  Swirl-switching is a low-frequency oscillatory phenomenon which affects the
Dean vortices in bent pipes and may cause fatigue in piping systems. Despite
thirty years worth of research, the mechanism that causes these oscillations
and the frequencies that characterise them remain unclear. Here we show that a
three-dimensional wave-like structure is responsible for the low-frequency
switching of the dominant Dean vortex. The present study, performed via direct
numerical simulation, focuses on the turbulent flow through a 90 \degree pipe
bend preceded and followed by straight pipe segments. A pipe with curvature 0.3
(defined as ratio between pipe radius and bend radius) is studied for a bulk
Reynolds number Re = 11 700, corresponding to a friction Reynolds number
Re_\tau \approx 360. Synthetic turbulence is generated at the inflow section
and used instead of the classical recycling method in order to avoid the
interference between recycling and swirl-switching frequencies. The flow field
is analysed by three-dimensional proper orthogonal decomposition (POD) which
for the first time allows the identification of the source of swirl-switching:
a wave-like structure that originates in the pipe bend. Contrary to some
previous studies, the flow in the upstream pipe does not show any direct
influence on the swirl-switching modes. Our analysis further shows that a
three- dimensional characterisation of the modes is crucial to understand the
mechanism, and that reconstructions based on 2D POD modes are incomplete.
