Browsing by Author "Wu, J-S"

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A temporal multi-scale algorithm for efficient fluid modeling of a one-dimensional gas discharge
(2014-12-01)
Gu, B-R
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Lin, K-M
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Hu, M-H
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Hung, C-T
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Wu, J-S
In this study, we present a temporal multi-scale algorithm (TMA) for efficient fluid modeling of a one-dimensional gas discharge with complex plasma chemistry. A helium dielectric barrier discharge driven by a power source with a frequency of 25 kHz is used as an example to demonstrate the superior capability of the TMA in accelerating fluid modeling simulations, while maintaining the same accuracy as compared to lengthy benchmarking fluid modeling using a single time-scale approach. The plasma chemistry considers 36 species and 121 reaction channels, which include some impurities such as nitrogen (25 ppm), oxygen (10 ppm) and water vapor (1 ppm), in addition to the helium itself. The results show that the runtime using the TMA can be dramatically reduced to 4% (25 times faster) with a relative difference of spatially averaged number densities generally less than 1% for all species between the TMA and the benchmarking cases when five initial cycles, five supplementary cycles and four repeated stages are used. Further reduction of the accuracy requirements to 44% for some specific species can lead to 92 times faster performance with the use of two initial cycles, two supplementary cycles and two repeated stages. The outlook for multi-dimensional fluid modeling considering a gas flow field is also described at the end of the paper.
Dayside-to-nightside dust coma brightness asymmetry and its implications for nightside activity at Comet 67P/Churyumov-Gerasimenko
(2020-11-15)
Gerig, S-B
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Pinzon-Rodriguez, O.
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Marschall, R.
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Wu, J-S
;
Thomas, N.
We have determined the dust coma brightness ratio between the dayside and the nightside (DS:NS) in OSIRIS images of comet 67P/Churyumov-Gerasimenko and compared them to results from numerical dust coma simulations to learn more about the dynamic processes that are involved in coma formation. The primary focus of this paper lies in the analysis of a subset of OSIRIS images acquired during one comet rotation on 11. April 2015 when the spacecraft was at a phase angle of 90 degrees and therefore directly above the terminator. The DS:NS ratio was found to be 2.49 +/- 0.18 on average - a very low value if insolation-driven sublimation of water dominates dust emission. We investigated two possible hypotheses: First, the influence of direct activity from non-illuminated (nightside) areas of the comet and second, the brightness contribution of large gravity-dominated particles in the innermost coma. For our numerical simulations, we used a combination of DSMC gas dynamics simulation and particle propagation by an equation of motion to simulate the dust coma. Our simulations show that direct activity from the nightside is preferred, contributing approximate to 10% of the total emission. We show that intensity profiles, used to quantify dust outflow behaviour, fit the observations better when nightside activity is present and we suggest that nightside gas emission by CO2 or CO is responsible for the observed dust flux. With the help of a simplified Keplerian modelling approach we exclude large particles on gravitationally bound or ballistic orbits from being the major contributor to the observed dust coma brightness. Additionally, we show the DS:NS ratio as a function of days to perihelion and observe that it is on a similar level as in the April OSIRIS time series from February to mid-June 2015, but increases towards a maximum of >= 4.07 +/- 0.49 shortly after perihelion passage. We suggest that this is correlated to the increasing importance of H2O production when approaching perihelion.
Development of parallel direct simulation Monte Carlo method using a cut-cell Cartesian grid on a single graphics processor
(2014-09-20)
Lo, M. -C.
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Su, C. -C.
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Wu, J-S
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Kuo, F. -A.
