Investigating munitions mobility in oscillatory flows with Inertial Measurement Units

Understanding the mobility, burial, and re-exposure of underwater munitions is important for the management and remediation of contaminated underwater sites. We have embedded nine degree-of-freedom Inertial Measurement Units (IMUs) into large caliber munitions to investigate their response to various oscillatory forcing and interaction with the seabed. We deployed an 81 mm mortar replica in a large oscillatory water tunnel on a fixed flat bed and measured its response to oscillatory flows with periods from 2 to 10 s and orbital velocity amplitudes of approximately 0.01 to 0.2 m/s. For each period, the orbital velocity was increased until the replica munition moved. As the orbital velocity approached the critical threshold velocity for motion, the replica mortar rocked back and forth from one fin to another before rolling down the tunnel a distance of up to 0.4 m. The processed rotation and translation values agreed well with the displacement obtained via optical tracking using a camera and LED to backlight the target. Following successful laboratory testing, a replica of the 155 mm Howitzer was fabricated and instrumented with two IMUs. The larger target was placed on the seabed in the nearshore off the coast of Dry Tortugas, Florida to investigate its response to waves and interaction with the sediment. The wave climate during the deployment was predominantly calm with a significant wave height of approximately 0.28 m, peak period of 5.9 s and peak near-bed wave orbital velocity amplitude of 0.07 m/s. The hydrodynamic forcing was not strong enough to mobilize the Howitzer replica, with the exception of one storm event, which produced significant wave heights of 0.9 m, peak period of 7.7 s and peak near-bed wave orbital velocity amplitude of 0.18 m/s. The 155 mm Howitzer replica was observed to wobble in response to the wave group forcing, but was never mobilized. Consequently, we believe the observed wave forcing represents the near threshold conditions for the onset of motion of replica munitions of the overall size, shape, and density of the 155 mm Howitzer.