Probabilistic Characterization of Sweep and Ejection Events in Turbulent
Flows: Insights from Direct Numerical Simulation Data
Abstract
Turbulent 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 of
large-scale motions in turbulent flows. This study focused 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, and the ejection and sweep events tend to
present near UMZ edges. On the basis of such observations, we considered
the effect of UMZ edges on the presence of ejection and sweep events. In
the current study, UMZ detection was employed to identify coherent
structures. Several criteria for identifying coherent structures are
revisited, and an integrated standard is applied to the available direct
numerical simulation (DNS) turbulent channel 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 maximum height,
wall-normal length and streamwise length. Physical insights from DNS
data such as joint probability density functions of wall-normal length
and streamwise length can be established. The attached and detached
features of the sweep and ejection coherent structures can then be
classified and characterized, respectively. Durations of sweep and
ejections events were demonstrated to follow a lognormal distribution in
this study. The occurrence ratio of sweep events in the large-scale
motions (LSMs) was quantified from the DNS data.