Three-Dimensional Flow Characteristics within the Scour Hole around Circular Uniform and Compound Piers

The experimental observations are presented on flow patterns and turbulence characteristics measured by an acoustic Doppler velocimeter within the developing (transient stage) scour hole around circular uniform and compound piers. Four series of experimental runs were conducted under the clear-water approach flow conditions. One experimental run was conducted around a uniform circular pier of diameter 114 mm, whereas the other three runs were conducted around a circular compound pier of diameter 114 mm and footing diameter 210 mm. In the series with a circular compound pier, the top surface of the footing was placed at three different elevations with respect to the general level of the channel bed, i.e., above the bed level, at the bed level, and below the level of the channel bed. Detailed measurements are presented on components of time-averaged velocity, turbulence intensity, and Reynolds shear stress around the pier in radial planes at 0°, 90°, and 180° from the flow axis. Flow structure around a circular compound pier in the presence of a scour hole was compared with flow structure similarly observed around a circular uniform pier by utilizing the observations made in radial planes at 0°, 30°, 60°, 90°, 120°, 150°, and 180° from flow axis. Diameter of the principal vortex upstream of the compound pier, when top surface of the footing is above the general level of the channel bed was 1.11 times as large as that for the circular uniform pier. However, size of the principal vortex is 0.85 times its size for the uniform pier, whereas the top surface of the footing was below the channel bed level. The components of turbulence intensities and Reynolds shear stress profiles in different radial planes are also compared around each of the pier models.