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Near-surface simulation methods for shallowly submerged underwater vehicles are necessary for the population of a variety of free-surface-affected, coefficient-based maneuvering and seakeeping models. Simulations vary in complexity and computational costs, often sacrificing accuracy for simplicity and speed. One particular simplifying assumption, the linearization of the free surface boundary conditions, is explored in this study by comparing the steady-state wave-making characteristics of a shallowly submerged prolate spheroid using two different simulation methods at several submergence depths and forward speeds. Hydrodynamic responses are compared between a time-domain boundary element method that makes use of a linearized free surface boundary condition and an inviscid, volume of fluid Reynolds-Averaged Navier–Stokes computational fluid dynamics code that imposes no explicit free surface boundary condition. Differences of up to 22.6%, 32.5%, and 33.3% are found in the prediction of steady state surge force, heave force, and pitch moment, respectively. The largest differences between the two simulation methods arise for motions occurring at small submergences and large wave-making velocities where linear free-surface assumptions become less valid. Nonlinearities that occur in such cases are revealed through physical artifacts such as wave steepening, wave breaking, and high-energy waves. A further examination of near-surface viscous forces reveals that the viscous drag on the vessel is depth dependent due to the changing velocity profile around the body.

A fixed time-step variational integrator cannot preserve momentum, energy, and symplectic form simultaneously for nonintegrable systems. This barrier can be overcome by treating time as a discrete dynamic variable and deriving adaptive time-step variational integrators that conserve the energy in addition to being symplectic and momentum-preserving. Their utility, however, is still an open question due to the numerical difficulties associated with solving the discrete governing equations. In this work, we investigate the numerical performance of energy-preserving, adaptive time-step variational integrators. First, we compare the time adaptation and energy performance of the energy-preserving adaptive algorithm with the adaptive variational integrator for Kepler's two-body problem. Second, we apply tools from Lagrangian backward error analysis to investigate numerical stability of the energy-preserving adaptive algorithm. Finally, we consider a simple mechanical system example to illustrate the backward stability of this energy-preserving, adaptive time-step variational integrator.

Es wird eine systematische Methode zum Entwurf synchronisierender Regler für Systeme in strenger Rückkopplungsform entwickelt. Dabei wird die Äquivalenz unter statischer Zustandsrückführung der gegebenen Systeme zu passiven Systemen ausgenutzt und eine neue Variante des Backstepping-Verfahrens vorgestellt. Es wird gezeigt, dass sich ein Kollektiv identischer Agenten synchronisiert, wenn das Kommunikationsnetzwerk zu allen Zeitpunkten stark zusammenhängend und ausgeglichen ist. Die Methode wird anhand zweier Beispiele illustriert.

This paper presents an experiment conducted to study the pisciform locomotion of an improved modular biolocomotion emulator in which the hydrodynamic and kinematic data are collected as the device mimics the body shape of real fishes in swimming action while being towed along a water tank at a steady speed. The results not only provide insights on how to select the optimal morphologies and locomotion gaits for such a biomimetic device but are also used to validate different proposed reduced-order model of the unsteady flow for the rapid development of future underwater autonomous vehicle.

In this study, hydrodynamic forces on a submerged vessel maneuvering near a free surface are determined using a reformulated Lagrangian nonlinear maneuvering and seakeeping model derived using Lagrangian mechanics under ideal flow assumptions. A Lagrangian mechanics maneuvering model is first reformulated to simplify the computation of parameters; then, incident wave effects are incorporated into the reformulation; finally, the parameters are computed using a medium-fidelity time-domain potential-flow panel code. Predictions from the reformulated Lagrangian nonlinear maneuvering and seakeeping model, whose parameters are computed using the methods described here, are compared with direct numerical computations in two steps for a prolate spheroid maneuvering in the longitudinal plane near the free surface. First, the hydrodynamic force and moment predicted by the model are compared with solutions from the panel code for sinusoidal motion in surge, heave, and pitch in calm water. Second, the hydrodynamic force and moment are investigated for cases where the spheroid maneuvers to approach the surface in calm water and in plane progressive waves. To conclude, a physically intuitive formulation of the Lagrangian nonlinear maneuvering and seakeeping model is presented for control applications and simulations.

We present a model-based approach to estimate the vertical profile of horizontal wind velocity components using motion perturbations of a multirotor unmanned aircraft system (UAS) in both hovering and steady ascending flight. The state estimation framework employed for wind estimation was adapted to a set of closed-loop rigid body models identified for an off-the-shelf quadrotor. The quadrotor models used for wind estimation were characterized for hovering and steady ascending flight conditions ranging between 0 and 2 m/s. The closed-loop models were obtained using system identification algorithms to determine model structures and estimate model parameters. The wind measurement method was validated experimentally above the Virginia Tech Kentland Experimental Aircraft Systems Laboratory by comparing quadrotor and independent sensor measurements from a sonic anemometer and two SoDAR instruments. Comparison results demonstrated quadrotor wind estimation in close agreement with the independent wind velocity measurements. However, horizontal wind velocity profiles were difficult to validate using time-synchronized SoDAR measurements. Analysis of the noise intensity and signal-to-noise ratio of the SoDARs proved that close-proximity quadrotor operations can corrupt wind measurement from SoDARs, which has not previously been reported.

