1,721,014 research outputs found
Influence of laminar-to-turbulent transition on model scale propeller performances. Part I: fully wetted conditions
No full textThe influence of the laminar-to-turbulent boundary layer transition on model scale propellers characteristics is investigated using the γ-Reθ model. OpenFOAM and StarCCM+ RANSE solversare used for the investigation, which is carried out for two test cases. Available measurements at a sufficiently low rate of shaft revolution (i.e. Reynolds number)are used to assess the capabilities of the model in improving model scale predictions. This first part of the study is focused on fully wetted conditions, considering the influence of grid resolution and inflow turbulent intensity on the transition onset predicted by the model. The influence of the laminar-to-turbulent transition on the inception and development of cavitation, and the capabilities of predicting such phenomena using a modified mass transfer model, are discussed in the second part of the study (Gaggero, 2020. Influence of Laminar-to-Turbulent transition on model scale propeller performances. Part II: cavitating conditions. Ship and Offshore Structures)
Influence of Laminar-to-Turbulent Transition on the Model Scale Propeller Performance and Induced Pressure Pulses in an Unsteady Case of Oblique Flow
No full textIn this paper, after the successful applications to open water propeller performance estimations, the influence of transition sensitive and modified mass transfer models tuned to account for the laminar flow in the prediction of the cavitation inception of marine propulsors is investigated from the point of view of the unsteady functioning and induced pressure pulses. The VP1304 (also known as PPTC) test case, for which dedicated data were collected during several workshops, is considered first. After preliminary analyses using RANS, also Detached Eddy Simulations (DES) are included to better account for the vortex dynamics and its influence on pressure pulses. Similarly to what observed in uniform inflow, results show a better agreement with the available measurements of propeller performances and confirm the reliability of the proposed approaches for unsteady, non-cavitating, model scale propeller predictions. The overall cavitation pattern is improved too by the application of the transition sensitive correction to the mass transfer model, but the complex dynamics of bubble cavitation observed in experiments prevents quantitatively better predictions in terms of thrust/torque breakdown and induced pressure pulses levels regardless the use of RANS or DES methods
Robust simulation-based design optimization of marine propellers
No full textMarine propellers often function under uncertain conditions such as variable inflow, rate of revolution, and manufacturing tolerances. A deterministic design
approach may result in excessive sensitivity to minor variations, leading to suboptimal performance in real-world scenarios. Then, quantifying these uncertainties, and leveraging their influence on propeller performance, is of fundamental importance to design and optimizing configurations less sensitive to input variations. In the context of a “robust” design of marine propellers through simulation-based design optimization methodologies, this paper explores both deterministic and nondeterministic design approaches for a conventional propulsor, accounting for the uncertainties in the nominal operating conditions. Since the quantification of uncertainties can be computationally very intensive, an efficient medium-fidelity Boundary Element Method (BEM) solver using equivalent steady-state cavitating analyses is employed in the optimization process. The optimal designs are finally validated through fully unsteady and cavitating BEM and RANSE calculations to demonstrate the advantages of the non-deterministic design approach
Rim driven propellers: Optimization based design approach using RANS calculations
No full textRIM driven propellers represent an unconventional, but underrated, propulsive solution which hydrodynamic design is still not obvious. In the last years, most of the attention has been devoted to the efficient coupling with electric motors, since only recently the development of permanent magnets allowed for the successful embedding of the driver directly inside the surrounding duct. From the hydrodynamic point of view, however, analysis and, in particular, design strategies are not yet ripe. In the light of the development of advanced design approaches for unconventional geometries, we propose a Simulation-Based Design Optimization tool based on RANS analyses of parametrically described geometries as a part of an automatic, multi-objective optimization loop. The SBDO is used to design a RIM driven propeller with an accelerating type duct, with improved performance simultaneously in terms both of efficiency and cavitation inception at different (design and quasi-bollard pull) functioning conditions
Influence of laminar-to-Turbulent transition on model scale propeller performances. Part II: cavitating conditions
No full textThe numerical study presented in Part I (Gaggero, 2020), focused on the prediction of the laminar-to-turbulent boundary layer transition on model scale propellers, is extended to cavitating conditions. In the previous study, the application of the γ-Reθ Local Correlation Transition Model allowed for significant improvements in model scale performance prediction and highlighted the role of the turbulence intensity on the transition onset. In this second part of the study, the influence of the same transition model on the inception and development of cavitation is considered. To this aim, a modification of the Schnerr-Sauer mass transfer model, useful for engineering-oriented applications, is proposed to account for the role of the laminar boundary layer in the inception and development of the leading edge sheet cavitation. The proposed analyses, compared with the available measurements at the cavitation tunnel, confirm the improvements of the numerical predictions using the modified model
Numerical design of a RIM-driven thruster using a RANS-based optimization approach
