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

Abstract

Large-eddy simulation of turbulent open-channel flow over threedimensional dunes ABSTRACT A large-eddy simulation study has been undertaken to investigate the turbulent structure of openchannel flow over three-dimensional (3D) dunes. The governing equations have been discretised using the finite volume method, with the partial cell treatment being implemented in a Cartesian grid form to deal with the 3D dune topography. The numerical model predicted free surface elevations, mean flow velocities and Reynolds shear stress distributions have been compared with experimental measurements published in the literature, with relatively close agreement being obtained between the two sets of results. The predicted mean velocity field and the associated turbulence structure are significantly different from those observed for flows over two-dimensional dunes. Due to the dune three-dimensionality, the model predictions show spanwise variations of mean flow fields, secondary currents and different distributions of vertical profiles of the double-averaged velocity. Furthermore, large-scale vortical structures, such as spanwise rollers and hairpin-like structures, are predicted in the simulations, with most of them being generated in the concave regions of the 3D dunes.

Large‐Eddy Simulation of Turbulent Oscillatory Flow Over Three‐Dimensional Transient Vortex Ripple Geometries in Quasi‐Equilibrium

“…The code used here has already been validated by simulating the oscillatory and combined oscillatory-current flows over a 2-D rippled bed, identical to the experimental studies of Fredsøe et al (1999); these validation results were presented in Grigoriadis et al (2012). In addition, LES of unidirectional flow over 3-D dune geometries, which was studied extensively in Maddux et al (2003) and Xie et al (2013), was also performed; these validation cases were presented in Chalmoukis and Dimas (2017) where the effect of the 3-D bed to the flow was reproduced accurately.…”

Large-eddy simulation of turbulent free surface flow over a gravel bed

“…The code Xdolphin3D was validated and applied for some LES studies of open-channel flows over well-defined regular roughness Xie et al, 2014;Xie, Lin, Falconer, & Maddux, 2013). In addition, the present model has been validated against available experimental data for a range of free surface flow problems, such as overturning waves over a sloping beach and a reef (Xie, 2012), periodic breaking waves in the surf zone (Xie, 2013), breaking solitary waves over 3D conical structures (Xie & Stoesser, 2020b) and complex topography (Xie, 2015), in which the overturning jet, air entrainment and splash-up have been captured during wave breaking.…”

“…In the simulation, same mesh sizes were used in the horizontal plane and they were approximately 2.1 times the mesh size in the vertical direction (∆x = ∆z ≈ 2.1∆y). Approximately 24 cells were selected to cover the height of the rough bed in the vertical direction, which has been shown to capture most of the turbulent structures in the rough boundary (Xie et al, 2014;Xie, Lin, Falconer, & Maddux, 2013). A time step with Courant-Friedrichs-Lewy (CFL) number of 0.1 was used in the simulation in order to accurately capture the free-surface dynamics.…”

A two-phase flow model for three-dimensional breaking waves over complex topography

“…The model can act as a complementary approach to experimental investigations to gain further insight into the kinematics and dynamics of three-dimensional breaking waves. It is worth noting that the VOF scheme can also be switched off in the present model when considering the rigid lid approximation in open-channel flows [76][77][78]. Further work will include the surface tension effect for bubble generation and adaptive mesh [79], which can reduce computational efforts without sacrificing accuracy.…”