Development of the two-fluid turbulence-resolving approach for sediment transport applications

Overview & Synopsis

Considering the importance of the cross-shore sand transport under intense flow conditions on the evolution of beaches' morphology, understanding physical mechanisms at small scales such as particle-turbulence interactions involved in sediment transport under waves is an important step toward accurate predictions of the coastline modifications at large scales. In this context, development of turbulence resolving modeling approaches is an extremely active topic among the scientific community. However, simulations using classical Lagrangian modeling methodologies, for which the trajectories of individual particles are tracked, are computationally unfeasible given the large amount of particles transported in the flow.In this thesis, an Eulerian turbulence-resolving two-phase flow model (or two-fluid model) is developed. In this modeling approach, both the carrier phase and the dispersed phase are seen as continua avoiding the limitations in term of number of particles. The model is successfully applied to oscillatory sheet flow configurations under a symmetric sinusoidal flow forcing. Numerical results allowed to confirm that bed instabilities and turbulence modulation by the presence of the particles contributes to unsteady effects, namely phase-lag effect and enhanced boundary layer thickness for fine sand.The two-fluid model is then applied to oscillatory sheet flow configurations involving velocity skewed and acceleration skewed flow forcing to study wave shape effects. Comparison between experimental measurements and numerical predictions of the wave averaged net sediment flux showed that the two-fluid model is able to accurately capture the flow dynamics, especially the change of sign of the net flux for fine sand under velocity skewed waves. These results allowed to further confirm the observations made from configurations involving a symmetric sinusoidal flow forcing.From simulations of configurations involving medium sand, some discrepancies regarding the concentration at flow reversal are observed. It is hypothesized that these discrepancies are the result of an absence of separation of scales between the particles and the smallest turbulent flow scales. For such particle and flow conditions, the classical interaction laws between the two phases do no longer hold and should be modified to take into account finite-size effects.To extend the range of applicability of the two-fluid model to configurations involving particles larger than the smallest turbulent flow scales, a finite-size correction model is proposed and validated against experimental configurations involving particle-laden unidirectional boundary layer flow in the dilute regime. This model is based on the assumption that the turbulent flow scales larger than the particle diameter will contribute to the resolved relative velocity between the two phases, whereas eddies smaller than the particle diameter will have two effects: (i) they will reduce the particle response time by adding a sub-particle scale eddy viscosity to the drag coefficient, and (ii) they will contribute to increase the production of granular temperature. Integrating finite-size effects in the two-fluid model allows us to quantitatively predict the concentration profile for heavier-than-fluid particles without any tuning parameter.