ETDs: Virginia Tech Electronic Theses and Dissertations

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Virginia Tech has been a world leader in electronic theses and dissertation initiatives for more than 20 years. On January 1, 1997, Virginia Tech was the first university to require electronic submission of theses and dissertations (ETDs). Ever since then, Virginia Tech graduate students have been able to prepare, submit, review, and publish their theses and dissertations online and to append digital media such as images, data, audio, and video.

University Libraries staff are currently digitizing thousands of pre-1997 theses and dissertations and loading them into VTechWorks. Most of these theses and dissertations are fully available to the public, but we will, in general, honor requests by the item's author to restrict access to Virginia Tech only. See our process for Requesting that Material be Amended or Removed.

To search all Virginia Tech print and digital theses and dissertations, use the University Libraries ETD resource guide.

Materials that are restricted to Virginia Tech only may be requested via your own university or public library's Interlibrary Loan program or through the VTechWorks request form that appears when you try to access the item. You might also be able to obtain a copy of the work through ProQuest's database of theses and dissertations. If you are on a Virginia Tech campus but are unable to find the pre-1997 thesis or dissertation you are seeking in VTechWorks, you may also be able to order a physical copy from library storage. Please check the library catalog at http://www.lib.vt.edu/ for physical copies.

The guidelines that apply to Virginia Tech's graduate students as ETD authors can be found at http://guides.lib.vt.edu/ETDguide.

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Now showing 1 - 20 of 273

Acceleration techniques for the radiative analysis of general computational fluid dynamics solutions using reverse Monte-Carlo ray tracing
Turk, Jeffrey A. (Virginia Tech, 1994)
A reverse Monte-Carlo ray trace capable of performing a radiative analysis on arbitrary multiple overlapping structured computational fluid dynamics solution sets is developed. In order to make effective use of time, a method based on a set of simplifying assumptions but using the same calculation procedures is developed for comparison and study purposes. Three acceleration techniques are tried. One acceleration technique reduces the grid dimensions to reduce the number of volumes intersected. The second acceleration technique develops a version of the code for execution in a parallel processing environment. The third acceleration technique mixes an orthogonal, evenly spaced grid with the computational fluid dynamics grids to obtain fast ray traversal of low variance areas while retaining the higher resolution of the computational fluid dynamics grids in the high variance areas. Two experimental data sets are used for comparison and as test cases during these studies: an exhaust plume from an auxiliary power unit, and a Boeing 747 in flight. Timing for the baseline and accelerated analyses is provided as well as numerical comparisons for a selected subset.
Acoustic influences on flow over an airfoil at low Reynolds numbers
Blanc, Philippe Francois (Virginia Polytechnic Institute and State University, 1989)
The dependence of an airfoil stall behavior upon the acoustic l environment was experimentally investigated at a Reynolds number of 200,000. The Wortmann FX-63-137-ESM airfoil section was used for the model with an aspect ratio of 4. Some acoustic disturbances could alter the transition process in the shear layer of the separation bubble on the upper surface of the airfoil. These disturbances could delay the deep leading edge stall or hasten stall recovery in some cases. A good agreement was found with the Crabtree criterion to predict the leading edge stall.
ACSYNT aerodynamic estimation: an examination and validation for use in conceptual design
Arledge, Thomas K. (Virginia Tech, 1993-05-27)
The aerodynamic prediction methodology available in ACSYNT is examined through comparison with aircraft data for a variety of classes of configurations. The predictions are a synthesis of the best empirical procedures currently available. The present work presents selected results obtained from the comparison, and shows how the basic capability can be enhanced by user supplied adjustments to represent changes in technology levels when considering advanced aircraft designs. The predictions and basis for adjustments are described for a supersonic cruise vehicle, a large subsonic transport, a typical fighter, an attack aircraft, and a typical business jet.
