VTechWorks Repository :: Browsing by Author "Kapania, Rakesh K."

Browsing by Author "Kapania, Rakesh K."

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Accelerating Structural Design and Optimization using Machine Learning
Singh, Karanpreet (Virginia Tech, 2020-01-13)
Machine learning techniques promise to greatly accelerate structural design and optimization. In this thesis, deep learning and active learning techniques are applied to different non-convex structural optimization problems. Finite Element Analysis (FEA) based standard optimization methods for aircraft panels with bio-inspired curvilinear stiffeners are computationally expensive. The main reason for employing many of these standard optimization methods is the ease of their integration with FEA. However, each optimization requires multiple computationally expensive FEA evaluations, making their use impractical at times. To accelerate optimization, the use of Deep Neural Networks (DNNs) is proposed to approximate the FEA buckling response. The results show that DNNs obtained an accuracy of 95% for evaluating the buckling load. The DNN accelerated the optimization by a factor of nearly 200. The presented work demonstrates the potential of DNN-based machine learning algorithms for accelerating the optimization of bio-inspired curvilinearly stiffened panels. But, the approach could have disadvantages for being only specific to similar structural design problems, and requiring large datasets for DNNs training. An adaptive machine learning technique called active learning is used in this thesis to accelerate the evolutionary optimization of complex structures. The active learner helps the Genetic Algorithms (GA) by predicting if the possible design is going to satisfy the required constraints or not. The approach does not need a trained surrogate model prior to the optimization. The active learner adaptively improve its own accuracy during the optimization for saving the required number of FEA evaluations. The results show that the approach has the potential to reduce the total required FEA evaluations by more than 50%. Lastly, the machine learning is used to make recommendations for modeling choices while analyzing a structure using FEA. The decisions about the selection of appropriate modeling techniques are usually based on an analyst's judgement based upon their knowledge and intuition from past experience. The machine learning-based approach provides recommendations within seconds, thus, saving significant computational resources for making accurate design choices.
Acoustic Analysis of Spacecraft Cavities using the Boundary Element Method
Marshall, Peter Johannes (Virginia Tech, 2018-06-05)
Spacecraft structures are subject to a series of load environments during their service life, with the most severe of these occurring during the spacecraft's launch and ascension through the atmosphere. In particular, acoustic loads imposed on stowed satellites within the launch vehicle fairing can result in high mechanical loads on sensitive spacecraft hardware. These acoustic loads have the potential to damage important components and as such it is necessary to accurately characterize and predict the acoustic launch environment for a given mission. This research investigates the Sound Pressure Level (SPL) that can be measured in and around spacecraft cavities resulting from a known excitation and the resultant structural responses. Linear finite element analysis (FEA) is coupled with the Boundary Element method (BEM) to analyze spacecraft acoustic environments and corresponding structural responses at low frequencies on the order of the structural modes. Analytical capability for predicting acoustic environments inside the launch vehicle has improved significantly in recent years; however, while it is easy to perform an analysis and obtain results, the modeling effort can become unnecessarily complicated and analytical data can be hard to interpret. This work seeks to alleviate unnecessary complexity in the low-frequency regime of acoustic modeling by examining the fundamentals of coupled BEM-FEM analysis and applying simplification to a spacecraft model where possible to achieve results verified against direct field acoustic testing (DFAT) methods.
Acoustic Tomography and Thrust Estimation on Turbofan Engines
Gillespie, John Lawrie (Virginia Tech, 2023-12-21)
Acoustic sensing provides a possibility of measuring propulsion flow fields non-intrusively, and is of great interest because it may be applicable to cases that are difficult to measure with traditional methods. In this work, some of the successes and limitations of this technique are considered. In the first main result, the acoustic time of flight is shown to be usable along with a calibration curve in order to accurately estimate the thrust of two turbofan engines (1.0-1.5%). In the second, it is shown that acoustic tomography methods that only use the first ray paths to arrive cannot distinguish some relevant propulsion flow fields (i.e., different flow fields can have the same times of flight). In the third result we demonstrate, via the first validated acoustic tomography experiment on a turbofan engine, that a reasonable estimate of the flow can be produced despite this challenge. This is also the first successful use of acoustic tomography to reconstruct a compressible, multi-stream flow.
