SHALLOW-WATER FLOW AROUND MODEL CONICAL ISLANDS OF SMALL SIDE SLOPE .1. SURFACE PIERCING

A series of experiments has been conducted to investigate recirculatin g shallow-water flow in the wakes of conical model islands with gently sloping sides. Four islands have been used with side slopes ranging f rom 33.1 to 8.0 degrees. For all tests reported here the water depth w as less than the island height; the islands are said to be surface pie rcing. Measurements of flow velocity have been made in the laboratory using a digital particle tracking velocimetry (PTV) system. This produ ces instantaneous, whole-field velocity vector maps of the wake flows. A ''wake stability parameter,'' S has been used to classify the islan d wakes into ''vortex shedding'' or ''unsteady bubble'' types. The sta bility parameter is a measure of the stabilizing effect of bed frictio n relative to the destabilizing influence of transverse shear. For sma ll values of S (<0.2) vortex shedding was well organized and vigorous in the island wakes. Vortex shedding was found to cease for larger val ues of S (0.35-0.40). These values are similar to those found by other researchers for the shallow wakes of vertically sided circular cylind ers. The influence of the island side slope angle on the wakes is disc ussed. Depth-averaged and three-dimensional (3D) numerical shallow-wat er flow models with the usual hydrostatic pressure assumption have bee n used to simulate the experimental flows. The models are based on a f ully coupled semi-implicit Lagrangian numerical method. For cases wher e vortex shedding occurred, the 3D model was found to predict a more 3 D far wake than was observed. The two-dimensional (2D) model produced good overall agreement with experimental results. For very shallow dep ths with the wake stabilized by bed friction, the 3D model demonstrate d closer agreement to the measured results than the 2D model.