Shear stress measurements over smooth and rough channel beds

Abstract

In fluvial hydraulics the knowledge of the fluid-bed interaction is an important factor to study the stability of fluvial beds. One way to characterize that interaction is through the evaluation of the shear stress at the wall. Indirect methods are often used to evaluate the shear stress. However, the use of those methods such as the velocity Log-Law, is not always straightforward: for example, in some flows it has to be proved that the law of the wall definition still holds and its parameters determined. In this paper an experimental study of the shear stress developed at a rectangular channel bed is presented and different methods are compared. Measurements were carried out in an open channel for different flow conditions, defined by a Reynolds number range from 1×104 to 5×104 and with Froude numbers between 0.2 and 1.7. Flows were established over smooth (perspex plate and sandpaper) and rough beds (uniform glass spheres). Direct measurements of the shear stress were obtained by means of an optical Doppler based shear stress probe. Indirect methods velocity profiles characterization was made by means of twocomponent Laser Doppler Anemometer measurements. Obtained results are compared and the limitations and advantages of each method are discussed.