combined DES/FWH-P approach are compared
with those directly computed by the hydrodynamic solver for observers placed in the near field.
Keywords: Propeller hydroacoustics, Oblique flow, Acoustic Analogy, Permeable surface tech-
nique
Analysis of the asymmetric behavior of propeller–rudder system of twin screw ships by CFD
No full textThe 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 (δ=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
Beyond the Great Wall. Win win Italy China partnership : collaborating for a mutual competitive advantage
No full textLAUREA SPECIALISTICAMotivazione: La Cina sta affrontando una grande trasformazione del settore imprenditoriale. Il Paese spera che questa trasformazione, di respiro mondiale, possa coinvolgere anche il settore del design.
La moda ed il design italiano sono noti in tutto il mondo, ma in seguito alla crisi economica la vendita di prodotti d'esportazione ha dovuto fronteggiare enormi difficoltà e ha dovuto, per questo, cercare nuove modalità di vendita: non solo fornire prodotti materiali, ma anche “idee” e servizi.
Problematiche: La Cina risulta essere un paese “chiuso” per molti aspetti, basti pensare alla politica o anche all’economia; il suo livello d’internazionalizzazione è ancora limitato, ed usi e costumi peculiari rendono questa barriera ancor più netta. Risulta, per questo, esser necessario porre basi culturali più profonde per una migliore conoscenza delle differenze e delle uguaglianze.
Proposta: Si propone qui di seguito un progetto in fase di realizzazione, il quale ha come attore protagonista il sistema Liò, prodotto di design d’illuminazione per esterni. L’obbiettivo è quindi elaborare intorno al prodotto ed alla sua possibile utenza anche una strategia di comunicazione, che qualifichi esso stesso ed il produttore. Contemporaneamente sperimentare le diverse possibilità e modalità di collaborazione e condivisione tra aziende e professionisti in Cina ed in Italia
Risultati: la semplice certezza che la piena condivisione del “know how” progettuale e produttivo dei due paesi, favorisce la crescita di entrambi.Facing a worldwide corporate restructuration, China is the stage of transforming from manufacturing factory of the world design centre. Since Italy is well known over the world for its design major, as the global crisis, the traditional export model of the product are also facing great challenges.
China is a relatively isolated country regard to its own unique cultural practices. Therefore, it is selective behind from the international level for some subjects, while the European understanding of China is limited; co-operation need to be established on the basis of mutual understanding between difference culture background.
Based on actual project - the design of outdoor lighting Lio, it extends to the corporate strategy and brand design. It explores the possible corporation opportunity between Italy and China.
A series of outdoor lighting design Lio, and through its series of lamps, it enhances Chinese enterprises by promoting the use of Italian design
Hydrodynamic Characterization of USV Vessels with Innovative SWATH Configuration for Coastal Monitoring and Low Environmental Impact
No full textAbstractThe high costs associated with the use of research oceanographic vessels and the maturity of the unmanned surface vehicles (USV) makes now possible to develop systems for monitoring coastal areas based on networks of independent USVs. This type of vessels is a valid alternative to conventional vessels, which have a limited mission profile due to their high environmental impact (conventional propulsion systems based on polluting fossil fuels) inhibiting their access to protected coastal regions. Moreover, conventional vessels have high hydrodynamic resistance (limiting the autonomy) producing high levels of noise that can dramatically influence the monitoring equipment shipped: beside the environmental impact reduction, there is also the necessity of low-resistance/low-noise hydrodynamic specification. Consequently, the coastal monitoring (of also protected regions) needs unconventional vessels able to address both the issues related to the environmental impact and the hydrodynamic performance.In this framework, this work aims to characterize the hydrodynamic performance of a system based on USV units able to launch and recover autonomous vehicles of different nature (gliders, AUVs, motor-gliders, wire-guided ROVs), and able to acquire environmental data (in the column water from free-surface to the sea floor), in order to meet the requirements of civil and military applications. The cutting-edge aspects that characterize the USV studied are the hull SWATH type (Small Waterplane Area Twin Hulls) non-conventional, optimized so as to ensure a unique seakeeping and a reduced resistance, along with the propulsion system with propellers in mantle, developed to combine propulsive efficiency and low noise. In the present paper, a SWATH-shaped USV designed for monitoring of protected coastal regions is numerically studied solving the Navier-Stokes equations on the fully appended vessels with several environmental conditions. An accurate hydrodynamic characterization will presented in order to investigate its performances and eventual maneuverability issues
