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 $p=w\rho$ with $w>-1/3$. 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 $-1/3<w\le 1$ 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 $K=0,\pm 1$ for perfect fluids with linear equation of state $p=w\rho$, where $\rho$ and $p$ 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 $K=0,-1$ and $-5/3<w<-1$ is a null non-scalar polynomial curvature singularity. We thus obtain the Penrose diagrams for all possible cases and identify $w=-5/3$ 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