Coherent structures in flow over two‐dimensional dunes

Abstract

[1] The instantaneous turbulent flow fields over a smooth bed and a bed containing largescale roughness elements are characterized by the presence of elongated low and high streamwise momentum regions or streaks. If the bed contains large-scale roughness elements (e.g., dunes), the size of the streaks increases and is of the order of the size of these elements and the flow depth. The present large eddy simulation (LES) study focuses on the case of developing flow within wide channels containing at the bottom a long array of spanwiseoriented sinusoidal 2-D dunes (2a/h ¼ 0.1, /h ¼ 1, is the wavelength, 2a is the dune height, and h is the mean flow depth) and an array of 2-D asymmetric dunes (2a/h ¼ 0.25, / h ¼ 3.75) of closer shape to the ones observed in natural streams. For the case of an incoming steady flow, the instantaneous flow fields, in the region where the flow transitions toward a fully developed turbulent flow regime, contain arrays of highly organized hairpin vortices, whose dimensions are larger than the dune height. The LES shows that for relatively shallow channels (e.g., channels with 2a/h ¼ 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. This paper explained the mechanism for the formation of these arrays of hairpin vortices and discussed the changes between a case with asymmetric dunes that are characterized by a large value of /2a (¼ 15) and a long upslope face and a case with symmetric dunes for which /2a ¼ 10, the upslope face is relatively short, and the rate of change of the bed curvature around the dune's crest is relatively small. The study discusses the main mechanisms through which large-scale hairpin form and how these mechanisms change between two dune geometries (sinusoidal versus asymmetric dunes). This study also shows that hairpin eddies play the primary role in the formation of the streaks over the region containing dunes and provides an estimation of the average dimensions of these streaks. The presence of resolved turbulence in the incoming flow reduces the streamwise distance needed for the streaks to develop over the region containing dunes, but does not qualitatively affect the transition process toward the fully developed flow regime nor the spacing of the streaks in the fully developed flow region.Citation: Chang, K. S., and G. Constantinescu (2013), Coherent structures in flow over two-dimensional dunes, Water Resour.

Large eddy simulation of the velocity-intermittency structure for flow over a field of symmetric dunes

“…While there are also examples of older numerical studies studying the flow structure in these environments [15,16], it is only recently that high resolution, eddy-resolving numerical studies have been performed [17,18], and flow structure generation mechanisms have been considered using numerical models [19][20][21][22]. In the context of the two-dimensional dunes that have tended to form the emphasis of previous experimental work [12,13,18,19,22], recent work has focused on the generation mechanisms describing the large-scale hairpin features in such flows, with a variety of mechanisms proposed:…”

“…In the context of the two-dimensional dunes that have tended to form the emphasis of previous experimental work [12,13,18,19,22], recent work has focused on the generation mechanisms describing the large-scale hairpin features in such flows, with a variety of mechanisms proposed:…”

“…• Chang and Constantinescu [22] examined this looking not only at a fully developed flow with periodic boundary conditions, but also at spatially developing transitional flow where the subdued activity of the smaller scales made identification of the mechanism responsible for the formation of the large-scale coherent structures much clearer. They found that the vortex tube induced by the dune upstream is transported above the SSL of the dune downstream, giving it a greater mean velocity, while retaining more coherence.…”

Flow resistance in natural, turbulent channel flows: The need for a fluvial fluid mechanics

“…Eddy-resolving simulation techniques such as large-eddy simulation (LES) and detached eddy simulation (DES), or lattice Boltzmann methods, then become necessary. The utility of eddy-resolving methods has been demonstrated by the explosion of the number of studies in the last few years, with applications to flow around channel obstacles and their associated scour holes [Kirkil et al, 2008;Koken and Constantinescu, 2009], flow through vegetation [Kim and Stoesser, 2011], as well as sediment transport [Nabi et al, 2012[Nabi et al, , 2013aSchmeeckle, 2014] and flow over bed forms [Nabi et al, 2013b;Chang and Constantinescu, 2013;Omidyeganeh and Piomelli, 2013], and flow through channel confluences [Constantinescu et al, 2011a]. See Keylock et al [2005] for an early introduction to the use of LES in channel flow hydraulics and Keylock et al…”

“…The characteristic equation for the velocity gradient tensor is A ij 5k 3 i 1Pk 2 i 1Qk i 1R50, where k i are the eigenvalues of A. Incompressibility means that P50, while both Q and R have a clear physical interpretation. The former gives the relative importance of strain and vorticity/ enstrophy [Hunt et al, 1988;Dubief and Delcayre, 2000] and, consequently, is now used commonly in numerical studies of fluvial processes for flow structure identification [Kirkil et al, 2009;Chang and Constantinescu, 2013]:…”