Practical Application of Viscous-flow Calculations for the Simulation of Manoeuvring Ships

The present work was initiated in order to improve traditional manoeuvring simulations based on empirical equations to model the forces and moments on the ship. With the evolution of viscous-flow solvers and their promising results in predicting forces and moments on ships, it was decided to develop a practical method to simulate the manoeuvrability of ships in which viscous-flow solvers are utilised and to investigate whether this improves the accuracy of manoeuvring predictions.
To achieve this goal, the virtual captive test approach is adopted, because of the efficient use of computational resources compared to other methods. This procedure mimics the approach for manoeuvring simulations in which experimental PMM is used to obtain the forces and moments on the ship. This study extends the work of other researchers by providing extensive verification and validation of the predicted forces and moments on the hull and a detailed study of the sensitivity of the manoeuvring characteristics of the ship to changes in the hydrodynamic coefficients in the simulation model.
Changes in the flow solvers were required to be able to calculate the flow around ships in rotational motion. These changes as well as the acceleration techniques that were developed to reduce the effort spent on grid generation and during the computations are discussed.
In this thesis, it is demonstrated that for a wide range of ship types, good predictions of the loads on the hull in manoeuvring motion can be obtained. The trends in the forces and moments as a function of the drift angle or yaw rate are simulated well.
The verification studies provide useful insight into the influence of grid density on the predicted forces and moments. In several cases, validation of the calculations failed, indicating modelling errors in the numerical results. In these cases, it was generally seen that the magnitude of the transverse force was under-predicted, while the magnitude of the yaw moment was over-predicted. For manoeuvring studies in the early design, the comparison errors are within acceptable levels. However, improvements remain desired and may be obtained using finer grids, larger domain sizes, different grid topologies with refinement in the wake of the ship, other turbulence models or incorporating free surface deformation.
The manoeuvring prediction program SurSim has been used to simulate the manoeuvrability of the HTC. A procedure to derive the hydrodynamic coefficients required to model the forces and moments on the bare hull is proposed. This procedure is chosen to enable accurate modelling of the linearised behaviour for course-keeping as well as realistic modelling of the harbour manoeuvring characteristics, and to enable the modelling of non-linear manoeuvres accurately.
To generate validation data for the manoeuvring predictions presented in this thesis, free sailing manoeuvring tests for the HTC were performed. This test campaign resulted in a very valuable data set which can be used for public validation studies. Besides obtaining general characteristics of the manoeuvrability of a single-screw container ship, unique information has been obtained on the drift angles and rates of turn combined with propeller and rudder forces. Furthermore, repeat tests have been conducted for selected manoeuvres and based on these tests, the uncertainty in the characteristic manoeuvring properties has been estimated.
By using hydrodynamic manoeuvring coefficients derived from the CFD calculations, it has been shown that it is possible to improve the prediction of ship manoeuvres compared to predictions using coefficients based on empirical equations, which was the objective of the present study. A considerable improvement in the turning circle predictions was obtained. The prediction of the yaw checking and course keeping and initial turning abilities based on zig-zag simulations improved as well, but further improvements are required for more reliable assessment of the manoeuvring performance.
The sensitivity of the manoeuvring predictions to changes in the hydrodynamic coefficients was studied. It was found that for accurate predictions of the manoeuvrability using coefficients derived from CFD calculations, accurate predictions of especially the yawing moment must be made.