Observations of flow separation by an isolated island

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Abstract
Remotely-sensed data from a series of overflights of the Clyde estuary in western Scotland are described. Thermal band images are presented to demonstrate the occurrence of flow separation at the tip of an isolated island in the estuary and the characteristics of the resulting downstream eddy are investigated. Field data are used to interpret the observed eddy structure and to confirm that the eddy is delineated in the images because of the existence of a source tracer of cold water upstream of the point at which the tidal stream separates from the tip of the island. Upwelling of cold water induced by the rotational motion of the attached eddy is shown to be inhibited by the stable density stratification existing in the estuary under summer conditions. The dynamics of the eddies generated at the island are discussed in terms of the non-dimensional parameters controlling the flow and it is demonstrated that the effects of the background rotation of the Earth upon the flow dynamics are weak. The usefulness of the so-called equivalent Reynolds number and the island wake parameter in describing estuarine flows is discussed and it is shown that the spatial structures of the observed eddies suggest a horizontal eddy viscosity for the flow of about 102 m2 s−, which is of the same order as the values adopted by other workers for similar cases.

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