Aerodynamic Analysis of Variable Geometry Raked Wingtips for Mid-Range Transonic Transport Aircraft
Jingeleski, David John (Virginia Tech, 2012-12-21)
Previous applications have shown that a wingtip treatment on a commercial airliner will reduce drag and increase fuel efficiency and the most common types of treatment are blended winglets and raked wingtips. With Boeing currently investigating novel designs for its next generation of airliners, a variable geometry raked wingtip novel control effector (VGRWT/NCE) was studied to determine the aerodynamic performance benefits over an untreated wingtip. The Boeing SUGAR design employing a truss-braced wing was selected as the baseline. Vortex lattice method (VLM) and computational fluid dynamics (CFD) software was implemented to analyze the aerodynamic performance of such a configuration applied to a next-generation, transonic, mid-range transport aircraft. Several models were created to simulate various sweep positions for the VGRWT/NCE tip, as well as a baseline model with an untreated wingtip. The majority of investigation was conducted using the VLM software, with CFD used largely as a validation of the VLM analysis. The VGRWT/NCE tip was shown to increase the lift of the wing while also decreasing the drag. As expected, the unswept VGRWT/NCE tip increases the amount of lift available over the untreated wingtip, which will be very beneficial for take-off and landing. Similarly, the swept VGRWT/NCE tip reduced the drag of the wing during cruise compared to the unmodified tip, which will favorably impact the fuel efficiency of the aircraft. Also, the swept VGRWT/NCE tip showed an increase in moment compared to the unmodified wingtip, implying an increase in stability, as well providing an avenue for roll control and gust alleviation for flexible wings. CFD analysis validated VLM as a useful low fidelity tool that yielded quite accurate results. The main results of this study are tabulated "deltas" in the forces and moments on the VGRWT/NCE tip as a function of sweep angle and aileron deflection compared to the baseline wing. A side study of the effects of the joint between the main wing and the movable tip showed that the drag impact can be kept small by careful design.
Aerodynamic Performance of High Turning Airfoils and the Effect of Endwall Contouring on Turbine Performance
Abraham, Santosh (Virginia Tech, 2011-08-31)
Gas turbine companies are always focused on reducing capital costs and increasing overall efficiency. There are numerous advantages in reducing the number of airfoils per stage in the turbine section. While increased airfoil loading offers great advantages like low cost and weight, they also result in increased aerodynamic losses and associated issues. The strength of secondary flows is influenced by the upstream boundary layer thickness as well as the overall flow turning angle through the blade row. Secondary flows result in stagnation pressure loss which accounts for a considerable portion of the total stagnation pressure loss occurring in a turbine passage. A turbine designer strives to minimize these aerodynamic losses through design changes and geometrical effects. Performance of airfoils with varying loading levels and turning angles at transonic flow conditions are investigated in this study. The pressure difference between the pressure side and suction side of an airfoil gives an indication of the loading level of that airfoil. Secondary loss generation and the 3D flow near the endwalls of turbine blades are studied in detail. Detailed aerodynamic loss measurements, both in the pitchwise as well as spanwise directions, are conducted at 0.1 axial chord and 1.0 axial chord locations downstream of the trailing edge. Static pressure measurements on the airfoil surface and endwall pressure measurements were carried out in addition to downstream loss measurements. The application of endwall contouring to reduce secondary losses is investigated to try and understand when contouring can be beneficial. A detailed study was conducted on the effectiveness of endwall contouring on two different blades with varying airfoil spacing. Heat transfer experiments on the endwall were also conducted to determine the effect of endwall contouring on surface heat transfer distributions. Heat transfer behavior has significant effect on the cooling flow needs and associated aerodynamic problems of coolant-mainstream mixing. One of the primary objectives of this study is to provide data under transonic conditions that can be used to confirm/refine loss predictions for the effect of various Mach numbers and gas turning. The cascade exit Mach numbers were varied within a range from 0.6 to 1.1. A published experimental study on the effect of end wall contouring on such high turning blades at high exit Mach numbers is not available in open literature. Hence, the need to understand the parametric effects of endwall contouring on aerodynamic and heat transfer performance under these conditions.
