With the aid of a recently developed high speed stereoscpic PIV (Particle Image Velocimetry) set-up we were able to observe flow patterns in the form of travelling waves in turbulent pipe flow. These flow patterns have previously been predicted in numerical calculations and are exact solutions to the Navier Stokes equations. They are belived to play an essential role in the transition to turbulence in pipe and related shear flows and it has been suggested that the turbulent state is organized around such unstable solutions.
We here report of an experimental investigation into the mechanism underlying the formation and regeneration of these periodic flow patterns. This mechanism involves vortices with a streamwise orientation and elongated streaks with a periodic modulation in the streamwise direction. Detailed analysis enables us to extract the wavelengths of periodically modulatd streaks in the turbulent flow and they are found to compare well with the wavelength predicted for travelling wave solutions. Overall our results confirm the relevance of travelling wave transients to turbulent pipe flow at moderate Reynolds numbers.