Browsing by Author "Dr. Ashok Gopalarathnam, Committee Member"

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Algorithmic Enhancements to the VULCAN Navier-Stokes Solver
(2003-08-15) Litton, Daniel; Dr. Jack R. Edwards, Committee Chair; Dr. D. Scott McRae, Committee Member; Dr. Ashok Gopalarathnam, Committee Member
VULCAN (Viscous Upwind aLgorithm for Complex flow ANalysis) is a cell centered, finite volume code used to solve high speed flows related to hypersonic vehicles. Two algorithms are presented for expanding the range of applications of the current Navier-Stokes solver implemented in VULCAN. The first addition is a highly implicit approach that uses subiterations to enhance block to block connectivity between adjacent subdomains. The addition of this scheme allows more efficient solution of viscous flows on highly-stretched meshes. The second algorithm addresses the shortcomings associated with density-based schemes by the addition of a time-derivative preconditioning strategy. High speed, compressible flows are typically solved with density based schemes, which show a high level of degradation in accuracy and convergence at low Mach numbers (M < 0.1). With the addition of preconditioning and associated modifications to the numerical discretization scheme, the eigenvalues will scale with the local velocity, and the above problems will be eliminated. With these additions, VULCAN now has improved convergence behavior for multi-block, highly-stretched meshes and also can accurately solve the Navier-Stokes equations for very low Mach numbers.
Analysis and Parameter Estimation of the Aerodynamic and Handling Qualities of the C-130A Modified With Wing Tip Tanks
(2004-12-02) Phillips, David; Dr. Charles E. Hall Jr., Committee Chair; Dr. Ashok Gopalarathnam, Committee Member; Dr. James Selgrade, Committee Member
This work presents the background, flight testing, and resulting change in the aerodynamic and handling qualities of a C-130A Hercules modified with wing tip tanks. The data collected during the baseline and modified flight tests of this aircraft demonstrated the potential aerodynamic benefits of a tip tank design that incorporates a greater aspect ratio and end plating effects. Wing mounted pressure belts measured a 24% increase in local Cl near the tip tanks. This local increase in lift contributed to a 38% increase in CL max for the airplane. The pressure and dynamic data was gathered using a LIFT (Linux In Flight Testing) system, and it laid the foundation for finding the longitudinal and lateral directional stability coefficients of the airplane. Then using MATLAB® and the System IDentification Programs for AirCraft (SIDPAC) to reduce this data, it was possible to generate aerodynamic, lateral, and longitudinal parameters that clearly proved the overall benefits of the design change. The demonstrated lift benefits of these uniquely designed tip tanks for the C-130A cargo transport proved that by capitalizing on the benefits of a combination tip tank and end plate design it is possible to generate increased lift without adversely affecting the stability and dynamic parameters of the aircraft.
Analysis of Taxi Test Data for an Unmanned Aerial VehicleImplemented with Fluidic Flow Control
(2006-07-07) Turner, Drew Patrick; Dr. Sharon Lubkin, Committee Member; Dr. Ashok Gopalarathnam, Committee Member; Dr. Charles E. Hall, Jr., Committee Chair
Serpentine inlet ducts are utilized in many aircraft where the inlet capture area is located off the thrust line or there is a desire to conceal the engine compressor face. Due to the curvature that characterizes a compact serpentine duct, issues with flow distortion and total pressure loss at the engine face arise leading to reduction in propulsion system performance. Computational analysis has shown that flow control implementing micro-fluidic vortex generators significantly reduces the losses. Previous work at North Carolina State University has demonstrated the benefits of a fluidic flow control of this type in a highly compact serpentine inlet duct through the design and experimental static testing of a propulsion system for an uninhabited aerial vehicle. With the implementation of flow control, engine face distortion was reduced and propulsion system performance was increased. This work continues the investigation of the effectiveness of the fluidic flow control by examining the performance of the system during dynamic situations through high speed taxi testing of an uninhabited aerial vehicle implemented with this technology. Additionally, the collected data was used to compare calculated takeoff parameters to values calculated using standard takeoff analysis.
Decentralized Autonomous Control of Aerospace Vehicle Formations
(2003-06-05) Levedahl, Blaine Alexander; Dr. Larry Silverberg, Committee Chair; Dr. Ashok Gopalarathnam, Committee Member; Dr. Edward Grant, Committee Member
Two approaches for the autonomous control of aerospace vehicle formations are developed. The development of the approaches relies on fundamental work in the areas of distributed control; specifically modal, robust, optimal, and decentralized control. The algorithms are shown to satisfy five separation principles that simplify design and enable the algorithms to be implemented reliably. The autonomous controllers uniformly dampen the modes of the formation (global control) using a decentralized approach and a nearest-neighbor approach. A numerical example illustrates robust formation changes from 9-vehicle (3 x 3) grids to V-type formations.
