• The Transactions of the Korea Information Processing Society
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• v.4 no.4
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• pp.923-932
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• 1997

In this paper, we propose a two-level hybrid signature file method(THM) to dffciently deal with large txt databases that use a term discrimination concept.In addition, we apply Yoo's clustering scheme to the two-level hybeid signature file method. The clustering schme groups similar signatures together according to the similarity of the highly discriminatiory tems so that we may achive better performance on retrival. The space-time ana-lyhtical model of the proposed two-level hybrid method is provided. Based on the analytical model and experiments, we compare it with the exsting methods, i.e. the bit-sliced method(BM), the-level method(TM), and the hybrid method(HM). As a result, we show that THM achives the best retrival performance in a large database with 100,000 records when the mumber fo matching records is less than 160.

• Journal of Power Electronics
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• v.17 no.1
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• pp.96-105
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• 2017

In this paper, a hybrid modular multilevel converter (MMC) topology with an improved nearest level modulation method is proposed for medium-voltage high-power applications. The arm of the proposed topology contains N series connected half-bridge submodules (HBSMs), one full-bridge submodule (FBSM) and an inductor. By exploiting the FBSM, half-level voltages are obtained in the arm voltages. Therefore, an output voltage with a 2N+1 level number can be generated. Moreover, the total level number of the inserted submodules (SMs) is a constant. Thus, there is no pulse voltage across the arm inductors, and the SM capacitor voltage is rated. With the proposed voltage balancing method, the capacitor voltage of the HBSM is twice the voltage of the FBSM, and each IGBT of the FBSM has a relatively low switching frequency and an equalized conduction loss. The capacitor voltage balancing methods of the two kinds of SMs are implemented independently. As a result, the switching frequency of the HBSM is not increased compared to the conventional MMC. In addition, according to a theoretical calculation of the total harmonic distortion of the electromotive force (EMF), the voltage quality with the presented method can be significantly enhanced when the SM number is relatively small. Simulation and experimental results obtained with a MMC-based inverter verify the validity of the developed method.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.2
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• pp.79-89
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• 2004

The viscous free surface flow around KRISO container ship (KCS) is computed using the finite volume based multi block RANS code, WAVIS developed at KRISO. The free surface is captured with the Level-Set method and the realizable k-$\varepsilon$ model is employed for turbulence closure. The computations are carried out at model scale. For accurate free surface solution and its stable convergence the computations are performed with a suitable grid refinement around the free surface by applying an implicit discretization method based on a finite volume method to the Level-Set formulation. In all computational cases the numerical results agree well with experimental measurements.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.2
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• pp.150-158
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• 2017

This study proposes a new pulse-width modulation switching pattern for the low conduction loss of a three-level neutral point clamped (NPC)-based dual-active bridge (DAB) converter. The operational principle for a bidirectional power conversion is a phase-shift modulation. The conventional switching method of the three-level NPC-based DAB converter shows a symmetric switching pattern. This method has a disadvantage of high root-mean-square (RMS) value of the coupling inductor current, which leads to high conduction loss. The proposed switching method shows an asymmetrical pattern, which can reduce the RMS value of the inductor current with lower conduction loss than that of the conventional method. The performance of the proposed asymmetrical switching method is theoretically analyzed and practically verified using simulation and experiment.

• The Journal of Korean Academy of Prosthodontics
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• v.44 no.1
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• pp.63-72
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• 2006

Purpose: The purpose of this study was to compare the accuracy of master cast fabricated by using different impression methods at the different impression levels. Material and Method: The master model used in this study was resin block having low implant analogs. Impression method studied were 1) direct method on future level (Group FIX-D), 2) Indirect method on fixture level(Group FIX-I), 3) Modified indirect method on fixture level(Group FD(-M), 4) Direct method on abutment level(Group AB-D) and 5) Indirect method on abutment level(Group AB-I). Each of the five groups took 10 impressions. Fifty impressions were made for master cast by using Impregum $F^(R)$ impression material loaded on individual tray. Three dimensional measuring microscope was used to measure the inter-implant distance. Error rate of each inter-implant distance were calculated and evaluated. Results: The results were as follows. 1. Group FIX exhibited higher accuracy than group AB. 2 In group FIX, modified indirect method showed the highest accuracy, while indirect method showed the lowest accuracy. In group Ab, indirect method showed the higher accuracy than direct method. 3. Group FIX showed larger horizontal error than group AB. But, group AB showed the larger vertical error than group FIX. 4. Group Fix-M showed smallest vertical and horizontal error.

