Characteristic scales and consequences of large-scale horizontal coherent structures in shallow open-channel flows
ABSTRACT: The nature and consequences of the largest horizontal coherent structures (HCS’s) of turbulence in a shallow open-channel flow are investigated on the basis of three series of velocity measurements. These were conducted in a 21 m long, 1 m wide channel, conveying a turbulent, sub-critical and uniform flow, with a flow depth of 4 cm. The bed surface was flat, and the bed shear stress was substantially below the threshold for initiation of motion. In two of the measurement series, small ‘discontinuities’ were introduced at the channel walls to regularize the shedding of turbulence structures. The filtered oscillograms of fluctuating velocity exhibited regular (nearly periodic) cycles of variation consistent with the presence of HCS’s affecting (occupying) the entire body of fluid. Moreover, these were consistent with the presence of persistent horizontal burst-sequences issued from (or promoted by) the discontinuities. A simple decomposition of the velocity records yielded an average horizontal burst length of approximately five times the flow width. This result was independently confirmed with the aid of different advanced techniques for the detection of coherent structures, including energy spectrum analysis, and analysis of velocity signals by continuous wavelet transform. A slight internal meandering of the flow caused by the superimposition of burst sequences on the mean flow was detectable. The resulting convective flow patterns were found to be capable of inducing bed deformation consistent with the trace of alternate bars.