chihyung wen

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Papers by chihyung wen

Numerical modeling of tubular reactor for catalytic hydrogen generation via ammonia decomposition

Proceedings, Oct 19, 2023

has been considered as a potential energy storage solution. However, the costly and technologyint... more has been considered as a potential energy storage solution. However, the costly and technologyintensive infrastructure required for H2 transportation and storage has hindered its widespread adoption. Ammonia (NH3)based energy storage system is thus emerging as a promising alternative for storing and producing H2 without carbon emissions. However, the investigation of practical scale NH3-to-H2 reactor systems is strongly impeded by their labourintensive and time-consuming features. Employing the computational fluid dynamic (CFD) simulation, we establish a twodimensional model of a typical tubular fixed-bed reactor for NH3 decomposition on our recently developed Ru/NiO composite catalyst. NH3 conversion and H2 production for fuel cell applications are theoretically and numerically predicted. Through a series of numerical simulations and experimental verifications, the underlying impact of catalyst porosity and inlet gas temperature on system performance is further investigated. The result shows that, at 500 °C , the NH3 conversion can enhance from 13% to 49% with increasing porosity from 0.2 to 0.8, and from 47% to 86% by preheating inlet gas to wall temperatures. This work provides potential solutions for developing a scalable process to produce H2, giving rise to a future reference and inspiration for clean energy development and applications.

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A Low-Cost Relative Positioning Method for UAV/UGV Coordinated Heterogeneous System Based on Visual-Lidar Fusion

Aerospace

Unmanned Ground Vehicles (UGVs) and Unmanned Aerial Vehicles (UAVs) are commonly used for various... more Unmanned Ground Vehicles (UGVs) and Unmanned Aerial Vehicles (UAVs) are commonly used for various purposes, and their cooperative systems have been developed to enhance their capabilities. However, tracking and interacting with dynamic UAVs poses several challenges, including limitations of traditional radar and visual systems, and the need for the real-time monitoring of UAV positions. To address these challenges, a low-cost method that uses LiDAR (Light Detection and Ranging) and RGB-D cameras to detect and track UAVs in real time has been proposed. This method relies on a learning model and a linear Kalman filter, and has demonstrated satisfactory estimation accuracy using only CPU (Central Processing Unit)- in GPS (Global Positioning System)-denied environments without any prior information.

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Broadband design of acoustic metasurfaces for the stabilization of a Mach 4 boundary layer flow

Advances in Aerodynamics, Apr 6, 2022

A piecewise acoustic metasurface is designed to suppress the first mode while marginally amplifyi... more A piecewise acoustic metasurface is designed to suppress the first mode while marginally amplifying the Mack second mode in a Mach 4 flat-plate boundary layer (BL) flow. The results of linear stability theory (LST) and the e N method demonstrate the stabilization effect and transition delay performance, respectively. However, the direct numerical simulation (DNS) results indicate that the designed broadband acoustic metasurface actually weakly excites the first mode with a slightly larger fluctuating pressure amplitude at the surface, which is in contrast to the analysis of LST. The discrepancies are found to lie in the 'roughness' effect caused by the recirculation zones inside the microslits and the alternating expansion and compression waves induced at the slit edges, which significantly amplifies the first mode. For further clarification of the competitive mechanism between the acoustic stabilization and 'roughness' destabilization effects of metasurfaces on the first mode, a carefully designed metasurface is installed at the maximum growth rate region, which excites the first mode on the metasurface but inhibits its development downstream.

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Establishment of Hypersonic Shock-Wave/Boundary-Layer Interaction over a Double Wedge

Investigation on Vibrational Nonequilibrium Effect on ZND Detonation Model

This paper reports a modified steady one-dimensional Zel’dovich–von Neumann–Doring detonation mod... more This paper reports a modified steady one-dimensional Zel’dovich–von Neumann–Doring detonation model that considers a vibrational nonequilibrium effect. The Landau–Teller model is adapted for the translational–rotational to vibrational mode exchange rate, and Park’s two-temperature model is applied in the single-step Arrhenius equation. The Millikan and White method is chosen to model the vibrational relaxation time. In this modified model, α is introduced and is defined as the ratio of the specific heat capacity related to the translational–rotational mode only versus the total specific heat capacity at constant pressure. Changes in half-reaction zone length and predicted postshock thermodynamic properties are observed in this modified profile across the postshock state to the Chapman–Jouguet state. The findings agree with a previous numerical simulation of gas detonation with detailed chemistry assessment, in which detonation cell size changes under a vibrational nonequilibrium assumption.

