1,721,018 research outputs found
Unsteady viscous flow with non linear free surface around oscillating SWATH ship sections
No full textShip motion problem is approached exploiting the assumption of linearity and subdividing forces acting on the hull in different components; in this paper the focus is on the radiation problem. A fully viscous solver, based on the numerical solution of the Navier-Stokes equation through the finite volume technique, is used to calculate radiation forces on hull surface, with non linear free surface evaluated through the volume of fluid method. Radiation forces are the most influenced by viscous effects, especially for particular hull geometries as SWATH ships. OpenFOAM libraries are used to solve the unsteady flow around one circular and two SWATH two dimensional sections forced to heave in calm water. The comparison with experiments evidence a very good agreement of the forces measured over a wide frequency range including irregular frequencies
Comparative CFD Investigation on the Performance of a New Family of Super-Cavitating Hydrofoils
No full textWe present a CFD characterization of a new type of super-cavitating hydrofoil section designed to have optimal performance both in super-cavitating conditions and in sub-cavitating conditions (including transitional regime). The basic concepts of the new profile family are first introduced. Lift, drag and cavity shapes at different cavitation numbers are calculated for a new foil and compared with those of conventional sub-cavitating and super-cavitating profiles. Numerical calculations confirm the superior characteristics of the new hydrofoil family, which is able to attain high lift and efficiency both in sub-cavitating and super-cavitating conditions. Numerical calculations are based on a multi-phase fully turbulent URANSE solver with a bubble dynamic cavitation model to follow the generation and evaporation of the vapor phase. The new profile family, initially devised for ultra-high speed hydrofoil crafts, may result useful for diverse applications such as super-cavitating or surface-piercing propellers or high-speed sailing boats
Influence of viscosity on radiation forces: A comparison between monohull, catamaran and swath
No full textThe behavior of the ship in waves should be a matter of hull optimization in order to maintain required levels of seaworthiness and operability in rough sea. Still nowadays the majority of ship motion prediction codes are based on the inviscid flow solvers; but in case of particular ship types, such as catamarans and SWATHs, the higher computational speed of these methods does not justify their inherent lack of accuracy. The paper aims to demonstrate this through a series of application examples of a new numerical method based on the unsteady solution of the Navier-Stokes equations with a mixture of fluid technique and a free surface capturing method to resolve for non-linear free surface effects. The proposed method overcomes the approximation introduced by empirical corrections factors in potential flow based solvers, allowing for accurate prediction on the real shape of the 2D hull section. The numerical method based on the the open source libraries of OpenFOAM, is used to evaluate the two dimensional radiation forces and the hydrodynamic coefficients of three different cross sections, representative of a monohull, a catamaran and a SWATH and it is validated against available experimental results also in terms of radiated wave amplitudes in the far field. The predicted viscous flow field characteristics are presented in detail in the paper, focusing the attention on the separation effects near the hulls and on the free surface breaking, especially in the inner area between the two hulls. Interesting comparison are finally drawn between the behavior of the three different cross section typologies. Copyright © 2013 by the International Society of Offshore and Polar Engineers (ISOPE)
A Multiphase RANSE-based Computational Tool for the Analysis of Super-Cavitating Hydrofoils
No full textHydrofoils have been traditionally used in marine systems for propulsion and stabilization purposes. During the twentieth century, planning craft started to be partially sustained by lift forces generated by immersed hydrofoils with the aim to decrease the hull wetted area, and hence the resistance. It is clear that hydrofoil design becomes a very important aspect for very high-speed craft. For this reason, flow details have to be accurately solved to capture the complex hydrodynamic phenomena. A complete simulation framework consisting of an automatic grid generation module, a high fidelity CFD solver, and a post-processing tool has been developed with the final goal to be included in a shape optimization process, specifically addressed to the design of cavitating or super-cavitating hydrofoils. The simulation framework has been conceived to deal with any foil geometry with the minimum required input data. The very complex fluid-dynamic aspects involved in hydrofoils design, such as cavitation, laminar-turbulent transition, flow separation, and vortex shedding are solved using a non-linear, fully viscous method based on URANS equations that has been carefully tuned for the solution of the flow around 2D foil geometries. Results post-processing is performed using algorithms specifically designed for the analysis of cavitating flow around hydrofoils, the output consists in time-averaged lift and drag coefficients, as well as pressure and friction coefficients over the hydrofoil surface. Flow velocity, pressure and cavity thickness are probed in specific locations defined in the input file. The grid strategy and the CFD solver setting have been specifically studied with the goal to obtain a relatively fast computational method while still maintaining a high level of accuracy. The simulation framework has been validated with two different geometries tested in Caltech's high-speed water tunnel over a wide range of cavitation indexes and different angles of attack. Interesting results are critically discussed describing the fully cavitating flow enveloping the whole hydrofoil pressure side (super-cavitating) and unsteady behavior of the hydrofoil working at partially cavitating conditions. The multiphase flow is numerically solved considering water and vapor as a single fluid whose characteristics depend on an indicator scalar function, as in the volume of fluid approach. Results have been verified on a successively refined grid to understand the influence of mesh resolution on capturing the dynamic of the cavity. The main advantage of these methods is that there is no boundary condition on the cavity surface and the vapor flow is fully resolved, allowing for a better solution of the pressure recovery at the cavity closur
Is command following unrelated to topo-down attention in consciousness disorders?
