Ian Teakle

Turbulent diffusion of momentum and suspended particles: A finite-mixing-length theory

A finite-mixing-length theory is presented for turbulent mixing. This theory contains Fickian diffusion as the limiting case for lm/L→0, where lm is the mixing length and L is the scale of the distribution under consideration. The new model is of similar generality to that of Taylor (1921), “Diffusion by continuous movements.” However, while Taylor’s model, being strictly Lagrangian, is difficult to apply to inhomogeneous scenarios, the new model is Eulerian and easily applicable to bottom boundary layers and other inhomogeneous flows. When applied to steady suspended sediment concentrations c(z), the theory predicts the observed trend of apparent Fickian diffusivities eFick=−wsc/(dc/dz) being larger for particles with larger settling velocity ws, in a given flow. The corresponding nature of the ratios between apparent Fickian sediment diffusivities and eddy viscosities, β=eFick/vt, for different particles in the same flow, is also revealed. That is, β is an increasing function of the particle settling ve...

Modelling sheet flow sediment transport using convolution integrals

Existing simple, practical models used to predict the transport rate of cohesionless sediment in the sheet flow regime have obvious shortcomings. They generally relate the transport rate directly to the velocity just outside the boundary layer or to an estimate of the instantaneous bed shear stress. Such models neglect important unsteady flow effects such as phase lags between the flow forcing and the bed shear stress and between the bed shear stress and the transport rate. Most also fail to account for the influence of the acceleration time history on the magnitude of the bed shear stress. In this paper a new simple, practical model is developed that aims to provide similar prediction accuracy as more complex process-based models without the computational expense. Convolution integrals are employed to obtain solutions for the governing differential equations in the time domain while accounting for the important unsteady flow effects.

Sheetflow Sediment Transport Modeling: Including Boundary Layer Streaming

A two-phase sheetflow sediment transport model is extended to account for boundary layer streaming due to the horizontal non-uniformity present in real-world progressive waves. The model predictions are compared with experimental results obtained in a large-scale wave flume facility (which include boundary layer streaming) and with results from wave tunnel facilities (which do not include boundary layer streaming). The experimental and modeling results both highlight the importance of boundary layer streaming for real-world sediment transport modeling.

Sheet flow sediment transport modelling using convolution integrals

A new method for the prediction of instantaneous sediment transport rates in the sheet flow regime is presented based on the use of convolution integrals. The convolution integral technique allows solution of the governing differential equations in the time domain while accurately representing important unsteady flow effects. This technique overcomes many of the limitations of simple quasi-steady sediment transport models and does not require the computational effort of more detailed process-based models. The present model has been parameterised using results from a more detailed process-based two-phase model. Preliminary results indicate that the technique is successful in predicting net transport rates in oscillatory flow tunnel experiments in the sheet flow regime. Extension of the model to other sediment transport regimes and incorporation of real-wave effects such as boundary layer streaming is possible.