Coastal Bottom Boundary Layers: Transport Phenomena and Grain-Scale Dynamics*

At the Coastal Boundary Layers Laboratory we use experimental fluid dynamics to better understand fundamental problems concerning unsteady turbulent boundary layers. While our team primarily focuses on coastal processes, our capabilities allow us to work on complex fluid mechanics problems involving rough boundary layers, transport and ablation phenomena. In this talk I will discuss three projects currently taking place at the lab. The first examines the combined effect of complex roughness scales and spatially varying mass sources on scalar transport across the sediment-water interface under oscillating flows. It builds upon previous findings that demonstrate a high variability of momentum fluxes across ripple wavelengths, and explores the effect of preferential organization of solutes on the overall mass exchange. Through a combination of detailed laboratory experiments (using PIV, PLIF, and an Oscillating Boundary Layer Apparatus) and a modeling component under the SedFOAM framework, this project sheds light into the complex intra-ripple mixing structure resulting from a combination of unsteady hydrodynamic forcing, bedform-induced coherent motions, and nonuniform mass sources. The second and third efforts focus on grain-scale dynamics within coastal environments. Predictive models of sand transport have strongly rely on formulations developed for spherical grains, which closely resemble silica sands. However, in tropical zones, sediments are usually composed of calcareous sand exhibiting coral, shells and other marine fragments with a variety of shapes and angular features. The irregular nature of these sediments increases intergranular friction thereby hindering sediment mobilization. In these projects, we quantify the effects of grain shape and angularity on the initiation of sediment motion, bedform development and evolution, bedload transport and munition mobility under the presence of waves.