Numerical Simulation of Flow over a Rough Bed

Singh, Krishna M.; Sandham, Neil D.; Williams, John

doi:10.1061/(asce)0733-9429(2007)133:4(386)

Cited by 60 publications

(50 citation statements)

References 17 publications

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“…For the two small spacing, high submergence cases (Figs 16b and c), u w increases approximately linearly with distance from the free surface, reaching a peak at the height of the roughness crests. This is consistent with other studies of turbulence in rough-bed flows (Bomminayuni & Stoesser, 2011;Manes et al, 2007;Singh, Sandham, & Williams, 2007;Stoesser & Nikora, 2008). Below the roughness crests u w decreases approximately linearly from the peak until it reaches zero at the channel bed, an observation also made by Stoesser and Nikora (2008).…”

Section: Resultssupporting

confidence: 91%

Free surface flow over square bars at intermediate relative submergence

McSherry

Chua

Stoesser

et al. 2018

Journal of Hydraulic Research

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Results from large-eddy simulations and complementary flume experiments of turbulent open channel flows over bed-mounted square bars at intermediate submergence are presented. Scenarios with two bar spacings, corresponding to transitional and k-type roughness, and three flow rates, are investigated. Good agreement is observed between the simulations and the experiments in terms of mean free surface elevations and mean streamwise velocities. Contours of simulated time-averaged streamwise, streamfunction and turbulent kinetic energy are presented and these reveal the effect of the roughness geometry on the water surface response. The analysis of the vertical distribution of the streamwise velocity shows that in the lowest submergence cases no logarithmic layer is present, whereas in the higher submergence cases some evidence of such a layer is observed. For several of the flows moderate to significant water surface deformations are observed, including weak and/or undular hydraulic jumps which affect significantly to the overall streamwise momentum balance. Reynolds shear stress, form-induced stress and form drag are analysed with reference to the momentum balance to assess their contributions to the total hydraulic resistance of these flows. The results show that form-induced stresses are dominant at the water surface and can contribute significantly to the overall drag, but the total resistance in all cases is dominated by form drag due to the presence of the bars.

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Section: Resultssupporting

confidence: 91%

Free surface flow over square bars at intermediate relative submergence

McSherry

Chua

Stoesser

et al. 2018

Journal of Hydraulic Research

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“…This is in line with general observations that the structure of turbulence is changed strongly only in the region close to the roughness elements, not in the outer layer (Birch & Morrison 2011;Singh et al 2007;Ashrafian & Andersson 2006).…”

Section: Structure Of the Velocity Fieldsupporting

confidence: 92%

Parametric forcing approach to rough-wall turbulent channel flow

Busse

Sandham

2012

J. Fluid Mech.

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The effects of rough surfaces on turbulent channel flow are modelled by an extra force term in the Navier-Stokes equations. This force terms contains two parameters, related to the density and the height of the roughness elements, and a shape function, which regulates the influence of the force term with respect to the distance from the channel wall. This permits a more flexible specification of a rough surface than a single parameter such as the equivalent sand grain roughness. The effects of the roughness force term on turbulent channel flow have been investigated for a large number of parameter combinations and several shape functions by direct numerical simulations. It is possible to cover the full spectrum of rough flows ranging from hydraulically smooth through transitionally rough to fully rough cases. By using different parameter combinations and shape functions it is possible to match the effects of different types of rough surfaces. Mean flow and standard turbulence statistics have been used to compare the results to recent experimental and numerical studies and a good qualitative agreement has been found. Outer scaling is preserved for the streamwise velocity both for the mean profile as well as its mean square fluctuations in all but extremely rough cases. The structure of the turbulent flow shows a trend towards more isotropic turbulent states within the roughness layer. In extremely rough cases spanwise structures emerge near the wall and the turbulent state resembles a mixing layer. A direct comparison with the study of Ashrafian et al. (2004) shows a good quantitative agreement of the mean flow and Reynolds stresses everywhere except in the immediate vicinity of the rough wall. The proposed roughness force term may be of benefit as a wall model for direct and large-eddy numerical simulations in cases where the exact details of the flow over a rough wall can be neglected.

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“…To the best knowledge of the authors, the few numerical analyses of turbulent flows over 3D rough surfaces were conducted only over regular rough geometries. Direct Numerical Simulation (DNS) of the turbulent flow over an idealized gravel bed generated with spheres of uniform size was performed by Singh et al (2007), where the fully rough regime was considered. The simulation confirmed the wall similarity hypothesis.…”

Section: Introductionmentioning

confidence: 99%

Effects of irregular two-dimensional and three-dimensional surface roughness in turbulent channel flows

Marchis

Napoli

2012

International Journal of Heat and Fluid Flow

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a b s t r a c tWall-resolved Large Eddy Simulation of fully developed turbulent channel flows over two different rough surfaces is performed to investigate on the effects of irregular 2D and 3D roughness on the turbulence. The two geometries are obtained through the superimposition of sinusoidal functions having random amplitudes and different wave lengths. In the 2D configuration the irregular shape in the longitudinal direction is replicated in the transverse one, while in the 3D case the sinusoidal functions are generated both in streamwise and spanwise directions. Both channel walls are roughened in such a way as to obtain surfaces with statistically equivalent roughness height, but different shapes. In order to compare the turbulence properties over the two rough walls and to analyse the differences with a smooth wall, the simulations are performed at the same Reynolds number Re s = 395. The same mean roughness height h = 0.05d (d the half channel height) is used for the rough walls.The roughness function obtained with the 3D roughness is larger than in the 2D case, although the two walls share the same mean height. Thus, the considered irregular 3D roughness is more effective in reducing the flow velocity with respect to the 2D roughness, coherently with the literature results that identified a clear dependence of the roughness function on the effective slope (see Napoli et al. (2008)), higher in the generated 3D rough wall. The analysis of higher-order statistics shows that the effects of the roughness, independently on its two-or three-dimensional shape, are mainly confined in the inner region, supporting the Townsend's wall similarity hypothesis. The tendency towards the isotropization is investigated through the ratio between the resolved Reynolds stress components, putting in light that the 3D irregular rough wall induces an higher reduction of the anisotropy, with respect to the 2D case.

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Numerical Simulation of Flow over a Rough Bed

Cited by 60 publications

References 17 publications

Free surface flow over square bars at intermediate relative submergence

Free surface flow over square bars at intermediate relative submergence

Parametric forcing approach to rough-wall turbulent channel flow

Effects of irregular two-dimensional and three-dimensional surface roughness in turbulent channel flows

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