No full textA simulation-based design optimization (SBDO) tool is proposed for the design of rim driven thrusters. The optimization framework consists of a parametric description of the rim blade geometry and a multi-objective optimization algorithm which makes use of the results from high-fidelity RANS calculations to drive the choice towards optimal blade shapes. Maximization of the propulsive efficiency and minimization of cavitation, monitored through the simplest cavitation inception criterion based on the analysis of the non-cavitating pressure distribution over the blade, are the contrasting objectives selected for the design. A constraint on the delivered thrust fixes the functioning condition of the devised propellers. Four distinct design runs, changing the number of blades, from three to six, are considered. The reference performance are those of a ducted propeller operating in a decelerating nozzle which is exactly used in the case of rim configuration. Results of the optimizations prove the flexibility and the reliability of the SBDO framework in dealing with unconventional configurations. A generalized reduction of the risk of cavitation is observed regardless the number of blades; five- and six-bladed propulsors ensure remarkable (about 40%) higher margins with respect to any (leading edge and midchord) cavitating phenomena at the cost only of a slight reduction of efficiency. Also, the structure of the tip vortexes results significantly modified
Design of pumpjet propulsors using RANS-based multi-objective optimization
No full textThe design of linear pumpjets is addressed through a simulation-based design optimization approach based on RANS analyses in the case of rotor/stator (i.e., post swirl) configurations, characterized by 5 rotor blades and 5 or 10 stator blades. The optimal geometries from a multi-objective optimization process aimed at maximizing the propulsive efficiency at the lowest possible cavitation inception index are compared to a reference ducted propeller with decelerating nozzle, which served as baseline during the activity. A significant increase of propulsive efficiency with a reduced risk of cavitation is observed. Fully unsteady cavitating analyses are used to assess the reliability of the design activity, which is necessary build upon some simplifying assumptions (i.e., rotor/stator coupling through a mixing plane) needed for an affordable numerical process. Detached Eddy Simulations (IDDES) are finally carried out to highlight, in addition to the performance improvements provided by the pumpjets, also the influence of the rotor/stator/nozzle interaction on the vortical structures shed by the propulsors
Comparison of different propeller boss cap fins design for improved propeller performances
No full textThe performances of different Propeller Boss Cap Fins devices, designed with the aid of a Simulation Based Design Optimization approach, are analysed and compared. A novel configuration, i.e., a PBCF inside a nozzle, is investigated in an effort to mitigate possible side effects of conventional Propeller Boss Cap Fins, such as secondary vortical structures from the fins tip, increased pressure pulses and radiated noise, without excessive worsening of the beneficial effects (increased propulsive efficiency) provided by this energy saving device. Detailed analyses reveal, instead, the destabilizing effect of the hub vortical structures on the blade tip vortex evolution, determining a substantial increase of the radiated noise of this type of propulsors
Improving Waterjet Performance Through Simulation-Based Optimization
No full textAxial waterjets are widely used for marine propulsion due to their efficiency and maneuverability. However, conventional design procedures heavily rely on empirical correlations and simplified models, limiting their ability to fully exploit the hydrodynamic performance potential of these devices. The study highlights how Simulation-Based Design Optimization (SBDO) approaches, coupled with the high-fidelity simulations required to hydrodynamically characterize the complex phenomena that occur in the case of waterjets, can enable the identification of non-intuitive design improvements over a wider design space that may be missed by traditional methods. In particular, the Reynolds-Averaged Navier–Stokes (RANS) equations are used to provide accurate performance predictions, capturing complex flow phenomena such as secondary flows (i.e., leakage vortices) and pressure distributions critical to waterjet design, of systematically varied configurations using a 42-dimensional parametric model. Simplified key performance indicators, in the specific cavitation inception obtained from the non-cavitating analysis, work in conjunction with the calculated hydraulic efficiency to identify geometries capable of improving (or not worsening) efficiency while postponing cavitation. The systematic and automated analysis of thousands of different configurations, iteratively modified by a genetic algorithm, is finally able to identify better waterjets, whose performances are confirmed by dedicated cavitating RANSE analyses. This demonstrates how RANS-based simulations, integrated with optimization algorithms, can lead to superior axial waterjet designs, providing a flexible, more robust, and effective methodology compared to conventional approaches
Pre-swirl fins design for improved propulsive performances: application to fast twin-screw passenger ships
No full textPre-swirl fins-based energy saving devices (ESDs) have been designed to improve the propulsive performances of twin-screw ships. To this aim, a combined BEM/RANSE method for efficient self-propulsion prediction is required. The approach is included in a framework for a design by optimization, where systematic variations of the ESD geometry have been used to explore the design space and maximize the energy-saving effect of the device. Surrogate models based on Ordinary Kriging are used too, with the aim of realizing an affordable design workflow for the very preliminary design of such devices. The results show encouraging improvements that reach promising energy-savings up to 3% at the design point and satisfactory savings also in off-design functioning conditions
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