Coherent structure models for open channel flows

Instantaneous velocities in open channel flows were measured using high-speed PIV in steady uniform flows at three different Reynolds numbers.Proper orthogonal decomposition(POD) was used to find the optimal models for the turbulence and turbulent coherent structures.The results show that the kinetic energy is concentrated in a few modes with the first mode containing 30% of the total energy.The Q2(Q4) events constitute the most prominent coherent motions in the flow.The Q2(Q4) events convey kinetic energy to other large-scale structures such as the hairpin vortex packets,but the mechanism of generating and maintaining the Q2(Q4) events remains elusive.Typical hairpin vortex packet structures incline at 10° to the wall,with the stream-wise length scale averaging 3.4h while the wall-normal scale ranged from the buffer layer to the free surface.Hairpin vortex packets contain 23% of the turbulent kinetic energy concentrated in the middle part of the structure in the logarithmic layer.Other less energetic large-scale structures include vortices near the surface,single and coupled vortex systems,and independent vortex packets.