Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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 give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
2 results
Search Results
Item A GHOST CELL BASED IMMERSED BOUNDARY METHOD FOR WALL-MODELED LARGE-EDDY SIMULATIONS(2023) Ganju, Sparsh; Brehm, Christoph; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Turbulence is one of the most important unsolved problems in classical physics as it strongly affects skin friction and heat transfer rates and, thus, the performance of aerospace systems. Due to the high cost associated with flight and wind tunnel tests, there exists an ever-increasing demand for computational tools for engineering analysis. Hence, accurately predicting turbulent flows around aerospace vehicles is crucial for lowering safety factors, achieving optimal performance, and for the development of novel designs. However, simulating turbulent flows around complex geometries is a challenging task, especially for off-design conditions, where the flow may not be fully attached to the vehicle surface, e.g., close to stall conditions. A key challenge is that resolving all relevant turbulent scales is computationally intractable in a day-to-day engineering environment even with currently available computing hardware. For these complex turbulent flows, higher-fidelity simulation approaches, such as large eddy simulation (LES), are employed as they are able to provide the desired accuracy. In recent years, with the advancement of simulation methodologies and the accessibility of modern computer hardware, there has been a growing consensus within the Computational Fluid Dynamics (CFD) community that higher-fidelity turbulent flow simulations (including DES and wall-modeled LES, or WMLES) will be used more frequently complimenting lower-fidelity CFD approaches, such as Reynolds Averaged Navier-Stokes (RANS) approaches, which are generally accurate enough at more benign flow conditions. While conventional wall-resolved LES approaches are still significantly more expensive than RANS-based methods, WMLES can provide accurate turbulent flow predictions at a fraction of the cost compared to direct numerical simulations (DNS) or wall-resolved large-eddy simulations (WRLES). This is achieved by modeling only the large-scale flow structures as in conventional LES and in the vicinity of the wall, where most of the grid points are concentrated in WRLES, a wall model is utilized to account for the very small near wall flow structures. All aforementioned CFD approaches, i.e. RANS, LES, and DNS, rely on the creation of high-quality body-fitted grids that can be a time-consuming task depending on the geometric complexity of the fluid system being analyzed. The use of immersed boundary methods (IBM) on Cartesian grids can completely eliminate the manual mesh generation process, thus, reducing the overall time-to-solution. This dissertation assesses the viability of an unconventional simulation approach eliminating the time-consuming mesh generation approach by combining the IBM with WMLES. It was shown that in cases where the Cartesian grid is not aligned with the immersed boundary, the details of the numerical scheme, \textit{i.e.}, the order of accuracy, boundary treatment, \textit{etc.} play an important role in obtaining accurate solutions. Schemes with higher orders of accuracy lead to reduced numerical errors in the vicinity of the boundary providing better solutions. Furthermore, a physics informed boundary operator was used to provide a better representation of the mean flow in the near-wall region by using the gradient from the wall-model solution. A series of test cases are used to demonstrate the capabilities of this method for real-world applications and provide comparisons with conventional body-fitted results.Item N-body Simulations with Cohesion in Dense Planetary Rings(2011) Perrine, Randall; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation is primarily focused on exploring whether weak cohesion among icy particles in Saturn's dense rings is consistent with observations--and if so, what limits can be placed on the strength of such cohesive bonds, and what dynamical or observable consequences might arise out of cohesive bonding. Here I present my numerical method that allows for N-body particle sticking within a local rotating frame ("patch")--an approach capable of modeling hundreds of thousands or more colliding bodies. Impacting particles can stick to form non-deformable but breakable aggregates that obey equations of rigid body motion. I then apply the method to Saturn's icy rings, for which laboratory experiments suggest that interpenetration of thin, frost-coated surface layers may lead to weak bonding if the bodies impact at low speeds--speeds that happen to be characteristic of the rings. This investigation is further motivated by observations of structure in the rings that could be formed through bottom-up aggregations of particles (i.e., "propellers" in the A ring, and large-scale radial structure in the B ring). This work presents the implementation of the model, as well as results from a suite of 100 simulations that investigate the effects of five parameters on the equilibrium characteristics of the rings: speed-based merge and fragmentation limits, bond strength, ring surface density, and patch orbital distance (specifically the center of either the A or B ring), some with both monodisperse and polydisperse particle comparison cases. I conclude that the presence of weak cohesion is consistent with observations of the A and B rings, and present a range of parameters that reproduce the observed size distribution and maximum particle size. The parameters that match observations differ between the A and B rings, and I discuss the potential implications of this result. I also comment on other observable consequences of cohesion for the rings, such as optical depth and scale height effects, and discuss the unlikelihood that very large objects are grown bottom-up from cohesion of smaller ring particles. Lastly, I include a brief summary of other projects in ring dynamics I have undertaken before and during my thesis work.