The project team presents findings from laboratory studies performed from a SERDP exploratory development project to explore bed fluidization in response to homogeneous isotropic turbulence. This study aims to quantify the penetration of energetic fluid motions into a porous sediment bed and to understand the relationship between turbulence, bed fluidization, and development of ripples. In turn, the project team aims to determine whether buried model unexploded ordnance (UXO) can be exhumed or exposed in such flow conditions, and whether there are changes to the bathymetry in the proximity of buried UXO.
Experiments were performed in a facility designed to generate homogeneous isotropic turbulence in the absence of mean shear, as an idealized fundamental component of nearshore swash flows. Turbulence was generated in water via a randomly actuated synthetic jet array, and a sediment bed was placed at the bottom of the tank. Non-invasive optical measurements were used to characterize the turbulent flow above the bed. Time lapse photography and video recordings are used to capture the development of bedforms and of sediment suspension events. Several methods were attempted for exploring within-bed dynamics, including pressure measurements, analog porosity measurements, and sediment freeze coring of layered colored sands. The project team made measurements both with and without buried model UXO for three different 3D printed models.
The project team performed experiments for five different turbulence cases and presents profiles of the flow statistics including mean flow strength and turbulent kinetic energy. The team saw evidence of suspension in the form of splats, or vortex ring impingement on the sediment bed, as well as bed-attached vortices. Ripple fields are quantified as they evolve in time and space; interestingly, the team did not observe differences to the generation of ripples for tests with shallowly-buried model UXO for the cases considered. Freeze coring analysis suggested that fluidization is occurring, yet may be limited in terms of the depth at which fluid motions and corresponding pressure gradients penetrate. Further investigations are required to provide more refined analysis of near-bed sediment dynamics.
The knowledge gained from this study allows for an improvement in predictions of burial depth and likelihood for munitions exposure in environmental settings dominated by turbulence above a non-cohesive sediment bed. Given evidence in complementary numerical and field studies suggesting munitions burial is influenced by instantaneous liquefaction and fluidization, in addition to burial by migrating sediments, it is necessary to understand the within-bed dynamics in highly turbulent flow conditions.