Numerical Study of Hydrogen–Air Detonation in Vibrational Non-equilibrium

The effects of vibrational non-equilibrium and vibration–chemistry coupling on hydrogen–air deton... more The effects of vibrational non-equilibrium and vibration–chemistry coupling on hydrogen–air detonation are numerically investigated by solving reactive Euler equations coupled with a multiple vibrational temperature-based model. Detailed hydrogen–air reaction kinetic is utilized, Landau–Teller model is adopted to solve the vibrational relaxation process, and the coupled vibration–chemistry vibration model is used to evaluate the vibration–chemistry coupling. It is shown that the relaxation process and vibration–chemistry coupling considerably influence the hydrogen–air detonation structure, highlighting the importance of correct treatment of vibrational non-equilibrium in detonation simulations.

Heat transfer and behavior of the Reynolds stress in Mach 6 boundary layer transition induced by first-mode oblique waves

Physics of Fluids

This paper investigates a Mach 6 oblique breakdown via direct numerical simulation in conjunction... more This paper investigates a Mach 6 oblique breakdown via direct numerical simulation in conjunction with stability and quadrant analyses. Particular emphasis is placed on, first, the heat transfer and mean flow distortion in the near-wall and outer transitional boundary layer, and, second, the flow events that are responsible for the production of the Reynolds stress. The energy budget reveals that enhancement of viscous dissipation due to mean flow distortion dominates the heat transfer overshoot, while the dissipation due to fluctuations is lesser but not negligible. Downstream of the location of the peak mean heat flux, the wall temperature gradient (non-dimensionalized by the freestream temperature and local boundary layer thickness) varies little, owing to the occurrence of breakdown and the establishment of self-similarity. Renormalized by the boundary layer thickness, a new correlation of the Stanton number shows no overshoot or difference between the original overshoot region ...

Autonomous Exploration of Unknown Indoor Environments for High-Quality Mapping Using Feature-Based RGB-D SLAM

Sensors

Simultaneous localization and mapping (SLAM) system-based indoor mapping using autonomous mobile ... more Simultaneous localization and mapping (SLAM) system-based indoor mapping using autonomous mobile robots in unknown environments is crucial for many applications, such as rescue scenarios, utility tunnel monitoring, and indoor 3D modeling. Researchers have proposed various strategies to obtain full coverage while minimizing exploration time; however, mapping quality factors have not been considered. In fact, mapping quality plays a pivotal role in 3D modeling, especially when using low-cost sensors in challenging indoor scenarios. This study proposes a novel exploration algorithm to simultaneously optimize exploration time and mapping quality using a low-cost RGB-D camera. Feature-based RGB-D SLAM is utilized due to its various advantages, such as low computational cost and dense real-time reconstruction ability. Subsequently, our novel exploration strategies consider the mapping quality factors of the RGB-D SLAM system. Exploration time optimization factors are also considered to se...

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On the Vibrational State-Specific Modeling of Radiating Normal Shocks in Air

AIAA Journal, Feb 11, 2022

A comprehensive state-to-state (StS) model for air was formed -including vibrationvibration-trans... more A comprehensive state-to-state (StS) model for air was formed -including vibrationvibration-translation (VVT) reactions. The generated VVT reaction rates were compared to available first principles calculations and reasonable agreement was obtained. The influence of multi-quantum transitions revealed that the possible reduction on the number of VVT transitions depends on the application. The influence of VV transitions revealed that the vibrational excitation becomes too fast if VVT transitions are reduced to VT transitions, invalidating this approximation. Comparisons were made with existing NO emission measurements in the UV and mid-IR spectrum. At velocities of 3 -4 km/s, some of the current O2 dissociation rates may be inaccurate. At a higher velocity of 6.81 km/s, the NO mole fraction predicted by the StS model is around an order of magnitude greater than that predicted by the two-temperature model. Finally, recommendations were given on the further development of the state-to-state model.

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On the conservative property of particle-based Fokker–Planck method for rarefied gas flows

Physics of Fluids, 2020

Boltzmann-based second-order constitutive models of diatomic and polyatomic gases including the v... more

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Nomenclature

The effect of nonequilibrium recombination after a curved two-dimensional shock wave in a hyperve... more The effect of nonequilibrium recombination after a curved two-dimensional shock wave in a hypervelocity dissociating flow of an inviscid Lighthill-Freeman gas is considered. An analytic solution is obtained with the effective shock values derived by Hornung (1976) and the assumption that the flow is ‘quasi-frozen ’ after a thin dissociating layer near the shock. The solution gives the expression of dissociation fraction as a function of temperature on a streamline. It can then provide a rule of thumb to check the validity of binary scaling for the experimental conditions and a tool to determine the limiting streamline which delineates the validity zone of binary scaling. The effects upon the nonequilibrium chemical reaction of the large difference in free stream temperature between free-piston shock tunnel and equivalent flight conditions are discussed. Numerical examples are presented and the results are compared with solutions obtained with two-dimensional Euler equations using Ca...

