  In this paper, the TR-PIV method is used to study the internal flow field
characteristics in U-shaped channels. The Reynolds number, based on the square
cross section channel hydraulic diameter is 8888,13333 and 17777. Mean flow,
Reynolds stress and POD are taken into consideration to investigate the flow
characteristic with three different turning sections. Through analysis, a
series of important conclusions have been drawn. For the main flow, the
structure of turning sections has obvious influence on the characteristics of
flow field. The size and number of vortices in the corner area are
significantly reduced, because the increase in Reynolds number makes the influx
impact stronger. It can be seen from the Reynolds stress distribution which is
obviously different in different turning sections that the pulsation caused by
the mixing of the main flow and the vortex is obviously stronger than that at
the boundary. The flow at the turning section is complex, the distribution of
the proportion of turbulent kinetic energy in the low-order mode is relatively
gentle, and there is an obvious wavy structure at the turning section of the
inner circle and outer circular passage, which matches the velocity field from
the POD.
