This project sought to quantify the incipient motion, transport and fate of underwater munitions in coastal environments comprised of mobile beds and/or hard bottoms (e.g., sandy and gravel/rock) under a range of relevant hydrodynamic conditions (e.g., waves, currents). The existing underwater phenomenology of munitions expects mobility to be maximized when munitions are proud (i.e., unburied). It has been suggested that the degree of mobility may be orders of magnitude larger when munitions are transported over a hard gravel-like substrate where there is little or no sediment cover (e.g., such as on coral reefs) versus a sandy or muddy bottom. However, there is a dearth of direct observations made under a wide range of controlled hydrodynamics conditions representative of waves and currents. Through an extensive set of detailed large-scale laboratory experiments we develop a more complete picture of the phenomena involved in the entrainment, transport, and fate of underwater munitions, especially on hard substrates of varying roughness.
In this study, the research team observed the incipient motion of surrogate munitions and canonical shapes (cylinders and spheres) through large-scale physical laboratory investigations. Numerous facilities in the Ven Te Chow Hydrosystems Laboratory (VTCHL) at the University of Illinois at Urbana-Champaign (UIUC) were used to examine munitions behavior under unidirectional and oscillatory flows. Through an extensive set of detailed large-scale laboratory experiments the research team developed a more complete picture of the phenomena involved in the entrainment, transport, and fate of underwater munitions, especially on hard substrates of various roughnesses. The laboratory experiments allowed for detailed measurements over a controlled range of conditions (e.g., hydrodynamic forcing, turbulence characteristics, bed composition, and properties of munitions) which are not practically possible to achieve in field experiments and cannot be completely simulated with numerical models due to the high Reynolds number and wide-range of bottom roughness observed under field conditions.
Work presented herein provides physical laboratory results for initiation of motion and transport of various surrogate munitions over various hard substrates having different roughness (smooth PVC, pitted steel, 1.4 cm marbles, 3.5 cm gravel, and 3.81cm spheres) in unidirectional flow and oscillatory flows. In addition, based on flows resulting in initiation of motion, particle image velocimetry (PIV) measurements were conducted in both unidirectional and oscillatory flows to resolve the flow structure and estimate drag forces to ultimately help develop relationships to predict initiation and transport of munitions. Leveraging the prior experience with scour and burial of mines, the results of the physical munition experiments presented herein provides critical data as well as newly developed initiation of motion predictors to improve existing field scale models for munitions mobility and eventually integration into the Underwater Munitions Expert System (UnMES), (Rennie and Brandt, 2015).
This project generated a comprehensive and detailed data set for the six degrees of freedom of various munitions as they roll, slide, and bounce over the substrate (across a broad range of forcing conditions and substrates). These findings significantly enhance ongoing SERDP field and numerical efforts by providing high-resolution, time-domain trajectories of munitions under well controlled experimental conditions. The laboratory data is a valuable complement to existing field experiments and augments the data available to test, calibrate, and validate predictive numerical models. The advantage of these laboratory experiments is greater experimental control of environmental conditions (both flow characteristics and substrate properties) as well as higher precision measurements than can be obtained in the field where the flow conditions are constantly changing with time.