Motion Coordination of Multiple Autonomous Vehicles in a Spatiotemporal Flowfield

The long-term goal of this research is to provide theoretically justified control strategies to operate autonomous vehicles in spatiotemporal flowfields. The specific objective of this dissertation is to use estimation and nonlinear control techniques to generate decentralized control algorithms that enable motion coordination for multiple autonomous vehicles while operating in a time-varying flowfield. A cooperating team of vehicles can benefit from sharing data and tasking responsibilities. Many existing control algorithms promote collaboration of autonomous vehicles. However, these algorithms often fail to account for the degradation of control performance caused by flowfields. This dissertation presents decentralized multivehicle coordination algorithms designed for operation in a spatially or temporally varying flowfield. Each vehicle is represented using a Newtonian particle traveling in a plane at constant speed relative to the flow and subject to a steering control. Initially, we assume the flowfield is known and describe algorithms that stabilize a circular formation in a time-varying spatially nonuniform flow of moderate intensity. These algorithms are extended by relaxing the assumption that the flow is known: the vehicles dynamically estimate the flow and use that estimate in the control. We propose a distributed estimation and control algorithm comprising a consensus filter to share information gleaned from noisy position measurements, and an information filter to reconstruct a spatially varying flowfield. The theoretical results are illustrated with numerical simulations of circular formation control and validated in outdoor unmanned aerial vehicle (UAV) flight tests

Multi-vehicle Control in a Strong Flowfield with Application to Hurricane Sampling

A major obstacle to path-planning and formation-control algorithms in multi-vehicle systems are strong flows in which the ambient flow speed is greater than the vehicle speed relative to the flow. This challenge is espe-cially pertinent in the application of unmanned aircraft used for collecting targeted observations in a hurricane. The presence of such a flowfield may inhibit a vehicle from making forward progress relative to a ground-fixed frame, thus limiting the directions in which it can travel. Using a self-propelled particle model in which each particle moves at constant speed relative to the flow, this paper presents results for motion coordination in a strong, known flowfield. We present the particle model with respect to inertial and rotating reference frames and provide for each case a set of con-ditions on the flowfield that ensure trajectory feasibility. Results from the Lyapunov-based design of decentralized control algorithms are presented for circular, folium, and spirograph trajectories, which are selected for their potential use as hurricane sampling trajectories. The theoretical results are illustrated using numerical simulations in an idealized hurricane model. Nomenclature N Number of particles in the system k Particle index k = 1,..., N rk Position of k th particle with respect to inertial frame r̃k Position of k th particle with respect to rotating fram

OBSERVABILITY-BASED SAMPLING AND ESTIMATION OF FLOWFIELDS USING MULTI-SENSOR SYSTEMS

The long-term goal of this research is to optimize estimation of an unknown flowfield using an autonomous multi-vehicle or multi-sensor system. The specific research objective is to provide theoretically justified, nonlinear control, estimation, and optimization techniques enabling a group of sensors to coordinate their motion to target measurements that improve observability of the surrounding environment, even when the environment is unknown. Measures of observability provide an optimization metric for multi-agent control algorithms that avoid spatial regions of the domain prone to degraded or ill-conditioned estimation performance, thereby improving closed-loop control performance when estimated quantities are used in feedback control. The control, estimation, and optimization framework is applied to three applications of multi-agent flowfield sensing including (1) environmental sampling of strong flowfields using multiple autonomous unmanned vehicles, (2) wake sensing and observability-based optimal control for two-aircraft formation flight, and (3) bio-inspired flow sensing and control of an autonomous unmanned underwater vehicle. For environmental sampling, this dissertation presents an adaptive sampling algorithm steering a multi-vehicle system to sampling formations that improve flowfield observability while simultaneously estimating the flow for use in feedback control, even in strong flows where vehicle motion is hindered. The resulting closed-loop trajectories provide more informative measurements, improving estimation performance. For formation flight, this dissertation uses lifting-line theory to represent a two-aircraft formation and derives optimal control strategies steering the follower aircraft to a desired position relative to the leader while simultaneously optimizing the observability of the leader's relative position. The control algorithms guide the follower aircraft to a desired final position along trajectories that maintain adequate observability and avoid areas prone to estimator divergence. Toward bio-inspired flow sensing, this dissertation presents an observability-based sensor placement strategy optimizing measures of flowfield observability and derives dynamic output-feedback control algorithms autonomously steering an underwater vehicle to bio-inspired behavior using a multi-modal artificial lateral line. Beyond these applications, the broader impact of this research is a general framework for using observability to assess and optimize experimental design and nonlinear control and estimation performance

