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Direct Shear Stress Measurements in Open Surface Flows

Author(s): E. Carvalho; R. Maia; M. F. Proenca

Linked Author(s): Elsa Maria Da Silva Carvalho

Keywords: Shear-stress; Direct measurements; Log-law

Abstract: In fluvial hydraulics the shear stress at the wall plays an important role in the characterization of the bed-fluid interaction. By definition, the shear stress is proportional to the velocity gradient at the bed whose direct measurement is not trivial. To determine it the use of indirect methods, like the logarithm law, are frequently used. Nevertheless these indirect methods have some limitations, e.g. only applicable when the log-law is valid. In this paper a technique based in the Doppler Effect is used to measure directly the shear stress in a non-intrusive way. This technique, which has a working principle similar to the Laser Doppler Anemometer, was tested in the metallic flume of the hydraulics laboratory of the Faculty of Engineering of Porto University. Six different flows over two different types of bed were considered and the results are compared with the ones obtained by an indirect method. The advantages and disadvantages of this technique are presented and discussed.


Year: 2011

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