Upscaling of bottomgenerated turbulence in largescale 3D models for sediment transport in estuaries and coastal zones
Toorman, E. A.; Widera, P.; Heredia, M.; Lacor, C. (2008). Upscaling of bottomgenerated turbulence in largescale 3D models for sediment transport in estuaries and coastal zones. Geophys. Res. Abstr. 10: EGU2008A03532
In: Geophysical Research Abstracts. Copernicus: KatlenburgLindau. ISSN 10297006; eISSN 16077962
 
Beschikbaar in  Auteurs 

Documenttype: Samenvatting

Auteurs   Top 
 Toorman, E. A.
 Widera, P.
 Heredia, M.
 Lacor, C.



Abstract 
Currently used 3D numerical sediment transport models still fail to make good quantitative predictions. To a great extent, this can be attributed to the inadequate description of physical processes which occur at the subgrid scale level. From flume experiments it is known that particleturbulence interactions near the bed significantly change the effective roughness experienced by the overlying water column. This results in different transport rates if not accounted for.From a theoretical perspective, bed load transport, sheet flow and fluid mud flow are all occurrences of supersaturated suspension flow in the inner nearbed layer comprising the viscous sublayer and the transient layer. Its thickness increases with sediment load, since particleparticle interactions (fourway coupling effects) consume considerable amounts of the available stream power. In order to know how much energy is left over to compute the transport capacity of the outer, fullydeveloped layer, it is necessary to quantify the energy budget in the inner layer.This is a difficult task. Every modelling approach has its drawbacks and limitations. Lagrangean particle tracking is hopeless, since the required number of particles to approach field conditions is much too high, and the volumes occupied by the particles cannot be neglected. Grain sizes are nonuniform in nature and concentrations near the bed very high, making it very difficult to give an accurate description of the momentum exchange between fluid and solid phase, which accounts for particle collisions. Therefore, in view of largescale applications, a onefluid approach is adopted. This implies that the momentum equation is solved for the suspension, together with a turbulence closure model and the sediment mass balance.Since the thickness of the supersaturated inner layer mostly is very small relative to the water depth and the vertical discretization in large scale applications, it is not possible to resolve this layer with a traditional lowReynolds model approach, which requires a very fine grid. A new approach is proposed, where a modified Prandtlmixing length (PML) model is used for the bed layer, and a new lowReynolds model is applied in the outer layers. In this way it is possible to obtain a correct behaviour for tidal oscillating flow in estuaries, where lowRe effects enter high in the water column during slack water.The correction factor for the PML eddy viscosity and the damping functions for the lowRe kepsilon turbulence model are constructed based on theoretical constraints, DNS and LES generated data, as well as experimental flume data. In parallel, LES and improved twolayer lowRe models are developed to simulate flow over rough bottoms without and with sediment, in order to generate data very close to the bed surface, where no measurements can be made. These additional data are used to help interpret experimental flume data, which always show relatively high experimental errors, and to extend the new damping functions for the cases with bottom roughness and suspended sediment.Preliminary results of the new coarse grid RANS model for openchannel flow with various roughness conditions without and with suspended sediment will be shown, compared to LES results for flow over a wavy bottom, lowReynolds RANS results over rough bottom and experimental flume data. 
