Experimental Study On Diverse Scale Coherent Structures In Open Channel Flows

Natural open channel flows contain multi-scale coherent structures, which governimportant turbulent processes, e.g., generation, transmission, and dissipation of kineticenergy. There exist complex interactions among these structures, as have been revealedin turbulent boundary layer and closed channel flow. In open channel flow, however,such interactions remain unclear partly due to the treatment of different structures asindependent elements in traditional research. With new progress in other wall-boundedturbulence, a unified framework of coherent structures now becomes possible in openchannel flows based on division of and interaction among turbulent scales.This study reports experimental results of diverse scale coherent structures andtheir interactions in open channel flows. The experiments employ high-frequency par-ticle image velocimetry to measure instantaneous velocity fields at various Reynoldsand Froude numbers. Major findings are as follows:1) As revealed by premultiplied spectral analysis, coherent structures can be classi-fied into small, large, and super scales in terms of statistical significance. Small-scalemotions are characterized by u*/ν, super-scale motions may extend to3-10h, andlarge-scale motions fall in between in size.2) The structured vorticity approach, which uses geometry information of stream-lines, proves to be effective for extracting spanwise vortex. Compared with otherwall-bounded turbulence, open channel flows witness more spanwise vortices with re-duced size and strength due to the presence of free surface. Average over instantaneousvelocity fields that contain hairpin-like spanwise vortices lends support to the hairpinvortex model in open channel flows.3) Proper orthogonal decomposition analysis indicates that hairpin vortex groupsconstitute large-scale structures while super-scale motions include Q2\Q4events ex-tending across the full water depth and low\high speed streaks in outer layer, bothstronger than those identified in other wall-bounded flows. The results agree with thesuper streamwise vortex model for super-scale motions.4) A closed-loop feedback cycle exists between super and large-scale structures, ashas been shown through analysis of consecutive velocity fields (based on Taylor’s fro- zen turbulence hypothesis) and joint distribution of vortex density and POD modecomponent. Hilbert transform and filtering techniques reveal close relationships be-tween different scales, e.g., amplitude modulation occurs between super-scale motionsin the log layer and small-scale motions in the inner layer, and super-scale motions in-duce hairpin vortices through mean shear.5) An overall model is proposed to unify coherent structures of various scales andto interpret their generation, evolution, and interactions in open channel flow.