Development of a Cartesian-grid-based solver for an oscillatory boundary layer over a rough wall

Graduation date: 2014Predominant models for predicting rates of sediment transport face acute shortcomings when applied to coastal boundary layers. This is due to a neglect of the web of stochastic variables governing the rate of sediment dislodgement. While stochastic models do exist, the parametric extent of their validity tends to be limited, and none have taken into account an understanding of phase dependence in application to oscillatory flow, likely because the existing knowledge of the evolution of flow properties throughout a cycle of wave motion is insubstantial. A detailed understanding of the statistical properties of sediment forces and motion is a precondition to the development of specific models for oscillatory flows. Experiments on such flows tend to be limited by the small length and time scales of the particles. Numerical simulations offer flexibility in measuring many properties simultaneously in hard-to-reach places without disturbing the delicate dynamics of particle ejection. Fully resolved simulations of purely oscillatory flow over an idealized sediment geometry were performed at moderate parameter ranges near the transition to turbulence. Due to the computational challenges posed by this flow type, a new structured, fully parallelized, incompressible-flow, finite-volume solver along with effective and generalized immersed-boundary tools was developed and validated against benchmark simulations. Turbulence statistics and their correlation with the statistics of forces on the sediment bed are analyzed. Large divergences from Gaussian behavior are found in the bed velocity during accelerating phases of the cycle, and the probability distribution functions of fluctuations in the bed-flow velocity, u²[subscript b], and lift appear to follow this trend. The results suggest that coherent structures thought to be linked to sediment ejection in laminar flow regimes have a diminished effect on particle forces in transitional and early turbulent regimes. The implications of these findings on model development will be discussed