Aeroelastic Analysis of Membrane Wings
Banerjee, Soumitra Pinak (Virginia Tech, 2007-08-22)
The physics of flapping is very important in the design of MAVs. As MAVs cannot have an engine that produces the amount of thrust required for forward flight, and yet be light weight, harnessing thrust and lift from flapping is imperative for its design and development. In this thesis, aerodynamics of pitch and plunge are simulated using a 3-D, free wake, vortex lattice method (VLM), and structural characteristics of the wing are simulated as a membrane supported by a rigid frame. The aerodynamics is validated by comparing the results from the VLM model for constant angle of attack flight, pitching flight and plunging flight with analytical results, existing 2-D VLM and a doublet lattice method. The aeroelasticity is studied by varying parameters affecting the flow as well as parameters affecting the structure. The parametric studies are performed for cases of constant angle of attack, plunge and, pitch and plunge. The response of the aeroelastic model to the changes in the parameters are analyzed and documented. The results show that the aerodynamic loads increase for increased deformation, and vice-versa. For a wing with rigid boundaries supporting a membranous structure with a step change in angle of attack, the membrane oscillates about the steady state deformation and influence the loads. For prescribed oscillations in pitch and plunge, the membrane deformations and loads transition into a periodic steady state.
Aeroelastic Analysis of Truss-Braced Wing Aircraft: Applications for Multidisciplinary Design Optimization
Mallik, Wrik (Virginia Tech, 2016-06-28)
This study highlights the aeroelastic behavior of very flexible truss-braced wing (TBW) aircraft designs obtained through a multidisciplinary design optimization (MDO) framework. Several improvements to previous analysis methods were developed and validated. Firstly, a flutter constraint was developed and the effects of the constraint on the MDO of TBW transport aircraft for both medium-range and long-range missions were studied while minimizing the take-off gross weight (TOGW) and the fuel burn as the objective functions. Results show that when the flutter constraint is applied at 1.15 times the dive speed, it imposes a 1.5% penalty on the take-off weight and a 5% penalty on the fuel consumption while minimizing these two objective functions for the medium-range mission. For the long-range mission, the penalties imposed by the similar constraint on the minimum TOGW and minimum fuel burn designs are 3.5% and 7.5%, respectively. Importantly, the resulting TBW designs are still superior to equivalent cantilever designs for both of the missions as they have both lower TOGW and fuel burn. However, a relaxed flutter constraint applied at 1.05 times the dive speed can restrict the penalty on the TOGW to only 0.3% and that on the fuel burn to 2% for minimizing both the objectives, for the medium-range mission. For the long-range mission, a similar relaxed constraint can reduce the penalty on fuel burn to 2.9%. These observations suggest further investigation into active flutter suppression mechanisms for the TBW aircraft to further reduce either the TOGW or the fuel burn. Secondly, the effects of a variable-geometry raked wingtip (VGRWT) on the maneuverability and aeroelastic behavior of passenger aircraft with very flexible truss-braced wings (TBW) were investigated. These TBW designs obtained from the MDO environment while minimizing fuel burn resemble a Boeing 777-200 Long Range (LR) aircraft both in terms of flight mission and aircraft configuration. The VGRWT can sweep forward and aft relative to the wing with the aid of a Novel Control Effector (NCE) mechanism. Results show that the VGRWT can be swept judiciously to alter the bending-torsion coupling and the movement of the center of pressure of wing. Such behavior of the VGRWT is applied to both achieve the required roll control as well as to increase flutter speed, and thus, enable the operation of TBW configurations which have up to 10% lower fuel burn than comparable optimized cantilever wing designs. Finally, a transonic aeroelastic analysis tool was developed which can be used for conceptual design in an MDO environment. Routine transonic aeroelastic analysis require expensive CFD simulations, hence they cannot be performed in an MDO environment. The present approach utilizes the results of a companion study of CFD simulations performed offline for the steady Reynolds Averaged Navier Stokes equations for a variety of airfoil parameters. The CFD results are used to develop a response surface which can be used in the MDO environment to perform a Leishman-Beddoes (LB) indicial functions based flutter analysis. A reduced-order model (ROM) is also developed for the unsteady aerodynamic system. Validation of the strip theory based aeroelastic analysis with LB unsteady aerodynamics and the computational efficiency and accuracy of the ROM is demonstrated. Finally, transonic aeroelastic analysis of a TBW aircraft designed for the medium-range flight mission similar to a Boeing 737 next generation (NG) with a cruise Mach number of 0.8 is presented. The results show the potential of the present approach to perform a more accurate, yet inexpensive, flutter analysis for MDO studies of transonic transport aircraft which are expected to undergo flutter at transonic conditions.
