  We show that a rather simple, steady modification of the streamwise velocity
profile in a pipe can lead to a complete collapse of turbulence and the flow
fully relaminarizes. Two different devices, a stationary obstacle (inset) and a
device to inject additional fluid through an annular gap close to the wall, are
used to control the flow. Both devices modify the streamwise velocity profile
such that the flow in the center of the pipe is decelerated and the flow in the
near wall region is accelerated. We present measurements with stereoscopic
particle image velocimetry to investigate and capture the development of the
relaminarizing flow downstream these devices and the specific circumstances
responsible for relaminarization. We find total relaminarization up to Reynolds
numbers of 6000, where the pressure drop in the downstream distance is reduced
by a factor of 3.4 due to relaminarization. In a smooth straight pipe the flow
remains completely laminar downstream of the control. Furthermore, we show that
transient (temporary) relaminarization in a spatially confined region right
downstream the devices occurs also at much higher Reynolds numbers, accompanied
by a significant drag reduction. The underlying physical mechanism of
relaminarization is attributed to a weakening of the near-wall turbulence
production cycle.