Active damping of a structure with low-frequency and closely- spaced modes: experiments and theory
Schamel, George C. (Virginia Polytechnic Institute and State University, 1985)
This thesis covers the investigation of active damping on a cruciform beam laboratory structure along with the development of this structure. Also important to this and other research was the development of a calibration apparatus that produces accurate, repeatable calibrations for several types of laboratory instruments. The cruciform beam model is developed out of a simpler beam-cable model with the addition of a crosspiece that produces a pair of closely-spaced modes. This model is developed theoretically and verified experimentally. Experimental verification is also obtained for theoretical results in the simultaneous design of a structure and control system. A spatial filtering method for determining the modal response of the structure from the physical response is also investigated.
An actively cooled floating element skin friction balance for direct measurement in high enthalpy supersonic flows
Chadwick, Kenneth Michael (Virginia Tech, 1992-12-14)
An investigation was conducted to design instruments to directly measure skin friction along the chamber walls of supersonic combustor models. Measurements were made in a combustor at the General Applied Science Laboratory (GASL) and in the Direct Connect Arcjet Facility (DCAF) supersonic combustor at the NASA AMES Research Center. Flow conditions in the high enthalpy combustor models ranged from total pressures of 275-800 psia (1900-5550 kPa) and total temperatures from 5800-8400 R (3222-4667 K). This gives enthalpies in the range of 1700-3300 BTU/Ib_m (3950-7660 KJ/kg) and simulated flight Mach number from 9 to 13. A direct force measurement device was used to measure the small tangential shear force resulting from the flow passing over a non-intrusive floating element. The floating head is mounted to a stiff cantilever beam arrangement with deflection due to the shear force on the order of 0.0005 in (0.0125 mm). This small deflection allows the balance to be a non-nulling type. Several measurements were conducted in cold supersonic flows to verify the concept and establish accuracy and repeatability. This balance design includes actively controlled cooling of the floating sensor head temperature through an internal cooling system to eliminate nonuniform temperature effects between the head and the surrounding chamber wall. This enabled the device to be suitable for shear force measurement in very hot flows. The key to this device is the use of a quartz tube cantilever with strain gages bonded at orthogonal positions directly on the surface at the base. A symmetric fluid flow was developed inside the quartz tube to provide cooling to the backside of the floating head. Bench tests showed that this did not influence the force measurement. Numerical heat transfer calculations were conducted for design feasibility and analysis, and to determine the effectiveness of the active cooling of the floating head. Analysis of the measurement uncertainty in cold supersonic flow tests show that uncertainty under 8% is achievable, but variations in the balance cooling during a particular test raised uncertainty up to 20% in these very hot flows during the early tests. Improvements to the strain gages and balance cooling reduced uncertainty for the later tests to under 15%.
Aerodynamic investigation of cylindrical and y-shaped building structures
Ghosh, Debasish (Virginia Polytechnic Institute and State University, 1982)
An aerodynamic investigation of cylindrical and Y-shaped building structures was carried out. Specific emphasis was directed towards determining the pressure distribution over three configurations: a flat top, a single dome and a dome with a counter-dome. The Y-model was tested in all three configurations while the cylindrical model was tested in dome/counter-dome configuration only. Both models were tested at Reynolds number(Re) of 360000 and 720000. Surface flow visualization was carried out to reveal regions of separation, recirculation and reattachment. It was found that a large region of negative pressure can be achieved with the dome counter-dome configuration. The maximum negative Cp for this configuration was about 50% higher than the maximum value observed with the flat top or single dome configuration. When the gap between the dome was decreased the maximum negative Cp increased; the increment being greater at the lower Re. Also, for the same Re and gap, the dome with higher curvature showed higher maximum negative Cp; the difference being more pronounced at the lower Re. The effect on the pressure distribution of an opening in the center of the lower dome and of an air flow through that opening was also investigated. It was observed that for injection rates corresponding to typical design flow rates required for ventilation of tall full scale buildings, the pressure distribution remains essentially unaltered.
