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장경식(K. Chang),G. Constant inescu 한국전산유체공학회 2012 한국전산유체공학회 학술대회논문집 Vol.2012 No.11
The present large eddy simulation (LES) study focuses on the case of developing flow within wide channels containing at the bottom wall an array of 2D dunes (2a/h=0.25, l/h=3.75, / is the wavelength, 2a is the dune height, h is the mean channel depth) of closer shape to the ones observed in natural streams. For the case of an incoming steady flow, the instantaneous flow fields are characterized by the formation of arrays of highly-organized hairpin vortices whose dimensions scale with the dune height. LES shows that for shallow channels (2a/h>0.25), the large-scale hairpins and the streaks penetrate regularly up to the free surface, thus affecting mass transport and mixing over the whole water column. LES shows that the hairpin eddies play the primary role in the formation of the streaks of high and low momentum over the region containing dunes. The presence of resolved turbulence in the incoming flow reduces the streamwise distance needed for the streaks to develop over the rough bed region, but does not affect qualitatively the transition process toward the fully developed flow regime, nor the spacing of the streaks in the fully-developed flow region.