UMD Theses and Dissertations
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Item NONLINEAR PROPAGATION OF ORBITAL ANGULAR MOMENTUM LIGHT IN TURBULENCE AND FIBER(2024) Elder, Henry; Sprangle, Phillip; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Light that carries orbital angular momentum (OAM), also referred to as optical vortices or twisted light, is characterized by a helical or twisted wavefront ∝exp[imφ]. In contrast to spin angular momentum (SAM), where photons are limited to two states, OAM allows for, in principle, an infinite set of spatially orthogonal states. OAM-carrying light has found applications ranging from quantum key distribution in free space and guided-wave communication systems, particle trapping and optical tweezers, nanoscopy, and remote sensing. Understanding how OAM light propagates through complex environments, and how to efficiently generate particular OAM states, is critical for any such application. In the first part of this dissertation, we describe how OAM light propagates through a turbulent atmosphere. We build analytic models which describe (1) the OAM mode mixing caused by turbulence, (2) the evolution of short, high-power OAM pulses undergoing the effects of self-phase modulation (SPM) and group velocity dispersion (GVD), and (3) the evolution of high-power Gaussian pulses including SPM, GVD, and turbulence. The models are validated against both experimental data and nonlinear, turbulent pulse propagation simulation programs, the latter of which we have made freely available. We also explore how self-focusing can minimize certain deleterious effects of turbulence for OAM light. The second part of this dissertation considers nonlinear effects of OAM light propagating in azimuthally symmetric waveguides. Such waveguides have so-called spin-orbit (SO) modes, which are quantized based on their total angular momentum (TAM). We develop a generalized theory of four wave mixing-based parametric amplification of SO modes and show that these processes conserve TAM, but under certain circumstances can be taken to conserve SAM and OAM independently. Our theory is validated against a nonlinear multimode beam propagation simulation program which we developed and, again, have made freely available.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 Increasing Helicity towards Dynamo Action with Rough Boundary Spherical Couette Flows(2022) Rojas, Ruben; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamo action is the process through which a magnetic field is amplified and sustained by electrically conductive flows. Galaxies, stars and planets, all exhibit magnetic field amplification by their conductive constituents. For the Earth in particular, the magnetic field is generated due to flows of conductive material in its outer core. At the University of Maryland, our Three-meter diameter spherical Couette experiment uses liquid sodium between concentric spheres to mimic some of these dynamics, giving insight into these natural phenomena. Numerical studies of Finke and Tilgner (Phys. Rev. E, 86:016310, 2012) suggest a reduction in the threshold for dynamo action when a rough inner sphere was modeled by increasing the poloidal flows with respect to the zonal flows and hence increasing helicity. The baffles change the nature of the boundary layer from a shear dominated to a pressure dominated one, having effects on the angular momentum injection. We present results on a hydrodynamics model of 40-cm diameter spherical Couette flow filled with water, where torque and velocimetry measurements were performed to test the effects of different baffle configurations. The selected design was then installed in the 3-m experiment. In order to do that, the biggest liquid sodium draining operation in the history of the lab was executed. Twelve tons of liquid sodium were safely drained in a 2 hours operation. With the experiment assembled back and fully operational, we performed magnetic field amplification measurements as a function of the different experimental parameters including Reynolds and Rossby numbers. Thanks to recent studies in the hydrodynamic scale model, we can bring a better insight into these results. Torque limitations in the inner motor allowed us to inject only 4 times the available power; however, amplifications of more than 2 times the internal and external magnetic fields with respect to the no-baffle case was registered. These results, together with time-dependent analysis, suggest that a dynamo action is closer than before; showing the effect of the new baffles design in generating more efficient flows for magnetic field amplification. We are optimistic about new short-term measurement in new locations of the parameter space, and about the rich variety of unexplored dynamics that this novel experiment has the potential to reach. These setups constitute the first experimental explorations, in both hydrodynamics and magnetohydrodynamics, of rough boundary spherical Couette flows as laboratory candidates for successful Earth-like dynamo action.Item ADAPTIVITY IN WALL-MODELED LARGE EDDY SIMULATION(2022) Kahraman, Ali Berk; Larsson, Johan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In turbulence-resolving simulations, smaller eddies account for most of the computational cost. This is especially true for a wall-bounded turbulent flow, where a wall-resolved large eddy simulation might use more than 99% of the computing power to resolve the inner 10% of the boundary layer in realistic flows.The solution is to use