Design and Modeling of a Mars Tumbleweed Rover
(2007-07-24) Wilson, Jamie Leigh Miss; Dr. Andre P. Mazzoleni, Committee Chair; Dr. Fred DeJarnette, Committee Member; Dr. Ashok Gopalarathnam, Committee Member
A Mars Tumbleweed Rover is a spherical, wind driven, planetary rover. Compared with conventional rovers, a tumbleweed rover can travel further faster and gain access to areas such as valleys and chasms that previously were inaccessible. This paper will present results of the design, testing, mathematical modeling, and computer simulation of a tumbleweed rover propelled by a constant wind loading over a rough surface representing the planet's surface. Results will show different trajectories for specific cases where initial conditions such as mass distribution and angular velocity are varied.
Development of a General Methodology for Optimizing Acoustic Treatment Using an Equivalent Source Method
(2007-11-19) Reimann, Craig Aaron; Dr. Robert Nagel, Committee Chair; Dr. Harvey Charlton, Committee Member; Dr. Terry Scharton, Committee Member; Dr. Ashok Gopalarathnam, Committee Member
Development of a Wind Tunnel Test Cell for Small Propellers with Application to the Plank Unmanned Aerial Vehicle
(2009-11-30) Bishop, Jason Thomas; Dr. Ashok Gopalarathnam, Committee Member; Dr. Pierre A. Gremaud, Committee Member; Dr. Charles E. Hall Jr., Committee Chair
The Plank unmanned aerial vehicle has been designed to be a test platform for an experimental bias angular moment flywheel designed by the NASA Guidance and Controls Branch. The baseline configuration of the aircraft is a flying wing with zero quarter-chord sweep. The benefits of this type of configuration include low structural weight and complexity and reduced drag compared to a standard wing-tail configuration. However, this type of configuration is inherently sensitive to pitch disturbances due to low pitch damping and moment of inertia. In order to remedy this sensitivity, NASA has proposed the use of a bias angular momentum flywheel with its rotational axis aligned with the z-axis of the aircraft. This momentum wheel will effectively stiffen the pitch axis of the aircraft by coupling it with the high damping, high moment of inertia roll axis. The aircraft was designed to be re-configurable allowing for a variety of flying qualities ranging from a well-behaved wing and aft tail configuration to the much more sensitive flying wing configuration. This range of flying qualities will allow for a more comprehensive performance evaluation of the momentum wheel system as well as contribute to risk mitigation during initial flight tests. An electric propulsion system has been chosen for this aircraft due its simple operation and low vibration characteristics. The AXi 5330/18 motor was chosen along with a Castle Creations Phoenix HV-110 electronic speed control. The system is powered by three FlightPower 5000 mAh 10s Evo Lite battery packs connected in parallel. A four-bladed propeller configuration was used in order to absorb the necessary power from the motor while still fitting between the tail booms of the aircraft. In order to experimentally evaluate the performance of the Plank propulsion system in the NCSU Subsonic Wind Tunnel, the Propeller Test Cell (PTC) was developed. The PTC consists of a strain gage based load cell that measures the thrust and torque generated by the propulsion system along with system operating conditions such as current, voltage, RPM, motor temperature, and battery consumption. The PTC was used to characterize the performance of three candidate propellers for the Plank aircraft: 16x10, 17x8, and 17x10. With this data, certain propulsion specific performance parameters of the aircraft were calculated including thrust and power available, rate of climb and climb angle, range, endurance, and take off performance. After analyzing the performance for each of the three propellers, the 17x8 propeller produced the most desired aircraft performance and was chosen to be paired with the AXi motor and the Plank aircraft.
