Large-Eddy Simulation of Turbulent Flow over a Fixed Two-Dimensional Dune

Large-eddy simulation ͑LES͒ is performed of a curved open-channel flow over topography based on the laboratory experiment by Blanckaert ͓"Topographic steering, flow circulation, velocity redistribution and bed topography in sharp meander bends," Water Resour. Res., doi:10.1029/ 2009WR008303 ͑in press͔͒. In the experiment, the large-scale bed topography had developed to a more or less stationary shape which was prescribed in the LES model as boundary conditions neglecting the small-scale dune forms by means of a straightforward immersed boundary scheme in combination with a simple wall-modeling approach. The small-scale dunes are accounted for in the numerical model by means of parametrization. Sensitivity of the flow to this roughness parametrization is examined by simulating the flow for three different roughness heights. It was found that, notwithstanding the coarse method of representing the dune forms, the qualitative agreement of the experimental results and the LES results is rather good. This good agreement is explained by the minor importance of turbulence stress gradients in the contribution to the transverse and streamwise momentum balance. Moreover, it is found that in the bend the structure of the Reynolds stress tensor shows a tendency toward isotropy which enhances the performance of isotropic eddy viscosity closure models of turbulence. This observation is remarkable since highly anisotropic turbulence might well be expected considering the complex nature of the geometry. Furthermore, the LES results reveal a pronounced recirculation zone near the convex inner bank of the flume due to the shallowness of the flow and strong curvature of the flume. At the interface between the recirculation zone and the main flow, a curved mixing layer is identified as well as strong upwelling flow motion that is accompanied with large production of turbulent kinetic energy.

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“…As an example, the open-channel flow over a fixed twodimensional dune studied by Yue et al 32 is chosen ͑see Fig. 16͒.…”

Section: Example: Open-channel Flow Over a Fixed Dunementioning

confidence: 99%

“…17 for the numerical results on the two grids as well as for the experimental results Ref. 32. The values are scaled by means of the free surface velocity at the dune crest U.…”

Section: -16mentioning

confidence: 99%

Large-eddy simulation of a curved open-channel flow over topography

2010

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“…The top surface is assumed to be rigid and free of shear stress: the vertical velocity is set to zero, as are the vertical derivatives of the streamwise and spanwise velocity components. The surface deformation is reported to be small, less than 2% of the flow depth in experiments (McLean et al 1994;Kadota & Nezu 1999;Maddux et al 2003b,a) and simulations (Yue et al 2006;Stoesser et al 2008) of 2D and 3D dunes, and it was shown that the free surface response to the 3D dune bed is 2D (Maddux et al 2003b;Omidyeganeh & Piomelli 2013). Therefore, the free-slip condition is expected to have small e↵ects on the statistics and on the evolution of significant structures.…”

Section: Problem Formulationmentioning

confidence: 97%

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

Omidyeganeh

Piomelli

2013

J. Fluid Mech.

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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

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“…This rigid lid approach has been used in various research papers to model the turbulent flow in channels [48][49][50]. It is also possible to model channels with a rigid but sloping lid [51,52], where the slope corresponds to a drop of 1/1000, for example, as observed in the field.…”

Section: Domainmentioning

confidence: 99%

Computational Fluid Dynamics and Visualisation of Coastal Flows in Tidal Channels Supporting Ocean Energy Development

et al. 2015

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Flow characteristics in coastal regions are strongly influenced by the topography of the seabed and understanding the fluid dynamics is necessary before installation of tidal stream turbines (TST). In this paper, the bathymetry of a potential TST deployment site is used in the development of the a CFD (Computational Fluid Dynamics) model. The steady state k-and transient Large Eddy Simulation (LES) turbulence methods are employed and compared. The simulations are conducted with a fixed representation of the ocean surface, i.e., a rigid lid representation. In the vicinity of Horse Rock a study of the pressure difference shows that the small change in height of the water column is negligible, providing confidence in the simulation results. The stream surface method employed to visualise the results has important inherent characteristics that can enhance the visual perception of complex flow structures. The results of all cases are compared with the flow data transect gathered by an Acoustic Doppler Current Profiler (ADCP). It has been understood that the k-method can predict the flow pattern relatively well near the main features of the domain and the LES model has the ability to simulate some important flow patterns caused by the bathymetry.

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Large-Eddy Simulation of Turbulent Flow over a Fixed Two-Dimensional Dune

Cited by 50 publications

References 27 publications

Large-eddy simulation of a curved open-channel flow over topography

Large-eddy simulation of a curved open-channel flow over topography

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

Computational Fluid Dynamics and Visualisation of Coastal Flows in Tidal Channels Supporting Ocean Energy Development

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