UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item Incorporation of Airfoil-Interactional Data to Improve the Accuracy of Stacked Rotor Performance Predictions in the Design Stage(2023) Costenoble, Miranda Banks; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, a methodology is presented for lower-fidelity modeling of stacked rotors, with improved accuracy compared to existing lower-fidelity rotor models. This methodology is built on a conventional prescribed vortex wake model of the rotor, with lifting-line airfoil blades. Such a lifting-line model cannot fully capture the aerodynamic interaction between the rotor blades, which are driven heavily by thickness and shape effects. To account for these effects, a high-fidelity 2D CFD code is used to model the airfoil-to-airfoil interaction along the span of the rotor. These 2D CFD loads are then injected into the lifting-line/prescribed wake rotor model, using an iterative technique to account for the changing deflections of the rotor blades. To accurately determine the airfoil-interactional loads along the span of the rotor, it is necessary to have some way to relate the conditions along the span of the rotor in 3D to the airfoil conditions in 2D, and vice-versa. Methods for parameterization of the airfoil system are presented, which account for both the geometry of the rotor/airfoil system and their aerodynamic conditions. Two different methods of relating the airfoil loads back to the rotor are presented, which offer different strategies depending upon the constraints of the underlying rotor model. Any rotor design must include selection of the airfoils on the blades, and stacked rotors are no different. To that end, 2D airfoil simulations are presented, which demonstrate both the necessity of the current methodology, and offer suggestions for future stacked rotor design. 2D airfoil loads are pre-computed (prior to the lower-fidelity rotor simulations) using an established 2D CFD code. Automation of this code allows for rapid generation of large data sets with minimal user input. The combined 2D CFD/3D prescribed wake methodology is presented and validated against recent experimental results. The baseline prescribed wake model is shown to significantly less variation of thrust and power with phase angle than the experiment. Inclusion of lifting-vortex airloads leads to improvements in thrust prediction, but with incorrect magnitude at small phase angles and incorrect power predictions. Only by including the 2D CFD airfoil-interactional airloads are the most accurate results achieved. Multiple inflow and coupling methods are also examined, with discussion of the strengths and limitations of each. Overall, it is believed that the current work should offer the potential for significantly faster and more accurate design of stacked rotors than was previously possible.Item Mesh Adaption for Tracking Vortex Structures in OVERTURNS Simulation of the S-76 Rotor in Hover(2016) Hayes, John Kaney; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The constant need to improve helicopter performance requires the optimization of existing and future rotor designs. A crucial indicator of rotor capability is hover performance, which depends on the near-body flow as well as the structure and strength of the tip vortices formed at the trailing edge of the blades. Computational Fluid Dynamics (CFD) solvers must balance computational expenses with preservation of the flow, and to limit computational expenses the mesh is often coarsened in the outer regions of the computational domain. This can lead to degradation of the vortex structures which compose the rotor wake. The current work conducts three-dimensional simulations using OVERTURNS, a three-dimensional structured grid solver that models the flow field using the Reynolds-Averaged Navier-Stokes equations. The S-76 rotor in hover was chosen as the test case for evaluating the OVERTURNS solver, focusing on methods to better preserve the rotor wake. Using the hover condition, various computational domains, spatial schemes, and boundary conditions were tested. Furthermore, a mesh adaption routine was implemented, allowing for the increased refinement of the mesh in areas of turbulent flow without the need to add points to the mesh. The adapted mesh was employed to conduct a sweep of collective pitch angles, comparing the resolved wake and integrated forces to existing computational and experimental results. The integrated thrust values saw very close agreement across all tested pitch angles, while the power was slightly over predicted, resulting in under prediction of the Figure of Merit. Meanwhile, the tip vortices have been preserved for multiple blade passages, indicating an improvement in vortex preservation when compared with previous work. Finally, further results from a single collective pitch case were presented to provide a more complete picture of the solver results.Item Numerical Simulation of Low-Pressure Explosive Combustion in Compartment Fires(2008-11-19) Hu, Zhixin (Victor); Trouve, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A filtered progress variable approach is adopted for large eddy simulations (LES) of turbulent deflagrations. The deflagration model is coupled with a non-premixed combustion model, either an equilibrium-chemistry, mixture-fraction based model, or an eddy dissipation model. The coupling interface uses a LES-resolved flame index formulation and provides partially-premixed combustion (PPC) modeling capability. The PPC sub-model is implemented into the Fire Dynamic Simulator (FDS) developed by the National Institute of Standards and Technology, which is then applied to the study of explosive combustion in confined fuel vapor clouds. Current limitations of the PPC model are identified first in two separate series of simulations: 1) a series of simulation corresponding to laminar flame propagation across homogeneous mixtures in open or closed tunnel-like configurations; and 2) a grid refinement study corresponding to laminar flame propagation across a vertically-stratified layer. An experimental database previously developed by FM Global Research, featuring controlled ignition followed by explosive combustion in an enclosure filled with vertically-stratified mixtures of propane in air, is used as a test configuration for model validation. Sealed and vented configurations are both considered, with and without obstacles in the chamber. These pressurized combustion cases present a particular challenge to the bulk pressure algorithm in FDS, which has robustness, accuracy and stability issues, in particular in vented configurations. Two modified bulk pressure models are proposed and evaluated by comparison between measured and simulated pressure data in the Factory Mutual Global (FMG) test configuration. The first model is based on a modified bulk pressure algorithm and uses a simplified expression for pressure valid in a vented compartment under quasi-steady conditions. The second model is based on solving an ordinary differential equation for bulk pressure (including a relaxation term proposed to stabilize possible Helmholtz oscillations) and modified vent flow velocity boundary conditions that are made bulk-pressure-sensitive. Comparisons with experiments are encouraging and demonstrate the potential of the new modeling capability for simulations of low pressure explosions in stratified fuel vapor clouds.