Galerkin spectral estimation of vortex-dominated wake flows

We propose a technique for performing spectral (in time) analysis of spatially-resolved flowfield data, without needing any temporal resolution or information. This is achieved by combining projection-based reduced-order modeling with spectral proper orthogonal decomposition. In this method, space-only proper orthogonal decomposition is first performed on velocity data to identify a subspace onto which the known equations of motion are projected, following standard Galerkin projection techniques. The resulting reduced-order model is then utilized to generate time-resolved trajectories of data. Spectral proper orthogonal decomposition (SPOD) is then applied to this model-generated data to obtain a prediction of the spectral content of the system, while predicted SPOD modes can be obtained by lifting back to the original velocity field domain. This method is first demonstrated on a forced, randomly generated linear system, before being applied to study and reconstruct the spectral content of two-dimensional flow over two collinear flat plates perpendicular to an oncoming flow. At the range of Reynolds numbers considered, this configuration features an unsteady wake characterized by the formation and interaction of vortical structures in the wake. Depending on the Reynolds number, the wake can be periodic or feature broadband behavior, making it an insightful test case to assess the performance of the proposed method. In particular, we show that this method can accurately recover the spectral content of periodic, quasi-periodic, and broadband flows without utilizing any temporal information in the original data. To emphasize that temporal resolution is not required, we show that the predictive accuracy of the proposed method is robust to using temporally-subsampled data.Comment: 35 pages, 12 figure

Modeling and simulation of inertial drop break-up in a turbulent pipe flow downstream of a restriction.

This work deals with the modeling of drop break-up in an inhomogeneous turbulent flow that develops downstream of a concentric restriction in a pipe. The proposed approach consists in coupling Euler–Lagrange simulations of the drop motion to an interface deformation model. First the turbulent flow downstream of the restriction is solved by means of direct numerical simulation. Single drop trajectories are then calculated from the instantaneous force balance acting on the drop within the turbulent field (one-way coupling). Concurrently, the interface deformation is computed assuming the drop to behave as a Rayleigh–Lamb type oscillator forced by the turbulent stress along its trajectory. Criterion for break-up is based upon a critical value of drop eformation. This model has been tested against experimental data. The flow conditions and fluids properties have been chosen to match those experimental investigations. Both turbulent flow statistics and break-up probability calculations are in good agreement with experimental data, strengthening the relevance of this approach for modeling break-up in complex unsteady flow

Rarefied gas dynamic simulations of planetary atmospheric systems

My doctoral research involves the advanced numerical simulation of rarefied (low-pressure) planetary atmospheres and volcanism with advanced physical modeling, in application of the Direct Simulation Monte Carlo (DSMC) method. This method is the approach of choice for modeling a wide range of continuum-to-rarefied systems - in which the average spacing between molecules in the flow becomes comparable to the flow length scales, and in which traditional means of computing fluid dynamics with the partial differential equations of continuum theory break down. DSMC is a probabilistic technique by which the motions and collisions of representative molecules are computed. Multiple gas species are modelled, along with non-equilibrium radiation, high speed collisions, photochemistry, and a wide range of other relevant physics. Comprehensive atmospheric simulations are computed in parallel on one- and three-dimensional domains that, depending on the scope of a particular project, can span entire atmospheric systems from planetary surface through vacuum. These projects are ongoing efforts in modeling and understanding global-scale atmospheric flows and the processes by which such flows are populated and propagated, and they represent advancements of the state-of-the-art in planetary atmospheric simulation. I have produced and presented research on four distinct topics: 1) simulations of the complete atmosphere of Jupiter's volcanic moon Io including sublimation and plasma-sputtering processes; 2) the creation of a novel neutral density model for Earth's upper-atmosphere in partnership with Los Alamos' ISR division; 3) multi-species simulations of the rarefied gas dynamic, transfer, and escape processes of the Pluto-Charon system; and 4) investigations of the canopy unsteadiness and development of transient filamentary structure as observed by the New Horizons probe at the Ionian Tvashtar plume site. In the course of these projects, and using my research group's existing planetary-science DSMC code as a foundation, I have developed a novel, generalized framework for rarefied atmospheric simulation that enables efficient and thorough construction of entire upper-atmospheric models. My dissertation offers an analysis of the methodology of rarefied gas dynamic planetary atmospheric simulation, in addition to discussion of each project's scientific context, the results of my simulations, and their relevance toward the explanation of various observed phenomena in planetary atmospheric science.Aerospace Engineerin

Experimental Investigation of Active Control of Bluff Body Vortex Shedding

Mean and fluctuating forces acting on a body are strongly related to vortex shedding generated behind it. Therefore, it is possible to obtain substantial reductions of at least the unsteady forces if vortex shedding is controlled or its regularity is reduced. While conventional active flow control methods are mainly concerned with direct interaction with, and alteration of, the mean flow about a body, modern techniques involve altering existing flow instabilities using relatively small inputs to obtain large-scale changes of mean flows. Aerodynamic flow control may be intended to delay or suppress boundary layer separation through creation of a boundary layer downstream from the control input that is able to withstand adverse pressure gradients imposed by the outer (global) flow. In the present work, aerodynamic characteristics of a circular cylinder at Re=156,000 and an axisymmetric body (ogive cylinder) at Re=170,000 are first analyzed using a proposed phase averaging technique for the Particle Image Velocimetry (PIV) data. Later, the effect of plasma actuators on the aerodynamic characteristics of these bodies is investigated. When plasma actuators were placed 10° upstream of the separation point on the circular cylinder, momentum addition, and maybe the effect of local heating, modified the streamwise pressure gradient, leading to the establishment of a thinner boundary layer downstream. Phase synchronization of vortex shedding was also obtained for Re=156,000 for a narrow frequency band of the carrier signal of the actuators when they operated with a 90° phase shift To the knowledge of the author no other method has been shown to achieve vortex shedding control up to this high a Reynolds number. Effects of the different configurations of plasma actuators on the circumference, on the base, and in a streamwise direction were investigated for the ogive cylinder. It was observed that direct alteration of the mean flow about a body was not as effective as the boundary layer flow control where the flow instabilities are exploited. Also, the three dimensionalities in this flow made it significantly more complex to analyze

