Timescale and structure of ejections and bursts in turbulent channel flows

Abstract

Burst structures in the near wall region of turbulent flows are associated with a large portion of the turbulent momentum transport from the wall. However, quantitative measures of the timescales associated with the burst event are not well defined, largely due to ambiguities associated with the methods used to detect a burst.In the present study, Eulerian burst-detection schemes were developed through extensions of the uv quadrant 2, VITA, and u-level techniques. Each of the basic techniques detects ejections. One or more ejections are contained in each burst and hence the key idea is to identify and to group those ejections from a single burst into a single-burst detection. When the ejection detections were grouped appropriately into burst detections, all of the extended techniques yielded the same average time between bursts as deduced from flow visualization for fully-developed channel flow in the range 8700 [les ] Reh [les ] 17 800. The present results show that inner variables (wall shear stress and kinematic viscosity) are the best candidates for the proper scaling of the average time between bursts. Conditional velocity sampling during burst and ejection detections shows that these burst events are closely correlated with slower-than-average moving fluid, moving both away from the wall and toward the wall.

Analysis of coherent structures and atmosphere-canopy coupling strength during the CABINEX field campaign

“…After events are detected with the Q-H method, multiple detections of the same event are grouped using a time frequency parameter tau (τ ), defined as the maximum time histogram or the intersections of the two asymptotic lines in the plot of the cumulative 860 probability distribution on a logarithmic scale against TE (Luchik and Tiederman, 1987;861 Tiederman, 1989). 862 863 Fig.…”

“…Sample histogram (left axis) and cumulative probability distribution (right axis) of time between ejections (T E (s)) for a single 30-min period (07:30 LT on 19 July 2009). τ , the maximum time between ejections of the same burst event, can be estimated by the minimum in the histogram or the intersections of the two asymptotic lines in the plot of the cumulative probability distribution on a logarithmic scale against T E (Luchik and Tiederman, 1987;Tiederman, 1989). between ejections from the same burst. τ is obtained by plotting the histogram or cumulative probability function of the time between events and visually detecting two distinctly different statistical regions: a region of multiple ejections within a single burst, and a region of independent detections from different bursts (Luchik and Tiederman, 1987;Tiederman, 1989) (Fig.…”

Three-dimensional structure of a low-Reynolds-number turbulent boundary layer

“…As recognized by Brown & Thomas (1977) and Praturi & Brodkey (1978) (among others), the essentially three-dimensional character of boundary-layer motions makes it very difficult to detect a complex process, such as the bursting cycle, unambiguously from a fixed measurement station. Part of the problem is no doubt because more than one fluid packet may be ejected from a given low-speed streak, so the identification of a single bursting event becomes ambiguous (Luchik & Tiederman 1987). Additionally, the different detection methods employed add to the confusion (e.g.…”

The Physics of Turbulent Boundary Layer Structures and Effects Due to Manipulation

“…Tmax was then taken as the point of intersection. Luchik & Tiederman (1987) used this method on their cumulative probability distribution, which was similar to the one obtained in this study.…”