• Geomechanics and Engineering
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• v.33 no.2
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• pp.195-202
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• 2023

In this study, the ground settlement in backside of retaining wall and the behavior of the retaining wall were analyzed according to the method of groundwater drawdown due to excavation by using two-dimensional(2D) finite element analysis. Numerical analysis was performed by applying 1) fixed groundwater level, 2) constant groundwater drawdown, and 3) transient groundwater drawdown. In addition, the behavior of the retaining wall according to the initial groundwater level, ground conditions, and surcharge pressure in backside of retaining wall was evaluated. Based on the numerical analysis results, it was confirmed that when the groundwater level is at 0.1H from the ground surface (H: Excavation soil height), the wall displacement and ground settlement are not affected by the method of groundwater drawdown, regardless of soil conditions (dense or loose) and surcharge pressure. On the other hand, when the groundwater level is at 0.5H from the ground surface, the method of groundwater drawdown was found to have a significant effect on wall displacement and ground settlement. In this case, the difference in ground settlement presents by up to 4 times depending on the method of groundwater drawdown, and the surcharge load could increase the ground settlement by up to 1.5 times.

• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.6
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• pp.114-121
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• 2013

BTC image compression is a simple and easy hardware implementation, is widely used in a video compression techniques required for LCD overdrive. In this paper, methods for reducing compression loss, a multi-level quantization BTC (MLQ-BTC) algorithm is proposed. The process of the MLQ-BTC algorithm is, a input image is compressed and decompressed by Quasi 8-level method and Advanced 2-level BTC method, and select the algorithm with the smallest compression loss. Simulation results show that the proposed algorithm is efficient as compared with PSNR and compression ratio of the existing BTC methods.

• Journal of Power Electronics
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• v.19 no.1
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• pp.119-133
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• 2019

Although the dual modulation wave method can solve the low-frequency neutral point potential (NPP) fluctuation problem for three-level neutral point clamped converters, it also increases the switching frequency and limits the zero-sequence voltage. That makes it harmful when dealing with the NPP drift problem if the converter suffers from a long dead time or asymmetric loads. By introducing two degrees of freedom (2-DOF), an NPP control based on a search optimization method can demonstrate its ability to cope with the above mentioned two types of NPP problems. However, the amount of calculations for obtaining an optimal 2-DOF is so large that the method cannot be applied to certain industrial applications with an inexpensive digital signal processor. In this paper, a novel optimal 2-DOF-based NPP control is proposed. The relationships between the NPP and the 2-DOF are analyzed and a method for directly determining the optimal 2-DOF is also discussed. Using a direct calculation method, the amount of calculations is significantly reduced. In addition, the proposed method is able to maintain the strongest control ability for the two types of NPP problems. Finally, some experimental results are given to confirm the validity and feasibility of the proposed method.

• Journal of the Earthquake Engineering Society of Korea
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• v.2 no.1
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• pp.95-111
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• 1998

In this study, the concepts and procedures of the seismic damage analysis methods are examined for both the element-level and the system-level. The seismic damage analysis at the element-level is performed for several example structures using existing method for structural elements or single-degree-of-freedom (SDOF) systems such as the Park and Ang method. In order to analyze seismic damage at the system-level, two types of procedures are used. In the first type of procedure, the system-level seismic responses can be estimated by using the system representative response method(SRRM), or the equivalent SDOF system response method (ESDOF-SRM). Then, the system-level seismic damage is analyzed from the system-level seismic responses using existing method for structural elements or SDOF systems. IN the second type of procedure, the system-level seismic damages are analyzed using the element damage combination method (EDCM) combing the element-level damage indices determined by existing method. To compare tendency of the seismic damage analysis using each methods, example analysis is accomplished for several cases of different structures and different earthquake excitation.

• The Journal of the Acoustical Society of Korea
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• v.22 no.2E
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• pp.87-92
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• 2003

In this paper, a new speech enhancement method using level adapted wavelet packet is presented. First, we propose a level adapted wavelet packet to alleviate a drawback of the conventional node adapted one in noisy environment. Next, we suggest an adaptive noise estimation method at each node on level adapted wavelet packet tree. Then, for more accurate noise component subtraction, we propose a new estimation method of spectral subtraction weight. Finally, we present a modified spectral subtraction method. The proposed method is evaluated on various noise conditions: speech babble noise, F-l6 cockpit noise, factory noise, pink noise, and Volvo car interior noise. For an objective evaluation, the SNR test was performed. Also, spectrogram test and a very simple listening test as a subjective evaluation were performed.