Aerodynamic pitch-up of cranked arrow wings: estimation, trim, and configuration design
Benoliel, Alexander M. (Virginia Tech, 1994-05-06)
Low aspect ratio, highly-swept cranked arrow wing planforms are often proposed for high-speed civil transports. These wing planforms offer low supersonic drag without suffering greatly from low lift/drag ratios in low-speed flight. They can, however, suffer from pitch-up at modest angles of attack (as low as 5Â°) during low-speed flight due to leading edge vortex influence, flow separation and vortex breakdown. The work presented here describes an investigation conducted to study past research on the longitudinal aerodynamic characteristics of highly-swept cranked wing planforms, the development of a new method to estimate pitch-up of these configurations, and the applications of this new method to the analysis of tail designs for trim at high lift coefficients. The survey of past research placed emphasis on 1) understanding the problem of pitch-up, 2) ascertaining the effects of leading and trailing edge flaps, and 3) determining the benefits and shortfalls of tail, tailless, and canard configurations. The estimation method used a vortex lattice method to calculate the inviscid flow solution. Then, the results were adjusted to account for flow separation on the outboard wing section by imposing a limit on the equivalent 2-D sectional lift coefficient. The new method offered a means of making low cost estimates of the nonlinear pitching moment characteristics of slender, cranked arrow wing configurations with increased accuracy compared to conventional linear methods. Numerous comparisons with data are included. The new method was applied to analyze the trim requirement of slender wing designs generated by an aircraft configuration optimization and design program. The effects of trailing edge flaps and horizontal tail on the trimmed lift coefficient was demonstrated. Finally, recommendations were made to the application of this new method to multidisciplinary design optimization methods.
Aeroelasticity of wings coupling Navier-Stokes aerodynamics with wing-box finite elements
MacMurdy, Dale E. (Virginia Polytechnic Institute and State University, 1994)
Strong interactions between flow about an aircraft wing and the wing structure can result in aeroelastic phenomena which significantly impact aircraft performance. Time-accurate methods for solving the unsteady Navier-Stokes equations have matured to the point where reliable results can be obtained with reasonable computational costs for complex non-linear flows with shock waves, vortices and separations. The ability to combine such a flow solver with a general finite element structural model is key to an aeroelastic analysis in these flows. Earlier work involved time-accurate integration of modal structural models based on plate elements. A finite element model was developed to handle three-dimensional wing boxes, and incorporated into the flow solver without the need for modal analysis. Static condensation is performed on the structural model to reduce the structural degrees of freedom for the aeroelastic analysis. Direct incorporation of the finite element wing-box structural model with the flow solver requires finding adequate methods for transferring aerodynamic pressures to the structural grid and returning deflections to the aerodynamic grid. Several schemes were explored for handling the grid-to-grid transfer of information. The complex, built-up nature of the wing-box complicated this transfer. Aeroelastic calculations for a sample wing in transonic flow comparing various simple transfer schemes are presented and discussed.
Afterbody drag prediction for conceptual aircraft design
Squire, Douglas J. (Virginia Tech, 1992)
With the increasing performance demands placed on aircraft, the nozzle is becoming a much more versatile component. This versatility is increasing the afterbody complexity and the difficulty in afterbody drag prediction. This thesis documents a computationally inexpensive method for estimating the afterbody drag of aircraft during the conceptual design stage. The design and creation of a new hybrid approach to afterbody drag prediction is discussed. This hybrid approach uses the Integral Mean Slope Truncated Method in place of an equivalent body of revolution pressure drag. Corrections or drag deltas are then applied to the Integral Mean Slope Truncated afterbody drag estimate. These drag deltas account for various geometric effects and three-dimensional effects. The resulting prediction method is applicable to both axisymmetric and 2-D nozzles. The hybrid approach has been incorporated into ACSYNT Version 2.0.0 (AirCraft SYNThesis), a conceptual aircraft design tool developed at NASA Ames Research Center and Virginia Polytechnic Institute and State University. Comparison of the predictions of the new hybrid approach with those of a more complex and much more computationally expensive method show generally good agreement.