Aeroelasticity of Morphing Wings Using Neural Networks
Natarajan, Anand (Virginia Tech, 2002-07-03)
In this dissertation, neural networks are designed to effectively model static non-linear aeroelastic problems in adaptive structures and linear dynamic aeroelastic systems with time varying stiffness. The use of adaptive materials in aircraft wings allows for the change of the contour or the configuration of a wing (morphing) in flight. The use of smart materials, to accomplish these deformations, can imply that the stiffness of the wing with a morphing contour changes as the contour changes. For a rapidly oscillating body in a fluid field, continuously adapting structural parameters may render the wing to behave as a time variant system. Even the internal spars/ribs of the aircraft wing which define the wing stiffness can be made adaptive, that is, their stiffness can be made to vary with time. The immediate effect on the structural dynamics of the wing, is that, the wing motion is governed by a differential equation with time varying coefficients. The study of this concept of a time varying torsional stiffness, made possible by the use of active materials and adaptive spars, in the dynamic aeroelastic behavior of an adaptable airfoil is performed here. A time marching technique is developed for solving linear structural dynamic problems with time-varying parameters. This time-marching technique borrows from the concept of Time-Finite Elements in the sense that for each time interval considered in the time-marching, an analytical solution is obtained. The analytical solution for each time interval is in the form of a matrix exponential and hence this technique is termed as Matrix Exponential time marching. Using this time marching technique, Artificial Neural Networks can be trained to represent the dynamic behavior of any linearly time varying system. In order to extend this methodology to dynamic aeroelasticity, it is also necessary to model the unsteady aerodynamic loads over an airfoil. Accordingly, an unsteady aerodynamic panel method is developed using a distributed set of doublet panels over the surface of the airfoil and along its wake. When the aerodynamic loads predicted by this panel method are made available to the Matrix Exponential time marching scheme for every time interval, a dynamic aeroelastic solver for a time varying aeroelastic system is obtained. This solver is now used to train an array of neural networks to represent the response of this two dimensional aeroelastic system with a time varying torsional stiffness. These neural networks are developed into a control system for flutter suppression. Another type of aeroelastic problem of an adaptive structure that is investigated here is the shape control of an adaptive bump situated on the leading edge of an airfoil. Such a bump is useful in achieving flow separation control for lateral directional maneuverability of the aircraft. Since actuators are being used to create this bump on the wing surface, the energy required to do so needs to be minimized. The adverse pressure drag as a result of this bump needs to be controlled so that the loss in lift over the wing is made minimal. The design of such a "spoiler bump" on the surface of the airfoil is an optimization problem of maximizing pressure drag due to flow separation while minimizing the loss in lift and energy required to deform the bump. One neural network is trained using the CFD code FLUENT to represent the aerodynamic loading over the bump. A second neural network is trained for calculating the actuator loads, bump displacement and lift, drag forces over the airfoil using the finite element solver, ANSYS and the previously trained neural network. This non-linear aeroelastic model of the deforming bump on an airfoil surface using neural networks can serve as a fore-runner for other non-linear aeroelastic problems. This work enhances the traditional aeroelastic modeling by introducing time varying parameters in the differential equations of motion. It investigates the calculation of non-conservative aerodynamic loads on morphing contours and the resulting structural deformation for non-linear aeroelastic problems through the use of neural networks. Geometric modeling of morphing contours is also addressed.
ALMO: Active Learning-Based Multi-Objective Optimization for Accelerating Constrained Evolutionary Algorithms
Singh, Karanpreet; Kapania, Rakesh K. (MDPI, 2024-10-31)
In multi-objective optimization, standard evolutionary algorithms, such as NSGA-II, are computationally expensive, particularly when handling complex constraints. Constraint evaluations, often the bottleneck, require substantial resources. Pre-trained surrogate models have been used to improve computational efficiency, but they often rely heavily on the model’s accuracy and require large datasets. In this study, we use active learning to accelerate multi-objective optimization. Active learning is a machine learning approach that selects the most informative data points to reduce the computational cost of labeling data. It is employed in this study to reduce the number of constraint evaluations during optimization by dynamically querying new data points only when the model is uncertain. Incorporating machine learning into this framework allows the optimization process to focus on critical areas of the search space adaptively, leveraging predictive models to guide the algorithm. This reduces computational overhead and marks a significant advancement in using machine learning to enhance the efficiency and scalability of multi-objective optimization tasks. This method is applied to six challenging benchmark problems and demonstrates more than a 50% reduction in constraint evaluations, with varying savings across different problems. This adaptive approach significantly enhances the computational efficiency of multi-objective optimization without requiring pre-trained models.