The transport of material through the atmosphere is an issue with wide ranging implications for fields as diverse as agriculture, aviation, and human health. Due to the unsteady nature of the atmosphere, predicting how material will be transported via the Earth’s wind field is challenging. Lagrangian diagnostics, such as Lagrangian coherent structures (LCSs), have been used to discover the most significant regions of material collection or dispersion. However, Lagrangian diagnostics can be time-consuming to calculate and often rely on weather forecasts that may not be completely accurate. Recently, Eulerian diagnostics have been developed which can provide indications of LCS and have computational advantages over their Lagrangian counterparts. In this paper, a methodology is developed for estimating local Eulerian diagnostics from wind velocity data measured by a single fixed-wing unmanned aircraft system (UAS) flying in a circular arc. Using a simulation environment, driven by realistic atmospheric velocity data from the North American Mesoscale (NAM) model, it is shown that the Eulerian diagnostic estimates from UAS measurements approximate the true local Eulerian diagnostics and also predict the passage of LCSs. This methodology requires only a single flying UAS, making it easier and more affordable to implement in the field than existing alternatives, such as multiple UASs and Dopler LiDAR measurements. Our method is general enough to be applied to calculate the gradient of any scalar field.

This report details the design and testing of a pneumatically propelled underwater glider. The vehicle was designed as a platform for motion control experimentation, and to explore the use of novel actuator designs to improve performance in shallow water and significant currents. The glider’s pneumatic buoyancy engine is capable of rapidly inflating an elastomeric bladder to 5 liters. (This displacement is an order of magnitude greater than that of legacy buoyancy engine designs.) The buoyancy engine was shown to operate reliably at 25 m depth. However, the compressibility of the bladder and associated change in tank weight (from exhausting air with each dive) presented significant challenges in trimming the vehicle. The attitude of the glider is controlled by translating and rotating a semi-annular mass. Because of the geometry of this mechanism, the glider is not restricted to a range of roll attitudes (i.e. the glider has unlimited roll authority and can “flip over”). By flipping over the glider may employ asymmetric hydrodynamic surfaces while preserving the same flow-relative geometry during both descents and ascents. Such asymmetric hydrodynamic surfaces (e.g. cambered hydrofoils, dihedral, wing twist) may be used to improve efficiency and performance. The ability to operate in both upright and inverted orientations requires reducing the contribution of the rigid body (minus the moving mass) to the bottom heaviness of the vehicle. A moving acoustic long-baseline ranging system was developed to position the glider while it was underway. The performance of this system was characterized experimentally in terms of ping success rate for various transducer geometries and depths in a shallow-water, rocky bottom lake.

In this report, we examine the controllability of a particular form of the equations of motion for spacecraft formation flying. These equations, the Tschauner-Hempel equations, rescale the formation flying equations to a domain in which the true anomaly is the independent variable. Using this form, we are able to compute an explicit, closed-form Gramian matrix for the period of one full orbit at arbitrary eccentricity. We do this for two cases: 1) the case in which there are three inputs to the system as well as 2) the restricted case where authority only exists in the in-track and cross-track directions. This Gramian is invertible and as a result the system is controllable for both cases. Since the transformation between the time-domain, linear equations of motion and the Tschauner-Hempel equations is bijective, we conclude that the linear equations of motion are also controllable.

In this report, we examine the controllability of a particular form of the equations of motion for spacecraft formation flying. These equations, the Tschauner-Hempel equations, rescale the formation flying equations to a domain in which the true anomaly is the independent variable. Using this form, we are able to compute an explicit, closed-form Gramian matrix for the period of one full orbit at arbitrary eccentricity. We do this for two cases: 1) the case in which there are three inputs to the system as well as 2) the restricted case where authority only exists in the in-track and cross-track directions. This Gramian is invertible and as a result the system is controllable for both cases. Since the transformation between the time-domain, linear equations of motion and the Tschauner-Hempel equations is bijective, we conclude that the linear equations of motion are also controllable.

Europa is an incredibly enticing target for exploration – the nearest reaches of what may be a vast new "habitable zone" of interior oceans warmed and stirred by tidal forces. Decades of NASA and National Academy studies including the most recent planetary science decadal survey have affirmed the preeminence of Europa as a destination for astrobiology research. This report provides a comprehensive technology roadmap and an assessment of current state of the art and future technologies to enable an under-ice mission to Europa. In this study, the authors provide an overview of key mission objectives, a profile of Europa, and a mission overview. The authors then delve into a discussion of the key fundamental science objectives and design tradeoffs to arrive at a comprehensive science traceability matrix and value system for design of a multi-vehicle, under-ice mission to Europa. The current state of the art is assessed and design alternatives discussed. The report culminates in a concept of operations for the mission and a recommended mission architecture utilizing three surface units, each deploying a single cryobot, with each cryobot carrying three biologically inspired, gliding under-ice hydrobots equipped with sensor packages that will characterize the physical and chemical state of Europa’s ocean over its entire depth.

The principal goal of this project is to develop the technology and algorithms that will enable an unmanned surface vehicle (USV) to operate fast and autonomously in unknown riverine environments, including tropical rivers. Robust autonomy requires that the USV senses the surface and subsurface environments, discriminates waterways that are navigable from those that are not, identifies stationary and moving obstacles, including other vessels, and then optimally plans and re-plan a route in realtime. Since speed is a vessel s principal defense, all of these tasks must be done as efficiently as possible to ensure successful operation at the greatest possible speed. This project is tightly coordinated with collaborators at the Naval Postgraduate School (NPS) whose work is conducted under a related project. All of the work reported herein was jointly developed with our NPS collaborators., The original document contains color images.

The principal goal of this project is to develop the technology and algorithms that will enable an unmanned surface vehicle (USV) to operate fast and autonomously in unknown riverine environments, including tropical rivers. Robust autonomy requires that the USV senses the surface and subsurface environments, discriminates waterways that are navigable from those that are not, identifies stationary and moving obstacles, including other vessels, and then optimally plans and re-plans a route in realtime. Since speed is a vessel’s principal defense, all of these tasks must be done as efficiently as possible to ensure successful operation at the greatest possible speed. This project is tightly coordinated with collaborators at the Naval Postgraduate School (NPS) whose work is conducted under a related project.