3-D Adaptive Eulerian-Lagrangian Method for Multiphase Flows with Spacecraft Applications.

Understanding interfacial dynamics and fluid physics is important in many engineering applications, including spacecraft. Under microgravity, the moving boundaries and associated interfacial transport processes significantly impact the vehicle dynamics, design, and missions. However, it is difficult to mimic the micro-gravity condition experimentally. Numerical simulations of such problems are also challenging due to multiple time/length scales, large variations in fluid properties, moving boundaries, and phase changes. A 3-D adaptive Eulerian-Lagrangian method is implemented for multiphase flow computations. The stationary (Eulerian) Cartesian grid is used to resolve the flow field, and the marker-based triangulated moving (Lagrangian) surface meshes are utilized to treat the phase boundaries. A main focus of the present study is to treat both fluid and solid phase boundaries in a unified framework with a contact line force model and a phase change model. The fluid interfaces are modeled using a continuous interface method which smoothes both the variations in material properties and the influences of surface tension. The solid boundaries are treated by a ghost cell-based sharp interface method. A dynamic contact line force model is applied to calculate the position and movement of the solid-fluid-fluid interface. The energy and mass transfer due to phase change is computed using Stefan condition across the interfaces. A multi-level adaptive grid method is devised so that different length scales of the flow field can be resolved effectively. Selected studies on the interfacial dynamics relevant to spacecraft fuel delivery applications are conducted and assessed with experimental measurements and scaling analysis. For liquid fuel draining under microgravity, depending on the relative influence between capillary force and inertia force, three different flow regimes are observed and liquid residuals are measured. The liquid fuel sloshing under varying acceleration results in a large shift in its center of mass and significant influence on the vehicle dynamics. For thrust oscillation studies, the liquid surface stability under vertically oscillating acceleration is investigated, and the threshold acceleration is correlated with the forcing frequency, surface tension, and viscosity. For thermo-fluid transport computations with phase changes, validation studies are conducted with natural convection flows, Stefan problems, and melting processes by convection/diffusion flows.Ph.D.Aerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/78953/1/honeypot_1.pd

3-D Multiscale Adaptive Eulerian-Lagrangian Method for Multiphase Flows with Phase Change

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83641/1/AIAA-2010-1295-103.pd

Computations of Multiphase Fluid Flows Using Marker-Based Adaptive, Multilevel Cartesian Grid Method

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76194/1/AIAA-2007-336-338.pd

Modeling of Fuel Vapor Jet Eruption Induced by Local Droplet Heating

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140419/1/6.2014-1017.pd

Debiased Automatic Speech Recognition for Dysarthric Speech via Sample Reweighting with Sample Affinity Test

Automatic speech recognition systems based on deep learning are mainly trained under empirical risk minimization (ERM). Since ERM utilizes the averaged performance on the data samples regardless of a group such as healthy or dysarthric speakers, ASR systems are unaware of the performance disparities across the groups. This results in biased ASR systems whose performance differences among groups are severe. In this study, we aim to improve the ASR system in terms of group robustness for dysarthric speakers. To achieve our goal, we present a novel approach, sample reweighting with sample affinity test (Re-SAT). Re-SAT systematically measures the debiasing helpfulness of the given data sample and then mitigates the bias by debiasing helpfulness-based sample reweighting. Experimental results demonstrate that Re-SAT contributes to improved ASR performance on dysarthric speech without performance degradation on healthy speech.Comment: Accepted by Interspeech 202

A Unified Adaptive Cartesian Grid Method for Solid-Multiphase Fluid Dynamics with Moving Boundaries

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76397/1/AIAA-2007-4576-676.pd

Adaptive thermo-fluid moving boundary computations for interfacial dynamics

In this study, we present adaptive moving boundary computation technique with parallel implementation on a distributed memory multi-processor system for large scale thermo-fluid and interfacial flow computations. The solver utilizes Eulerian-Lagrangian method to track moving (Lagrangian) interfaces explicitly on the stationary (Eulerian) Cartesian grid where the flow fields are computed. We address the domain decomposition strategies of Eulerian-Lagrangian method by illustrating its intricate complexity of the computation involved on two different spaces interactively and consequently, and then propose a trade-off approach aiming for parallel scalability. Spatial domain decomposition is adopted for both Eulerian and Lagrangian domain due to easy load balancing and data locality for minimum communication between processors. In addition, parallel cell-based unstructured adaptive mesh refinement (AMR) technique is implemented for the flexible local refinement and even-distributed computational workload among processors. Selected cases are presented to highlight the computational capabilities, including Faraday type interfacial waves with capillary and gravitational forcing, flows around varied geometric configurations and induced by boundary conditions and/or body forces, and thermo-fluid dynamics with phase change. With the aid of the present techniques, large scale challenging moving boundary problems can be effectively addressed. © The Chinese Society of Theoretical and Applied Mechanics and Springer-Verlag Berlin Heidelberg 2012

Experimental Investigation of the Compression Ignition Process of High Reactivity Gasoline Fuels and E10 Certification Gasoline using a High-Pressure Direct Injection Gasoline Injector

Gasoline compression ignition (GCI) technology shows the potential to obtain high thermal efficiencies while maintaining low soot and NOx emissions in light-duty engine applications. Recent experimental studies and numerical simulations have indicated that high reactivity gasoline-like fuels can further enable the benefits of GCI combustion. However, there is limited empirical data in the literature studying the gasoline compression ignition process at relevant in-cylinder conditions, which are required for further optimizing combustion system designs. This study investigates the temporal and spatial evolution of the compression ignition process of various high reactivity gasoline fuels with research octane numbers (RON) of 71, 74 and 82, as well as a conventional RON 97 E10 gasoline fuel. A ten-hole prototype gasoline injector specifically designed for GCI applications capable of injection pressures up to 450 bar was used. Vapor and liquid penetration from high speed optical visualizations, as well as combustion measurement were studied in an optically accessible constant volume spray and combustion chamber. Near simultaneous shadowgraph and Mie scattering images were captured to investigate the spray characteristics. OH∗ chemiluminescence and natural luminosity images were recorded simultaneously to characterize the ignition process through two high-speed cameras. The experiments were conducted under a wide range of ambient charge gas conditions, including temperatures from 900 to 1200 Kelvin, charge gas pressures from 50 to 100 bar, oxygen levels from 10-21% to represent 0-50% exhaust gas recirculation (EGR) levels. The fuel was injected at 300 and 450 bar injection pressure. Results show that vapor penetration of the E10 and high reactivity gasoline fuels are similar, and the liquid penetration is related to the fuel density. With the OH∗ chemiluminescence images analysis, the ignition delay decreases, and the flame lift-off length moves upstream towards the injector tip with increasing ambient temperature, increasing charge gas pressure, increasing cetane number and decreasing EGR level. A gasoline ignition delay correlation and a lift-off length correlation considering the charge gas conditions and the fuel properties have been achieved