Sediment transport, in rivers and coastal regions, affects large-scale geomorphic processes of dune formation, beach erosion and landform evolution that can displace human settlements, as well as destroy vegetation and agricultural infrastructure with strong socio-economic impact. Large-scale predictive computations of large-scale sediment-laden flows typically employ simplistic models for (i) incipient motion and resultant the bedload transport, and (ii) suspended sediment transport. These models are based on quasi-steady models based on averaged bed-shear stress for erosion onset and advective-diffusive transport based continuum models, neglecting particle inertia and are limited in their ability to accurately predict sediment erosion, entrainment and transport. Lack of accurate criteria for onset of incipient motion and sediment pickup function remain two of the biggest hurdles in developing better predictive models for sediment transport.
A numerical investigation of sediment-bed-turbulence interactions is proposed in oscillatory turbulent boundary layers representative of coastal environments. The primary research objective is to quantify the effect of these interactions on the onset of erosion, entrainment, suspension, and deposition of sediments. The broad range of spatio-temporal scales associated with particle-particle and particle-fluid interactions makes modeling of sediment transport at practical scales extremely challenging. The proposed research builds upon the following main hypotheses: (i) the dynamics of near-wall turbulence structure and resultant variations in the magnitudes and time-scales of the destabilizing drag and lift forces on sediment grains are critical in formulating predictive criteria for onset of erosion, and (ii) particle inertia and variations in suspended sediment concentration due to fluidization and settling of sediments substantially affects the spatio-temporal evolution of the wall-events in the outer as well as inner regions of a turbulent boundary layer. Use of high-fidelity numerical simulations and modeling are proposed to test these hypotheses.
Intellectual Merit: The novelty of this research is in the development and use of a fully resolved simulation (FRS) approach based on first principles, without requiring models for drag and lift forces, for the study of sediment incipient motion. This work will, for the first time, provide data on the temporal variations in the magnitude of drag and lift forces on sediment grains, the time-scales associated with these variations, and their correlation to the sweep-burst events in turbulent boundary layers. Effect of temporal variations in drag and lift forces, due to the sweep-burst turbulence events, on incipient motion will be revealed, identifying the roles of bed shear stress (Shield's criterion) and local accelerations (Sleath's parameter), and impulse (Diplas' concept) on onset of erosion. The FRS studies varying the grain size, flow Reynolds number, and volume loading, will also yield flow parameterizations for development of better analytical and engineering predictions of erosion, scour, transport, and deposits based on continuum approaches. These flow parameterizations obtained from fully resolved numerical experiments have the potential to transform current practices in modeling sediment transport on practical scales.
Broader Impact: The research methodologies and models are applicable to several other fluid-particle systems, such as fluid-structure interactions in biological applications and flapping wings in micro-air vehicles, inertial flow through porous media, oxy-coal combustion, among others. This research will yield a Numerical Water/Wind Tunnel (NWT), a virtual educational tool for fluid-particle systems. Several aspects of the NWT will be integrated into graduate courses and University Honors College (UHC) curriculum. Training workshops for local school teachers and demonstrations to students will be held on the role of simulation-based engineering and science in predictions of river and coastal sediments during the ongoing K-12 outreach activities.
