The Department of Defense (DoD) is faced with the challenge of identifying and characterizing locations in U.S. coastal and inland waters where underwater military munitions (UWMM) are present and developing safe and cost-effective means to remediate these sites as an important step towards comprehensive range clearance. The underwater environment is one of the most challenging and dynamic operating environments for both man and machine, making UWMM location and recovery especially difficult. One of the more critical and technically challenging environments is the surf-zone.
ARA studied two key engineering concepts that directly affect the ability of a robotic system to operate in the surf-zone: platform hull shape and propulsion. To address platform hull shape, ARA studied an existing arthropod’s (horseshoe crab) carapace as a biomimetic representation for the hull shape of a robotic system. This work was used to develop a hydro-dynamically advantageous shape for a robotic system. To address locomotive factors, ARA completed a research and technical study based on an Archimedes screw drive as the mode of propulsion to assess platform traction and mobility in the near shore environment, very shallow water (VSW) and surf-zone (SZ).
An applied research approach was used to study stability, mobility, and traction. To study stability, a horseshoe crab’s carapace served as the basis for a biomimetic hull shape that would theoretically provide the appropriate balance between lift and drag for a robotic platform operating in the VSW/SZ. The use of a biomimetic hull as a hydrodynamic design shape was chosen in order to direct water flow and wave energy in such a way as to aid in tractive potential and stability. Several biomimetic hulls were modeled and underwent simulated and empirical testing in a water channel. The empirical testing was used to validate the data obtained from the Computational Fluid Dynamics (CFD) simulations used to identify a more effective hull shape.
To study mobility and traction, a propulsion system based on an Archimedes screw drive was used. A drive design based on an Archimedes screw was chosen because of its ability to operate in various mediums with varying flow rates. A test bed was designed and assembled in order to measure the speed and power consumption created by the screw drive when interacting with various mediums. Several screw drive designs with different design parameters were empirically tested to record efficacies in a range of mediums, including water, sand, and pebbles.
CFD analysis and empirical testing has demonstrated the ability for a robot to operate in the VSW/SZ. A shell was optimized to reduce drag and lift. Fluent modeling was used to compare different potential hull shapes for optimizing the design of a future robot. The horseshoe crab shell was used a baseline model. Two other similar shapes were modeled with improved drag reduction characteristics. The CFD modeling confirmed that the smooth shell had the lowest drag in comparison to all other tested hull shapes.
Empirical flow testing was conducted to confirm the results of the Fluent CFD analysis. Three 3D printed hulls were tested at the same flow rates as CFD modeling for comparison. The experiments confirm that the smooth hull had the lowest drag of any shape tested. There is also strong agreement between the data from the Fluent CFD modeling and the results of the empirical experiments. This gives high confidence in the Fluent results. All testing confirms that it is possible to design a hull shape to improve stability in the dynamic wave conditions found in the VSW/SZ.
An ideal Archimedes screw drive was determined and is sufficient to be used across different mediums such as sand, water, and gravel. Testing of multiple screw profiles was conducted in a custom test bed capable of holding the above mediums. The ideal screw outperformed all of the designs in every experiment.
Results of this research lay the groundwork for the follow-on development of a prototype unmanned Surf-zone Underwater Robotic Demonstration Platform (SURDP) capable of operating in and around the near shore environment. The SURDP will enable DoD to conduct detailed and comprehensive underwater surveys in the VSW/SZ by providing a platform with the necessary stability and mobility to employ the sensors required to locate and classify UWMM.