SUMMARYThe boundary integral equation method constitutes the basis of a number of computer programs used for the solution of wave-obstacle interaction problems. For the case of obstacles in a constant depth fluid, the method assumes that the velocity potential at any point in the fluid may be represented by a distribution of Green's function sources over the immersed surface of the obstacle. Application of the obstacle kinematic boundary condition gives rise to an integral equation which may be solved, using numerical discretization, for the unknown source strength distribution function. Subsequent evaluation of the discretized velocity potential permits evaluation of the hydrodynamic interaction parameters.A series of numerical solutions have been carried out for a range of substantially rectangular obstacles, in a two-dimensional domain, using varying levels of immersed profile discretization. The results, presented in the form of fixed and floating mode wave reflection and transmission, together with the motion response of the floating obstacle, demonstrate the significant sensitivity of the evaluated parameters to variations in the level of discretization.

SUMMARYNumerical solutions of a non-linear oscillator have been produced by various methods. Some unexpected discrepancies have been found in the solutions obtained. This behaviour is reported herein, and the implications to the application of numerical methods to non-linear dynamical systems are discussed.

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Active flow control using automated gates and weirs aims to utilise available dispersed storage within sewer systems to alleviate the severity and frequency of localised flooding incidents. Whilst a previous study has demonstrated its potential, a key operational concern before implementation was sedimentation. An experimental programme was designed to investigate the sediment deposition created when using a flow control device. Tests were also undertaken to examine the potential for rapid gate opening to flush away any resulting deposits. In catchments dominated by fine material in suspension, the use of an active flow control device can result in a uniformly thick deposit upstream of the gate. Rapid gate opening results in deposited material eroding in large sections starting at the gate and moving in an upstream direction. Granular sediment forms a series of discrete bedforms which are fairly uniform regardless of the flow conditions and a larger deposit further upstream. The potential for flushing granular deposits is limited and modification of the operation of the gate has shown little potential for increasing the effectiveness. Therefore, active flow control using a single downstream gate may only be suitable in systems with fine material moving in suspension during dry weather flow and not where there is significant granular sediment.

An experimental investigation into the behavior of pulsed vortex flows in water at a pipe orifice is outlined. An experimental apparatus is described whereby flow pulsations are generated using an electronically controlled motor-piston arrangement. Preliminary investigations are made into the unpulsed system to determine the natural vortex shedding frequency for various Reynolds numbers and orifice diameters. The downstream development of initially axisymmetric, periodically pulsed vortices shed from the orifice plate are then examined. The growth, evolution, and eventual breakdown of the initially axisymmetric vortices are investigated for a range of flow control parameters. Various flow regimes are discerned and an attempt is made to categorize them.