Experimental investigation of turbulent flow in a two-pass channel with different U-shaped bends

Time-resolved particle image velocimetry is used to study internal flow field characteristics in U-shaped channels of square cross section and different structures of the bend section. The Reynolds number based on the hydraulic diameter of the channel is 8888, 13 333, or 17 777. The mean flow and Reynolds stress are considered, and proper orthogonal decomposition (POD) is used to investigate the flow characteristics. A series of important conclusions are drawn from the results. For the main flow, the structure of the bend section has an obvious influence on the flow field characteristics. The size and number of vortices in the corner area are significantly reduced because the increase in the Reynolds number makes the impact of the influx stronger. It can be seen from the clear differences in the Reynolds stress for different bend sections that the fluctuations caused by the mixing of the main flow and the vortices are significantly stronger than those at the boundary. The flow in the bend section is complex, there is a relatively high proportion of turbulent kinetic energy in the low-order modes, and there is an obvious stripe-like structure in the bend section of the channel in which the bend has both inner and outer circular walls, which matches the velocity field from the POD.