Linear Parameter-Varying Control of an F-16 Aircraft at High Angle of Attack
(2005-02-02) Lu, Bei; Dr. Ashok Gopalarathnam, Committee Member; Dr. Fen Wu, Committee Chair; Dr. Paul I. Ro, Committee Member; Dr. Mo-Yuen Chow, Committee Member
To improve the aircraft capability at high angle of attack and expand the flight envelope, advanced linear parameter-varying (LPV) control methodologies are studied in this thesis with particular applications of actuator saturation control and switching control. A standard two-step LPV antiwindup control scheme and a systematic switching LPV control approach are derived, and the advantages of LPV control techniques are demonstrated through nonlinear simulations of an F-16 longitudinal autopilot control system. The aerodynamic surface saturation is one of the major issues of flight control in the high angle of attack region. The incorporated unconventional actuators such as thrust vectoring can provide additional control power, but may have a potentially significant pay-off. The proposed LPV antiwindup control scheme is advantageous from the implementation standpoint because it can be thought of as an augmented control algorithm to the existing control system. Moreover, the synthesis condition for an antiwindup compensator is formulated as a linear matrix inequality (LMI) optimization problem and can be solved efficiently. By treating the input saturation as a sector bounded nonlinearity with a tight sector bound, the synthesized antiwindup compensator can stabilize the open-loop exponentially unstable systems. The LPV antiwindup control scheme is applied to the nonlinear F-16 longitudinal model, and compared with the thrust vectoring control approach. The simulation results show that the LPV antiwindup compensator improves the flight quality, and offers advantages over thrust vectoring in a high angle of attack region. For a thrust vectoring augmented aircraft, the actuator sets may be different at low and high angles of attack. Also due to different control objectives, a single controller may not exist over a wide angle of attack region. The proposed switching LPV control approach based on multiple parameter-dependent Lyapunov functions provides a flexible design method with improved performance. A family of LPV controllers are designed, each suitable for a specific angle of attack region. They are switched according to the trajectory of angle of attack so that the closed-loop system remains stable and its performance is optimized. Two switching logics, hysteresis switching and switching with average dwell time, are examined. The control synthesis conditions for both switching logics are formulated as generally non-convex matrix optimization problems. To make the switching LPV control approach more applicable, two convexified methods are given according to the state of the controller is reset or not at switching time. The thrust vectoring augmented F-16 longitudinal model with different control objectives at low and high angles of attack is used to validate the results of the switching control scheme.
Methods for Improvements in Airworthiness of Small UAS
(2008-11-17) Cline, Charles Benjamin; Dr. Charles Hall Jr., Committee Chair; Dr. Ashok Gopalarathnam, Committee Member; Dr. Pierre Gremaud, Committee Member
CLINE, CHARLES BENJAMIN. Methods for Improvements in Airworthiness of Small UAS. (Under the direction of Dr. Charles Hall Jr.) Unmanned Aircraft Systems are a relatively new addition to the myriad of different objects in the skies above us. The use of UAS is desirable over manned aircraft in many cases where their utility and adaptability at low cost allow these platforms to be more cost effective. However, this low cost is often linked to a reduction in overall system reliability and safety, and the question becomes how to increase the airworthiness of these systems without significantly increasing costs. UAS possess distinctive characteristics derived from a broad range of sizes and configurations which have been tailored to specific intended missions. The additional challenges brought on by the lack of an onboard pilot and the need to retain a certain level of cost effectiveness make addressing the airworthiness of these systems a unique problem apart from simply applying the requirements established for manned aviation. This research proposes a method for determining the airworthiness of UAS by a quantitative approach which can be tailored to a specific platform and its intended mission. The proposed set of airworthiness requirements derived from this methodology is presented within this work as the Quantitative Airworthiness Scheme (QAS). QAS is a system level approach to airworthiness linked to hazard analysis where design, manufacture, and testing techniques intended to mitigate failures present in the system are awarded points depending on their effectiveness. These points are then summed to ascertain the overall airworthiness of the system itself. Failure Modes and Effects Analysis (FMEA) provides the underlying structure of the system, increasing efficiency through organization enabling QAS users to apply failure mitigations more acutely. The focus of this research has sought to address the airworthiness of small UAS by proposing a certification system capable of adapting to these platforms, which vary in size from 50 lbs to 350 lbs, thoroughly addressing the failures which plague them, and efficiently improving safety and reliability by suggesting low cost yet effective and appropriate mitigation techniques.
A Simplified Visible/Near-Infrared Spectrophotometric Approach to Blood Typing for Automated Transfusion Safety
(2005-05-02) Anthony, Steven R; Dr. Ashok Gopalarathnam, Committee Member; Dr. Kara J. Peters, Committee Member; Dr. M.K. Ramasubramanian, Committee Chair
A new technique has recently been introduced for objectively quantifying the agglutination of red blood cells in a blood typing procedure using ultraviolet/visible spectroscopy. The technique involves analyzing the spectra of red blood cell suspensions in saline between the 665 nm and 1000 nm range, where the relative slope between a control and antibody treated sample are entered into a simple algorithm to form a so called agglutination index (A.I.). The proposal of this research is to simplify the detection method by replacing the spectral imaging of the diode array spectrophotometer with a discrete series of LED and photodiode pairs within the wavelength range of interest, in the forward scattering direction. The scattering theory involved in this phenomenon is investigated, and a simplified experimental sensor is designed and evaluated. The resulting experimentation shows a significant recreation of the spectrophotometer results by the simplified design with a promising potential for improvement. Optoelectronic design considerations are discussed for maximizing the sensitivity of this technique for use in a cost effective, automated device for transfusion safety.
