Hydrodynamic optimization of ship hull forms in shallow water

A computational method for improving hull form in shallow water with respect to wave resistance is presented. The method involves coupling ideas from two distinct research fields: numerical ship hydrodynamics and nonlinear programming techniques. The wave resistance is estimated by means of Morinos panel method, which is extended to free surface flow and considers the influence of finite depth on the wave resistance of ships. This is linked to the optimization procedure of the sequential quadratic programming (SQP) technique, and an optimum hull form can be obtained through a series of iterations giving some design constraints. Sinkage is an important factor in shallow water, and this method considers sinkage as a hydrodynamic design constraint. The optimization procedure developed is demonstrated by selecting a Wigley (C _B = 0.444) hull and the Series 60 (C _B = 0.60) hull, and new hull forms are obtained at Froude number 0.316. The Froude number specified corresponds to a lower than critical speed since most of the ships operating in shallow water move below their critical speed. The numerical results of the optimization procedure indicate that the optimized hull forms yields a reduction in wave resistance.     

Numerical modeling of breaking waves generated by a ship’s hull

A new model for the simulation of spilling breaking waves in naval flows is presented. The hydrostatic pressure is used in order to mimic the weight of the breaker on the underlying flow, as in the model of Cointe and Tulin, whereas the algorithm for detecting the breaking inception and the definition of its geometry are completely new and are suitable for the simulation of three–dimensional flows around ships hulls. The model has been implemented in a finite-volume code developed for naval flows, and its performances have been validated against experimental data for a submerged profile, an S60 hull in drift motion, and the US Combatant DTMB 5415 model on a straight course.     

A 2D+T VOF fully coupled formulation for the calculation of breaking free-surface flow

We present the results of simulations obtained with a free-surface flow solver based on the following method. The free surface is simulated by the volume-of-fluid interface capturing method. This code solves the Navier–Stokes equations using a finite-volume method adapted to a structured or unstructured mesh. The system is constructed using a fully coupled approach. This global approach allows the simulation of complex flow as a breaking or merging wave. Moreover, with the use of a 2D+T decomposition, it is possible to simulate three-dimensional steady flow.     

Investigation of the effectiveness of tandem oil fences under currents

The behavior of the flow passing a tandem oil fence, and the performance of the fence, were investigated by experimental and numerical methods. The flow characteristics between tandem fences were measured by the particle image velocimetry (PIV) method for the rigid and open free surface between the two fences in order to gather reference data for numerical investigations. A method of assessing a tandem fence by tracing the movement of an oil droplet around the fence is introduced. The effect of the current speed, the separation distance between the two fences, the relative draft of the two fences, and the water depth on the oil containment between the fences was investigated.     

An anti-wind-up controller design based on a two-degrees-of-freedom servosystem design approach

In actual control systems, many types of restrictions or nonlinearities exist. For example, owing to nonlinearities in the actuators or sensors, the controller may not be applicable in some practical situations. Such a nonlinearity is amplitude saturation in actuators. Although it may sometimes be ignored, failure to consider actuator saturation may severely degrade the performance of a closed-loop system, and even lead to instability. On the other hand, limiting the controller gain to avoid saturation sacrifices control effort, and may lead to loss of performance. Many approaches have been tired in order to overcome these undesirable problems, and some good results have been obtained. The saturation has been replaced with a gain which is calculated as a function of the variance of the signal at its inputs. Based on this assumption, some attractive control theories have been introduced. Here, we introduce a useful anti-wind-up control system. In other words, the control system proposed has a very simple design process and can guarantee good control performance, as it is based on the assumption that the saturation is replaced with a varying gain. The validity of the proposed control system will be shown by comparing some simulation results.     

Nonlinear analysis of parametric rolling in longitudinal and quartering seas with realistic modeling of roll-restoring moment

Parametric rolling of a containership in longitudinal and quartering seas is examined by applying nonlinear dynamics to a 1DOF mathematical model with realistic modeling of the wave effect on roll-restoring moment. In our previous work, we confirmed that a mathematical model with a roll-restoring moment in waves calculated with the Froude–Krylov assumption could considerably overestimate the danger of capsizing associated with parametric rolling. Therefore, in the present work, all numerical calculations based on nonlinear analysis were carried out with the direct aid of a measured roll-restoring moment in waves. For this purpose, captive model experiments were conducted for various sets of wavelengths in longitudinal seas. This experiment demonstrates that the Froude–Krylov prediction could not explain the wavelength effect on restoring moment as the wave-steepness effect. Using the numerical model with the aid of this measured roll-restoring moment, the Poincaré mapping technique was applied to identify bifurcation structures of roll motions not only in longitudinal seas, but also in quartering seas. As a result, it was confirmed that capsizing associated with parametric rolling is more likely to occur in following seas than in quartering seas. However, period-doubling and chaos appeared in quartering seas. Finally, an averaging method assuming a period-2 orbit was applied to the same model with the same conditions as the Poincaré map. Reasonably good agreement was obtained between the numerical results with a Poincaré map and those with the averaging method in longitudinal seas, but the averaging method has limited capability in quartering seas.     

Investigation of cavitation inception characteristics of hydrofoil sections via a viscous approach

Unlike traditional approaches based on potential theory, this work investigated the cavitation inception characteristics of hydrofoil sections by solving Reynolds-averaged Navier–Stokes (RANS) equations. The viscous effects at high Reynolds numbers were taken into account through calculation of the eddy viscosity using a k- model. This viscous approach delivers the lift and drag forces in a more self-contained way, which cannot be obtained directly from the potential theory. A comparison with smooth surface measurements validates the proposed numerical method. Two approaches to include the roughness effects are also described. The predictions of cavitation bucket characteristics based on potential theory are more conservative than those reached by the proposed viscous computations. Hydrofoil sections with different camber ratios, foil thicknesses, and Reynolds numbers were studied numerically. A decrease in the Reynolds number tends to increase the cavitation-free region without changing the shape of the steep region. An increase in foil thickness gives a range increase in the angle of attack where no cavitation is expected. An increase in the camber ratio moves the cavitation-free location of the angle of attack, and slightly increases its range.     

Redefinition of roles and responsibilities in U.S. transportation

Major shifts in U.S. Federal transportation policy are occurring which are realigning the roles and responsibilities of the Federal, State and local governments, and the private sector. These shifts include a decentralization of control to State and local governments, a larger role for the private sector, and reduced emphasis on construction of new facilities and more on rehabilitation and utilization of existing facilities. Although some reaction to these changes is already apparent, it is unclear at this time what the effect will be on improving the efficiency and effectiveness of transportation systems.     

Forecasting downtime event durations for the Morgantown people mover

A model was developed to forecast the duration of emergency shut downs of the Morgantown People Mover. An extensive data base of downtime events for a 2 1/2 year period were analyzed. Analysis of Variance (ANOVA) methods were applied to determine the significance of five variables which were hypothesized to influence downtime duration, including physical sub-system, restartablity, location of failure, number of vehicles in the system, and level of demand. Results of the analysis enable MPM system operators to provide improved information to system users during downtime events. The forecasting methodology also enables operators to evaluate alternative user management strategies during downtimes.