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currents from any direction and they are designed using bluff cross sections, such as the circle and the square. Hence the offshore structure designer needs to have some knowledge of bluff body hydrodynamics.

Within the field of naval architecture ideal flow theory has been used very successfully to estimate wave-induced motions and loads on large floating structures. However, it cannot be used to model realistically phenomena resulting from the viscous nature of water such as boundary layers, flow separation and vortex generation and shedding. Bluff body hydrodynamics is concerned with real fluids and with understanding the effects of flow separation. Traditionally bluff body flows have been studied experimentally with relatively simple models developed to provide ways of correlating data. As computer power has increased more and more CFD studies of bluff body flows are becoming available, based on numerical simulations of the unsteady Navier Stokes equations. In some applications, such as predicting the slow drift response of moored structures, predictions from ideal flow theory are being combined with results from bluff body research. The drag arising from viscous effects is treated as a form of hydrodynamic damping which is considered as independent of the primary loading which is developed by waves. This is an attractive approach but there is a need to validate how successfully flows can be superposed in this way.

Circular cross-section members are a common form used in offshore structures and hence it is fortunate that the circular cylinder is also the most studied bluff body shape. It has geometric simplicity enabling models to be easily manufactured for basic experimental research. Its simple form also makes it attractive as a representative bluff body for CFD investigations. Much of this paper will be concerned with the circular cylinder but such a great deal of research has been carried out using this shape that it is impossible to provide a comprehensive review within the confines of a single paper. Basic questions that will be addressed include: does a bluff body with a two-dimensional geometry placed in a uniform stream generate a two-dimensional flow, what new phenomena appear when a bluff body is placed in waves and how successfully can CFD be used to predict loading and response of bluff bodies?

Figure 1 Base Pressure Coefficient versus Reynolds Number for a Circular Cylinder (Roshko (1))



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