Analyses of Ship Collisions: Determination of Longitudinal Extent of Damage and Penetration
Sajdak, John Anthony Waltham (Virginia Tech, 2004-07-29)
The overall objective of this thesis is to develop, validate and assess a probabilistic collision damage model to support ongoing work by the Society of Naval Architecture and Marine Engineering (SNAME) Ad Hoc Panel #6 and IMO working groups. It is generally agreed that structural design has a major influence on tanker oil outflow and damaged stability in grounding and collision, but crashworthiness is not considered in present regulations. The proposed methodology provides a practical means of considering structural design in a regulatory framework, and when implemented would improve the safety and environmental performance of ships. This thesis continues the development and applies a Simplified Collision Model (SIMCOL) to calculate damage extent (transverse, vertical and longitudinal) and oil outflow in ship collisions. The primary contribution of this thesis is the development and validation of a theory for the determination of energy absorbed in longitudinal extent of damage, and the implementation of the theory within SIMCOL. SIMCOL is sufficiently fast to be applied to thousands of collision cases as is required for a probabilistic analysis. The following specific tasks were completed using SIMCOL in support of this project: Completed the development of SIMCOL Version 3.0 including: 1) Deformable Bow sub model 2) Implementation and validation of theory for the determination of energy absorbed in longitudinal extent of damage. • Developed the capability to model collision events using LSDYNA. • Validated Virginia Tech LSDYNA ship collision modeling procedure. • Validated SIMCOL using real collision data, and probabilistic collision data for penetrating collisions.
Analysis and Design of a Morphing Wing Tip using Multicellular Flexible Matrix Composite Adaptive Skins
Hinshaw, Tyler (Virginia Tech, 2009-07-01)
The material presented in this thesis uses concepts of the finite element and doublet panel methods to develop a structural-aerodynamic coupled mathematical model for the analysis of a morphing wing tip composed of smart materials. Much research is currently being performed within many facets of engineering on the use of smart or intelligent materials. Examples of the beneficial characteristics of smart materials might include altering a structure's mechanical properties, controlling its dynamic response(s) and sensing flaws that might progressively become detrimental to the structure. This thesis describes a bio-inspired adaptive structure that will be used in morphing an aircraft's wing tip. The actuation system is derived from individual flexible matrix composite tube actuators embedded in a matrix medium that when pressurized, radical structural shape change is possible. A driving force behind this research, as with any morphing wing related studies, is to expand the limitations of an aircraft's mission, usually constrained by the wing design. Rather than deploying current methods of achieving certain flight characteristics, changing the shape of a wing greatly increases the flight envelope. This thesis gives some insight as to the structural capability and limitations using current numerical methods to model a morphing wing in a flow.
Analysis of Adiabatic Shear Banding in a Thick-Walled Steel Tube by the Finite Element Method
Rattazzi, Dean J. (Virginia Tech, 1996-09-02)
The initiation and propagation of adiabatic shear bands is analyzed numerically for an impulsively loaded thick-walled steel tube. A circumferential V-notch located at the outer surface of the center of the tube provides a stress concentration. The material is modeled as strain hardening, strain-rate hardening and thermal softening. The dynamic loading conditions considered are pure torsion, axial pressure combined with torsion, and internal pressure combined with torsion. Because of the stress concentration, a shear band will first initiate in an element adjoining the notch tip and propagate radially inwards through the thickness of the tube. The speed of propagation and the amount of energy required to drive a shear band through the material are calculated. The effects of the pressure preload and the depth of the notch are studied. Also, the influence of thermal softening is investigated by modeling it after a relation proposed by Zhou et al. [Vita removed July 18, 2008 CK/GMc 2/2/2012]
Analysis of Pressurized Arch-Shells
Goh, Julian Kok Seng (Virginia Tech, 1998-11-19)
A pressurized arch-shell structural component made of flexible material is considered. The component is inflated with high internal pressure. The behavior of similar types of structures, such as a pair of leaning pressurized arches and pressurized arch-supported membrane shelters, has been investigated in the past. More recently, several types of pressurized structures have been incorporated as part of the framework for a variety of structural systems. Particularly, the U.S. Army has been investigating the use of large lightweight and transportable pressurized arch-shell structures to be used as maintenance shelters for vehicles, helicopters, and airplanes. The formulated equations using thin shell theory are applied to a pressurized arch-shell component. A numerical investigation based on the Rayleigh-Ritz method is utilized to determine the behavior of arch-shells under various types of loading. The types of loading include a uniformly distributed vertical load representing snow, a wind load, and a horizontal side load distributed along the arc length. Deflections, stress resultants, and moments at various locations are computed for two types of shapes: circular and non-circular arch-shells.
