1,721,014 research outputs found
Automatic computer driven optimization of innovative hull forms for marine vehicles
No full textThe paper deals withthe theoretical and numerical aspects of a complete automatic optimization procedure which has been specifically devised to optimize the unconventional hull form design for a new USV (Unmanned Surface Vehicle). The related computational procedure integrates a parametric generation module for innovative and unconventional SWATH (Small Waterplane Area Twin Hull) vessel geometry, a multi-objective, global convergent and constrained, optimizationalgorithm and a Computational Fluid Dynamic (CFD) solver.The integrated process is able to find the best shape of the submerged hull of the vessel, subject to volume and other design constraints. The hydrodynamic computation is carried out by means of a free surface potential flow method and it is addressed to find the value of wave resistance of each hull variant.Results of the application of the described computational procedure are presented for two optimization cases and the obtained best shapes are compared with a conventional one,featuring a typical torpedo-shaped body, proving the effectiveness of the method in reducing the resistance by a considerableextent
Application of a Vortex Lattice Method to the Analysis of Sail Plans in Upwind Condition
No full textTo study new sail plans it is a common practice to use advanced CFD tools which are able to simulate in a accurate way almost all the aerodynamic complex phenomena typical of those systems. However, these analisys are very time consuming and this could make prohibitive their massive usage for long series of analysis. A way to solve this drawback could be found in Vortex Lattice Methods (VLM): with this technique it is possible to develop a lot of analysis, in a relative short time, in order to choose which will be the best configurations to test in detail. In the present article the results of analysis of sail plans, made with a in house software based on the VLM theory (Katz & Plotkin, 1991 and Fiddes & Gaydon, 1996) are shown. It is included an algorithm for wake relaxation. The validation of the method has been done comparing the results of the computation with those derived by the theory of elliptic wing and with available experimental tests at the wind tunnel on the sail rig of a modern racing yacht. Finally, the fluid dynamic code has been integrated with a non linear membrane FEM and an example of this application is presented
A three-dimensional vortex method for the hydrodynamic solution of planing cambered dihedral surfaces
No full textA new numerical approach based on the Vortex Lattice Method (VLM) for the solution of the hydrodynamic performances of cambered hulls in steady planing is formulated and validated. Due to its fully 3D formulation, the method can be applied to both cambered and un-cambered dihedral planing surfaces of any shape without any further approximation. The exact three-dimensional wetted surface of the hull is where the body boundary condition is fulfilled. The sprays region detaching both in front of the stagnation root line and from the wet portion of the chine are modeled in the numerical scheme by means of additional vortex lattice regions. The dynamic boundary condition at the stern of the hull is non-linear with respect to the perturbation potential. Results show the dynamic pressure consistently accounts for the 3D features of the flow especially in the case of cambered planing surfaces. The numerical method is verified by a systematic analysis against semi-empirical methods and it is finally validated with experimental results on prismatic as well as cambered dihedral planing surfaces. Excellent correlations are found for both types of planing surfaces that range in the same confidence interval of higher fidelity numerical models, such as RANSE solvers
Resistance and seakeeping numerical performance analyses of a semi-small waterplane area twin hull at medium to high speeds
No full textThe hydrodynamic analysis of a new semi-small waterplane area twin hull (SWATH) suitable for various applications such as small and medium size passenger ferries is presented. This may be an attractive crossover configuration resulting from the merging of two classical shapes: a conventional SWATH and a fast catamaran. The final hull design exhibits a wedge-like waterline shape with the maximum beam at the stern; the hull ends with a very narrow entrance angle, has a prominent bulbous bow typical of SWATH vessels, and features full stern to arrange waterjet propellers. Our analysis aims to perform a preliminary assessment of the hydrodynamic performance of a hull with such a complex shape both in terms of resistance of the hull in calm water and seakeeping capability in regular head waves and compare the performance with that of a conventional SWATH. The analysis is performed using a boundary element method that was preliminarily validated on a conventional SWATH vessel
Ship synthesis model for the preliminary design of a fleet of compressed natural gas carriers
No full textThe transportation by sea of compressed natural gas is a very recent subject, prompted by recent changes in the perspectives of a possible economic exploitation of relatively small quantities of this hydrocarbon, available as by-product of oil extraction in off-shore fields. An automatic and integrated preliminary design procedure has been established to generate and evaluate feasible technical solutions for a trade of this kind. A first trial application has been carried out for a specific case, for which various feasible solutions corresponding to fleets composed of a different number of equal ships are generated.
