A. James Clark School of Engineering

Permanent URI for this communityhttp://hdl.handle.net/1903/1654

The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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    Response of hypersonic boundary-layer disturbances to compression and expansion corners
    (2021) Butler, Cameron Scott; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    An experimental campaign was conducted at the University of Maryland - College Park to examine the impact of abrupt changes in surface geometry on hypersonic boundary-layer instability waves. A model consisting of a 5-degree conical forebody was selected to encourage the dominance of second-mode wavepackets upstream of the interaction region. Interchangeable afterbody attachments corresponding to flow deflections of -5-degree to +15-degree in 5-degree increments were considered. The adverse pressure gradient imposed by the +10-degree and +15-degree configurations caused the boundary layer to separate upstream, creating a region of recirculating flow. High-speed schlieren (440-822 kHz) was employed as the primary means of flow interrogation, with supplemental surface measurements provided by PCB132B38 pressure transducers. A lens calibration was applied to the images to provide quantitative fluctuations in density gradient. The high frame rate made possible the use of spectral analysis techniques throughout the entire field of view. This analysis reveals complex growth and decay trends for incoming second-mode disturbances. Additional, low-frequency content is generated by the deflected configurations. This is most pronounced for the separated cases where distinct, shear-generated disturbances are observed. Spectral proper orthogonal decomposition (SPOD) is demonstrated as a powerful tool for resolving the flow structures tied to amplifying frequencies. Nonlinear interactions are probed through bispectral analysis. Resonance of low-frequency structures is found to play a large role in nonlinear energy transfer downstream of the compression corners, particularly for the separated cases. Concave streamline curvature appears to result in concentrated regions of increased nonlinearity. These nonlinear interactions are shown to be spatially correlated with coherent flow structures resolved through SPOD. Finally, a limited computational study is carried out to demonstrate the ability of linear stability theory and the parabolized stability equations to reproduce experimental results obtained for the +10-degree extension. The development of the second-mode and shear-generated disturbances resolved by the computational analysis shows excellent agreement with the experimental results.
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    Development of Hybrid Air-Water Rotor Transition Thrust Prediction and Control
    (2020) Semenov, Ilya Yevgeniyevich; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Hybrid vehicles are able to function in some combination of aerial, underwater, and terrestrial environments, which greatly expands the scope of missions a vehicle can perform. Hybrid aerial-water (HAW) vehicles are a promising subcategory that are designed to operate in two vastly different fluid mediums. Multirotor HAW vehicles configurations have advantages in maneuverability, but pose a challenge in the water entry or water exit transitions. The interaction of a powered rotor with the air-water interface and its performance in a mixed air-water medium are poorly understood. Previous HAW vehicle strategies avoid a powered rotor with additional propulsion and buoyancy systems, constraining the design space. A custom test stand was constructed to better understand rotor performance during the air-water transition. By recording powered rotor performance during controlled water entries and exits in a large tank, several novel observations were made. Previously unrecorded phenomenon such as the gradual height and RPM dependent transition and the underwater ceiling effect are determined. These observations inform the development of the Transition Index TI, a novel metric that indicates the transition state of the rotor, without the need for specialized sensors or computationally intensive modeling. TI is applied to experimental data to make further observations, and is also used in a novel thrust prediction formulation. The first known low-order prediction of thrust through the transition is validated against experimental data, and allows for the development of a TI based controller. A preliminary controller implementation shows promising results in maintaining constant thrust through the air-water transition. Finally, a HAW vehicle to apply this controller is built. Careful consideration to the waterproofing and motor choice is shown and preliminary flight tests are demonstrated. Future expansion on the application of the novel TI and thrust prediction has great potential to advance the capabilities of hybrid aerial-water vehicles.
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    An Experimental Investigation of Hypersonic Boundary-Layer Transition on Sharp and Blunt Slender Cones
    (2019) Kennedy, Richard Edward; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Understanding the instabilities leading to the laminar-to-turbulent transition of a hypersonic boundary layer is a key challenge remaining for the design of efficient hypersonic vehicles. In the present study, experiments are performed in three different facilities at freestream Mach numbers between 6 and 14 to characterize instability mechanisms leading to transition on a 7-degree half-angle slender cone. Second-mode instability waves are visualized using a high-speed schlieren setup with the camera frame rate and spatial resolution optimized to allow individual disturbances to be tracked. In order to facilitate quantitative time-resolved measurements, a method of calibrating the schlieren system and novel image-processing algorithms have been developed. Good agreement is observed between the schlieren measurements, surface pressure measurements, and parabolized stability equation computations of the second-mode most-amplified frequencies and N factors. The high-frequency-resolution schlieren signals enable a bispectral analysis that reveals phase locking of higher harmonic content leading to nonlinear wave development. Individual disturbances are characterized using the schlieren wall-normal information not available from surface measurements. Experiments are also performed to investigate the effect of nose-tip bluntness. For moderate to large bluntness nose tips, second-mode instability waves are no longer visible, and elongated structures associated with nonmodal growth appear in the visualizations. The nonmodal features exhibit strong content between the boundary-layer and entropy-layer edges and are steeply inclined downstream. Simultaneously acquired surface pressure measurements reveal high-frequency pressure oscillations typical of second-mode instability waves associated with the trailing edge of the nonmodal features.
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    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.
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    Morphotropic Phase Boundaries in Tb1-xDyxFe2 Alloys
    (2013) Bergstrom Jr., Richard Eaton; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Magnetostrictive alloys, materials that change in dimension under an applied magnetic field, are desired candidates for transducers. Unfortunately, common magnetostrictive metals, alloys, and oxides produce such small strains that they are not a viable option. In the early 1960's rare earths were found to possess extraordinary magnetostriction values at cryogenic temperatures. When alloyed with traditional transition metals they form a Laves phase compound of the form AB2. These Laves phase compounds have shown large magnetostriction values, up to 2500με in TbFe2. A major drawback to using these materials as transducers is their huge magnetocrystalline anisotropy constants, K1 and K2. However, it was found that TbFe2 and DyFe2 have opposing signs of K1 and K2. A pseudo-binary alloy, Tb1-xDyxFe2 (Terfenol-D) TDFx, was formed to decrease the total magnetocrystalline anisotropy. The anisotropy reached a room temperature minimum for TDF73. It is suspected that this minimum of the anisotropy is accompanied by a morphotropic phase boundary (MPB) at which the crystal structure changes from tetragonal to rhombohedral. Unraveling the nature of the temperature and composition dependence of the magnetic and crystalline properties along this MPB is the primary focus of this thesis. The structure of the TDF alloys was probed through macroscopic and microscopic techniques. The maximum in the DC magnetization at the transition temperature from tetragonal to rhombohedral broadens as the transition temperature is increased. This is attributed to decreasing anisotropy at increased temperature. Synchrotron and neutron powder diffraction are utilized to elucidate the microscopic changes in the structure and magnetism. Neutron powder diffraction results were somewhat inconclusive but were sufficient to produce magnetic moments that were invariant, within experimental error, across the transition region. Synchrotron powder diffraction was used to probe the structure at temperatures across the MPB. Reitveld refinement of the structure in TDF65 reveals that large strain gradients exist across the MPBs. This was supplemented by temperature dependent scans of various TDF alloys showing a broadening of the phase transition with increasing temperature which we attribute a widening of the meta-stable [100] and [111] easy directions.