Browsing by Author "Huang, J. -C."

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A true-direction reconstruction of the quiet direct simulation method for inviscid gas flows
(2013-11-01)
Lin, Y. -J.
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Smith, M. R.
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Kuo, F. -A.
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Cave, H. M.
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Huang, J. -C.
In this paper, a true-direction flux reconstruction of the second-order quiet direct simulation (QDS-2N) Smith et al. (2009)[3] as an equivalent Euler equation solver, called QDS-N-2, is proposed. Because of the true-directional nature of QDS, where volume-to-volume (true-direction) fluxes are computed, as opposed to fluxes at cell interfaces as employed by traditional finite volume schemes, a volumetric reconstruction is required to reach a totally true-direction scheme. The conserved quantities are permitted to vary (according to a polynomial expression) across all simulated dimensions. Prior to the flux computation, QDS particles are introduced using properties based on weighted moments taken over the polynomial reconstruction of the conserved quantity fields. The resulting flux expressions are shown to exactly reproduce the existing second-order extension for a one-dimensional flow, while providing a means for true multi-dimensional reconstruction. The new reconstruction is demonstrated in several verification studies. These include a shock-bubble interaction problem, an Euler-four-shock interaction problem, and the advection of a vortical disturbance. These results are presented, and the increased computation time and the effect of higher-order extension are discussed in this paper. The results show that the proposed multi-dimensional reconstruction provides a significant increase in the accuracy of the solution. We show that, despite the increase in the computational expense, the computational speed of the proposed QDS-N-2 method is several times higher than that of the previously proposed QDS-2N scheme for a fixed degree of numerical accuracy, at least, for the test problem of the advection of vertical disturbances. (C) 2013 Published by Elsevier B.V.
ENHANCEMENT OF HEAT TRANSFER OF MIXING CONVECTION IN A VERTICAL CHANNEL BY A MOVING BLOCK
(2013-03-01)
Fu, W. -S.
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Huang, J. -C.
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Wang, Y. -Y.
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Huang, Y.
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機械工程學系
Enhancement of a heat transfer rate of mixed convection flow in a three-dimensional vertical channel with insertion of a moving slender block is investigated numerically. A slender block is installed along the direction of the channel flow, and the movement of the slender block is in periodic motion and transverse to the channel flow. The interaction between the moving block and the channel flow destroys and suppresses the velocity and thermal boundary layers on the heat surface periodically. Various ratios of the Richardson numbers (Gr/Re-2) are simulated. The results show that under a higher velocity of the channel flow and a lower magnitude of Gr/Re-2, the enhancement of heat transfer rate is better. Oppositely, under a lower velocity of the channel flow and a higher magnitude of Gr/Re-2, the effect of natural convection driven by the buoyancy force is stronger and it is unfavorable to the heat transfer. A counter effect of the heat transfer rate is observed. These phenomena which are seldom analyzed before by numerical simulation are carried out in this study.
General Higher Order Extension to the Quiet Direct Simulation Method
(2011)
Lin, Y. -J.
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Smith, M. R.
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Cave, H. M.
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Huang, J. -C.
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Wu, J. -S.
Presented is the multi-dimensional extension of the Quiet Direct Simulation (QDS) method to general, higher order spatial accuracy. Due to the true directional nature of QDS, where volume-to-volume fluxes are computed as opposed to fluxes at cell interfaces, a volumetric reconstruction is required. In this approach, the conserved quantities are permitted to vary (according to a polynomial expression) across all simulated dimensions. Prior to flux computation, QDS particles are introduced using properties based on weighted moments taken over the polynomial reconstruction of conserved quantity fields. The resulting flux expressions are shown to exactly reproduce existing 2(nd) order extensions (for one-dimensional flows) while providing means for true multi-dimensional reconstruction and general higher order extensions. Here we will show a typical comparison among different spatial order extensions of QDS and other WENO (5(th) order accurate) based numerical scheme. The extension is demonstrated up to 5(th) order spatial accuracy and is shown to be comparable to or more accurate than the other higher order schemes.
Implementation of unsteady sampling procedures for the parallel direct simulation Monte Carlo method
(2008-06-01)
Cave, H. M.
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Tseng, K. -C.
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Wu, J. -S.
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Jermy, M. C.
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Huang, J. -C.
An unsteady sampling routine for a general parallel direct simulation Monte Carlo method called PDSC is introduced, allowing the simulation of time-dependent flow problems in the near continuum range. A post-processing procedure called DSMC rapid ensemble averaging method (DREAM) is developed to improve the statistical scatter in the results while minimising both memory and simulation time. This method builds an ensemble average of repeated runs over small number of sampling intervals prior to the sampling point of interest by restarting the flow using either a Maxwellian distribution based on macroscopic properties for near equilibrium flows (DREAM-I) or output instantaneous particle data obtained by the original unsteady sampling of PDSC for strongly non-equilibrium flows (DREAM-II). The method is validated by simulating shock tube flow and the development of simple Couette flow. Unsteady PDSC is found to accurately predict the flow field in both cases with significantly reduced run-times over single processor code and DREAM greatly reduces the statistical scatter in the results while maintaining accurate particle velocity distributions. Simulations are then conducted of two applications involving the interaction of shocks over wedges. The results of these simulations are compared to experimental data and simulations from the literature where there these are available. In general, it was found that 10 ensembled runs of DREAM processing could reduce the statistical uncertainty in the raw PDSC data by 2.5-3.3 times, based on the limited number of cases in the present study. (c) 2008 Published by Elsevier Inc.
Stable AlGaN/GaN high electron mobility transistors with tungsten nitride gate metallisation
(2009-12-03)
Lu, C. -Y.
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Chang, E. Y.
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Huang, J. -C.
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Chang, C. -T.
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Lee, C. -T.
An AlGaN/GaN high electron mobility transistor (HEMT) with tungsten nitride (WNx) Schottky gate fabricated on a sapphire substrate is presented. Gate forward current stress was chosen to evaluate the stability of the Schottky gate. After stress, this WNx HEMT remains stable, while the conventional Ni/Au HEMT shows performance degradation and failure. The maximum output power density from this device is 5 W/mm at 2 GHz. A combination of these findings indicates the robust performance of this WNx material and its potential as a Schottky gate for AlGaN/GaN HEMTs.