an approximate model in the inner 10% of the boundary layer where the turbulence is expected to exhibit universal behavior, a technique generally called wall-modeled large eddy simulation. Wall-modeled large-eddy simulation introduces a modeling interface (or exchange location) separating the wall-modeled layer from the rest of the domain. The current state-of-the-art is to rely on user expertise when choosing where to place this modeling interface, whether this choice is tied to the grid or not. This dissertation presents three post-processing algorithms that determine the exchange location systematically. Two algorithms are physics-based, derived based on known attributes of the turbulence in attached boundary layers. These algorithms are assessed on a range of flows, including flat plate boundary layers, the NASA wall-mounted hump, and different shock/boundary-layer interactions. These algorithms in general agree with what an experienced user would suggest, with thinner wall-modeled layers in nonequilibrium flow regions and thicker wall-modeled layers where the boundary layer is closer to equilibrium, but are completely ignorant to the cost of the simulation they are suggesting. The third algorithm is based on the sensitivity of the wall-model with the predicted wall shear stress and a model of the subsequent computational cost, finding the exchangelocation that minimizes a combination of the two. This algorithm is tested both a priori and a posteriori using an equilibrium wall model for the flow over a wall-mounted hump, a boundary layer in an adverse pressure gradient, and a shock/boundary-layer interaction. This third algorithm also produces exchange locations that mostly agree with what an experienced user would suggest, with thinner layers where the wall-model sensitivity is high and thicker layers where this sensitivity is low. This suggests that the algorithm should be useful in simulations of realistic and highly complex geometries.Item Turbulent Transport and Mixing of Unconfined and Sloped Fire-Induced Flows Using a Laser-Assisted Saltwater Modeling Technique(2019) Maisto, Pietro; Gollner, Michael J.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The present work investigates turbulent, buoyant fire-induced flows using an experimental scaling technique known as saltwater modeling — a methodology enabling quantitative analysis of fire plumes built upon the analogy with saltwater (plume) flowing into the ambient water (air). The investigation, conducted by means of velocimetry (PIV) and concentration (PLIF) laser-based techniques, concerns unconfined plume mixing and transport, characterization of ceiling jet flows under sloped ceilings and activation of suppression devices in these sloped configurations. Flow imaging provides detailed measurements of velocity and saltwater concentration within the entire spatial and temporal domain of a planar section of the plume. In analogy with low-pass filtering in large eddy simulation (LES), a virtual, pixel-binning grid of varying size is overlaid on images to compute statistical moments representative of the larger and smaller scales. By leveraging actual measurements, converged statistics (first, second, higher-order) enables selection of cutoff resolutions, useful for validation and development of computational fluid dynamics (CFD) simulations. The saltwater plume's subsequent impingement onto a sloped plate generates a ceiling jet flowing both streamwise (up- and downslope) and spanwise with respect to the impingement point. Such flow is investigated to first build correlations predicting velocity and temperature along a sloped ceiling and second to analyze slope-related suppression device (sprinkler) activation. For the first task, single-planar, streamwise measurements are employed; for the second, multiple orthogonal laser sheets crossing the plate are used to generate a virtual grid of measured points. Transport characteristics are implemented into an activation model, modified to predict a dimensionless response time spatial distribution. At increasing slopes, the delay in the activation between upslope (faster) and downslope (delayed) devices progressively increases at increasing ceiling angles. This also occurs between sprinklers symmetrically located upslope and spanwise. From the response spatial distribution, the streamwise-to-spanwise correlation for the delay time (thermal responsiveness) is determined using the saltwater front arrival times. The analysis for the lag time reveals that the delay in thermal responsiveness between two sprinklers with the same activation time located up- and downslope, respectively, increases exponentially compared to that found for sprinklers located spanwise, at a quadratic rate with increasing angles.Item Integrated Field Inversion and Machine Learning With Embedded Neural Network Training for Turbulence Modeling(2019) Holland, Jonathan Richard; Baeder, James D; Duraisamy, Karthik; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A rich set of experimental and high fidelity simulation data is available to improve Reynolds Averaged Navier Stokes (RANS) models of turbulent flow. In practice, using this data is difficult, as measured quantities cannot be used to improve models directly. The Field Inversion and Machine Learning (FIML) approach addressed this challenge through an inference step, in the form of an inverse problem, which treats inconsistencies between the models and the data in a consistent manner. However, a separate learning algorithm