Investigation of wake flow characteristics for tandem hydraulic rotors using time-resolved PIV

To study the influence of the downstream rotor on the wake of the upstream rotor, the time-resolved particle image velocimetry (TR-PIV) was used to measure the flow between two hydraulic Bach rotors. For different flow velocities, the influence of the setting angle of the downstream rotor on the upstream rotor wake was considered, and wake characteristics associated with various kinds of rotor boundaries were compared and analyzed. The results indicate that as the upstream velocity increases, the velocity recovery zone extends towards the upstream rotor. As the setting angle of the downstream rotor is smaller than 108°, velocity decreases as the setting angle increases. Such a tendency is overturned as the setting angle exceeds 108°. The positions of the vortex cores in the wake are shifted up and down with the variation in the setting angle, and the vortices are stretched and flattened at certain setting angles. Meanwhile, streamlines are deflected with respect to the main flow, which is significantly different from the situation without the downstream rotor. At some setting angles, with the increase of the upstream flow velocity, high-vorticity regions gradually develop in the streamwise direction and towards the wake center. Meanwhile, the number of sparsely distributed small-scale vortices increases continuously. Large-scale vortex structures in the wake are involved in the first three orders of proper orthogonal decomposition (POD) modes, while the high-order POD modes are featured by small-scale flowstructures.