Measurement of Dynamic Pressure Gradients on the Surface of Short Cylinders

Military testing and training around the United States has resulted in over 10 million acres of property in underwater environments, potentially containing military weapons. The weapons, or munitions, are difficult to locate, capable of sudden movement, and a danger to marine life and the public. Improved understanding of their mobility in underwater environments is vital for safe and cost-effective munition recovery. A pressure-mapped model munition (PMM) was designed and fabricated to resolve the role of dynamic pressure gradients on munition mobility. The PMM is an untethered instrument, containing all electronics necessary to retrieve, time, and store data. The PMM is capable of detecting and measuring surface pressure gradients, orientation and positional changes, and uses an acoustic tracker for retrieval after deployments.

The surface pressure mapping was accomplished using an array of 16 small diaphragm pressure sensors. Orientation changes were recorded using an Internal Measurement Unit (IMU). All data is stored to an on-board microSD card and recorded on the same time stamp. The PMM was evaluated through several full scale laboratory and field experiments to determine its accuracy in detecting hydrostatic pressure changes, orientation changes, passing waves, and environmental changes, such as being submerged in a sand bed. The PMM was able to resolve both the overlying progressive wave signal as well as the deviations due to vortex shedding around the cylinder. The PMM showed a negative pressure region in the lee of the vortex twice per wave period. When partially buried the PMM showed the expected signal attenuation and a decrease of the pressure signatures associated with vortex shedding. These results suggest that vortex shedding may significantly contribute to the burial of free standing objects.