Two-dimensional scour hole problem: Role of fluid structures

An experimental program was carried out to further understand the scour caused by a plane wall jet. To this end, a two-dimensional laser Doppler anemometer was used to characterize the velocity field at various stations in the scour hole region. Present observations indicate that different types of flow structures influence scour at different time periods. Following vigorous digging caused principally by jet shear forces and impingement at the start of the test, the flow was characterized by the presence of longitudinal axial vortices, turbulent bursts and movement of the jet impingement point during the later stages. Attempts have been made to identify the role of these structures at near asymptotic conditions. The scour hole region was characterized by the presence of randomly forming and disappearing streaks, longitudinally located concave shaped depressions, rolling and ejection of sand grains. Through analysis of higher order moments and quadrant decomposition, sweep and ejection type events were observed. Such events contribute to scour and need to be incorporated in future modeling work.