School of Mechanical Engineering Prof. Alex Yakhot (BGU)

School of Mechanical Engineering Seminar
Wednesday, March14, 2018 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

TURBULENCE IN A LOCALIZED PUFF IN A PIPE

Alexander Yakhot

Department of Mechanical Engineering, Ben-Gurion University, Beersheva, Israel

We have performed direct numerical simulations of transient turbulence in a pipe flow for Re=2,250 which has been recognized as a threshold for an equilibrium puff. We investigated the structure of an individual puff by considering three-dimensional snapshots over a long time period. To assimilate the velocity data, we applied a conditional sampling based on the location of the maximum energy of the transverse (turbulent) motion. Specifically, at each time instance, we followed 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 flow is intermittent and transitional. The velocity field inside the puff shows the dynamics of a developing turbulence. The localized puff is about 12-15 pipe diameters long and the flow regime upstream of its trailing edge and downstream of its leading edge is almost laminar. In the puff core, despite the low Reynolds number, the turbulence statistics becomes similar to a fully-developed turbulent pipe flow and the velocity profile becomes flat in the pipe core and logarithmic near the wall. It is shown that this “fully-developed turbulent gap” is very narrow being about two pipe diameters long.

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