Aircraft cruise performance optimization using chattering controls
Bhardwaj, Pradeep (Virginia Tech, 1986-07-05)
Aircraft Cruise Performance is examined by using energy-state modelling to investigate fuel-range optimal trajectories. Chattering controls are considered appropriate when the hodograph is non-convex. Classical steady-state cruise, simple chattering-cruise and the extended chattering-cruise models are studied as constrained parameter-optimization problems. The term "extended chattering" refers to vehicle system modelling extended to maintain vertical equilibrium only on the average. Numerical solution is obtained using a variable-metric gradient-protection algorithm and computational results are presented for three different aircraft. This study shows that simple chattering cruise for certain specific energies can result in substantial fuel savings over classical steady-state cruise. However extended chattering cruise results in only marginal fuel savings when compared to simple chattering cruise.
Aircraft departure resistance prediction using structured singular values
York, Brent W. (Virginia Tech, 1993)
Research has been conducted in recent years to determine the dynamic behavior of aircraft in unusual flight attitudes, particularly at very high angle-of-attack or post-stall conditions. The possibility that future advanced fighter aircraft will have the ability to perform controlled maneuvers at such attitudes is indicated by the current military aircraft flying qualities specification, MIL-STD-1797. As it becomes more important to understand the dynamics of aircraft at such flight conditions, the need for a meaningful and useful assessment of aircraft departure resistance in varying attitudes will increase proportionally. This thesis surveys some of the measures of departure susceptibility currently in use and examines a candidate for a new departure resistance criterion which offers distinct advantages over the traditional metrics. The new departure resistance criterion, called DP_SSV is essentially a measure of how much uncertainty the nominally stable plant can tolerate before being driven unstable. DP_SSV is calculated using structured singular values. In this thesis, DP_SSV is calculated over various flight conditions for a typical high-performance fighter aircraft which is represented by a full six degree of freedom, nonlinear simulation. The results are compared with those obtained by using a traditional departure susceptibility metric and by examining the eigenvalues of linearized forms of the aircraft model. The new criterion DP_SSV is shown to provide more information about the departure susceptibility of an aircraft than C_{ηβ_DYN} traditional metric, and to produce results in good agreement with the eigenvalue analysis of the stability of the aircraft for the conditions studied. The interpretation of DP_SSV is discussed, and suggestions for future investigation are also presented.
Aircraft simulation validation using an instrumental variable approach
Weekley, Christopher D. (Virginia Tech, 1992)
A procedure is developed which offers the potential to validate aircraft simulation models using noisy flight test measurements. The proposed validation procedure is based on the instrumental variable parameter identification method. The instrumental variable method requires a choice of "instruments." For this research, the "instruments" are chosen using the response predicted by an available simulation model. With the “instruments” chosen from the predicted response, it is shown that the parameter estimates are correlated with only the measured input noise vector. In contrast, the generally used least-squares approach is shown to be correlated with both the state and input noise vectors. Several studies are presented to demonstrate the utility of the validation procedure. These studies include input variations and noise variations. The method is demonstrated using longitudinal and lateral/directional axis cases derived from a nonlinear simulation of a high performance fighter aircraft. The results are presented using time response comparisons, eigenvalue comparisons, and identified stability derivative comparisons. The case study results confirm that the instrumental variable method performs better than the least-squares technique when the state noise level is high and the input noise level is relatively low.
Aircraft stability and departure prediction using Eigenvalue Sensitivity analysis
Abbott, Troy D. (Virginia Tech, 1995-09-15)
A stability analysis and departure prediction method has been developed and coded in a MATLAB®-based software package called the Stability And Departure Analysis Tool using Eigenvalue Sensitivity (SADATES). Using eigenvalue and eigenvector analysis, SADATES is capable of performing a full-envelope stability analysis, returning both quantitative and qualitative data regarding the stability of the airplane at a static reference condition. SADATES not only supplies the analyst with information describing where and when an aircraft is likely to depart, but also information about the departure characteristics, enabling the analyst to design for better departure resistance. While the eigenvalue and eigenvector approach is straightforward, it is a broader approach than many traditional stability parameters, yielding more accurate and reliable results than traditional methods. SADATES also analyzes the aircraft dynamics from a standpoint of eigenvalue sensitivity. Using this feature, the analyst may directly study the impact of data uncertainty and non-zero angular rates on the nominal stability of the aircraft. Of particular interest are the effects of the dynamic damping derivatives, as these derivatives are particularly difficult to estimate. In addition, the effect of an unsteady reference condition may be examined by studying the sensitivity of the eigenvalues to changes in angular rates, thereby using a static approach to give answers to a dynamic problem. Given the development of eigenvalue sensitivity data, the analyst is able to determine the margin of error on nominal aircraft stability. The utility of the SADATES package is tested using aerodynamic data of the McDonnell-Douglas F / A-18C Hornet. Bare airframe, controls fixed stability is analyzed, and its sensitivity to data uncertainty and to non-zero angular rates is examined. The Hornet's bare airframe stability characteristics are then compared to those using an active feedback control system to drive an automatic leading and trailing edge flap schedule, demonstrating the accuracy and versatility of the program.
