The isotropic-nematic phase transition in hard, slightly curved, lens-like particles.

Monte Carlo numerical simulations are used to study in detail how the characteristics of the isotropic-nematic phase transition change as infinitely thin hard platelets are bent into shallow lens-like particles. First, this phase transition in the former reference model system is re-examined and more accurately located. Then, it is shown quantitatively that this already quite weak but distinctly first-order phase transition weakens further upon curving the platelets to such an extent that, thanks to the thinness of these particles that does not favor its pre-emptying by a transition to a (partially) positionally ordered phase, an isotropic-nematic tricritical point limit can be arbitrarily closely approached

EFD and CFD Design and Analysis of a Propeller in Decelerating Duct

Ducted propellers, in decelerating duct configuration, may represent a possible solution for the designer to reduce cavitation and its side effects, that is, induced pressures and radiated noise; however, their design still presents challenges, due to the complex evaluation of the decelerating duct effects and to the limited amount of available experimental information. In the present paper, a hybrid design approach, adopting a coupled lifting line/panel method solver and a successive refinement with panel solver and optimization techniques, is presented. In order to validate this procedure and provide information about these propulsors, experimental results at towing tank and cavitation tunnel are compared with numerical predictions. Moreover, additional results obtained by means of a commercial RANS solver, not directly adopted in the design loop, are also presented, allowing to stress the relative merits and shortcomings of the different numerical approaches

Ship propeller side effects: pressure pulses and radiated noise

The present paper deals with the side effects of propellers cavitation, i.e. pressure pulses and radiated noise. These effects are gaining more and more importance for commercial ships for different reasons. Pressure pulses significantly affect comfort onboard, thus their reduction is of utmost importance for all ships carrying passengers. As regards the underwater radiated noise, in the last decade interest has shifted from navy applications to commercial ships, due to the concern for the rising background noise in the oceans. The propellers, generating noise directly in water, represent one of the main contributions to the overall underwater noise emitted from ships. Due to the complexity of the mechanisms of propeller noise generation, different complementary strategies have to be followed to properly analyze the problem, ranging from induced pressure pulses to broadband noise and cavitation. In the present work, part of the activities carried out in the framework of the collaborative EU FP7 project AQUO (Achieve QUieter Oceans by shipping noise footprint reduction, www.aquo.eu) are reported. The paper presents the investigations carried out on a specific test case represented by a single screw research vessel, which is analyzed with three different strategies: numerical calculations, model scale investigations and full scale measurement

Large Attractive Depletion Interactions in Soft Repulsive-Sphere Binary Mixtures

We consider binary mixtures of soft repulsive spherical particles and calculate the depletion interaction between two big spheres mediated by the fluid of small spheres, using different theoretical and simulation methods. The validity of the theoretical approach, a virial expansion in terms of the density of the small spheres, is checked against simulation results. Attention is given to the approach toward the hard-sphere limit, and to the effect of density and temperature on the strength of the depletion potential. Our results indicate, surprisingly, that even a modest degree of softness in the pair potential governing the direct interactions between the particles may lead to a significantly more attractive total effective potential for the big spheres than in the hard-sphere case. This might lead to significant differences in phase behavior, structure and dynamics of a binary mixture of soft repulsive spheres. In particular, a perturbative scheme is applied to predict the phase diagram of an effective system of big spheres interacting via depletion forces for a size ratio of small and big spheres of 0.2; this diagram includes the usual fluid-solid transition but, in the soft-sphere case, the metastable fluid-fluid transition, which is probably absent in hard-sphere mixtures, is close to being stable with respect to direct fluid-solid coexistence. From these results the interesting possibility arises that, for sufficiently soft repulsive particles, this phase transition could become stable. Possible implications for the phase behavior of real colloidal dispersions are discussed.Comment: 31 pages, 8 figures; version accepted for publication in the Journal of Chemical Physic

Model Scale Investigation of Blade Root Cavitation Erosion on a Set of Marine Propellers

The present study is focused on the experimental analysis of the erosion caused by cavitation occurring at the blade root for a set of three model scale marine propellers. The experimental method is based on the adoption of soft paint technique together with cavitation observations. Cavitation dynamics and erosion damage patterns are recorded using three standard cameras and one high speed camera. Standard cameras are fixed on the top of test section to continuously monitor the effect of erosion damage on the blade root, the high-speed camera has been placed at different positions to investigate detailed evolution and collapse of bubbles on pressure and suction side of propeller blades. The soft paint test damage patterns have been simultaneously analysed with the high- speed videos, and results showed remarkable agreement between the occurrence of damage on the blades and the bubble collapse of cavitation. The damage pattern and cavities collapse seem to be inversely related with the inception time of damage

Space vectors and pseudo inverse matrix methods for the radial force control in bearingless multi-sector permanent magnet machines

Two different approaches to characterize the torque and radial force production in a Bearingless Multi-Sector Permanent Magnet (BMSPM) machine are presented in this work. The first method consists of modelling the motor in terms of torque and force production as a function of the stationary reference frame α-β currents. The current control reference signals are then evaluated adopting the Joule losses minimization as constrain by means of the pseudo inverse matrix. The second method is based on the control of the magnetic field harmonics in the airgap through the current Space Vector (SV) technique. Once the magnetic field harmonics involved in the torque and force production are determined, the SV transformation can be defined to obtain the reference current space vectors. The methods are validated by numerical simulations, Finite Element Analysis (FEA) and experimental tests. The differences in terms of two Degrees of Freedom (DOF) levitation performance and efficiency are highlighted in order to give the reader an in-depth comparison of the two methods