Multilayer Mapping Kit for Autonomous UAV Navigation

IEEE Access, 2021

Mapping, as the back-end of perception and the front-end of path planning in the modern UAV navig... more Mapping, as the back-end of perception and the front-end of path planning in the modern UAV navigation system, draws our interest. Considering the requirements of UAV navigation and the features of the current embedded computation platforms, we designed and implemented a novel multilayer mapping framework. In this framework, we divided the map into three layers: awareness, local, and global. The awareness map is constructed in cylindrical coordinate, enabling fast raycasting. The local map is a probability-based volumetric map. The global map adopts dynamic memory management, allocating memory for the active mapping area, and recycling the memory from the inactive mapping area. We implemented this mapping framework in three parallel threads: awareness thread, local-global thread, and visualization thread. Finally, we evaluated the mapping kit in both the simulation environment and the real-world scenario with the vision-based sensors. The framework supports different kinds of map outputs for the global or local path planners. The implementation is open-source for the research community.

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A Numerical Study of Droplet Impingement for In-Flight Ice Accretion Prediction

Volume 1A, Symposia: Turbomachinery Flow Simulation and Optimization; Applications in CFD; Bio-Inspired and Bio-Medical Fluid Mechanics; CFD Verification and Validation; Development and Applications of Immersed Boundary Methods; DNS, LES and Hybrid RANS/LES Methods; Fluid Machinery; Fluid-Structu..., 2016

Droplet impingement is the basic module in both ice accretion and anti-icing numerical calculatio... more Droplet impingement is the basic module in both ice accretion and anti-icing numerical calculation. A three dimensional finite volume approach with the capacity of modeling the in-flight droplet impingement on a wide range of subsonic regime is therefore established in this research, using OpenFOAM®. The Eulerian model is applied to estimate the droplet flow field with the same computational grid sets as those of the air flow calculation. The roughness effect caused by ice accretion is considered in the wall function modeling. Thus, the collection efficiency could be investigated for further icing numerical simulations. This approach is validated on both cylinder and sphere benchmark cases. The results are compared with the corresponding experimental and LEWICE (LEWis ICE accretion program) simulation data.

State-Specific Study of Air in the Expansion Tunnel Nozzle and Test Section

AIAA Journal, 2022

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Thermal Effect on the Performance of an Alternating-Current Dielectric-Barrier-Discharge Plasma Actuator

AIAA Journal, 2020

The dielectric barrier discharge (DBD) plasma actuator is a popular technology for active flow co... more The dielectric barrier discharge (DBD) plasma actuator is a popular technology for active flow control; however, the influence of the heat generated by the actuator on its performance is seldom mentioned. In this work, an experimental investigation is conducted to evaluate the interaction between spontaneous heat generation and the performance of an AC-DBD plasma actuator. The characteristics of the AC-DBD plasma actuator are examined temporally in quiescent air, including the profile of the induced flow, capacitance properties, power consumption, plasma light emission, and surface temperature. The particle image velocimetry shows that the velocity profile of the induced flow increases temporally, indicating enhanced momentum injection by the AC-DBD plasma actuator. The capacitance, power consumption, plasma brightness and surface temperature increase with the operation time analogously to exponential curves (f (x) = a − b exp −cx). And values of these properties are proportional to 3.5 power of the applied voltage. The dielectric surface is categorized into three typical streamwise regions according to the heat generation characteristics: the plasma region, the insulated electrode region, and the far field region. The dominant heat generation occurs in the plasma region due to the plasma discharge. The temperature increase of the local dielectric and the gas-plasma mixture enlarges the actuator capacitance, benefits the local induced electric field, and results in longer mean free paths of particles and stronger discharges accordingly. Thus, the spontaneous heat generation affects the induced ionic wind, and the performance of the AC-DBD plasma actuator is time dependent during the early period of the operation.

