1,720,969 research outputs found
Powering performance of a self-propelled ship in waves
No full textThe ability to accurately predict the powering performance of a ship when travelling in waves is of high importance for the design of new ships. Almost a century of experience exists regarding how to predict the mean resistance increase in waves compared to calm water. Despite this, improvements in numerical models are still in high demand. Traditionally, the mean increase together with the calm water resistance and propeller open water curves are used to determine the powering performance. This thesis argues that, to achieve better predictions, a more holistic approach can be taken. A RANS based numerical approach to predicting the performance of a self propelled ship in waves is presented. The model is supported by a review of previous literature as well as new experiments to determine what phenomena need to be modelled. It is concluded that the surge force amplitude in waves is something that is not well studied but that has an impact on the propeller performance. The experiments show that this is likely to be harder to predict than the mean increase. Furthermore, the inclusion of RPM control in the model is seen as important to make it better suited for predicting the performance. In developing the numerical model, it is shown that the amplitude and phase of the viscous surge force are affected to some extent by the way the RANS equations are solved numerically. Recommendations on the choice of schemes are given based on several comparative studies where a limited TVD scheme is found to give the best representation of the flow. Furthermore, detailed analysis on how the boundary layer is affected by the passing waves is presented. A framework for coupling the RANS solver with a simplified propeller model is presented. This is a powerful tool that allows for a broad range of present and future studies regarding propeller modelling and RPM control for self propelled simulations in waves. The implementation of Blade Element Momentum theory in the framework is outlined and a correction able to achieve a satisfactory run time coupling in terms of identifying the propeller induced velocities from the total wake is presented. The coupled solver is found to be a computationally efficient tool for studying ship performance in waves. It is applied to study the propulsive performance of the KCS in unsteady inflow conditions. Reasonable agreement with experiments is found both for resistance and for propeller performance. Overall, the findings and methods presented here represent a contribution towards better predictions of the performance of self propelled ships in waves
Anti roll tanks in pure car and truck carriers
No full textThis is a master thesis conducted at KTH Centre for Naval Architecture in collaboration with Wallenius Marine AB. Rolling motions is something that is undesired in all kinds of seafaring. In terms of propulsion resistance, comfort and route planning it would be desirable to reduce these motions. This thesis is an investigation on how different roll stabilising systems affect the performance of an 8000 unit PCTC vessel, special emphasis is put on investigating the performance of anti roll tanks. The ship in question has a recorded incidence of parametric rolling and the ability of the tanks to countervail this phenomenon is also investigated. The tank and fin stabilising systems are relatively equal when it comes to roll damping performance related to changes in the required forward propulsion power. The tanks however, have a higher potential for improvements, addition of features such as heeling systems and parametric roll prevention systems. The tank performance is also independent of the speed of the ship. The tanks are easier to retrofit and do not require the ship to be put in dry dock during installation. The conclusion of this thesis is that a combined anti roll and heeling system should be installed but that a further study has to be made on the performance of active rudder stabilisation. It is shown that passive tanks are efficient at preventing parametric rolling in some sea states. A proposal is made for a further study on a control system that could achieve the same performance for all sea states.HTML clipboard Detta är ett examensarbete utfört på KTH Marina System i samarbete med Wallenius Marine AB. Rullningsrörelser är något som är oönskat i all form av sjöfart. Framsteg kan göras i både framdrivningsmotstånd, komfort och ruttplanering om dessa rörelser kunde minskas. Detta examensarbete består av en undersökning hur olika system för rulldämpning påverkar prestandan hos ett 8000 enheters PCTC-fartyg. Speciell vikt har lagts vid att undersöka prestandan hos antirulltankar. Det undersökta fartyget har en dokumenterad incident med parametrisk rullning och tankarnas förmåga att motverka detta fenomen undersöks. Tank- och fenstabilisatorer är i princip likvärdiga vad det gäller dämpningsprestanda relaterat till erforderliga ändringar i framdrivningseffekten. Tankarna har dock en större potential för förbättring och tillägg av ytterligare inslag som krängningshämmare och system för motverkan av parametrisk rullning. Tankarnas prestanda är också oberoende av fartygets fart. Tankarna är lättare att installera i efterhand och kräver inte att fartyget läggs i torrdocka under installationen. Slutsatsen av detta arbete är att en kombinerad antirull- och krängningshämmande tank bör installeras men att en vidare studie måste göras på prestandan hos aktiva roderstabiliseringssystem. Det visas att passiva tankar kan motverka parametrisk rullning i vissa sjötillstånd. Ett förslag om en vidare studie på reglersystem som skulle kunna ge samma prestanda vid alla sjötillstånd ges.
Predicting powering performance changes for ships in offshore conditions from small design modifications
No full textThis paper addresses the prediction of ship performance in waves by means of RANS-based CFD. Lately, much attention has been given to modelling the complex geometry (moving hull and rotating propeller) which can sometimes distort or suppress the importance of the underlying physics. The approach here is to subject a fixed hull to waves and study how the flow around the hull is affected and what this means for the inflow to the propeller and the resulting propulsive performance of the ship. This study provides a straightforward approach for gaining insight into how hull design can influence the performance of ships in waves
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
Wireless sensor network for determining boat motions and hydroelastic responses
No full textA flexible segmented ship model has been tested in a series of head sea wave conditions including regular, irregular and a rogue wave. The model has a typical frigate form and is constructed using a flexible central spine against which four segments are mounted. A three node wireless sensor system, which currently acquires data at up to 30Hz was also attached to the model. These wireless development sensors were supplied as part of the ESPRIT programme and incorporate 9 degrees of freedom using rotational and translational accelerometers and a 3 component magnetometer. The data acquired was compared to those obtained using the traditional pitch and heave potentiometers and a standard heave accelerometer. Overall, the system, although still requiring further development to improve data acquisition rate, performed well with good correlation observed between the various measurement components. The practical advantages are the low mass and low power requirements of such a wireless sensor networ
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