1,721,124 research outputs found
Simulations of a self propelled autonomous underwater vehicle
Full text linkThe missions being proposed for autonomous underwater vehicles (AUVs), by both marine scientists and industry, are becoming increasingly complex and challenging. In order to meet these demands the next generation of AUVs will need to be faster, operate for longer and be more manoeuvrable than existing vehicles. It is therefore vital that the hydrodynamic forces and moments acting on a self propelled manoeuvring AUV can be predicted accurately at the initial design stage. The flow around a typical AUV is both turbulent and three dimensional with significant interactions between the hull, propeller and control surfaces. An unsteady computational fluid dynamics analysis based on the Reynolds Averaged Navier Stokes (RANS) equations is too expensive for AUV design. In order to capture the action of the propeller at an acceptable computational cost, a novel method of coupling a commercial RANS solver with a body force propeller model based on blade element momentum theory has been developed. This discretises the propeller plane into a series of radial and circumferential sectors. The local axial and tangential inflow conditions at each sector of the propeller plane can then be considered. This allows analysis of non-uniform propeller inflow conditions due to the interaction of hull, propeller and control surfaces. During a manoeuvre the hull boundary layer may separate due to the adverse pressure gradient, resulting in free vortex sheets which roll up to form a pair of body vortices. An adaptive mesh strategy is required to ensure a suitable mesh structure and density to capture these flow features. Modifications to a vortex capture algorithm (VORTFIND) are proposed, optimising it as a tool for identifying the path of vortex structures. This enables it to be used as part of an iterative meshing strategy, capturing vortical flow features more accurately and consequently their influence on the pressure loading experienced by the hull. To demonstrate the pertinence of the numerical methods developed in this work a series of case studies has been analysed. These include: determining the hydrodynamic derivatives of an AUV, propeller-rudder interaction studies, steady state manoeuvring performance of the self propelled KVLCC2, and in-service straight line performance prediction of Autosub 3. These highlight the roles of the numerical methodologies in the design process for future AUVs. The techniques developed in this work enable the designer to accurately predict the hydrodynamic loading acting on a self propelled manoeuvring AU
Influence of drift angle on the computation of hull–propeller-rudder interaction
Full text linkThe operation of the propeller dominates the flow interaction effects on the upstream hull and a downstream rudder. An investigation is carried out into the sensitivity with which these effects can be resolved when an angle of drift is applied as well as the length of an upstream body is varied. The computed results are compared to a detailed wind tunnel investigation which measured changes in propeller thrust, torque and rudder forces. Variation of the upstream body length and drift angle effectively varies the magnitude of the crossflow and wake at the propeller plane. The time resolved flow was computed around the hull-propeller–rudder configuration using the Reynolds averaged Navier Stokes (RANS) equations and an Arbitrary Mesh Interface (AMI) model to account for the motion of the propeller. A mesh sensitivity study quantifies the necessary number of mesh cells to adequately resolve the flow field. Overall, good agreement is found between the experimental and computational results when predicting the change in propulsive efficiency, flow straightening and rudder manoeuvring performance. However, it can be seen that there is a significant computational expense associated with a time resolved propeller interaction and that alternative body force based methods are likely to still be required with the computation of self-propelled ship manoeuvres
Numerical investigation of the influence of propeller to the interference drag of twin prolate spheroids at various longitudinal offsets and transverse separations.
No full textThe purpose of this paper is to provide guidance for operators on suitable spacing for multiple vehicles’ missions. This paper investigates the combined drag of a pair of propelled prolate spheroids and compared to the towed models for the length Reynolds Number of 3.2×106. The model has a length-diameter ratio of 6:1. A series of configuration of a pair of spheroids is simulated at various longitudinal offsets and transverse separations. Three-dimensional simulations are performed using a commercial Reynolds Averaged Navier Stokes (RANS) Computational Fluid Dynamics code ANSYS CFX 12.1 with the SST turbulence closure model. In each case, the fluid domain has a mesh size of approximately nine million cells including inflated prism layers to capture the boundary layer. Mesh convergence is tested and then validated with wind tunnel test results. The drag of each spheroid is compared against the benchmark drag of a single hull. The three-dimensional cylinder is modelled to simulate the thrust distribution of propeller. The drag of the propelled model is compared against the single bare hull model. The results show that the transverse separations and longitudinal offsets determine the interaction drag between both hulls. The increasing of separation results in lower interference drag. The decreasing of offset results in higher drag reduction. By implementing the body force propeller, the combined drag and drag of the follower is interfered by the accelerated flow. Based on the numerical information, operators can determine the optimal configurations in transvers separation and longitudinal offset based on energy consideration
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Ship speed prediction based on machine learning for efficient shipping operation
Full text linkOptimizing ship operational performance has generated considerable research interest recently to reduce fuel consumption and its associated cost and emissions. One of the key factors to optimize ship design and operation is an accurate prediction of ship speed due to its significant influence on the ship operational efficiency. Traditional methods of ship speed estimation include theoretical calculations, numerical modeling, simulation, or experimental work which can be expensive, time-consuming, have limitations and uncertainties, or it cannot be applied to ships under different operational conditions. Therefore, in this study, a data-driven machine learning approach is investigated for ship speed prediction through regression utilizing a high-quality publicly-accessible ship operational dataset of the ‘M/S Smyril’ ferry. Employed regression algorithms include linear regression, regression trees with different sizes, regression trees ensembles, Gaussian process regression, and support vector machines using different covariance functions implemented in MATLAB and compared in terms of speed prediction accuracy. A comprehensive data preprocessing pipeline of operational features selection, extraction, engineering and scaling is also proposed. Moreover, cross validation, sensitivity analyses, correlation analyses, and numerical simulations are performed. It has been demonstrated that the proposed approach can provide accurate prediction of ship speed under real operational conditions and help in optimizing ship operational parameters
Experimental testing and simulations of an autonomous, self-propulsion and self-measuring tanker ship model
Full text linkImproving the energy efficiency of ships has generated significant research interest due to the need to reduce operational costs and mitigate negative environmental impacts. Numerous hydrodynamic energy saving technologies have been proposed. Their overall performance needs to be assessed prior to implementation. A new approach to this evaluation is investigated at model scale which applies an approach comparable to that applied for the performance monitoring of a full scale ship. That is long duration testing that measures power consumption for given environmental and ship operating conditions and can use statistical analysis of the resultant large amount of data to identify performance gains. As a demonstration of the approach, an autonomous, self-propelled and self-measuring free running ship model of an Ice Class tanker is developed. A series of lake based and towing tank tests experiments have been conducted which included bollard pull, shaft efficiency, naked-hull, self-propulsion, and manoeuvrability tests. These investigated the efficiency improvement resulting from changing the ship operational trim and testing different bow designs. An associated mathematical model for the time domain simulation of the autonomous ship model provides an effective tool for data analysis. It has been demonstrated that the use of a suitably instrumented self-propelled autonomous ship model can provide long duration tests that incorporates the influence of varying environmental conditions and thereby identify marginal gains in ship energy efficiency.</p
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
