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UBC Theses and Dissertations

A numerical model for vortex shedding from sharp wedges in oscillatory flow Wong, Lok Haw


This thesis describes a numerical simulation and some flow visualization of vortex shedding from sharp edges in normal oscillatory flow. The modelling of vortex shedding from sharp edges has been done using a discrete vortex method; the separated shear layer issuing from the separation point is represented by a system of discrete two dimensional vortices. In Chapter Two, a finite wedge is modelled by considering the flow near the edge as the inner region of an oscillatory flow around an infinite wedge. This can be done if the Keulegan-Carpenter number is low, i.e. if the vortex hedding takes place mostly in the vicinity of the edge and is independent of shedding from any other edge(s). The mathematical formulation of this problem, although based on the combination of recent work of other researchers, represents a somewhat different approach when examined in detail. Each new vortex, called the nascent vortex, is introduced into the flow at a position not fixed in advance. Its position is dependent on the edge angle, the time step used in the numerical simulation and the influence of all the other vortices in the field. The expression describing the position of the nascent vortex can be derived as a natural development of the formulation. Therefore, it is not necessary to use empirical formulae to define the initial position of the nascent vortex and/or to fix this position for all time throughout the numerical simulation. Lamb vortices are used in the present study to delay the onset of instability in the numerical calculations. This results in very stable computations. Numerical modelling results concerning vortex induced forces are presented in Chapter Three. These results are then compared to those obtained numerically and experimentally by other researchers. Flow visualization experiments of vortex shedding from finite sharp wedges in an oscillatory flow are described in Chapter Four. The flow was produced using a sloshing tank, and visualized by hydrogen bubbles produced by the electrolysis of water. All results were recorded on video tape and photographs of flow visualizations have been produced through the use of a mirco-computer based frame grabber. The kinematics of the numerical modelling are compared to those obtained from flow visualizations. An application of the model to the roll decay of a simplified geometry of a single chine west coast trawler is presented in Chapter Five. No firm conclusions regarding the accuracy of the numerical prediction of roll decay can be drawn due to the gross simplification of the vessel section. However, the results do indicate that, with the absence of other forms of roll damping, vortex induced forces alone was able to cause roll extinction in the vessel. Therefore, it can be said that the prediction of roll extinction given by the present model is of an acceptable order of magnitude when compared to experimental roll decay results from previous work done in this department.

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