Airplane trajectory expansion for dynamics inversion
Munro, Bruce C. (Virginia Tech, 1992)
In aircraft research, there is keen interest in the procedure of determining the set of controls required to perform a maneuver from a definition of the trajectory. This is called the inverse problem. It has been proposed that if a complete set of states and state time derivatives can be derived from a trajectory then a model-following solution can allocate the controls necessary for the maneuver. This paper explores the problem of finding the complete state definition and provides a solution that requires numerical differentiation, fixed point iteration and a Newton's method solution to nonlinear equations. It considers trajectories that are smooth, piecewise smooth, and noise ridden. The resulting formulation was coded into a FORTRAN program. When tested against simple smooth maneuvers, the program output was very successful but demonstrated the limitations imposed by the assumptions and approximations in the development.
The analysis and control of nonlinear systems using Lyapunov stability theory
Thomas, Neil B. (Virginia Tech, 1996)
Techniques based on Lyapunov theory for the stability analysis and control of nonlinear systems are developed. In the first part of this work, procedures for determining conditions of guaranteed asymptotic stability are developed for nonlinear, uncontrolled systems. A second order model of an aircraft which is potentially unstable in pitch is used to demonstrate these methods. This approach is then expanded for use with systems of arbitrary order and applied to the investigation of a nonlinear model describing the lateral-directional motion of a departure prone aircraft. These investigations show that concepts based on Lyapunov stability can be used to effectively analyze nonlinear systems. A systematic method of finding an efficient controller is then developed for systems having controls which behave nonlinearly. These control techniques are demonstrated on a generic aircraft which exhibits nonlinear elevator behavior at high angles of attack. Although a conventional controller based on linear theory results in an unstable divergence, a Lyapunov based controller intelligently uses the available elevator power to augment the stability of the aircraft. A Lyapunov controller is then developed for use with the orbital Clohessy-Wiltshire equations of relative motion. When an engine unpredictably fails, this controller automatically accounts for the new conditions and the desired rendezvous is thus, still obtained. These Lyapunov based controllers are shown not only to perform well under highly nonlinear circumstances but to also maintain a high level of efficiency under more linear conditions.
Analysis and design of composite curved frames
Mason, Brian H. (Virginia Tech, 1994)
In this work, methods for rapid analysis and design of composite curved C-section frames subjected to axial tensile loading are developed. Failure is predicted using polynomial in-plane and interlaminar failure criteria. Interlaminar stresses can be directly computed only from three-dimensional finite element models, but the computational expense of these models is prohibitive. Therefore, approximate two-dimensional analysis methods are used here to predict interlaminar stresses in the curved corner regions between the web and flanges and at the free edges of the flanges. A response surface design approach is used to approximate the failure response using a minimum number of finite element analyses. Large degree of freedom 2D/3D global/local finite element models are selectively used in conjunction with the smaller 2D shell element models in the design process to improve the response surface polynomials. This combined use of simple and complex analyses is known as variable complexity modeling. Two design case studies are conducted, one with two design variables and one with five design variables. Three different objective function formulations are used in the two design variable case, minimum weight, maximum strength, and combined minimum weight and maximum strength. Only the minimum weight formulation is used in the five design variable case due to the complexity of the design space. The design studies demonstrate the accuracy and efficiency of the proposed approach.
