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Bosschers J.,MARIN Maritime Research Institute Netherlands
Journal of Physics: Conference Series | Year: 2015

Cavitating propellers generate pressure fluctuations on the hull of the ship. These pressure fluctuations are usually analyzed in the frequency domain using FFTs and the spectrum is composed of tonals at multiples of the blade passage frequency and a broadband part. The two are often considered separately but a relation between the two exists which has been investigated by theoretical signal analysis. It will be shown that the broadband part is related to the variability of the signal between blade passages and a simple procedure is proposed to quantify the variability in terms of amplitude and phase angle. The procedure has been applied to a data set obtained at sea trials. Source

Gkikas G.D.,MARIN Maritime Research Institute Netherlands
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2015

A nonlinear response amplitude operator (NRAO) that is able to model a wide class of nonlinear systems and structures, usually found in maritime and offshore applications, is developed. Its functional scheme is of the Volterra-series type and its order is explicitly dictated from the order of nonlinearity of the actual dynamical system. In addition, the proposed method is not bounded by any amplitude or frequency constrains and it can compute the response of a nonlinear system even for multi-chromatic excitations that consist of modes with significantly, or not, different amplitudes and frequencies. Copyright © 2015 by the International Society of Offshore and Polar Engineers (ISOPE). Source

Van Der Ploeg A.,MARIN Maritime Research Institute Netherlands
MARINE 2015 - Computational Methods in Marine Engineering VI | Year: 2015

A procedure for optimizing the aft body of a ship for minimal power and best wake field quality is described, based on full-scale RANS computations possibly including free- surface effects. A flexible and effective definition of parametric hull form variations is used, based on interpolation between a limited number of basis hull forms. This keeps the dimension of the search space low, which enables to do systematic variations. For a test case in which the ship's wave making can be neglected it is shown that the grid dependence in the computed trends is low, and for a test case including free-surface effects it is demonstrated how the search space can be set up in such a way that a considerable decrease in required power can be obtained without spoiling the wake quality. Source

Grin R.,MARIN Maritime Research Institute Netherlands
Journal of Ship Production and Design | Year: 2015

There is continuous research on analytical, numerical, and (semi)empirical methods to predict wave-added resistance. Most of this research focuses on a particular area, like motion-induced wave added resistance, wave-added resistance in short waves, or is limited to head seas only. The practical application of most methods is therefore often limited. Moreover, most methods require detailed information on hull lines and results are rather sensitive to the discretization of those hull lines. Since 2006, MARIN has been investigating the feasibility of empirical methods that do not have those limitations. They only require the main particulars to predict wave-added resistance. Within the Sea Trial Analysis Joint Industry Project (STA-JIP), a method was developed for the correction of wave-added resistance in head seas covering both the motion-induced and the wave reflection-induced component. This method was further refined and extended to all wave directions within the service performance analysis JIP (SPA-JIP) in 2008. This article presents the results of the comparison between the prediction methods and model tests for almost 50 different ships, comprising more than 1500 tests in regular and irregular seas. Source

Drummen I.,MARIN Maritime Research Institute Netherlands | Holtmann M.,DNV GL Maritime Advisory Services
Ocean Engineering | Year: 2014

Throughout the maritime world, considerable efforts have been spent on predicting loads associated with slamming. Up to now, little attention has, however, been paid to the accuracy of the translation from these loads to the structural responses. An important reason for this is that, in general, it is assumed that the uncertainties in the modeling of the hydrodynamic properties are larger than those related to the structural responses. To address this topic, the ISSC 2012 Dynamic Response committee, performed a benchmark study. The goal of this benchmark was twofold: on the one hand, the degree of variation in estimates produced by different methods and organizations was revealed; on the other hand, the deviations of the analyses were investigated by comparison with responses measured during model tests. From the results presented, it may amongst others be concluded that the shapes and frequencies of the two and three node, dry and wet and horizontal and vertical flexural vibration modes determined by the participants, were well in line with experimental results for four of the six participants. Computations considering an impulse induced by a regular head wave showed significant differences between the experiment, the different participants, and applied methods. © 2014 Elsevier Ltd. All rights reserved. Source

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