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SPECIAL SECTION ON RANS/LES/DES/DNS: THE FUTURE PROSPECTS OF TURBULENCE MODELING

# Coherent Structures In Open-Channel Flows Over a Fixed Dune

[+] Author and Article Information
Wusi Yue

Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore, Maryland 21218yue@jhu.edu

Ching-Long Lin, Virendra C. Patel

Department of Mechanical and Industrial Engineering, and IIHR-Hydroscience and Engineering, The University of Iowa, Iowa City, Iowa 52242

J. Fluids Eng 127(5), 858-864 (Feb 27, 2005) (7 pages) doi:10.1115/1.1988345 History: Received July 22, 2004; Revised February 27, 2005

## Abstract

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free surface motion is simulated using level set method. Two subgrid scale models, namely, dynamic Smagorinsky model and dynamic two-parameter model, are employed for assessing model effects on the free surface flow. The present numerical predictions of mean flow field and turbulence statistics are in good agreement with experimental data. The mean flow can be divided into two zones, an inner zone where turbulence strongly depends on the dune bed geometry and an outer layer free from the direct influence of the bed geometry. Streaky structures are observed in the wall layer after flow reattachment. Quadrant two events are found to prevail in near-wall and near-surface motions, indicating the predominance of turbulence ejections in open-channel flows. Large-scale coherent structures are produced behind the dune crest by a strong shear layer riding over the recirculation zone. These quasistreamwise tubelike vortical structures are transported downstream with the mean flow and most are destructed before arriving at the next crest. Free surface deformation is visualized, demonstrating complex patterns of upwelling and downdraft.

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## Figures

Figure 1

Schematic of geometry of open-channel flow over a two-dimensional dune (not to scale)

Figure 2

Grid used in numerical simulations

Figure 3

Comparison of mean streamwise velocity profiles at selected longitudinal stations. Dashed-dotted lines represent predicted free surface positions.

Figure 4

Comparison of mean vertical velocity profiles at selected longitudinal stations. Dashed-dotted lines represent predicted free surface positions.

Figure 5

Comparison of streamwise component of turbulence intensities at selected longitudinal stations. Dashed-dotted lines represent predicted free surface positions.

Figure 7

Comparison of spanwise component of turbulence intensities at selected longitudinal stations. Dashed-dotted lines represent predicted free surface positions.

Figure 11

Instantaneous isosurface of λ2=−200. Shadow areas represent instantaneous free surface positions. Labels A, B, and C mark different large-scale structures.

Figure 10

Instantaneous velocity fluctuation fields of u′ and w′ in the middle plane of the channel. Dashed lines represent the instantaneous free-surface positions. Q2 and Q4 stand for quadrant two and four events, respectively.

Figure 9

Contours of time-averaged Reynolds shear stress at the middle plane of the channel. The dashed lines represent time-averaged free surface positions.

Figure 8

Contours of time-averaged turbulent kinetic energy at the middle plane of the channel. The dashed lines represent time-averaged free surface positions.

Figure 6

Comparison of vertical component of turbulence intensities at selected longitudinal stations. Dashed-dotted lines represent predicted free surface positions.

Figure 12

Close-up view of structures A, B, and C in Fig. 1

Figure 13

Contours of three components of velocity fluctuations at the wall layer zb+=9. (a) u′; (b) w′.

Figure 14

Instantaneous free-surface patterns. U and D stand for upwelling and downdraft, respectively.

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