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.
Analysis, Design and Testing of a Wind Tunnel Model to Validate Fiber-Optic Shape Sensing Systems
Montero, Ryan M. (Virginia Tech, 2013-06-14)
The ability to collect valuable data concerning the stress, strains, and shape profiles of aircraft and aircraft components during flight is important to fields such as structural health monitoring, gust alleviation, and flutter control. A research interest in the form of a NASA Phase I SBIR called for possible systems that would be able to take accurate shape sensing data on a flexible wing aircraft. In a joint venture between Luna Technologies Inc. and Virginia Polytechnic Institute and State University a flexible wing wind tunnel model was designed and constructed as a test article for the Luna Technologies Inc. fiber optic shape sensing system. In order to prove the capability of a fiber optic shape sensing system in a wind tunnel environment a flexible wing test article was constructed. The wing deflections and twists of the test article were modeled using a vortex lattice method called Tornado combined with simple beam theories. The beam theories were linear beam theories and the stiffness of the composite bodies was supplied by static testing of the test articles. The code was iterative in that it ran the VLM code to estimate the forces and moments on the wing and these were applied to a linear beam which gave the wing a new geometry which in turn was run through the VLM. The wind tunnel model was constructed at Virginia Tech using 3-D printing techniques for the fuselage and foam and fiberglass for the wings. On the bottom surface of the wings the Luna Technologies Inc. fiber optic shape sensing fiber was bonded along the leading and tailing edges. The swept-wing test article was experimentally tested in the Virginia Tech 6'x6' Stability Wind Tunnel at various airspeeds and the VLM based code results were in agreement, within margins of error and uncertainty, with the experimental results. The agreement of the analytical and experimental results verified the viability of using an iterative VLM code in combination with simple beam theories as a quick and relatively accurate approximation method for preliminary design and testing. The tests also showed that a fiber optic shape sensing system can be sufficiently tested in a wind tunnel environment, and if applied carefully could perhaps in the future provide useful shape and strain measurements.
Analysis, Design, and Experimentation of Beam-Like Structures
Miglani, Jitish (Virginia Tech, 2022-03-23)
Significant research is ongoing in the world to meet the needs of social and environmental crisis by harnessing wind and solar energy at high altitudes. One such approach is the use of an inflatable High Altitude Aerial Platform (HAAP). In the presented work, such periodically supported beam-like structures are analyzed using various mathematical models primarily modeling them as an equivalent beam using one-dimensional theories. The Euler-Bernoulli Theory (EBT) has been widely used for high aspect ratio beams, whereas the First Order Shear Deformation Theory (FSDT), or the Timoshenko beam theory, considers transverse shear effects and hence is superior in modeling low aspect ratio beams. First, an Isogeometric Analysis (IGA) is conducted using both FSDT and EBT to predict thermal buckling of periodically supported composite beams. Isogeometric analysis overcomes the limitations of the Gibbs phenomenon at discontinuities for a periodically supported beam using a higher order textit{k}-refinement. Next, an Integral Equation Approach (IEA) is implemented using EBT to obtain natural frequencies and buckling loads of periodically supported non-prismatic beams. Ill-conditioning errors were alleviated using admissible orthogonal Chebychev polynomials to obtain higher modes. We also present the prediction of the onset of flutter instability for metal plate and inflatable wing shaped foam test articles analyzed using finite element analysis (FEA). FEA updating based on modal testing and by conducting a geometrically nonlinear analysis resulted in a good agreement against the experiment tests. Furthermore, a nonlinear co-rotational large displacement/rotation FEA including the effects of the pressure as a follower forces was implemented to predict deformations of an inflatable structures. The developed FEA based tool namely Structural Analysis of Inflatables using FEA (SAIF) was compared with the experiments and available literature. It is concluded that the validity of the developed tool depends on the flexibility of the beam, which further depends upon the length of the beam and the bending rigidity of the beam. Inflatable structures analyzed with materials with high value of the Young's modulus and low to medium slenderness ratio tend to perform better against the experimental data. This is attributed to the presence of wrinkling and/or the Brazier effect (ovalling of the cross section) for flexible beams. The presented work has applications in programmable buckling, uncertainty quantification, and design of futuristic HAAP models to help face the upcoming environmental crises and meet the societal needs.
