Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 2. Flow structures

Abstract

This is the accepted version of the paper.This version of the publication may differ from the final published version. We performed large-eddy simulations of the flow over a series of three-dimensional (3D) dunes at laboratory scale. The bedform three-dimensionality was imposed by shifting a standard two-dimensional (2D) dune shape in the streamwise direction according to a sine wave. The turbulence statistics were discussed in Part 1 of this article [M. Omidyeganeh, U. Piomelli, J. Fluid Mech. 721, pp. 454-483, (2013)]. Coherent flow structures and their statistics are discussed concentrating on two cases with the same crestline amplitudes and wavelengths but di↵erent crestline alignments: in-phase and staggered. The present paper shows that the induced large-scale mean streamwise vortices are the primary factor that alters the features of the instantaneous flow structures. Wall turbulence is insensitive to the crestline alignment; alternating high-and low-speed streaks appear in the internal boundary layer developing on the stoss side, whereas over the node plane (the plane normal to the spanwise direction at the node of the crestline), they are inclined towards the lobe plane (the plane normal to the spanwise direction at the most downstream point of the crestline) due to the mean spanwise pressure gradient. Spanwise vortices (rollers) generated by Kelvin-Helmholtz instability in the separated shear-layer appear regularly over the lobe with much larger length scale than those over the saddle (the plane normal to the spanwise direction at the most upstream point of the crestline). Rollers over the lobe may extend to the saddle plane and a↵ect the reattachment features; their shedding is more frequent than in 2D geometries. Vortices shed from the separated shear-layer in the lobe plane undergo a three-dimensional instability while advected downstream, and rise toward the free surface. They develop into a horseshoe shape (similar to the 2D case) and a↵ect the whole channel depth, whereas those generated near the saddle are advected downstream and toward the bed. When the tip of such a horseshoe reaches the free surface, the ejection of flow at the surface causes "boils" (upwelling events on the surface). Strong boil events are observed on the surface of the lobe planes of 3D dunes more frequently than in the saddle planes. They also appear more frequently than in the corresponding 2D geometry. The crestline alignment of the dune alters the dynamics of the flow structures, in that they appear in the lobe plane and are advected towards the saddle plane of the next dune, where they are dissipated. Boil events occur at a higher frequency in the staggered alignment, but with less intensity than in the in-phase alignment. Permanent repository link

Numerical simulation of sand waves in a turbulent open channel flow

“…Therefore, our results suggest that LES over frozen bedforms, such as those reported by Omidyeganeh & Piomelli (2013b), constitutes a viable and more economical approach to study the structure of turbulence during various stages of bedform development. The coupled hydro-morphodynamic method we have developed herein can be used to obtain realistic 3D sand wave topologies, which can then be used as input to high-resolution LES to study the details of the turbulent flow over the frozen bed and during various stages of sand wave development.…”

Direct numerical simulation of the flow around an aerofoil in ramp-up motion

“…24,25,30 However, to further corroborate the predictive capabilities of the code, we report a new validation of the flow over an aerofoil, where we compare our numerical results with the ones obtained by Lehmkuhl et al 34 and by Rodriguez et al 35 In particular, we have considered a NACA 0012 aerofoil at a chord Reynolds number Re c = 5 × 10 4 at angles of attack α = 5…”

“…Equations (1) and (2) are discretised on a collocated grid using a well-established curvilinear finite volume code. 24,25 The fluxes are approximated by a second-order central formulation, and the method of Rhie and Chow 26 is used to avoid pressure oscillations. The equations are advanced in time by a second-order semi-implicit fractional-step procedure, 27 where the implicit Crank-Nicolson scheme is used for the wall normal diffusive terms, and the explicit Adams-Bashforth scheme is employed for all the other terms.…”

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:…”