is not always able to be learned from the generated inverse problem data accurately. Two new methods of incorporating higher fidelity data into RANS turbulence models via machine learning are proposed and applied for the first time in this thesis. Both build on the FIML framework by performing learning during the inference step, instead of considering the inference and learning steps separately as in the classic FIML approach. The first new approach embeds neural network learning into the RANS solver, and the second trains the weights of the neural network directly. Additionally, for the first time, the inverse problem can incorporate higher fidelity data from multiple cases simultaneously, promising to improve the generalization of the augmented model. The two new methods and the classic approach are demonstrated with a simple model problem, as well as a number of challenging RANS cases. For a 2D airfoil case, all three FIML augmentations are shown to improve predictions, with the new methods demonstrating increased regularization. Additionally, a model augmentation is generated by considering seven angles of attack of an airfoil in the inference step, and the augmentation is shown to improve predictions on a different airfoil. Additional cases are considered including a transonic shock wave boundary layer interaction and the NASA wall-mounted hump. In all cases, the inference is shown to improve predictions. For the first time, the inverse problem accounts for the limitations of the learning procedure, guaranteeing that the model discrepancy is optimal for the chosen learning algorithm. The results in this thesis prove that learning during the inference step provides additional regularization, and guarantees the inference produces learnable model discrepancy.Item Application of Uncertainty Quantication of Turbulence Intensity on Airfoil Aerodynamics(2017) Salahudeen, Atif; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Traditional CFD results have a number of freestream inputs. In the physical world, these input conditions often have some uncertainty associated with them. However, this uncertainty is often omitted from the CFD results. The effects of uncertainty in CFD can be determined through application of Uncertainty Quantification (UQ). The primary objective of the present work is to determine the effect of uncertainty in freestream turbulence intensity (FSTI) on the coefficients of lift, drag, and moment for four different airfoils: S809, NACA 0012, SC1095, and RC(4)-10. In this work, the Monte Carlo method is used to calculate the sensitivities of the aerodynamic coefficients to Gaussian distributions of uncertainty in FSTI over a range of angles of attack (AOA) at various Reynolds numbers and Mach numbers. However, the Monte Carlo method would require hundreds of thousands of CFD calculations in order to converge to the correct results. A surrogate surface is therefore generated using a parametric study using the in-house flow solver OVERTURNS. Rather than run a separate CFD run for each Monte Carlo run, all of the results can be attained virtually instantaneously via the surrogate surface. The UQ analysis shows how varying these parameters affects the sensitivies of the aerodynamic coefficients to uncertainty in FSTI. In most cases, the response is nearly Gaussian and the mean response is not too dierent from the discrete FSTI response without uncertainty. However, the output standard deviation for drag and pitching moment can become large when the transition location changes rapidly with changing FSTI.Item NONLINEAR SELF-CHANNELING OF HIGH-POWER LASERS THROUGH TURBULENT ATMOSPHERES(2018) DiComo, Gregory Putnam; Antonsen, Thomas; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A variety of laser applications have been considered which depend on long-distance atmospheric propagation of the beam to attain practical utility. The effectiveness of these applications is limited to some extent by beam distortions caused by atmospheric optical turbulence. Often the limiting factor is the instantaneous beam spreading due to turbulence, which makes it impossible to create a small laser spot at the receiver. In the absence of turbulence, laser beams of sufficient peak power propagating in atmosphere have been shown to undergo nonlinear self-guiding, in which the beam size remains constant over multiple Rayleigh lengths. Recent research suggests that self-guiding beams of sufficiently small diameter might exhibit resistance to turbulent spreading, in a propagation mode known as nonlinear self-channeling. Presented here is an experimental demonstration of such self-channeling through an artificially controlled turbulent atmosphere, with investigation into the region of parameter space over which it can occur. This research makes use of a distributed-volume turbulence generator and long propagation ranges at the Naval Research Laboratory and the Air Force Research Laboratory in order to produce a controlled propagation environment suitable for the study of high-power beams. Nonlinear self-channeling is found to resist the diffractive effects of turbulence, with its effectiveness decreasing significantly as the inner scale of turbulence decreases below the size of the beam.Item Assessment of Turbulence Length Scales in Hybrid RANS-LES Methods(2017) Jain, Nishan; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Separated flows are common in many scenarios of practical interest. Key examples of these scenarios include static stall over fixed wing aircraft