Author(s): Kyoungsik Chang; George Constantinescu
Linked Author(s): George Constantinescu
Keywords: Open channel; Rough bed; Dunes; Large eddy simulation; Coherent Structures
Abstract: An important three-dimensional feature of the turbulent flow fields in the flow over a rough surface is the formation of long streamwise-oriented streaks of high and low streamwise velocity, similar to those observed in the vicinity of the bed in turbulent flow over a smooth surface. However, in the case of a rough surface, the size of the streaks scales with the size of the roughness elements (e. g., dunes). Large eddy simulation (LES) is performed for the case of developing flow within wide channels containing at the bottom wall a series of identical sinusoidal symmetric 2D dunes (2a/h=0. 1, λ/h=1, λ is the wavelength, 2a is the dune height, h is the mean channel depth) and a series of 2D asymmetric dunes (2a/h=0. 25, λ/h=3. 75). In simulations performed with an incoming steady flow, the instantaneous flow fields in the region where the flow transitions toward a fully developed turbulent flow regime over the dunes are characterized by the formation of spanwise arrays of highly-organized hairpin vortices whose dimensions are larger than the dune height. LES shows that for sufficiently large dunes (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. The hairpin eddies play the primary role in the formation of the streaks of high and low momentum over the region containing dunes. LES is used to estimate the average sizes of these streaks and to reveal the differences in the structure of the mean flow and turbulence over symmetrical and asymmetrical dunes.