Propeller geometry optimization for pressure pulses reduction: an analysis of the influence of the rake distribution

The evaluation of pressure pulses is a current issue for any high-performance propeller design. It has been addressed experimentally, by means of model tests, and numerically but in most cases the analysis has been limited to the verification of a given geometry (or, at least of few configurations) identified at the end of a traditional design loop. A more direct inclusion of pressure pulses evaluation in the design procedure, for instance by very attractive multi-objective optimization approaches, could be beneficial, especially if more accurate codes may be exploited. Among the others, BEM represent an acceptable compromise between computational costs and accuracy with the further advantage, with respect to lower fidelity approaches, to account for effects of geometrical haracteristics (such as rake distribution) which are often defined only according to designer experience and special needs. However, if the ability of the BEM methods to predict propeller performance and cavitation extension is well documented, the direct computation of pressure pulses may be less reliable, especially in correspondence to heavy cavitating conditions, requiring further validations in particular when the influence of characteristics such as rake distribution, hardly addressed in literature also from the experimental point of view, are considered. Cavitation tunnel test, BEM and RANS calculations have been consequently carried out for two propellers, designed for the same functioning conditions with different rake distributions, in order to stress the capabilities and the limitations of the numerical approaches in dealing with cavitation, pressure pulses predictions and the capability to discriminate between slightly different geometries in the light of their possible application in a design by optimization procedure

Numerical cavitation noise prediction of a benchmark research vessel propeller

This paper presents the preliminary results of a numerical study for noise prediction of a benchmark propeller in open water/uniform flow conditions. The experimental benchmark test data for the research vessel, “The Princess Royal”, were used for validation purposes. The numerical analyses were implemented by using a viscous solver based on the finite volume method while the experimental data were obtained from model tests conducted at the Genova University Cavitation Tunnel. The main aim of the study is to predict propeller hydro-acoustic performance under cavitating conditions. The hydrodynamic flow field was solved using a RANS (Reynolds-averaged Navier-Stokes) solver. The Schnerr-Sauer cavitation model based on a reduced Rayleigh-Plesset equation together with a VOF approach was used to model sheet cavitation on the propeller blades. The computed hydrodynamic characteristics and sheet cavity patterns were shown to be in good agreement with the Genoa experimental data, thus providing a firm basis for cavitating noise predictions. The hydro-acoustic performance of the model propeller was predicted by using a hybrid method. In the noise simulations, RANS equations were equipped with a porous FW-H (Ffowcs Williams-Hawkings) formulation. The different propeller operational conditions were simulated using this hybrid method. The numerical results were also validated with the experimental data for the propeller hydro-acoustic performance. Whereas such validations showed promising results by means of overall noise spectrum with the benchmark test cases in the low-frequency range, the numerical prediction overestimated the 1st BPF values (around 20 dB) in five loading conditions. Besides, in some loading conditions, especially between 200 and 800 Hz, the difference between numerical predictions and the experiment was found around 5–10 dB

Embodied Language Learning and Cognitive Bootstrapping:Methods and Design Principles

Co-development of action, conceptualization and social interaction mutually scaffold and support each other within a virtuous feedback cycle in the development of human language in children. Within this framework, the purpose of this article is to bring together diverse but complementary accounts of research methods that jointly contribute to our understanding of cognitive development and in particular, language acquisition in robots. Thus, we include research pertaining to developmental robotics, cognitive science, psychology, linguistics and neuroscience, as well as practical computer science and engineering. The different studies are not at this stage all connected into a cohesive whole; rather, they are presented to illuminate the need for multiple different approaches that complement each other in the pursuit of understanding cognitive development in robots. Extensive experiments involving the humanoid robot iCub are reported, while human learning relevant to developmental robotics has also contributed useful results. Disparate approaches are brought together via common underlying design principles. Without claiming to model human language acquisition directly, we are nonetheless inspired by analogous development in humans and consequently, our investigations include the parallel co-development of action, conceptualization and social interaction. Though these different approaches need to ultimately be integrated into a coherent, unified body of knowledge, progress is currently also being made by pursuing individual methods

Experimental investigation on the effect of heterogeneous hull roughness on ship resistance

There has been an increasing attention to the effect of hull roughness on ship resistance and powering. In conventional studies, the hull surfaces have been treated as uniform rough surfaces while the real ships' hulls are exposed heterogeneous fouling accumulation. The work described here presents an experimental investigation into the effect of heterogeneous hull roughness on ship resistance. A series of towing tests were conducted using a ship model of the Wigley hull with various hull roughness conditions, including homogeneous conditions (i.e. smooth and full-rough conditions) and heterogeneous conditions (i.e. ¼-bow-rough, ¼-aft-rough, ½-bow-rough and ½-aft-rough conditions). The bow-rough conditions (e.g. ¼-bow-rough and ½-bow-rough) showed larger added resistance than aft-rough conditions (e.g. ¼-aft-rough and ½-aft-rough) with the same wetted surface area of the rough region. This finding suggests that the hull roughness of the forward part of the hull is more significant than the others in terms of the added resistance. Finally, a new method was proposed to predict the added resistance due to the heterogeneous hull roughness based on Granville's similarity law scaling and the predictions were compared with the experimental result