Distribution and motion characteristics of Q-events for open-channel uniform flow

To understand the time-averaged transient motion characteristics of Q-events for open-channel uniform flow (first-quadrant (Q1), second-quadrant(Q2), third-quadrant(Q3), fourth-quadrant(Q4)), a high-frequency particle image velocimetry system was used to measure three two-dimensional instantaneous velocity vector fields of turbulent open-channel flows. The Reynolds stress, occurrence frequency, and measured area of Q-events in fluctuating velocity fields in open-channel flow and the relationship between Q-events and vortex motions were statistically analyzed. The results indicated that the time-averaged characteristics of Q1 and Q3 were similar, with low occurrence frequency, small area, and negative instantaneous fluctuating momentum flux. The time-averaged characteristics of Q2 and Q4 were opposite to those of Q1 and Q3. In addition, the occurrence frequency of Q4 increased with water depth to reach a maximum at about 0.8H, whereas the occurrence frequency of Q2 reached a maximum near the walls and decreased with water depth. The transient motion characteristics of the open channel were evident mainly in the downstream movement of Q4 resulting from the water surface area and in the extrusion, uplift, and climbing behavior of the Q2 structure in the bed region accompanying the transient development process of its deformation.