Browsing by Author "Pitt, Jonathan"

Now showing 1 - 11 of 11
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    Accelerating Conceptual Design Analysis of Marine Vehicles through Deep Learning
    Jones, Matthew Cecil (Virginia Tech, 2019-05-02)
    Evaluation of the flow field imparted by a marine vehicle reveals the underlying efficiency and performance. However, the relationship between precise design features and their impact on the flow field is not well characterized. The goal of this work is first, to investigate the thermally-stratified near field of a self-propelled marine vehicle to identify the significance of propulsion and hull-form design decisions, and second, to develop a functional mapping between an arbitrary vehicle design and its associated flow field to accelerate the design analysis process. The unsteady Reynolds-Averaged Navier-Stokes equations are solved to compute near-field wake profiles, showing good agreement to experimental data and providing a balance between simulation fidelity and numerical cost, given the database of cases considered. Machine learning through convolutional networks is employed to discover the relationship between vehicle geometries and their associated flow fields with two distinct deep-learning networks. The first network directly maps explicitly-specified geometric design parameters to their corresponding flow fields. The second network considers the vehicle geometries themselves as tensors of geometric volume fractions to implicitly-learn the underlying parameter space. Once trained, both networks effectively generate realistic flow fields, accelerating the design analysis from a process that takes days to one that takes a fraction of a second. The implicit-parameter network successfully learns the underlying parameter space for geometries within the scope of the training data, showing comparable performance to the explicit-parameter network. With additions to the size and variability of the training database, this network has the potential to abstractly generalize the design space for arbitrary geometric inputs, even those beyond the scope of the training data.
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    Anisotropic Turbulence Models for Wakes in an Active Ocean Environment
    Wall, Dylan Joseph (Virginia Tech, 2021-07-13)
    A set of second-moment closure turbulence models are implemented for the study of wake evolution in an oceanic environment. The effects of density stratification are considered, and the models are validated against laboratory experiments mimicking the stratified ocean environment, and against previous experimental study of wakes subjected to a density stratification. The turbulence models are found to reproduce a number of important behaviors which differentiate stratified wakes from those in a homogeneous environment, including the appropriate decay rates in turbulence quantities, buoyant suppression of turbulence length scales, and canonical stages in wake evolution. The existence of background turbulence is considered both through the introduction of production terms to the turbulence model equations and the replication of scale-resolved simulations of wakes embedded in turbulence. It is found that the freestream turbulence causes accelerated wake growth and faster decay of wake momentum. Wakes are then simulated at a variety of Re and Fr representative of full-scale vehicles operating in an ocean environment, to downstream distances several orders of magnitude greater than existing RANS studies. The models are used to make some general predictions concerning the dependence of late-wake behavior on these parameters, and specific insights into expected behavior are gained. The wake turbulence is classified using "fossil turbulence" and stratification strength criteria from the literature. In keeping with experimentally observed behavior, the stratification is predicted to increase wake persistence. It is also predicted that, regardless of initial Re or F r, the wake turbulence quickly becomes a mixture of overturning eddies and internal waves. It is found that the high Re wakes eventually become strongly affected by the stratification, and enter the strongly-stratified or LAST regime. Additional model improvements are proposed based on the predicted late wake behavior.
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    A Coupled OpenFOAM-WRF Study on Atmosphere-Wake-Ocean Interaction
    Gilbert, John; Pitt, Jonathan (MDPI, 2020-12-30)
    This work aims to better understand how small scale disturbances that are generated at the air-sea interface propagate into the surrounding atmosphere under realistic environmental conditions. To that end, a one-way coupled atmosphere-ocean model is presented, in which predictions of sea surface currents and sea surface temperatures from a microscale ocean model are used as constant boundary conditions in a larger atmospheric model. The coupled model consists of an ocean component implemented while using the open source CFD software OpenFOAM, an atmospheric component solved using the Weather Research and Forecast (WRF) model, and a Python-based utility foamToWRF, which is responsible for mapping field data between the ocean and atmospheric domains. The results are presented for two demonstration cases, which indicate that the proposed coupled model is able to capture the propagation of small scale sea surface disturbances in the atmosphere, although a more thorough study is required in order to properly validate the model.
