Turbulent Bed Shear Stress under Symmetric and Asymmetric Waves: Experimental and Numerical Results

Abstract

A new instrument built to directly measure the bottom shear stress under different conditions of periodic waves is presented. Similar to classical Taylor’s cylindrical viscosimeter, the instrument is formed by two concentric cylinders, and the space between them is filled with water. The internal cylinder is fixed whereas the external cylinder, with a specific roughness, rotates following a given unsteady velocity time history. The shear stress at the outer cylinder wall is measured by using a torque meter. Both monochromatic and sawtooth-shaped waves have been tested, and the experimental shear stress time histories were compared with the results obtained from a numerical model.