Experimental and Numerical Investigation of Grain Shape Effects on Munition Mobility
Sylvia Rodriguez-Abudo | University of Puerto Rico - Mayaguez
Munition mobility, burial and reemergence are closely related to the dynamics of sediment transport occurring at the wave-bottom boundary layer of the littoral zone. For the case of non-cohesive sediment beds, predictive models of sediment dynamics have strongly relied on formulations developed for rounded grains, which closely resemble silica sands. However, in tropical littoral zones, non-cohesive 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 types of sediments increases intergranular friction thereby hindering sediment mobilization. The goal of this investigation is to accurately quantify the effects of grain shape and angularity on munition mobility under the presence of oscillating fluid motion. This investigation is organized around the following scientific objectives:
1) Quantify the role of grain shape and angularity on munition mobility, burial and reemergence.
2) Assess the validity and/or propose modifications to current predictive models addressing munition mobility for irregularly shaped grains.
3) Resolve the physics of flow entrainment at the grain-scale and characterize its fundamental differences for spherical vs irregularly shaped substrates.
Detailed full-scale laboratory observations of the nearbed velocity field, bed geometry, and munition movement will be collected at University of Puerto Rico Mayaguez’s oscillating boundary layer apparatus to accurately quantify the effects of grain shape and angularity on the incipient motion, burial and reemergence of munitions, and assess the hydrodynamic and munition characteristics that dictate these processes in irregularly-shaped grains. The experimental program will be complemented by a numerical modeling effort by the University of Texas-Dallas team that seeks to study the grain-scale physics that cannot be resolved in the experimental facility. Simulations will be performed using a Direct Numerical Simulation-Large Eddy Simulation scheme with immersed boundaries and two-way coupling of the fluid and solid phases. The two groups will work in synergy to inform numerical simulations based on: 1) a thorough sediment characterization of the control and variable groups focused on grain shape and angularity; and 2) assessments of key experimental results that require further understanding of the fundamental physics at the grain-scale level.
This investigation focuses on the effects of grain angularity and shape on the incipient motion, bedload fluxes, bed form development, evolution, and resulting nearbed hydrodynamics under oscillating flow conditions. This study focuses on the fundamental understanding of these phenomena in coastal regions, as it pertains to appropriate predictions of munition mobility, burial and reemergence. The knowledge gathered through this study seeks to inform the way current efforts, such as the Underwater Munitions Expert System, predict mobility, burial and re-exposure of underwater munitions in tropical settings. Attention should be given to the fact that angular sediment substrates are common in tropical areas. Such areas of particular interest to the Navy include: Puerto Rico’s Vieques and Culebra islands, where decades of Navy training left behind thousands of unexploded ordnances and numerous ship wrecks; active bases in Hawaii, Guam, Singapore, Bahrain, and Cuba; and any other new sites of potential interest.