Full text linkRole of fatigue/alertness fluctuactions in P300 evocation in DOC patient
Case report: Intensive rehabilitation program delivered before and after single-event multilevel surgery in a girl with diplegic cerebral palsy
No full textIntroductionDiplegic cerebral palsy (CP) is often associated with musculoskeletal disorders that contribute to worsen walking function. The standard care in these cases is single-event multilevel surgery (SEMLS) followed by rehabilitation. Our aim was to investigate whether a rehabilitation program starting even before SEML could add a benefit with respect to standard postoperative programs considered by previous research.MethodsFrom 2 months before to 13 months after SEMLS (except for the first month after surgery), the participant underwent a motor training focused on ROM exercises with tactile and kinaesthetic feedback. Walking performance, walking capacity, and quality-of-life were assessed before and after SEMLS at different follow-up times.ResultsWalking capacity improved 3 months after SEMLS (i.e., earlier than in current literature) and walking performance improved 12 months after SEMLS (instead of simply returning to baseline as previously reported), with a positive impact on quality-of-life.ConclusionsThis case suggests that a rehabilitation program starting even before SEMLS could add benefits over walking function and quality-of-life of children with diplegic CP compared to postoperative programs only
A reduced order approach for optimal design of efficient marine propellers
No full textA proper orthogonal decomposition (POD)-based method is proposed to reduce the dimensions of the design space for the shape optimisation of marine propellers. The effectiveness of the proposed approach is proven in the case of the INSEAN-E779A propeller, which blade shape is modified to maximise efficiency while reducing suction side cavitation. The 23-dimensions design space defined by the conventional shape representation is reduced by the POD method to 5, 12 and 15 dimensions, retaining up to the 98% of the geometric variance of the original space. A multi-objective optimisation algorithm drives the simulation-based design optimisation (SBDO) process in the new design spaces using BEM for the hydrodynamic predictions. Finally, optimal designs are verified using RANSE to assess the correlation between the performance improvements, the dimensionality reduction and the corresponding geometric variance. The effectiveness of the proposed POD-SBDO framework is discussed with respect to a design by optimisation process relying on the conventional parametric representation of the blade geometry
Influence of viscous effects on numerical prediction of motions of SWATH vessels in waves
No full textThe accurate prediction of motion in waves of a marine vehicle is essential to assess the maximum sea state vs. operational requirements. This is particularly true for small crafts, such as Autonomous Surface Vessels (ASV). Two different numerical methods to predict motions of a SWATH-ASV are considered: an inviscid strip theory initially developed at MIT for catamarans and then adapted for SWATHs and new a hybrid strip theory, based on the numerical solution of the radiation forces by an unsteady viscous, non-linear free surface flow solver. Motion predictions obtained by the viscous flow method are critically discussed against those obtained by potential flow strip theory. Effects of viscosity are analyzed by comparison of sectional added mass and damping calculated at different frequencies and for different sections, RAOs and motions response in irregular waves at zero speed. Some relevant conclusions can be drawn from this study: influence of viscosity is definitely non negligible for SWATH vessels like the one presented: amplitude of the pitch and heave motions predicted at the resonance frequency differ of 20% respectively and 50%; in this respect, the hybrid method with fully non-linear, viscous free surface calculation of the radiation forces turns out to be a very valuable tool to improve the accuracy of traditional strip theories, without the burden of long computational times requested by fully viscous time domain three dimensional simulations
Physics-Based Design by Optimization of Unconventional Supercavitating Hydrofoils
No full textA computational framework to design a new family of unconventional supercavitating (SC) hydrofoils with optimized hydrodynamic performance is developed. A low‐order boundary element method is used to solve for the steady potential flow over the hydrofoil predicting its hydrodynamic characteristics, including the vapor–cavity interface. Shape variations are obtained by an ad hoc parametrization scheme by composite B‐spline curves whose control points represent the design variables to the hydrodynamic optimization problem. The accuracy of the Computational Fluid Dynamics (CFD) tools is also preventively validated on the experimental characteristics of a conventional SC hydrofoil. A computational test case is performed to maximize the efficiency of a SC hydrofoil accounting for both shape and angle of attack variations. The new hydrofoil leads to 40% improvement on the lift over drag ratio compared to the initial reference shape. This result is confirmed by high‐fidelity unsteady multiphase viscous solver
Comparative Performance of Optimum High Speed SWATH and Semi-SWATH in Calm Water and in Waves
No full textThe hydrodynamic performance of unconventional SWATH and Semi-SWATH for high speed applications are analyzed
and compared in this paper. Bare hull resistance in calm water is estimated by an inviscid boundary element method
with viscous corrections and verified by a fully turbulent, multiphase unsteady RANSE solver. Motions response in
head waves, calculated by a frequency domain 3D panel method with forward speed effects are also evaluated and
compared. Both considered hulls are the best designs coming from full parametric hull form optimization procedures,
based on CFD solvers for the estimation of their hydrodynamic performance and driven by evolutionary minimization
algorithms. The SWATH has twin parabolic struts and an unconventional underwater shape, the semi-SWATH has a
slender triangular waterline, a bulbous shape in the entrance body which gradually morph into a U-section with a
shallow transom in the run body. In general, as expected, the Semi-SWATH hull shows a lower drag at high speeds
while the single strut SWATH is superior at lower speeds. As regards seakeeping, the SWATH shows unbeatable lower
pitch and heave motions in shorter waves, where the Semi-SWATH evidences a double peaked RAO. More detailed
analysis and conclusion are drawn in the paper
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