567 research outputs found
High-fidelity Simulations for Rotating Detonation Engines
RDEs have drawn increased attention throughout the world as a viable technique for pressure gain combustion. An annular cylindrical combustor is used to drive a detonation wave azimuthally, which provides a continuous detonation process. RDEs provide a promising route to substantially increasing cycle efficiency compared to traditional cycles because of their ability to use shock-based compression to increase the pressure of the fluid in the combustor. Due to these characteristics, it is expected to bring revolutionary advancements to aviation and aerospace propulsion systems such as rocket engines, ramjet engines, and turbojet engines.
The goal of this dissertation is to provide the RDE community with a comprehensive database of full-scale RDE calculations for a variety of injector designs and operating conditions which enables design teams to make rapid progress for the realization. The main design challenge emerges from a non-premixed feed system where the fuel and oxidizer are injected separately into the combustion chamber. A non-premixed injection scheme is employed not only for safety and controllability, but also for an air-breathing RDE where the air stream will not come from a plenum, but rather through an intake. The main design challenge at this stage is developing a non-premixed fuel feed system that achieves adequate mixing and minimizes pressure losses while ensuring a reliable and safe detonation process. In order to rapidly accelerate such engineering design, comprehensive RDEs physics including chemistry, effects of complex geometry on detonation structures, and the complexity of the injection scheme need to be understood. With this mind, my dissertation will focus on the detailed detonation structure affected by the mixing process with a variety of injection geometries.
To perform large scale simulations of realistic RDEs geometry, a finite volume method (FVM)-based solver, named as UMdetFOAM, with following three key features is developed in this work: (1) implementation of schemes to reduce dispersive/dissipative errors at the detonation front where a spatial discontinuity exists, (2) the capability of dealing with complex geometries, and (3) the ability to incorporate user-specified chemical kinetics by coupling the FVM solver with a chemistry solver. These large-scale simulations using thousands of cores, validated in conjunction with the experimental group at U of M, provide detailed understanding into the performance of such detonation processes.
One of the main outcomes of this work is the development of a solver that enables the simulation of RDEs with the practical geometry. Furthermore, this dissertation demonstrated the effect of mixing-limited detonations on engine performance by identifying key sources of spurious losses. In particular, it was shown that turbulent mixing of fuel and air control the detonation processes. But, additional mixing with products of detonation can lead to premature ignition and parasitic losses. It was identified that the differential recovery of the injectors is the prime reason for the mixing-induced losses. These features were also found in other experimental studies, which validates the hypothesized flame processes.PHDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163166/1/takusato_1.pd
Future Person Localization in First-Person Videos
We present a new task that predicts future locations of people observed in
first-person videos. Consider a first-person video stream continuously recorded
by a wearable camera. Given a short clip of a person that is extracted from the
complete stream, we aim to predict that person's location in future frames. To
facilitate this future person localization ability, we make the following three
key observations: a) First-person videos typically involve significant
ego-motion which greatly affects the location of the target person in future
frames; b) Scales of the target person act as a salient cue to estimate a
perspective effect in first-person videos; c) First-person videos often capture
people up-close, making it easier to leverage target poses (e.g., where they
look) for predicting their future locations. We incorporate these three
observations into a prediction framework with a multi-stream
convolution-deconvolution architecture. Experimental results reveal our method
to be effective on our new dataset as well as on a public social interaction
dataset.Comment: Accepted to CVPR 201
Conformally Schwarzschild cosmological black holes
We thoroughly investigate conformally Schwarzschild spacetimes in different
coordinate systems to seek for physically reasonable models of a cosmological
black hole. We assume that a conformal factor depends only on the time
coordinate and that the spacetime is asymptotically flat
Friedmann-Lema\^{\i}tre-Robertson-Walker universe filled by a perfect fluid
obeying a linear equation state with . In this class of
spacetimes, the McClure-Dyer spacetime, constructed in terms of the isotropic
coordinates, and the Thakurta spacetime, constructed in terms of the standard
Schwarzschild coordinates, are identical and do not describe a cosmological
black hole. In contrast, the Sultana-Dyer and Culetu classes of spacetimes,
constructed in terms of the Kerr-Schild and Painlev\'{e}-Gullstrand
coordinates, respectively, describe a cosmological black hole. In the
Sultana-Dyer case, the corresponding matter field in general relativity can be
interpreted as a combination of a homogeneous perfect fluid and an
inhomogeneous null fluid, which is valid everywhere in the spacetime unlike
Sultana and Dyer's interpretation. In the Culetu case, the matter field can be
interpreted as a combination of a homogeneous perfect fluid and an
inhomogeneous anisotropic fluid. However, in both cases, the total
energy-momentum tensor violates all the standard energy conditions at a finite
value of the radial coordinate in late times. As a consequence, the
Sultana-Dyer and Culetu black holes for cannot describe the
evolution of a primordial black hole after its horizon entry.Comment: 58 pages, 10 figures, 8 tables; v3, this version corrects the
published version according to the corrigendum (2023 Class. Quantum Grav. 40,
079501). The main results remain unchange
Complete classification of Friedmann-Lema\^{i}tre-Robertson-Walker solutions with linear equation of state: parallelly propagated curvature singularities for general geodesics
We completely classify the Friedmann-Lema\^{i}tre-Robertson-Walker solutions
with spatial curvature for perfect fluids with linear equation of
state , where and are the energy density and pressure,
without assuming any energy conditions. We extend our previous work to include
all geodesics and parallelly propagated curvature singularities, showing that
no non-null geodesic emanates from or terminates at the null portion of
conformal infinity and that the initial singularity for and
is a null non-scalar polynomial curvature singularity. We thus
obtain the Penrose diagrams for all possible cases and identify as a
critical value for both the future big-rip singularity and the past null
conformal boundary.Comment: 21 pages, 7 figures, major revision, published in Class. Quantum Gra
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