  The dynamical behavior of propagating structures, determined from a
Karhunen-Lo`eve decomposition, in turbulent pipe flow undergoing reverse
transition to laminar flow is investigated. The turbulent flow data is
generated by a direct numerical simulation started at a fully turbulent
Reynolds number of Re_\tau=150, which is slowly decreased until Re_\tau=95. At
this low Reynolds number the high frequency modes decay first, leaving only the
decaying streamwise vortices. The flow undergoes a chugging phenomena, where it
begins to relaminarize and the mean velocity increases. The remaining
propagating modes then destabilize the streamwise vortices, rebuild the energy
spectra, and eventually the flow regains its turbulent state. Our results
capture three chugging cycles before the flow completely relaminarizes. The
high frequency modes present in the outer layer decay first, establishing the
importance of the outer region in the self-sustaining mechanism of wall bound
turbulence.

Fluid Dynamics