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Turbulence in a Localized Puff in a Pipe

DateMonday, December 18, 2017


DetailsDepartment of Mechanical Engineering Ben-Gurion University

LecturerProf. Alex Yakhot

LocationEdna and K.B. Weissman Building of Physical Sciences

AbstractWe have performed direct numerical simulations of transient turbulence in pipe flow for Re=2,250 which is established as a threshold of an equilibrium puff. We investigate the structure of an individual puff by considering three-dimensional snapshots across a long time-period. To assimilate the velocity data, we apply a conditional sampling based on the location of the maximum energy of the transverse (turbulent) motion. Specifically, at each time instance, we follow a turbulent puff by a three-dimensional moving-window centered at that location. We collected a snapshot-ensemble of the velocity fields acquired over 10,000 time instances (snapshots) inside the moving-window. The considered fow is intermittent and transitional. The velocity field inside the puff shows the dynamics of a developing turbulence. The localized puff is about of 12-15 pipe diameters long with almost laminar trailing and leading edges. In the puff core, despite the low Reynolds number, along the moving-window, it takes the downstream short distance of several pipe diameters to form the state of developed turbulence, when the turbulence statistics becomes similar to fully-developed turbulent pipe flow; the velocity profile becomes flat in the pipe core and logarithmic near the wall. It is shown that this “fully-developed turbulent slot” is very narrow, about two pipe diameters; it is localized and moves with a puff.

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