and dynamic stall over rotorcraft blades. During rotor operation at high advance ratio, the stall events lead to loss in performance of the rotorcraft and may cause severe aerodynamic loads. In order to mitigate vibratory loads, it is important to evaluate the involved flow physics as accurately as possible. It is well known that a complex rotor flow field involving separation and reverse flow cannot be numerically predicted reliably by classical RANS model. At the other end, using high-fidelity approaches such as DNS and LES to resolve the rotor flow-field at practical Reynolds number is beyond the current computational capabilities. Therefore, the main objective of this work is to develop a high-fidelity modeling framework for capturing flow features that are important for predicting stall events while remaining computationally affordable. The framework employs and refines DES type hybrid RANS-LES methods along with specialized numerical techniques from literature to accurately resolve incipient separated flows under static and dynamic conditions. A baseline computational framework comprising of well established laminar-turbulent transition model, adverse pressure gradient (APG) correction and a low Mach number correction is selected as a starting point. By conducting simulations of flow over SC1095 airfoil at near-stall regime using the baseline framework, the importance of regulating eddy viscosity in the outer part of the shear layer is realized. Sub-grid length scales from the literature are implemented into the in-house computational solvers and their sensitivity in generating the eddy viscosity is investigated. A novel length scale called SSM length scale is proposed based on the properties of available length scales and the grid requirements in mildly separated flows. Proposed length scale demonstrated good predictive capabilities in mildly separated flows under static conditions by reducing eddy viscosity levels at the outer region boundary layer. Three-dimensional dynamic stall simulations are also conducted on flow over the modified VR12 airfoil. With SSM length scale, DDES method predictions agreed well with experimental data and captured the cycle-to-cycle variation of integrated aerodynamic quantities. The undesirable weakening of conventional shielding is observed due to proposed length scale in a highly resolved computational domain. A novel and stronger shielding formulation are proposed based on the properties of available length scales. The combination of new shielding and SSM length scale demonstrated good predictive capabilities in near stall regime without any undesirable effects. The combination also eliminated the need for adverse pressure gradient correction. The final computational framework proved to be robust towards grid resolution and varying flow separation and provided highly accurate aerodynamic characteristics for rotorcraft airfoils exhibiting stall events in the complete angle of attack range.Item THE EFFECTS OF SURFACE GRAVITY WAVES ON AIR-SEA MOMENTUM TRANSFER AND VERTICAL MIXING IN A FETCH-LIMITED, ESTUARINE ENVIRONMENT(2017) Fisher, Alexander William; Sanford, Lawrence P.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Surface gravity waves are the principal pathway through which momentum and energy are transferred from the atmosphere to the ocean. Recent studies have contributed to a growing recognition that wind events can be of leading-order importance for mixing and circulation in estuaries, yet the specific nature of air-sea momentum transfer in coastal environments remains relatively understudied. As part of a collaborative investigation of wind-driven estuarine physics, this dissertation addresses the role that surface gravity waves play in the transfer of momentum from the air to the oceanic surface boundary layer in a fetch-limited, estuarine environment. Using a combination of direct field observations and numerical simulations, the role of surface gravity waves in structuring momentum transfer and vertical mixing were examined for a range of wind, wave, and stratification conditions. Results indicate that inclusion of surface gravity waves in bulk parameterizations of wind stress reduced bias to below 5% for nearly all observed wind speeds and that up to 20% of wind stress variability within Chesapeake Bay was directly attributable to surface wave variability. Furthermore, the 10-meter neutral drag coefficient was shown to vary spatially by more than a factor of two over the extent of Chesapeake Bay as a result of combined wind and wave variability. Anisotropic fetch-limitation resulted in dominant wind-waves that were commonly and persistently misaligned with local wind forcing. Direct observations of stress above and below the water surface demonstrated that, within the oceanic surface layer, stress was more aligned with wave forcing than wind forcing. Accounting for the surface wave field was needed to close the local momentum budget between the atmosphere and the mean flow. Directly observed turbulent profiles showed that breaking waves dominated the transfer of momentum and energy and resulted in a three-layer turbulent response consisting of a wave transport layer, surface log layer, and stratified bottom boundary layer. Comparisons to commonly employed second-moment turbulence closures suggest that the presence of breaking waves homogenized the surface layer to a greater extent than predicted by present parameterizations of turbulent kinetic energy transport away from a source at the surface.