Investigation of the upstream and downstream flow in a bent pipe using time-resolved three-dimensional particle tracking velocimetry

Turbulent flow through a 90◦ bend with a curvature ratio 𝛾 = 0.3 has been investigated experimentally using time-resolved three-dimensional particle tracking velocimetry (PTV) at 𝑅𝑒 = 12800. Fluorinated ethylene propylene tubes were installed upstream and downstream an opaque bend, and index-matching was achieved using a mixture of 94.1% of water and 5.9% of glycerol. The PTV measurements have been performed both before and after the bent section in order to provide further understanding of bent flow in general, and the swirl-switching phenomenon in particular. From the streamline pattern in the upstream tangent it is shown that the effect of the bend is minor for distances exceeding about two pipe diameters upstream from the bend. In the downstream tangent, elongated quasi-streamwise vortices have been identified using the 𝜆2-criterion on time-averaged data, and it is conjectured that the dynamics of these vortices might play an important role in the dynamics of the swirl-switching. This is in line with previously performed direct numerical simulations (DNS) of flow in toroidal pipes. To further characterize swirl-switching, two and three-dimensional proper orthogonal decomposition have been performed. It is shown that the cross-stream in-plane mode shapes do not differ significantly between the two approaches. Similarly as found in DNS, the mode shapes in the bend symmetry plane e