Transition to turbulence in pipe flow is one of the most fundamental and longest-standing problems in fluid dynamics. Stability theory suggests that the flow remains laminar for all flow rates, but in practice pipe flow becomes turbulent even at moderate speeds. This transition drastically affects the transport efficiency of mass, momentum, and heat. On the basis of the recent discovery of unstable traveling waves in computational studies of the Navier-Stokes equations and ideas from dynamical systems theory, a model for the transition process has been suggested. We report experimental observation of these traveling waves in pipe flow, confirming the proposed transition scenario and suggesting that the dynamics associated with these unstable states may indeed capture the nature of fluid turbulence.

Stereoscopic particle image velocimetry (SPIV) is applied to measure the instantaneous three component velocity field of pipe flow over the full circular cross-section of the pipe. The light sheet is oriented perpendicular to the main flow direction, and therefore the flow structures are advected through the measurement plane by the mean flow. Applying Taylor's hypothesis, the 3D flow field is reconstructed from the sequence of recorded vector fields. The large out-of-plane motion in this configuration puts a strong constraint on the recorded particle displacements, which limits the measurement accuracy. The light sheet thickness becomes an important parameter that determines the balance between the spatial resolution and signal to noise ratio. It is further demonstrated that so-called registration errors, which result from a small misalignment between the laser light sheet and the calibration target, easily become the predominant error in SPIV measurements. Measurements in laminar and turbulent pipe flow are compared to well established direct numerical simulations, and the accuracy of the instantaneous velocity vectors is found to be better than 1% of the mean axial velocity. This is sufficient to resolve the secondary flow patterns in transitional pipe flow, which are an order of magnitude smaller than the mean flow.