System Level Airworthiness Tool: A Comprehensive Approach to Small Unmanned Aircraft System Airworthiness.
(2010-04-30) Burke, David Alexander; Dr. Nelson Couch, Committee Member; Dr. Stephen Cook, Committee Member; Dr. Paul Ro, Committee Member; Dr. Ashok Gopalarathnam, Committee Member; Dr. Charles E. Hall Jr., Committee Chair
One of the pillars of aviation safety is assuring sound engineering practices through airworthiness certification. As Unmanned Aircraft Systems (UAS) grow in popularity, the need for airworthiness standards and verification methods tailored for UAS becomes critical. While airworthiness practices for large UAS may be similar to manned aircraft, it is clear that small UAS require a paradigm shift from the airworthiness practices of manned aircraft. Although small in comparison to manned aircraft these aircraft are not merely remote controlled toys. Small UAS may be complex aircraft flying in the National Airspace System (NAS) over populated areas for extended durations and beyond line of sight of the operators. A comprehensive systems engineering framework for certifying small UAS at the system level is needed. This work presents a point based tool that evaluates small UAS by rewarding good engineering practices in design, analysis, and testing. The airworthiness requirements scale with vehicle size and operational area, while allowing flexibility for new technologies and unique configurations.
Unscented Kalman Filtering for Real-Time Atmospheric Thermal Tracking
(2010-04-09) Hazard, Matthew Wesley; Dr. Charles E. Hall, Committee Chair; Dr. Ashok Gopalarathnam, Committee Member; Dr. Fen Wu, Committee Member
The increasing use of unmanned air vehicles in military and civilian applications has been accompanied by a growing demand for improved endurance and range. These demands have been largely met by advances in aerodynamic and structural efficiency, improved battery technology, and the ongoing miniaturization of onboard computing and payload systems. Recently, more attention has been paid to the extraction of energy from the atmosphere. Aircraft can make use of atmospheric updrafts, or thermals, to gain altitude without expenditure of onboard fuel stores. By intelligently tracking thermals, an unmanned aircraft can extend its range or loiter time without carrying additional fuel or specialized sensors. Prior research has focused on the `big picture' concepts associated with autonomous soaring - determining when to stop and soar in a thermal, what speed to fly, when to return to the desired course, and so on. Finding and tracking thermals is only a single component of the complete soaring system. However, because the high-level decision making tasks rely on estimates of the thermal parameters, the accuracy and computational cost of the thermal tracking algorithm set the upper performance limit of the entire system. So, this research reformulated batch regression thermal finding algorithms used in past efforts into an efficient Unscented Kalman Filter. Open-loop simulation results showed the filter was capable of accurately estimating thermal position, strength, and size with low computational cost for a variety of realistic flight paths. Closed-loop simulation reaffirmed this statement in the presence of realistic aircraft, sensor, and thermal dynamics. Further, the algorithm was embedded into the ALOFT soaring platform (a 4.3m wingspan unmanned glider) for flight testing, which demonstrated its ability to track real-world thermals during cross-country flights exceeding 5 hours flight time over a 70 mile course.
Vehicle Control in Full Unsteady Flow Using Surface Measurements
(2010-03-16) Levedahl, Blaine Alexander; Dr. Ashok Gopalarathnam, Committee Member; Dr. Fred Dejarnette, Committee Member; Dr. Larry Silverberg, Committee Chair; Dr. Winser Alexander, Committee Member
This dissertation is the first comprehensive attempt to address a new engineering problem: control of a vehicle maneuvering in a full unsteady flow field. The approach to the solution is focused in three main areas: modeling of a vehicle in full unsteady flow, control of a vehicle in full unsteady flow, and synthesizing the fluid loads for use in control of a vehicle maneuvering in a full unsteady flow field. To model a vehicle maneuvering in a full unsteady flow field this dissertation develops the Coupled Fluid Vehicle (CFV) model in which the fluid, which is a sum of a finite number of spatially dependent velocity fields whose contributions vary with time, is coupled to the vehicle rigid-body equations of motion. To control a vehicle maneuvering in a full unsteady flow field this dissertation develops the Fluid Compensation Control (FCC) strategy which gives the designer an opportunity to include the fluid states, in addition to the vehicle states, in the control law and an opportunity to balance reducing the fluid dynamic load through compensation and reducing the state error through regulation. To synthesize the fluid loads this dissertation has attempted to forward current work on the prediction of fluid loads from stagnation and separation point measurements using the Kutta principle, which says that the velocity around a vehicle is a smoothly varying function and that it is determined up to a multiplicative constant by its nodes (stagnation, separation, and reattachment points/lines), and by conducting an experiment to attempt to determine the correlation of the fluidic loads from the orientation and separation lines on a 3-dimensional bluff body.