Effect of bed dunes on spatial development of open-channel flow

Abstract

The spatial development of turbulent, sub-critical open-channel flow over five identical dunes is studied by the numerical solution of the RANS equations utilizing the VOF free-surface formulation and the k-ε or Spalart-Allmaras turbulence models. Results are presented for smooth and rough walls and several dune dimensions. One of the cases was also studied experimentally. The flow separation at each dune crest generates a recirculation in the dune lee-side and reattachment at a distance, which increases with increasing dune height and decreasing dune length. Turbulence does not fully develop over the dunes, while the majority of turbulent kinetic energy production takes place in the recirculation region. The spatially mean, free-surface level decreases in the flow direction over the dunes, while the free-surface amplitude increases with increasing dune height. The dune drag coefficient increases with increasing dune height, while the contribution of form resistance on the drag increases with increasing dune height and decreasing dune length. RÉSUMÉLe développement spatial de l'écoulement turbulent et fluvial dans un canal à surface libre avec cinq dunes de fond identiques est étudié par la résolution numérique des équations RANS utilisant la formulation VOF à surface libre et les modèles de turbulence k-ε ou de Spalart-Allmaras. Les résultats sont présentés pour des parois lisses et rugueuses et plusieurs dimensions de dunes. Un des cas a été également étudié expérimentalement. La séparation de l'écoulement à chaque crête dunaire produit une recirculation à l'aval de la dune et un recollement à une distance, qui augmente avec la hauteur de la dune et quand sa longueur diminue. La turbulence ne se développe pas entièrement au-dessus des dunes, alors que la majorité de la production d'énergie cinétique turbulente a lieu dans la région de recirculation. Le niveau moyen de surface libre diminue dans le sens de l'écoulement au-dessus des dunes, alors que l'amplitude de la surface libre augmente avec la taille dunaire. Le coefficient de traînée augmente avec la hauteur de la dune, alors que la contribution de la résistance de forme à la trainée augmente avec la hauteur de la dune et quand sa longueur décroit.

Reshaping of the hyporheic zone beneath river restoration structures: Flume and hydrodynamic experiments

“…[14] The CFD model has been verified for flume surface hydraulics around complex bedforms in our prior research [Zhou and Endreny, 2012]; however, we further verified the CFD for turbulent surface flow using experimental data from van Mierlo and de Ruiter's [1988] flume experiment ''Run T6.'' These data have been widely adopted for CFD turbulent model verifications [e.g., Cardenas and Wilson, 2007;Dimas et al, 2008;Yoon and Patel, 1996]. The Run T6 flume experiment investigated the flow surface elevation, velocity profiles, and turbulence fluctuations over a fixed, impermeable, repeating dune geometry.…”

“…These data have been widely adopted for CFD turbulent model verifications [e.g., Cardenas and Wilson, 2007;Dimas et al, 2008;Yoon and Patel, 1996]. The Run T6 flume experiment investigated the flow surface elevation, velocity profiles, and turbulence fluctuations over a fixed, impermeable, repeating dune geometry.…”

Large-eddy simulation of turbulent open-channel flow over three-dimensional dunes

“…Overall, the modelled Reynolds shear stresses u w ¢ ¢ were in reasonable agreement with the experimental measurements, although the values were overpredicted in the separation region and wake layer. This discrepancy might be partly attributed to the high velocity gradient predicted in the flow in the separation zone and partly due to the fact that the periodic boundary conditions used in the simulation were different from the actual flow condition in the experiment, which was not exactly periodic in the streamwise direction (Maddux et al, 2003b) and which could have enhanced the turbulence levels in the simulations, with this phenomena having been observed in a previous study of flow over 2D dunes by Dimas et al (2008). Figure 9 shows a comparison between the predicted streamwise-averaged mean streamwise velocities and secondary currents and the corresponding experimental measurements, along with dashed lines denoting the height and phase change of the two dune crest lines.…”

The Impact of Nonequilibrium Flow on the Structure of Turbulence Over River Dunes

“…The length of flow separation over dunes has previously been shown to vary with hydraulic and geometric conditions, such as dune height and crestal shape (Araújo et al, 2013;Dimas et al, 2008;Engel, 1981;Kwoll et al, 2016;Simpson, 1989), leeside angle (Best & Kostaschuk, 2002;Kwoll et al, 2016), flow depth (Balachandar et al, 2007), flow velocity (Araújo et al, 2013;Engel, 1981), downstream bedform shape and distance (Dimas et al, 2008;Engel, 1981), and crestline curvature (Venditti, 2007). Such an array of controls has resulted in the parametrization of flow separation length tending to be achieved using empirically defined relationships derived from specific data sets, and thus not necessarily providing a fundamental description of the multitude of processes involved (e.g., Lefebvre et al, 2014;Paarlberg et al, 2007).…”