In the procedure, a parent hull shape is adopted in order to evaluate, through systematic variations of the main dimensions, all the elements necessary to perform the classical steps of the design spiral (like weight evaluation, buoyancy, trim and stability checks and motion resistance prediction) accounting for the mutual interaction between the various items. When possible and necessary, parts of the procedure have been calibrated with data from existing ships, in order to ensure realism in the predictions. This has been done with reference e.g. to the evaluation of weight items and in particular to hull weight. On the other hand, when setting boundaries to the
range of variation in the ship dimensions, care has been given not to constrain the investigation domain only to existing dimensional ratios, but to let the procedure explore a wider range of solutions. A particular feature of the procedure is to include in the evaluation process the seakeeping performances in terms of added resistance in waves. This aspect interacts with the other part of the design process and influences the final outcome in terms of performance of the solution. The main implications of accounting for the increase in motion resistance are the capability of evaluating the reduction in the average speed, the corresponding reduction in the annual cargo delivery and the increase in fuel consumption (all aspects evaluated in respect to nominal conditions). The procedure, accordingly, is able to provide, in addition to several technical output identifying the various solutions (such as main dimensions, weight, power installed, seakeeping performances), indicators of the performances of the fleet in terms of annual cargo
delivery, CapEx and OpEx, that can be used to rank the various solutions
Automatic Optimization Computational Method for Unconventional S.W.A.T.H. Ships Resistance
No full textSimulation based design of a fleet of ships under power and capacity variations
No full textAchieving the right balance between fixed and operating costs is a key but non-trivial task for optimizing the characteristics of a fleet of ships for a new trade at a very early stage of the design. A wide range of design choices as well as a significant number of uncertainties related to the operation of the fleet affects those two expense items. In the proposed analysis, operative uncertainties rising from rough sea conditions are taken into account in a Simulation Based Design (SBD) framework for fleet generation and evaluation. The costs-benefits analysis is aimed at understanding the effects of variations in the installed power and in the ship capacity on the overall fleet performances. These two major characteristics of the ship design (and, accordingly, of the fleet of ships realizing the trade) are introduced in the numerical model considering the sea margin and the nominal capacity as free design parameters instead of fixing them as commonly done in practice. From a design point of view, this corresponds to the introduction of two new variables, enlarging the dimensions of the design space to be explored, better than relaxing the boundaries of a simpler design space. The validity of this approach is demonstrated by a practical application to the design of a fleet of Compressed Natural Gas (CNG) carriers. In this context, the effects of variations in the fuel price and in the voyage length on the optimal fleet characteristics are analyzed and the results are discussed in comparison to those coming from the reference scenario
A Reformulated Lifting Line Theory for Supercavitating Hydrofoil Design
No full textA new computational procedure based on a reformulated lifting line theory has been developed to represent the spanwise circulation distribution of super-cavitating hydrofoils. The vortex lattice method can consider fully submerged as well as surface piercing hydrofoils, incorporating an approximate model with free surface effects. The proposed method exactly allows for the nonlinear behavior of the hydrofoil sections which change, transitioning from a fully wetted or base cavitating regime to a partial cavitating or fully cavitating regime. In this respect, a reformulated numerical lifting line method has been developed with a new non-linear boundary condition consistent with the variable slope
The method proves to be enough accurate to be used for the design of the angle of attack spanwise distribution of supercavitating surface piercing hydrofoils when applied to design a super-cavitating hydrofoil, which has been then tested at the cavitation tunnel, at different angles of attack and different cavitation indexes
Resistance and Seakeeping Optimization of a Fast Multihull Passenger Ferry
No full textSemi-Small Waterplane Area Twin Hull (Semi-SWATH) ferry is optimized by an automatic procedure with respect
to seakeeping and resistance at different speeds. The numeric procedure is based on a fully parametric model of the
unconventional hull form, a 3D linear Rankine sources panel method for the steady wave resistance and the seakeeping
problem, and a multiobjective global convergence genetic algorithm. Both global and local shape variations are allowed in
order to explore highly unconventional hull shapes. Interesting results are obtained from the optimization considering
resistance at the two very different speeds and including or not the seakeeping merit functions. Considerable reductions of drag
and vertical acceleration can be obtained from the optimization procedure on the order of 15% on drag at high speeds and
30% on vertical acceleration in most exposed passenger areas
Seakeeping and Added Resistance of a Fast Semi-Swath Ship
No full textMulti-hull vessels, including SWATH vehicles, are interesting design solutions for a number of marine
applications, from coastal ferries to workboats. Some hybrid and unconventional configurations, able to combine
good quality of more classic designs, appear to be also attractive. This is the case of Semi-SWATHs, that merge
the stern shapes of high speed catamarans with the bow shapes of conventional SWATHs.
The aim of the proposed work is to go deeper into its seakeeping behavior also evaluating the added resistance.
The analysis is carried out by means of a three dimensional Boundary Element Method (BEM), that makes use of
a Rankine source distribution over the hull and the free surface. Incoming head waves only are considered in this
study. Both the response amplitude operators of heave and pitch motions and the added resistance of the high
speed catamaran are compared with available experimental data. The same analysis is carried out on the proposed
Semi-SWATH vessel and results are critically discussed
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