Analysis of a target defense game
Migliore, Anthony T. (Virginia Polytechnic Institute and State University, 1989)
A maneuvering reentry vehicle (evader) and missile interceptor (pursuer) engagement is modeled in two dimensions, with constant velocity vehicles subject to bounded accelerations. Parameterized guidance laws are constructed for both players and a continuous, but finite-dimensional game is formulated. The related Majorant and Minorant game formulations are analyzed. Several modifications are provided in order to smooth performance contours. A comparison of the Majorant and Minorant game formulation shows the game is not strictly determined and there is a significant advantage to knowing the opponent’s strategy.
An analysis of curved flow wind tunnel testing
Mutchler, Mack Steele (Virginia Polytechnic Institute and State University, 1974)
The theory used to develop curved flow as a method of obtaining dynamic stability derivatives is presented including an analysis of the flows involved in the curved flow wind tunnel and in curved flight. Equations for the forces and moments for each of these flows are presented and then used to develop equations for the corrections to the forces and moments obtained in curved flow wind tunnel tests. An example of the physical setup and of the testing procedure for curved flow testing is also presented with some of the results from a typical test. The principles involved in several other methods of testing that are also used to obtain the dynamic stability derivatives are discussed so that a comparison may be made with the curved flow method.
Analysis of Stresses in Metal Sheathed Thermocouples in High-Temperature, Hypersonic Flows
Powers, Sean W. (Virginia Tech, 2020-04-17)
Flow temperature sensing remains important for many hypersonic aerodynamics and propulsion applications. Flight test applications, in particular, demand robust and accurate sensing, making thermocouple sensors attractive. Even for these extremely well-developed sensors, the prediction of stresses (hoop, radial, and axial) within thermocouple sheaths for custom-configured probes remains a topic of great concern for ensuring adequate lifetime of sensors. In contemporary practice, high-fidelity simulations must be run to prove if a new design will work at all, albeit at significant time and expense. Given the time and money it takes to run high-fidelity simulations, rapid optimization of sensor configurations is often impossible, or at a minimum, impractical. The developments presented in this Thesis address the need for hypersonic flow temperature sensor structural predictions which are compatible with rapid design iteration. The derivation and implementation of a new analytical, low-order model to predict stresses (hoop, radial, and axial) within the sheath of a thermocouple are provided. The analytical model is compared to high-fidelity ANSYS mechanical simulations as well as simplified experimental data. The predictions using the newly developed structural low-order model are in excellent agreement with the numerically simulated results and experimental results with an absolute maximum percent error of approximately 4% and 9.5%, respectively, thus validating the model. A MATLAB GUI composed of the combination of a thermal low-order model outlined in additional references [1] through [6] and the new structural low-order model for thermocouples was developed. This code is capable of solving a highly generalized version of the 1-D pin fin equation, allowing for the solution of the temperature distribution in a sensor taking into account conduction, convection, and radiation heat transfer which is not possible with other existing analytical solutions. This temperature distribution is then used in the analytical structural low-order model. This combination allows for the thermal and structural performance of a thermocouple to be found analytically and compared quickly with other designs.
An analysis of the aerodynamics of a fiber optic mortar projectile
Robertson, Edward Angus (Virginia Polytechnic Institute and State University, 1989)
In December of 1987 tests were conducted in the Virginia Tech Stability Wind Tunnel on a full-scale model of a fiber optic mortar projectile. The desired model configurations were sting-mounted on the Stability Tunnel STO-1 strain gauge balance. The sting was mounted on a streamlined vertical pylon which provided remote rotation in both pitch and yaw while maintaining the center of the balance along the tunnel centerline. The model inputs included the six-component force and moment data in body coordinates and the pressure data from the five pressure taps located within the model. The tunnel inputs were the static temperature, static pressure, and dynamic pressure. The angle of attack and yaw angle were input manually by the tunnel operator. The data analysis for the preliminary test program was intended to define the aerodynamic qualities of various components and configurations to aid in the redesign of the projectile.

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Browsing ETDs: Virginia Tech Electronic Theses and Dissertations by Department "Aerospace Engineering"

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