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機械工程學系
This study developed a parallel two-dimensional direct simulation Monte Carlo (DSMC) method using a cut-cell Cartesian grid for treating geometrically complex objects using a single graphics processing unit (GPU). Transient adaptive sub-cell (TAS) and variable time-step (VTS) approaches were implemented to reduce computation time without a loss in accuracy. The proposed method was validated using two benchmarks: 2D hypersonic flow of nitrogen over a ramp and 2D hypersonic flow of argon around a cylinder using various free-stream Knudsen numbers. We also detailed the influence of TAS and VTS on computational accuracy and efficiency. Our results demonstrate the efficacy of using TAS in combination with VTS in reducing computation times by more than 10x. Compared to the throughput of a single core Intel CPU, the proposed approach using a single GPU enables a 13-35x increase in speed, which varies according to the size of the problem and type of GPU used in the simulation. Finally, the transition from regular reflection to Mach reflection for supersonic flow through a channel was simulated to demonstrate the efficacy of the proposed approach in reproducing flow fields in challenging problems. (C) 2014 Elsevier Ltd. All rights reserved.
Effect of Free Electrons on Nanoparticle Charging in a Negative Direct Current Corona Charger
(2009)
Aliat, A.
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Tsai, C-J
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Hung, C-T
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Wu, J-S
;
機械工程學系
A 2D numerical model is proposed to study the electric charge that will provide nanoparticles within a cylindrical corona discharge with a wire along the axis. The wire-tube electrical charger is subjected to a negative DC voltage and filled with air at atmospheric pressure at ambient temperature. A numerical application is given by considering spherical nanoparticles with a diameter of 65 nm within a wire-tube charger with a radius of 2 cm and 26cm in length. Numerical results obtained are in very good agreement with the experimental data. These results show that free electrons contribute greatly to the electrical charging efficiency mechanism of the corona charger in particular at high Nt-product, which explains very well the large discrepancy found in earlier models which not considered electrons.
Effects of Oxygen Addition and Treating Distance on Surface Cleaning of ITO Glass by a Non-Equilibrium Nitrogen Atmospheric-Pressure Plasma Jet
(2010-10-01)
Chiang, M-H
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Liao, K-C
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Lin, I-M
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Lu, C-C
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Huang, H-Y
Effects of oxygen addition and treating distance on cleaning organic contaminants on stationary and non-stationary (1-9 cm/s) ITO glass surfaces by a parallel-plate nitrogen-based dielectric barrier discharge (DBD) are investigated experimentally; the DBD is driven by a 60 kHz bipolar quasi-pulsed power source. The results show that two regimes of favorable operating condition for improving the hydrophilic property of the surface (reducing the contact angle from 84 degrees to 25-30 degrees) are found. The measured spatial distribution of NO-gamma UV emission, O(3) concentration and OES spectra are shown to strongly correlate with the measured hydrophilic property. At the near jet downstream locations (z < 10 mm), the metastable N(2)(A(3)Sigma(+)(mu)) and photo-induced dissociation of ozone play dominant roles in cleaning the ITO glass surface; while at the far jet downstream locations (z > 10 mm), where the ratio of oxygen to nitrogen is lower, only the long-lived metastable N(2)(A(3)Sigma(+)(mu) ) plays a major role in cleaning the ITO glass surface.
Effects of Oxygen Addition and Treating Distance on Surface Cleaning of ITO Glass by a Non-Equilibrium Nitrogen Atmospheric-Pressure Plasma Jet
(2010-10-01)
Chiang, M-H
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Liao, K-C
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Lin, I-M
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Lu, C-C
;
Huang, H-Y
Effects of oxygen addition and treating distance on cleaning organic contaminants on stationary and non-stationary (1-9 cm/s) ITO glass surfaces by a parallel-plate nitrogen-based dielectric barrier discharge (DBD) are investigated experimentally; the DBD is driven by a 60 kHz bipolar quasi-pulsed power source. The results show that two regimes of favorable operating condition for improving the hydrophilic property of the surface (reducing the contact angle from 84 degrees to 25-30 degrees) are found. The measured spatial distribution of NO-gamma UV emission, O(3) concentration and OES spectra are shown to strongly correlate with the measured hydrophilic property. At the near jet downstream locations (z < 10 mm), the metastable N(2)(A(3)Sigma(+)(mu)) and photo-induced dissociation of ozone play dominant roles in cleaning the ITO glass surface; while at the far jet downstream locations (z > 10 mm), where the ratio of oxygen to nitrogen is lower, only the long-lived metastable N(2)(A(3)Sigma(+)(mu) ) plays a major role in cleaning the ITO glass surface.
