Journal ArticleDOI
Numerical simulation of sand waves in a turbulent open channel flow
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TLDR
In this paper, a coupled hydro-morphodynamic numerical model was developed for simulation of stratified, turbulent flow over a mobile sand bed. The model is based on the curvilinear immersed boundary approach of Khosronejad et al. and is applied to simulate sand wave initiation, growth and evolution in a mobile bed laboratory open channel.Abstract:
We develop a coupled hydro-morphodynamic numerical model for carrying out large-eddy simulation of stratified, turbulent flow over a mobile sand bed. The method is based on the curvilinear immersed boundary approach of Khosronejad et al. (Adv. Water Resour., vol. 34, 2011, pp. 829–843). We apply this method to simulate sand wave initiation, growth and evolution in a mobile bed laboratory open channel, which was studied experimentally by Venditti & Church (J. Geophys. Res., vol. 110, 2005, F01009). We show that all the major characteristics of the computed sand waves, from the early cross-hatch and chevron patterns to fully grown three-dimensional bedforms, are in good agreement with the experimental data both qualitatively and quantitatively. Our simulations capture the measured temporal evolution of sand wave amplitude, wavelength and celerity with good accuracy and also yield three-dimensional topologies that are strikingly similar to what was observed in the laboratory. We show that near-bed sweeps are responsible for initiating the instability of the initially flat sand bed. Stratification effects, which arise due to increased concentration of suspended sediment in the flow, also become important at later stages of the bed evolution and need to be taken into account for accurate simulations. As bedforms grow in amplitude and wavelength, they give rise to energetic coherent structures in the form of horseshoe vortices, which transport low-momentum near-bed fluid and suspended sediment away from the bed, giving rise to characteristic ‘boil’ events at the water surface. Flow separation off the bedform crestlines is shown to trap sediment in the lee side of the crestlines, which, coupled with sediment erosion from the accelerating flow over the stoss side, provides the mechanism for continuous bedform migration and crestline rearrangement. The statistical and spectral properties of the computed sand waves are calculated and shown to be similar to what has been observed in nature and previous numerical simulations. Furthermore, and in agreement with recent experimental findings (Singh et al., Water Resour. Res., vol. 46, 2010, pp. 1–10), the spectra of the resolved velocity fluctuations above the bed exhibit a distinct spectral gap whose width increases with distance from the bed. The spectral gap delineates the spectrum of turbulence from the low-frequency range associated with very slowly evolving, albeit energetic, coherent structures induced by the migrating sand waves. Overall the numerical simulations reproduce the laboratory observations with good accuracy and elucidate the physical phenomena governing the interaction between the turbulent flow and the developing mobile bed.read more
Citations
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A numerical approach for simulating fluid structure interaction of flexible thin shells undergoing arbitrarily large deformations in complex domains
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Journal ArticleDOI
Immersed boundary methods for fluid-structure interaction: A review
Woojin Kim,Haecheon Choi +1 more
TL;DR: In this paper, the authors introduce immersed boundary (IB) methods for fluid-structure interactions (FSIs) of rigid and elastic bodies, which impose momentum forcing on an Eulerian mesh to satisfy boundary conditions on the interface between fluid and structure, which enables us to use a non-body conforming grid system for complex-shaped moving bodies.
Journal ArticleDOI
Formation of sediment patterns in channel flow: minimal unstable systems and their temporal evolution
TL;DR: In this paper, the authors investigated the phenomenon of sediment pattern formation in a channel flow by performing simulations which resolve all the relevant length and time scales of the problem, and found that the cutoff length for pattern formation lies in the range 75-100 times the particle diameter (3-4 times the clear fluid height).
Turbulence in Open-Channel Flows
TL;DR: A review of open channel turbulence, focusing especially on certain features stemming from the presence of the free surface and the bed of a river, can be found in this article, where the statistical theory of turbulence and coherent structures in open channel flows and boundary layers are discussed.
Journal ArticleDOI
Influence of permeable beds on hydraulically macro-rough flow
TL;DR: In this article, the influence of bed permeability on the turbulence dynamics and structure is investigated, and it was observed that the scales of the spanwise vortical structures over more permeable granular beds are larger than those over less permeable beds.
References
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