Analysis, shape sensitivities and approximations of modal response of generally laminated tapered skew plates
Singhvi, Sarvesh (Virginia Tech, 1991-05-06)
not available until scanned
Analytical and experimental comparison of deterministic and probabilistic optimization
Ponslet, Eric (Virginia Tech, 1994)
The probabilistic approach to design optimization has received increased attention in the last two decades. It is widely recognized that such an approach should lead to designs that make better use of the resources than designs obtained with the classical deterministic approach by distributing safety onto the different components and/or failure modes of a system in an optimal manner. However, probabilistic models rely on a number of assumptions regarding the magnitude of the uncertainties, their distributions, correlations, etc. In addition, modelling errors and approximate reliability calculations (first order methods for example) introduce uncertainty in the predicted system reliability. Because of these inaccuracies, it is not clear if a design obtained from probabilistic optimization will really be more reliable than a design based on deterministic optimization. The objective of this work is to provide a partial answer to this question through laboratory experiments — such experimental validation is not currently available in the literature. A cantilevered truss structure is used as a test case. First, the uncertainties in stiffness and mass properties of the truss elements are evaluated from a large number of measurements. The transmitted scatter in the natural frequencies of the truss is computed and compared to experimental estimates obtained from measurements on 6 realizations of the structure. The experimental results are in reasonable agreement with the predictions, although the magnitude of the transmitted scatter is extremely small. The truss is then equipped with passive viscoelastic tuned dampers for vibration control. The controlled structure is optimized by selecting locations for the dampers and for tuning masses added to the truss. The objective is to satisfy upper limits on the acceleration at given points on the truss for a specified excitation. The properties of the dampers are the primary sources of uncertainties. Two optimal designs are obtained from deterministic and probabilistic optimizations; the deterministic approach maximizes safety margins while the probability of failure (i.e. exceeding the acceleration limit) is minimized in the probabilistic approach. The optimizations are performed with genetic algorithms. The predicted probability of failure of the optimum probabilistic design is less than half that of the deterministic optimum. Finally, optimal deterministic and probabilistic designs are compared in the laboratory. Because small differences in failure rates between two designs are not measurable with a reasonable number of tests, we use anti-optimization to identify a design problem that maximizes the contrast in probability of failure between the two approaches. The anti-optimization is also performed with a genetic algorithm. For the problem identified by the anti-optimization, the probability of failure of the optimum probabilistic design is 25 times smaller than that of the deterministic design. The rates of failure are then measured by testing 29 realizations of each optimum design. The results agree well with the predictions and confirm the larger reliability of the probabilistic design. However, the probabilistic optimum is shown to be very sensitive to modelling errors. This sensitivity can be reduced by including the modelling errors as additional uncertainties in the probabilistic formulation.
Analytical and experimental study of control effort associated with model reference adaptive control
Messer, Richard Scott (Virginia Tech, 1992)
During the past decade, researchers have shown much interest in control and identification of Large Space Structures (LSS). Our inability to model these LSS accurately has generated extensive research into robust controllers capable of maintaining stability in the presence of large structural uncertainties as well as changing structural characteristics. In this work the performance of Model Reference Adaptive Control - (MRAC) is studied in numerical simulations and verified experimentally, to understand how differences between the plant and the reference model affect the control effort. MRAC is applied analytically and experimentally to a single-degree-of-freedom system and analytically to a multi-degree-of-freedom system with multi-inputs and multi-outputs. Good experimental and analytical agreement is demonstrated in control experiments and it is shown that MRAC does an excellent job of controlling the structures and achieving the desired performance even when large differences between the plant and ideal reference model exist. However, it is shown that reasonable differences between the reference model and the plant significantly increase the required control effort. The effects of increased damping in the reference model are considered, and it is shown that requiring the controller to provide increased damping actually decreases the required control effort when differences between the plant and reference model exist. This result is very useful because one of the first attempts to counteract the increased control effort due to differences between the plant and reference model might be to require less damping, however, this would actually increase the control effort. The use of optimization to successfully improve performance and reduce control effort is shown to be limited, because the actual control-structure system can not realize all the performance improvements of the analytical optimal system. Finally, it is shown that very large sampling rates may be required to accurately implement MRAC.