Numerical and experimental investigation of light emissions of a planar nitrogen atmospheric-pressure dielectric barrier discharge due to addition of ammonia considering oxygen impurity
(2013-12-01)
Li, F-L
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Hung, C-T
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Lin, K-M
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Wei, T-C
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Wu, J-S
In this paper, the mechanisms of light emissions, including NO-gamma, NO-beta and N-2-SPS, produced in a N-2/NH3 atmospheric-pressure dielectric barrier discharge considering realistic oxygen impurity (30 ppm) are investigated numerically and experimentally. Self-consistent, one-dimensional fluid modeling is used to numerically simulate the discharge process with 48 species and 235 reaction channels. An optical emission spectrometer (OES) is used to measure the relative intensities of the light emission. The simulations of the light emission intensities for the above-mentioned OES lines generally reproduce the trends observed in the experiments caused by changes in the NH3 concentration. All of the predicted intensities of NO-gamma, NO-beta and N-2-SPS decrease with increasing amount of NH3 caused by various reaction mechanisms. The former is due to the loss of N-2(A) and NO(A) by the reaction of NH3 with N-2(A) and NO(A), respectively. The decrease in NO-beta is due to the depletion of N and O because of NH3, and the decrease in N-2-SPS is due to electron attachment to NH3 and a weaker metastable-metastable associative ionization of N-2. All of the simulated results demonstrate that the discharges are typically Townsend-like because the ions outnumber the electrons and the electric field across the gap is distorted only slightly by the charged particles during the breakdown. Finally, a reduced chemical kinetics model for a planar atmospheric-pressure N-2/O-2/NH3 dielectric barrier discharge is proposed and validated by benchmarking against the above complete chemical kinetics. This results in a reduced chemical kinetics consisting of 33 species and 87 reactions with a very limited loss of accuracy of discharge properties, while it is 2.1 times faster in computational time as compared with the complete version.
One-dimensional simulation of nitrogen dielectric barrier discharge driven by a quasi-pulsed power source and its comparison with experiments
(2011-01-01)
Cheng, K-W
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Hung, C-T
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Chiang, M-H
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Hwang, F-N
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Wu, J-S
Parallel-plate nitrogen dielectric barrier discharge (DBD) driven by a quasi-pulsed power source (60 kHz) is simulated using a fully-implicit 1D self-consistent fluid modeling code Simulated discharged currents of gap distances (0 5 07 and 1 0 mm) agree very well with measured data while simulated discharged currents of wider gap distance (1 2 mm) fail to reproduce the measured data It is found that the discharge mode is homogeneous Townsend-like for the former case while it is filamentary-like for the latter case based on the experimental observation These findings demonstrate that previous numerical studies showing mode transition by changing the gap distance may require further investigation (C) 2010 Elsevier B V All rights reserved
Three-dimensional simulation of the femto-second pulsed laser interacting with a nitrogen molecule
(2011-01-01)
Lee, Y-M
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Jiang, T-F
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Su, Z-Y
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Wu, J-S
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機械工程學系
Interaction of femto-second pulsed laser with a nitrogen molecule at various angles is investigated numerically in the present study We propose a three-dimensional time-dependent Schrodinger equation (TDSE) solver using finite volume method (FVM) with single-active-electron (SAE) assumption We construct the model potential by combining the soft-Coulomb potential and Yukawa potential and fitting both the first ionization energy and orbital type of highest-occupied molecular orbital (HOMO) of a N(2) molecule The ratio of simulated maximum (chi = 0 degrees) to minimum (chi = 90 degrees) Ionization yield is 1 9 and it agrees more favorably with the converted experimental data (2 3-3 3) than the MO-ADK model does (10 0) This shows that the proposed model potential in the 3D TDSE solver can faithfully predict the ionization of a nitrogen molecule under strong pulsed laser incidence and is a very useful tool for future application in similar analysis of more complicated molecules (C) 2010 Elsevier BV All rights reserved