Rotorcraft Flight Dynamics and Control in Wind for Autonomous Sampling of Spatiotemporal Processes

In recent years, there has been significant effort put into the design and use small, autonomous, multi-agent, aerial teams for a variety of military and commercial applications. In particular, small multi-rotor systems have been shown to be especially useful for carrying sensors as they have the ability to rapidly transit between locations as well as hover in place. This dissertation seeks to use multi-agent teams of autonomous rotorcraft to sample spatiotemporal fields in windy conditions. For many sampling objectives, there is the problem of how to accomplish the sampling objective in the presence of strong wind fields caused by external means or by other rotorcraft flying in close proximity. This dissertation develops several flight control strategies for both wind compensation, using nonlinear control techniques, and wind avoidance, using artificial potential-based control. To showcase the utility of teams of unmanned rotorcraft for spatiotemporal sampling, optimal algorithms are developed for two sampling objectives: (1) sampling continuous spatiotemporal fields modeled as Gaussian processes, and (2) optimal motion planning for coordinated target detection, which is an example of a discrete spatiotemporal field. All algorithms are tested in simulation and several are tested in a motion capture based experimental testbed

Direct computations of a synthetic jet actuator

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Synthetic jet actuators have previously been defined as having potential use in both internal and external aerodynamic applications. The formation of a jet flow perpendicular to the surface of an aerofoil or in a duct of diffuser has a range of potential flow control benefits. These benefits can include both laminar to turbulent transition control, which is associated with a drag reduction in aerodynamic applications. The formation and development of zero-net-mass-flux synthetic jets are investigated using highly accurate numerical methods associated with the methodology of Direct Numerical Simulation (DNS). Jet formation is characterised by an oscillating streamwise jet centreline velocity, showing net momentum flux away from the jet orifice. This momentum flux away from the orifice takes the form of a series of vortex structures, often referred to as a vortex train. Numerical simulations of the synthetic jet actuator consist of a modified oscillating velocity profile applied to a wall boundary. The Reynolds numbers used vary from 85 ≤ Re ≤ 300. A complete numerical study of both axisymmetric and fully three-dimensional jet flow is performed. A parametric axisymmetric simulation is carried out in order to study the formation criterion and evolution of zero-net-mass-flux synthetic jets under variations in actuator input parameters. From the results of these simulations the conditions necessary for the formation of the synthetic jet along with the input parameters that provide an optimal jet output are deduced. Jet optimisation is defined by the mass flow, vortex strength and longevity of the vortex train as it travels downstream. Further investigations are carried out on a fully three-dimensional DNS version of the optimised axisymmetric case. Comparisons between the jet evolution and flow-field structures present in both the axisymmetric and three-dimensional configurations are made. This thesis examines the vortex structures, the jet centreline velocities along with time dependent and time averaged results in order to deduce and visualise the effects of the input parameters on the jet formation and performance. The results attained on altering the oscillation frequency of the jet actuator indicated that synthetic jets with zero mean velocity at the inflow behave significantly differently from jets with non-zero mean velocity at the inflow. A study into the evolution and formation of the train of vortex structures associated with the formation of a synthetic jet is performed. This study is accompanied with a series of time averaged results showing time dependent flow-field trends. The time history of the jet centreline velocity, showing the net momentum flux of the fluid away from the orifice of a fully developed synthetic jet, is analysed for both axisymmetric and three-dimensional cases. Differences in the fluid dynamics between the idealised axisymmetric configuration and the three-dimensional case have been identified, where three-dimensional effects are found to be important in the region near the jet nozzle exit. The effect of a disturbance introduced into the three-dimensional simulation in order to break its inherent symmetr around the jet centreline is examined by altering the input frequency of the disturbance. It was found that the effect of this relatively minor disturbance had a major effect on the jet flow field in the region adjacent to the orifice. The effect of which was deemed to be caused by discontinuities in the surface of the jet orifice due to manufacturing tolerances. Although the effects of these disturbances on the jet flow-field are large, they seem to have been neglected from numerical simulations to date. The effect of a synthetic jet on an imposed cross-streamwise velocity profile was examined. It was found that the synthetic jet flow-field resulted in a deformation of the velocity profile in the region downstream of the synthetic jet. It is suggested that this region of deformed flow could interact with coherent structures in a transitional boundary layer in order to delay flow transition to turbulence. The effect of varying the Strouhal number of a synthetic jet in a cross-flow is also analysed. It is clear from the results presented that, in the presence of a cross-flow velocity the Strouhal number effect on the synthetic jet flow field evolution, while dominant in a quiescent fluid is surpassed by the effect of the cross-flow