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    Experimental and Numerical Investigations of the Aerodynamics of Flexible Inflatable Wings
    Desai, Siddhant Pratikkumar (Virginia Tech, 2022-06-22)
    With a look towards the future, which involves a push towards utilizing renewable energy sources and cementing energy independence for future generations, the design of more efficient aircraft and novel energy systems is of utmost importance. This dissertation looks at leveraging some of the benefits offered by inflatable wings for use in tethered kite-like systems towards the goal of designing a High Altitude Aerial Platform (HAAP). Uses of such a system include Airborne Wind Energy Systems (AWES), among others. The key bene- fit offered by such wings is their lightweight construction and durability, but challenges to aerodynamic performance arise out of their flexible nature and non-standard airfoil profile. Studying the aerodynamic behavior of such wings forms the critical focus of this research. This effort primarily encompasses an experimental investigation of two swept, tethered, inflatable wings conducted in the Virginia Tech Stability Wind Tunnel, and numerical CFD computations of these wings. The experiment was conducted in the modular wall configuration of the anechoic test section at speeds ranging from 15 − 32.5 m/s for three different tether attachment configurations and wings constructed out of two different fabric materials. Along with static aeroelastic deformation data using a 3D photogrammetry system, aerodynamic measurements were taken in the form of Pitot and static pressure measurements in the wake of the wing, force and moment measurements at the base of the mount, and tension measurements at the tether attachment locations. This provides a data set for validating static aeroelastic modeling approaches for such a system and highlights the dramatic effect of the variability in test configuration on the wing's aerodynamics. In addition to the wind tunnel tests, 3D steady RANS CFD computations of the rigid 3D scanned inflatable wing geometry were conducted in the wind tunnel environment for these configurations to validate the CFD modeling approach and highlight the level of detail necessary to accurately characterize the wing aerodynamic performance. Static aeroelastic deformation data from the 3D photogrammetry system, at a speed of 27.5 m/s, were also used to deform the 3D scanned inflatable wing geometry, and RANS CFD computations of this deformed inflatable wing were conducted at a wind tunnel speed of 27.5 m/s. Several turbulence models were investigated and comparisons were made with the wind tunnel test data. Good agreement was found with experimental data for the forces and moments and wake Pitot pressure coefficient contours. Comparisons were also made with the rigid wing CFD computations at the same tunnel speed of 27.5 m/s to illustrate the effect of static aeroelastic deformations on the aerodynamic performance, wake Pitot pressure coefficient contours and wing-tip vortex structures, of these flexible inflated wings. In effect, this research utilizes the synergy be- tween wind tunnel experiments and numerical CFD computations to study the flow behavior over inflatable wings and provide a comprehensive verification and validation approach for modeling such complex systems.
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    Machine Learning and Data Fusion of Simulated Remote Sensing Data
    Higgins, Erik Tracy (Virginia Tech, 2023-07-27)
    Modeling and simulation tools are described and implemented in a single workflow to develop a means of simulating a ship wake followed by simulated synthetic aperture radar (SAR) and infra-red (IR) images of these ship wakes. A parametric study across several different ocean environments and simulated remote sensing platforms is conducted to generate a preliminary data set that is used for training and testing neural network--based ship wake detection models. Several different model architectures are trained and tested, which are able to provide a high degree of accuracy in classifying whether input SAR images contain a persistent ship wake. Several data fusion models are explored to understand how fusing data from different SAR bands may improve ship wake detection, with some combinations of neural networks and data fusion models achieving perfect or near-perfect performance. Finally, an outline for a future study into multi-physics data fusion across multiple sensor modalities is created and discussed.
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    Measuring the Communicative Constitution of Partial Organizations as Complex Systems
    Schwing, Kyle Michael (Virginia Tech, 2023-05-11)
    Communicative acts constitute organizations as social entities. I build upon the most structured previous analysis of this process, the four flows framework, by introducing a complex systems model of how organization emerges along a continuum, thereby enabling measurement of the growth and decline of partial organizations. I validate my approach using simulated data from two stochastic agent-based models and 30 historical case studies of insurgency. I show that the four flows may be used to assess the historical victor of a conflict, or to track the emergence of an organization from real-time communication network data. My results demonstrate the complex interrelationship of the four flows, and how they relate to social phenomena such as information asymmetry, individual versus group interest, governance, and the development of community structure. I reaffirm the centrality of these flows to the phenomenon of organization, while challenging the minimum requirements for it to begin, by showing that organization spontaneously emerges in a population as a result of markers of affiliation and human cognitive biases.
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    Multi-Scale Localized Perturbation Method for Geophysical Fluid Flows
    Higgins, Erik Tracy (Virginia Tech, 2020-09-01)
    An alternative formulation of the governing equations of a dynamical system, called the multi-scale localized perturbation method, is introduced and derived for the purpose of solving complex geophysical flow problems. Simulation variables are decomposed into background and perturbation components, then assumptions are made about the evolution of these components within the context of an environmental flow in order to close the system. Once closed, the original governing equations become a set of one-way coupled governing equations called the "delta form" of the governing equations for short, with one equation describing the evolution of the background component and the other describing the evolution of the perturbation component. One-way interaction which arises due to non-linearity in the original differential equations appears in this second equation, allowing the background fields to influence the evolution of a perturbation. Several solution methods for this system of equations are then proposed. Advantages of the delta form include the ability to specify a complex, temporally- and spatially-varying background field separate from a perturbation introduced into the system, including those created by natural or man-made sources, which enhances visualization of the perturbation as it evolves in time and space. The delta form is also shown to be a tool which can be used to simplify simulation setup. Implementation of the delta form of the incompressible URANS equations with turbulence model and scalar transport within OpenFOAM is then documented, followed by verification cases. A stratified wake collapse case in a domain containing a background shear layer is then presented, showing how complex internal gravity wave-shear layer interactions are retained and easily observed in spite of the variable decomposition. The multi-scale localized perturbation method shows promise for geophysical flow problems, particularly multi-scale simulation involving the interaction of large-scale natural flows with small-scale flows generated by man-made structures.
