Development Of Overset Grid Technique For Hull-Propeller-Rudder Interactions

With the developments of computer techniques and numerical methods, Computational Fluid Dynamics(CFD) has become one of the most popular research tools in the field of ship and ocean engineering. However, the development of CFD are impeded by several bottlenecks. One of the most critical issues is the dynamic grid technique. In practical ship engineering, ships are appended with propellers, rudders and other appendages that have great influence on the ship motions. In real situation, the movement of ship hull, rudders and propellers are fully coupled. The ship has full 6DoF motions in free surface. In the meanwhile, the rudders and propellers are moving or rotating with respect to the ship hull. The conventional dynamic grid approaches, such as deforming and sliding grid methods are difficult and inefficient to handle the movements of ship hull, rudders and propellers simultaneously. The overset grid approach is an effective solution to the problems described above. The simulation of hull-propeller-rudder interactions can be handled by this method. Overset grid method removes the restrictions of the mesh topology among different objects and allows grids to move independently within the computational domain. The computations such as self-propulsion and free maneuvers with moving propellers and rudders in calm water or waves, which are difficult to be handled by traditional dynamic grid approaches, can be solved by the overset grid method.The objective of this dissertation is to develop the overset grid technique based on the open source code OpenFOAM. The implementation of the overset grid module is based on the numerical methods from Open FOAM including the cell-centered scheme and unstructured grids. SUGGAR program is utilized to generate the domain connectivity information(DCI) for the overset grid interpolation.The implementation consists of two parts. The first part is static overset grid method, in which the module is fully parallelized and the interface is unified so that it can be applied to all of the governing equations in OpenFOAM. In parallelization, the DCI is renumbered and decomposed. Each block of DCI is sent to the specific processor and the size of the block is minimized in order to optimize the scalability. In addition, the interface of the overset gird module is unified by an abstract class based on the idea of the object-oriented programming in C++ language. Almost all the governing equations in OpenFOAM can be used for the overset grid method using the unified module interface.The second part is dynamic overset grid method, where a full 6DoF motion module with a hierarchy of bodies is developed and the communication between OpenFOAM and SUGGAR is optimized. In the hierarchy of bodies, the ship hull(parent) has 6Do F motions with respect to the earth on the free-surface while the rudders and propellers(children) can move relatively to the ship hull. Besides, OpenFOAM and SUGGAR are running simultaneously in different processors using the lagged mode, which effectively resolve the idle time issue for CPU processors in conventional dynamic overset grid approaches.Based on the overset grid module and OpenFOAM, a new solver named naoeFoamos-SJTU is developed. The new solver has the overset grid capability and a full 6Do F module with a hierarchy of bodies, allowing the ship and its appendages to move simultaneously. This solver can be applied for the investigations of hull-propeller-rudder interactions.In order to validate the developed overset grid module and naoe Foam-os-SJTU solver, three categories of validation cases including self-propulsion, free maneuvering and seakeeping are performed. The first one is a self-propulsion test of the KCS model with a rotating propeller. The self-propulsion factors are obtained through full CFD computations including open-water test, towed condition and self-propulsion. The second category is free maneuvers with moving propellers and rudders. All of the propellers and rudders are modelled by overset grids. Two ship models are investigated in the free maneuvers. The first one is the KCS model, on which 15/1 and 10/10 zigzag maneuvers are carried out. The second ship model is fully appended DTMB 5415 M, where one turning circle maneuver and one 20/20 zigzag maneuver are performed. The third category includes incident waves. The computations of KCS model with a rotating propeller in head waves are carried out. In all cases, the rudders and propellers are fully discretized by overset grids. These appendages are able to rotate with respect to the ship hulls and provide the thrust forces and turning moments for the ship. The experimental data are available for validation in all cases. The comparisons show good agreements with measurements, indicating the fine accuracy and robustness of the overset grid method. The validation cases show that the overset grid method is able to handle complex ship hydrodynamic problems such as hull-propellerrudder interactions, which cannot be handled by traditional dynamic mesh methods. The results indicate that the overset grid technique can extend the applications of CFD methods.