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Theoretical Modeling of Porous Coatings with Simple Microstructures for Hypersonic Boundary-Layer Stabilization

AIAA Journal, 2019

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Design and Analysis of a Valveless Impedance Pump for a Direct Sodium Borohydride–Hydrogen Peroxide Fuel Cell

Journal of Electrochemical Energy Conversion and Storage, 2020

A valveless impedance pump is designed and applied for the first time to supply the liquid fuels ... more A valveless impedance pump is designed and applied for the first time to supply the liquid fuels for a direct sodium borohydride–hydrogen peroxide fuel cell (DBHPFC). This valveless pump consists of an amber latex rubber tube, which is connected at both ends to rigid stainless-steel tubes of different acoustic impedance, and a simple actuation mechanism with a small direct control (DC) motor and a cam combined. The cam is activated by the motor and periodically compresses the elastic tube at a position asymmetrical from the tube ends. The traveling waves emitted from the compression combine with the reflected waves at the impedance-mismatched rubber tube/stainless-steel tube interfaces. The resulting wave interference creates a pressure gradient and generates a net flow. When connected to a DBHPFC with an active area of 25 cm2, the pump can deliver the fuel at a maximum pumping rate of 30 ml/min, resulting in corresponding DBHPFC maximum power and a current of 13.0 W and 25.5 A, res...

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Numerical investigation of hypervelocity shock-wave/boundary-layer interactions over a double-wedge configuration

International Journal of Heat and Mass Transfer, 2019

Hypervelocity flows of nitrogen and air over a 30°-55° double-wedge configuration are numerically... more Hypervelocity flows of nitrogen and air over a 30°-55° double-wedge configuration are numerically investigated under the condition corresponding to recent experiments conducted with total enthalpy of 8.0 MJ/kg. Time-accurate twodimensional and three-dimensional simulations are performed to resolve the unsteady shock interaction process. For the nitrogen flow, it is found that the three-dimensional simulation predicts a much smaller separation bubble and reduced surface heat flux and pressure peaks. Good agreement can be observed with the experiments in terms of the shock location, the flow structure, and the time-averaged surface heat flux when the three-dimensional effects are considered. For the air flow, the shock interaction mechanisms are similar to those in nitrogen. The real-gas effects tend to decrease the separation bubble and reduce the standoff distance of the detached shock induced by the second wedge, leading to a lower surface heat flux peak compared with the nitrogen result. However, the trend of the experimental heat flux data shows the opposite. To explain the discrepancies, effects of thermochemical nonequilibrium models are investigated. The results indicate that the air flow under the current condition is insensitive to air chemistry and vibration-dissociation coupling models. Suggestions for further study are presented.

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Model Predictive Control for Path Tracking of a VTOL Tailsitter UAV in an HIL Simulation Environment

2018 AIAA Modeling and Simulation Technologies Conference, 2018

This paper investigates the application of Model Predictive Control (MPC) for path tracking of a ... more This paper investigates the application of Model Predictive Control (MPC) for path tracking of a vertical takeoff and landing (VTOL) tail-sitter unmanned aerial vehicle (UAV) in hover flight. In this work, the nonlinear dynamic model of a quad-rotor tail-sitter UAV including the aerodynamic effect of the wing, propellers, and slipstream was developed. The cascaded MPC controllers were then built upon linearized dynamic models. Path tracking simulations were conducted in a hardware-in-loop (HIL) environment where the UAV model and controllers were running on a PC and a flight computer independently. The simulation results show that the proposed MPC controllers enable the UAV to perform good path tracking and the ability of disturbance rejection under limited onboard computation resource.

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Transition Optimization for a VTOL Tail-Sitter UAV

IEEE/ASME Transactions on Mechatronics, 2020

This paper focuses on the transition process optimization for a vertical takeoff and landing (VTO... more This paper focuses on the transition process optimization for a vertical takeoff and landing (VTOL) tail-sitter unmanned aerial vehicle (UAV). For VTOL UAVs that can fly with either hover or cruise mode, transition refers to the intermediate phases between these two modes. This work develops a transition strategy with trajectory optimization method. The strategy is a reference maneuver enabling the vehicle to perform transition efficiently by minimizing the cost of energy and maintaining a small change of altitude. The simplified 3-degree-of-freedom (3-DOF) longitudinal aerodynamic model is used as a dynamic constraint. The transition optimization problem is then modeled by nonlinear programming (NLP) and solved by the collocation method to get the reference trajectory of the pitch angle and throttle offline. Simulations with the Gazebo simulator and outdoor flight experiments are carried out with the optimized forward (hover-cruise) and backward (cruise-hover) transition solutions. The simulation and experimental results show that the optimized transition strategy enables the vehicle to finish transition with less time and change of altitude compared with that by using traditional linear transition methods.

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