HYDRODYNAMIC PERFORMANCES OF SMALL SIZE SWATH CRAFT

The good seakeeping characteristics of SWATH hull form are very interesting for small working craft and pleasure boats. Intrinsic limitations as the low values of weight per inch of immersion and transversal and longitudinal instability, can be acceptable and successfully managed when the mission profile does not ask for significant load variation and shift. The exploitation of SWATH concept is limited by the craft size, but if main dimensions allow enough static stability, this configuration appears very promising. SWATH behaviour in rough sea at zero and low speed have led to consider this hull form within the small craft design research program in progress at University of Naples Federico II. The design of small size SWATH working/pleasure craft has to begin from the consideration of strut waterplane areas that are the key factor to get acceptable static and dynamic stability. Displacement has to be reduced as most as possible to increase static stability, as shown by last design trends. The results of CFD analysis concerning SWATH resistance and propulsion, aspects are presented. A numerical evaluation of the hull-propeller interactions is performed, through simulations of self-propulsion tests with a simplified method (Actuator Disk model) to discretize the propeller effect. The effective wake coefficient, the thrust deduction fraction and hull efficiency are provided. To validate CFD resistance results a comparison with experimental tests performed by Authors is reported. The presented work highlights different hydrodynamic aspects, comments advantages and critical issues of SWATH concept and reports detailed CFD modelling procedure with the aim to provide a reference for SWATH small craft design

Experimental ship motion and load measurements in head and beam seas

This contribution looks at experimental ship motion and load measurements in head and beam sea

Optimizing NUCA organizations and wiring alternatives for large caches with CACTI 6.0

Journal ArticleA significant part of future microprocessor real estate will be dedicated to L2 or L3 caches. These on-chip caches will heavily impact processor performance, power dissipation, and thermal management strategies. There are a number of interconnect design considerations that influence power/performance/area characteristics of large caches, such as wire models (width/spacing/repeaters), signaling strategy (RC/differential/transmission), router design, etc. Yet, to date, there exists no analytical tool that takes all of these parameters into account to carry out a design space exploration for large caches and estimate an optimal organization. In this work, we implement two major extensions to the CACTI cache modeling tool that focus on interconnect design for a large cache. First, we add the ability to model different types of wires, such as RC-based wires with different power/delay characteristics and differential low-swing buses. Second, we add the ability to model Non-uniform Cache Access (NUCA). We not only adopt state-of-the-art design space exploration strategies for NUCA, we also enhance this exploration by considering on-chip network contention and a wider spectrum of wiring and routing choices. We present a validation analysis of the new tool (to be released as CACTI 6.0) and present a case study to showcase how the tool can improve architecture research methodologies

PLANING HULL SEAKEEPING IN IRREGULAR HEAD SEAS

The paper presents the results of planing hull seakeeping tests in irregular seas. The tested model belongs to a small systematic series developed at the University of Naples; it is a prismatic hull very similar to the well-known Fridsma’s models. The 16.7 degrees deadrise angle, length-to-breadth and load coefficient are representative of modern hull forms of pleasure boats. Tests in irregular waves have been performed at three speeds for one displacement in three sea states. The measured heave, pitch, acceleration at the centre of gravity and at bow have been analysed in the time domain and the results are presented in terms of significant values (the mean of 1/3rd highest values). They are given in tabular and graphical form. Furthermore, the obtained results are commented with respect to the state of the art in planing hull seakeeping, and compared with the available experimental data from literature. The conclusions highlight the applicability of these data in design practice, commenting on trends and the range of significant parameters

A Boundary Element Method for Motions and Added Resistance of Ships in Waves

The accurate prediction of ship resistance in waves is nowadays of increased importance since it greatly influences ship performance regarding sustainable service speed and fuel consumption in seaways. Added resistance is considered as the longitudinal component of the second order mean force acting on a ship in waves and can be calculated from the first order ship motions by integrating the corresponding second-order pressure on the body surface. The purpose of this paper is to present a methodology for the prediction of motions and added resistance by a three dimensional Rankine panel method and to discuss and validate its results by comparing them with experimental data. The prediction in the short wave range, where forces due to wave reflection dominate, has been made applying semi-empirical corrections proposed by Kuroda. Experimental data for the heave, pitch, and added resistance of an ITTC benchmark KRISO container ship have been compared with numerical ones, and the applicability of the proposed method is discussed

Verification and validation of numerical modelling of DTMB 5415 roll decay

The paper presents a numerical roll damping assessment of the intact DTMB 5415 naval ship at zero speed. Free model motions from four experimental roll decays with initial heeling angle of 4.0, 13.5, 19.58 and 24.50 deg, performed previously at the University of Strathclyde, Glasgow, have been analysed and the one with 19.58 deg initial heeling has been chosen for the Computational Fluid Dynamic (CFD) analysis. All calculations are performed using CD Adapco Star CCM+ software investigating the accuracy and efficiency of the numerical approach for case of high initial heeling angle of bare hull. In the numerical procedure the verification analysis of mesh refinement and time step was performed with the aim to investigate the numerical error/uncertainty. For grid refinement and time step, validation and verification procedure has been performed according to the Grid Convergence Index (GCI) method. Moreover, to verify the main source of the modelling error/uncertainty, the effect of degrees of freedom are evaluated, comparing the numerical results with the experimental results. Conclusions are identifying best practice for roll decay simulations commenting the accuracy of numerical results and required calculation time

Stability and Seakeeping of Marine Vessels

Stability has always been the main safety issue for all marine vessels, and static stability evaluation is adequate for ship service [...

Added Mass and Damping Coefficient Prediction - Results of Different Methods

Added Mass And Damping Coefficient Prediction - Results Of Different Methods Ermina Begovic, University of Naples Federico II, Naples/Italy, [email protected] Guido Boccadamo, University of Naples Federico II, Naples/Italy, [email protected] Abstract The evaluation of ship motions and loads obtained through seakeeping calculations is continuing to be one of the most important research subject. Numerical procedures used for this purpose are generally validated by ship motions experiment; evaluation of motion equation coefficients is carried out experimentally by forced motions and measurement of exciting forces on restrained model in regular seaway. Even if the motion prediction by some numerical method is satisfactory, the predicted values of particular coefficients from the motion equation (added mass and damping coefficients) and of exciting forces are not always satisfactory. This can affect heavily loads assessment that is fundamental for structure scantlings. In particular, for the widely studied Wigley based hulls (Journee (1992)), big discrepancies between numerical and experimental values have been noted. In this work the review of experimental results for added mass and damping coefficients for heave and pitch available in literature is given. Fo r two Wigley models (Journee (1992)) and for high speed Blok and Beukelmann Model 5 (Keuning (1990)) heave and pitch added mass and damping coefficients are calculated by 2 ½ D high speed theory by Faltinsen and Zhao (1990) with and without cross-flow corrections as proposed by Authors in previous works. Results are compared with numerical results of similar works where different 3D time domain calculation methods were used. Significant differences in some coefficients are found, calling for further investigation on the matter