An experimental investigation has been carried out in a curved duct of rectangular cross section in order to study the development of flow instability in such geometries. Hot wire anemometry was used to obtain detailed measurements of velocity on the symmetry plane of the duct for different curvature ratios. As the duct Dean number is increased, a centrifugal instability develops and the Dean vortices are seen to oscillate along the inner wall. To understand the contribution of these vortices to the laminar-turbulent transition, time histories and spectra of the flow were taken on the symmetry plane of the duct for different Reynolds numbers. These data reveal a time-periodic motion along the inner wall where the secondary flows originating from the side wall boundary layers collide. The bend angle where this instability develops depends on the Reynolds number while the frequency of the instability depends on the curvature ratio of the bend.

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