Abstract-An unmanned vehicle is being developed for highspeed aerial ingress to a target shallow water environment after which it will transition to underwater low-speed operations. This paper describes the design and analysis of a bio-inspired robotic fin for use as an underwater propulsion and control mechanism, and the effect this fin has on the aerodynamic characteristics of the air-deployed vehicle platform. Building on previous fin research, both computational fluid dynamics (CFD) simulation results and experimental data are used to evaluate the hydrodynamic thrust of a flapping fin, as well as the aerodynamic lift of a static fin. This analysis validates the fin design for use on a hybrid air-underwater vehicle.
The Naval Research Laboratory (NRL) Vehicle Research Section (VRS) has developed a small unmanned vehicle that is capable of transitioning from flight to underwater operation. This allows standoff emplacement of underwater vehicles and their payloads to areas where access may be denied for traditional UUVs. A few design concepts were analyzed and tested that explore the trade space between flight performance, swimming performance, and system complexity. The resulting vehicle, Test Sub, features an axi-symmetric hull with non-folding flight surfaces sized for cruise speeds around 60 kts. Test Sub demonstrated mixed-mode operation via three test flights and dozens of swim tests. This paper documents the design methodology, construction, and testing of this new class of hybrid mixed-mode vehicles.
Ceramic recuperators could enable microturbines to achieve higher fuel efficiency and specific power. Challenges include finding a suitable ceramic fabrication process, minimizing stray heat transfer and gas leakage, mitigating thermal stress, and joining the ceramic parts to neighboring metal components. This paper describes engine and recuperator design concepts intended to address these obstacles. The engine is sized to produce twelve kilowatts of shaft power, and it has a reverse-flow compressor and turbine. Motivations for this layout are to balance axial thrust forces on the rotor assembly; to minimize gas leakage along the rotating shaft; to reduce heat transfer to the compressor diffuser; to enable the use of a simple, single-can combustor; and to provide room for lightweight ceramic insulation surrounding all hot section components. The recuperator is an annular, radial counterflow heat exchanger with the can combustor at the center. It is assembled from segmented wafers made by ceramic injection molding (CIM). These are housed in a pressure vessel to load the walls mainly in compression, and are joined together by flexible adhesives in the cool areas to accommodate thermal expansion. A representative wafer stack was built by laser-cutting, laminating, and sintering tapecast ceramic material. The prototype was tested at temperatures up to 675°C, and the results were used to validate analytical and computational fluid dynamics (CFD) models, which were then used to estimate the effectiveness of the actual design. Turbomachinery efficiencies were also calculated using CFD, and allowances were made for additional losses like bearing friction and gas leakage. Based on these component performance estimates, a cycle model indicates the engine could achieve a net fuel-to-electrical efficiency of 21%, at a core weight including the recuperator of 11 kg, or about 1 kg/kW electric output.
An unmanned vehicle has been developed for dual use as both an aircraft and a submersible. To achieve long-range emplacement of a highly maneuverable underwater asset to a target environment, the Flimmer (Flying-Swimmer) vehicle is designed for both high-speed flight and low-speed swimming. Building on previous research in bioinspired propulsion and control systems, the vehicle employs a unique set of artificial flapping fins for underwater maneuvering, which must be considered when evaluating the flight and water landing capabilities. This paper describes the computational analysis and experimental results for all three phases of vehicle operation—flight, landing, and swimming. Computational fluid dynamics simulation results predict aero- and hydrodynamic characteristics and demonstrate landing loads on and trajectory of the vehicle. Experimental data demonstrate flight and swimming performance and validate the computational results, and experimental testing of water landing provides a comparison with computations. Results and analyses of the Flimmer vehicle performance demonstrate the operational capabilities of an unmanned hybrid vehicle for long-range flight and low-speed swimming.
The development of a hybrid unmanned aerial and underwater vehicle (UAV/UUV) for transit to a shallow water zone is described. The computational fluid dynamics study is focused on the aerodynamic characteristics of the vehicle during the glide phase, the thrust production for underwater propulsion, and predicting the forces on the vehicle during the landing phase of the operation. The effect of size, position, and camber of the front fins acting as canards on the pitch stability of the vehicle is investigated. The hydrodynamic thrust of the flapping fins for underwater propulsion shows that a vehicle speed of 1kt can be achieved. The trajectory and the computed forces and moments using coupled 6-dof simulations during the landing phase of the vehicle provide confidence that the fin and the vehicle will survive a water landing.
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