Time evolution of uniform momentum zones in a turbulent boundary layer

Abstract

Time-resolved planar particle image velocimetry was used to analyse the structuring of a turbulent boundary layer into uniform momentum zones (UMZs). The instantaneous peak-detection method employed by Adrian et al. (J. Fluid Mech., vol. 422, 2000, pp. 1–54) and de Silva et al. (J. Fluid Mech., vol. 786, 2016, pp. 309–331) is extended to account for temporal coherence of UMZs. The resulting number of zones detected appears to follow a normal distribution at any given instant. However, the extreme cases in which the number of zones is either very high or very low, are shown to be linked with two distinct flow states. A higher than average number of zones is associated with a large-scale $Q2$ event in the log region which creates increased small-scale activity within that region. Conversely, a low number of zones corresponds to a large-scale $Q4$ event in the log region and decreased turbulent activity away from the wall. The residence times, within the measurement plane, of zones belonging to the latter scenario are shown to be on average four times larger than those of zones present during higher than average zone structuring states. For both cases, greater residence times are observed for zones of higher momentum that are generally closer to the free stream.

On the mixing length eddies and logarithmic mean velocity profile in wall turbulence

“…Studies of coherent structures in high-Reynolds-number flows have revealed the presence of spatial regions with relatively uniform streamwise velocity (Meinhart & Adrian 1995). These structures are known as uniform momentum zones (UMZs) and have been identified both at the laboratory scale (Adrian et al 2000;de Silva et al 2016;Saxton-Fox & McKeon 2017;Laskari et al 2018) and in the atmosphere (Morris et al 2007;Heisel et al 2018). Individual UMZs are separated by relatively thin regions where a large percentage of the overall shear and vorticity are concentrated (Priyadarshana et al 2007;Eisma et al 2015;de Silva et al 2017).…”

A self-sustaining process theory for uniform momentum zones and internal shear layers in high Reynolds number shear flows

“…The largest UMZs remain coherent in the streamwise direction for distances approaching the boundary-layer (BL) thickness or channel half-height h for times estimated to be O(h/U ∞ ), where U ∞ is the free-stream or mean channel-centreline velocity (Laskari et al 2018). Both laboratory experiments (de Silva et al 2016) and analyses of the mean momentum equation (Klewicki 2013a,b) indicate that the number of UMZs grows logarithmically with the friction Reynolds number Re τ ≡ u τ h/ν, where u τ is the wall friction velocity and ν is the kinematic viscosity of the fluid, and that the typical change in streamwise velocity across individual fissures is a few multiples of u τ .…”

Modeling Suspended Sediment Transport Under Influence of Turbulence Ejection and Sweep Events

“…The values of mean flow velocity , turbulence intensities σ f = ( σ fu , σ fv ), the mean period of event occurrences , and the probability distribution of the event duration T used in simulations are taken from the flow data of wall‐bounded turbulence studies ( Dennis & Nickels, ; Laskari et al, ; Noguchi & Nezu, ). The values of sediment particle settling velocity w s = (0, − w s ) depend on the properties of simulated suspended sediment particles.…”

“…Probability density functions of the residence time of large‐scale Q2 events measured by Laskari et al (). In this work, the residence time is treated as the duration for ejection and sweep events.…”

“…In Figure 13 of Laskari et al (2018), it can be seen that two kinds of events have similar residence time distributions. As the first attempt to consider the turbulence coherent structure in the illustration of particle trajectory in suspended sediment transport, in addition to the mean time period between ejection/sweep event occurrences, this study also assumes that the duration distribution of ejection and sweep events be the same for simplicity.…”