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    Multi-Scale Localized Perturbation Method in OpenFOAM
    Higgins, Erik; Pitt, Jonathan; Paterson, Eric G. (MDPI, 2020-12-19)
    A modified set of governing differential equations for geophysical fluid flows is derived. All of the simulation fields are decomposed into a nominal large-scale background state and a small-scale perturbation from this background, and the new system is closed by the assumption that the perturbation is one-way coupled to the background. The decomposition method, termed the multi-scale localized perturbation method (MSLPM), is then applied to the governing equations of stratified fluid flows, implemented in OpenFOAM, and exercised in order to simulate the interaction of a vertically-varying background shear flow with an axisymmetric perturbation in a turbulent ocean environment. The results demonstrate that the MSLPM can be useful in visualizing the evolution of a perturbation within a complex background while retaining the complex physics that are associated with the original governing equations. The simulation setup may also be simplified under the MSLPM framework. Further applications of the MSLPM, especially to multi-scale simulations that encompass a large range of spatial and temporal scales, may be beneficial for researchers.
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    OpenFOAM Implementation of Microbubble Models for Ocean Applications
    Harris, David Benjamin (Virginia Tech, 2021-07-27)
    An investigation was carried out on the current state of the art in bubble modelling for computational fluid dynamics, and comparisons made between the different methods for both polydisperse and monodisperse multiphase flows. A multigroup method for polydisperse bubbly flows with the bubbles binned in terms of mass was selected from the various alternatives, which included other multigroup models and moment methods. The latter of these involve the integration of moments of the bubble number density function and transport of these quantities. The equations from this multigroup solver were then changed to more accurately and efficiently model cases involving extremely small bubbles over significant amounts of time, as the original model which was subsequently adapted had, as its primary purpose, simulation of larger bubbles over shorter periods of time. This was done by decoupling the gas and liquid momentum equations and adding an empirical rise velocity term for the bubbles. This new model was then partially implemented into OpenFOAM. The functioning of this new solver was confirmed by comparisons between the results and basic analytical solutions to the problems, as well as by means of comparison with another similar multiphase CFD solver (pbeTransportFoam). Following this confirmation of its functionality, the bubble model was implemented into another solver specifically designed for modelling wakes. Finally, the newly created solver was used to run some cases of interest involving a submerged wake.
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    Spacecraft-ns3: Spacecraft Discrete-Event Network Simulation
    Evans, Julianna Marie (Virginia Tech, 2020-06-24)
    As near-Earth space becomes more populated with large constellations of satellites and research into spacecraft autonomy and disaggregation becomes more prevalent, it will be increasingly important to design effective communication procedures between satellites to efficiently share resources and avoid collisions. Though there have been several space networking simulation tools created in recent years, they all lack rigorous astrodynamics models or use high-fidelity but bulky and computationally taxing commercial software. This research presents Spacecraft-ns3, an extension to the ns-3 network simulator. Using a modular approach, Spacecraft-ns3 propagates orbit state, plans discrete events, and analyzes network metrics and flows. A case study using Spacecraft-ns3 is presented for exploratory space network analysis.
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    Turbulent Simulations of a Buoyant Jet-in-Crossflow
    Martin, Christian Tyler (Virginia Tech, 2020-01-08)
    A lack of complex analysis for a thermally buoyant jet in a stratified crossflow has motivated the studies presented. A computational approach using the incompressible Navier--Stokes equations (NSE) under the Boussinesq approximation is utilized. Temperature and salinity scalar transport equations are utilized in conjunction with a linear equation of state (EOS) to obtain the density field and thus the buoyancy forcing. Comparing simulation data to experimental data of a point heat source in a stratified environment provides general agreement between the aforementioned computational model and the physics studied. From the literature surveyed, no unified agreement was presented on the selection of turbulence models for the jet--in--crossflow (JICF) problem. For this reason, a comparison is presented for a standard Reynolds--Averaged Navier--Stokes (RANS) and a hybrid Reynolds--Averaged Navier--Stokes/large eddy simulation (HRLES) turbulence model. The mathematical differences are outlined as well as the implications each model has on solving a buoyant jet in stratified crossflow. The RANS model provides a general over prediction of all flow quantities when comparing to the HRLES models. Studies involving the removal of the thermal component inside the jet as well as varying the environmental stratification strength have largely determined that these affects do not alter the near-field in any significant way, at least for a high Reynolds number JICF. The velocity ratio of the jet being the ratio of the jet velocity to the free--stream flow velocity. Deviating from a velocity ratio of one has provided information on the variability of the forcing on the plate the jet exits from, as well as in the integrated energy quantities far downstream of the jet's exit. The departures presented here show that any deviation from the unity value provides an increase in the overall forces seen by the plate. It was also found that the change in the integrated potential and turbulent kinetic energies is proportional to the deviation from a unity velocity ratio.