Probabilistic Characterization of Sweep and Ejection Events in Turbulent Flows and Its Implications on Sediment Transport

Abstract

AbstractTurbulent boundary layers are populated by a hierarchy of recurrent structures normally referred to as “coherent structures.” Among others, ejection and sweep events are critical coherent structures responsible for sediment transport. This study focuses on gaining a better understanding of the spatial‐–temporal probabilistic characteristics of sweep and ejection events. The existence of uniform momentum zones (UMZs) is demonstrated to affect the spatial distribution of large‐scale motions (LSMs), and the ejection and sweep events tend to present near UMZ edges where interfacial layers of high shears exist. In this study, UMZ detection is employed to identify coherent structures. The conditional velocity decomposition is then applied to the available direct numerical simulation (DNS) flow data after UMZ edges were determined. Based on the integrated criterion for distinguishing ejection and sweep events, one can determine the probabilistic characteristics of coherent structures such as the center point, wall‐normal length, and streamwise length. Physical insights such as joint probability density functions of wall‐normal length and streamwise length can be established. The attached and detached features of the sweeps and ejections can then be classified and characterized, respectively. Durations of sweep and ejection events are demonstrated to follow a lognormal distribution in this study. The occurrence ratio of sweep events in the (LSMs) is quantified from the DNS data. The implications of the results on sediment transport are discussed. The contributions of coherent structures with respect to wall‐normal height and the intermittency and geometrical properties of the coherent structures are presented.