Ravi Ramamurti

Computation of the 3-D Unsteady Flow Past Deforming Geometries

A 3-D incompressible unsteady flow solver based on simple finite elements with adaptive remeshing and grid movement for both moving and deforming surfaces is described. We demonstrate the combination of adaptive remeshing techniques with the incompressible flow solver with the computation of flow past an eel in 2-D and a blue-fin tuna in 3-D. The flow past a swimming tuna was computed for two extreme cases of the caudal fin frequency and swimming speed. A grid refinement study was performed and a grid converged solution for the force produced by the caudal fin was obtained.

A LOAD BALANCING ALGORITHM FOR UNSTRUCTURED GRIDS

Abstract We present a general, parallelizable, load balancing algorithm for unstructured grid-based problems that belong to the so-called diffusion class and employ a give-and-take concept among neighbouring subdo-mains. The algorithm is found to converge very quickly to almost perfect load balance while minimizing the surface-to-volume ratio of the domains. The algorithm can be used for problems whose cost grows nonlinearly with the number of elements, because it measures continuously the computational cost to be incurred for each subdomain. This is an advantage over the recursive bisection algorithms currently in use, which assume a linear relationship between the computational cost and the number of elements. The load balancing algorithm is applied in conjunction with parallel heat transfer and parallel incompressible flow solvers for the solution of 2-D and 3-D problems, employing several hundred processors on a MIMD machine.

Fuzzy logic PID based control design and performance for a pectoral fin propelled unmanned underwater vehicle

This paper describes the modeling, simulation, and control of a UUV in six degree-of-freedom (6-DOF) motion using two NRL actively controlled-curvature fins. Computational fluid dynamic (CFD) analysis and experimental results are used in modeling the fin as part of the 6-DOF vehicle model. A fuzzy logic proportional-integral-derivative (PID) based control system has been developed to smoothly transition between preprogrammed sets of fin kinematics in order to create a stable and highly maneuverable UUV. Two different approaches to a fuzzy logic PID controller are analyzed: weighted gait combination (WGC), and modification of mean bulk angle bias (MBAB). Advantages and disadvantages of both methods at the vehicle level are discussed. Simulation results show desirable system performance over a wide range of maneuvers.

Design, Development, and Testing of Flapping Fins with Actively Controlled Curvature for an Unmanned Underwater Vehicle

This paper describes the design, construction, and testing of a biomimetic pectoral (side) fin with actively controlled curvature for UUV propulsion. It also describes the development of a test UUV and the design of a fin control system for vertical plane motion. A 3D unsteady computational fluid dynamics (CFD) analysis has been carried out to computationally optimize the fin design including a full study of the primary design parameters. The fin has been constructed and it can reproduce any specified deformation time-history. The full dynamics of the proposed vehicle have been modeled and the forces produced by the flapping fins computed. Finally, the stability of motion in the vertical plane has been analyzed and a control system has been designed.

Robotic pectoral fin thrust vectoring using weighted gait combinations

A method was devised to vector propulsion of a robotic pectoral fin by means of actively controlling fin surface curvature. Separate flapping fin gaits were designed to maximize thrust for each of three different thrust vectors: forward, reverse, and lift. By using weighted combinations of these three pre-determined main gaits, new intermediate hybrid gaits for any desired propulsion vector can be created with smooth transitioning between these gaits. This weighted gait combination WGC method is applicable to other difficult-to-model actuators. Both 3D unsteady computational fluid dynamics CFD and experimental results are presented.

FOUR-FIN BIO-INSPIRED UUV: MODELING AND CONTROL SOLUTIONS

This paper describes the modeling and control development of a bio-inspired unmanned underwater vehicle (UUV) propelled by four pectoral fins. Based on both computational fluid dynamics (CFD) and experimental fin data, we develop a UUV model that focuses on an accurate representation of the fin-generated forces. Models of these forces span a range of controllable fin parameters, as well as take into account leading-trailing fin interactions and free stream flow speeds. The vehicle model is validated by comparing open-loop simulated responses with experimentally measured responses to identical fin inputs. Closed-loop control algorithms, which command changes in fin kinematics, are tested on the vehicle. Comparison of experimental and simulation results for various maneuvers validates the fin and vehicle models, and demonstrates the precise maneuvering capabilities enabled by the actively controlled curvature pectoral fins.Copyright

Scaling studies for an actively controlled curvature robotic pectoral fin

Scaling studies for an actively controlled curvature robotic pectoral fin are presented in detail. Design, development, and analysis of the fin are conducted using a combination of computational fluid dynamics tools and experimental tests. Results include a Generation 2 (Gen2) fin design with approximately 3x more surface area and a slightly larger aspect ratio compared with our Generation 1 (Gen1) version. The Gen2 fin demonstrates 9x more thrust production than the Gen1 fin, validating the computational studies. Additionally, changes to the structural design of the ribs and actuation of the rib angles leads to a power savings and a more efficient fin.

Development of a robotic fin for hydrodynamic propulsion and aerodynamic control

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.

3-D Unsteady Computations of Flapping Flight in Insects, Fish, and Unmanned Vehicles

3-D unsteady computations have been carried out for a swimming tuna with an oscillating caudal fin, the flapping flight of the fruit fly and a pectoral fin swimmer, the bird wrasse, and a variety of unmanned air vehicles. Such computations for creatures or vehicles with moving and deforming surfaces can provide information on the dynamics of force production that is quite useful for vehicle design. Novel biomimetic vehicles have been designed and built and their performance is described. As vehicle size decreases there has also been a need for incorporation of novel materials, sensors, and control systems. Computational challenges for coupling novel sensor designs, vehicle timevarying force and moment computations, and self-consistent vehicle trajectory computations are discussed.

Thrust magnitude and efficiency studies for multiple robotic flapping fins

The development of multiple generations of robotic flapping pectoral fins has warranted a closer study of the relative thrusts and efficiencies exhibited by these fins. This paper describes the differences between the various fins - including stroke frequency, stroke amplitude, aspect ratio, and effective angle of attack - and investigates the effects these differences have on thrust generation and propulsive efficiency. Drawing from and building on previous fin results, both computational fluid dynamics (CFD) simulation results and experimental data are used to evaluate the thrust, power draw, and efficiency, and to find useful relationships between these values and parameters such as Strouhal number and advance ratio. This analysis has implication for future fin design and development of reduced order models for flapping propulsion mechanisms.