Analytical Modeling of the Mechanics of Nucleation and Growth of Cracks
Goyal, Vinay K. (Virginia Tech, 2002-11-15)
With the traditional fracture mechanics approaches, an initial crack and self-similar progression of cracks are assumed. In this treatise, theoretical and numerical tools are developed to mathematically describe non-self-similar progression of cracks without specifying an initial crack. A cohesive-decohesive zone model, similar to the cohesive zone model known in fracture mechanics as Dugdale-Barenblatt model, is adopted to represent the degradation of the material ahead of the crack tip. This model unifies strength-based crack initiation and fracture based crack progression. The cohesive-decohesive zone model is implemented with an interfacial surface material that consists of an upper and lower surface connected by a continuous distribution of normal and tangential nonlinear elastic springs that act to resist either Mode I opening, Mode II sliding, Mode III sliding, or mixed mode. The initiation of fracture is determined by the interfacial strength and the progression of fracture is determined by the critical energy release rate. The material between two adjacent laminae of a laminated composite structure or the material between the adherend and the adhesive is idealized with an interfacial surface material to predict interfacial fracture. The interfacial surface material is positioned within the bulk material to predict discrete cohesive cracks. The proper work-conjugacy relations between the stress and deformation measures are identified for the interfacial surface theory. In the principle of virtual work, the interfacial cohesive-decohesive tractions are conjugate to the displacement jumps across the upper and lower surfaces. A finite deformation kinematics theory is developed for the description of the upper and lower surface such that the deformation measures are invariant with respect to superposed rigid body translation and rotation. Various mechanical softening constitutive laws thermodynamically consistent with damage mechanics are postulated that relate the interfacial tractions to the displacement jump. An exponential function is used for the constitutive law such that it satisfies a multi-axial stress criterion for the onset of delamination, and satisfies a mixed mode fracture criterion for the progression of delamination. A damage parameter is included to prevent the restoration of the previous cohesive state between the interfacial surfaces. In addition, interfacial constitutive laws are developed to describe the contact-friction behavior. Interface elements applicable to two dimensional and three dimensional analyses are formulated for the analyses of contact, friction, and delamination problems. The consistent form of the interface element internal force vector and the tangent stiffness matrix are considered in the formulation. We investigate computational issues related to interfacial interpenetration, mesh sensitivity, the number of integrations points and the integration scheme, mathematical form of the softening constitutive law, and the convergence characteristics of the nonlinear solution procedure when cohesive-decohesive constitutive laws are used. To demonstrate the predictive capability of the interface finite element formulation, steadystate crack growth is simulated for quasi-static loading of various fracture test configurations loaded under Mode I, Mode II, Mode III, and mixed-mode loading. The finite element results are in agreement with the analytical results available in the literature and those developed in this work. A progressive failure methodology is developed and demonstrated to simulate the initiation and material degradation of a laminated panel due to intralaminar and interlaminar failures. Initiation of intralaminar failure can be by a matrix-cracking mode, a fiber-matrix shear mode, and a fiber failure mode. Subsequent material degradation is modeled using damage parameters for each mode to selectively reduce lamina material properties. The interlaminar failure mechanism such as delamination is simulated by positioning interface elements between adjacent sublaminates. The methodology is validated with respect to experimental data available in the literature on the response and failure of quasi-isotropic panels with centrally located circular cutouts. Very good agreement between the progressive failure analysis and the experiments is achieved if the failure analyses includes the interaction of intralaminar and interlaminar failures in the postbuckling response of the panels. In addition, ideas concerning the implementation of a fatigue model incorporated with a cohesive zone model are discussed.

Browsing by Author "Kapania, Rakesh K."

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