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트랜스포머의 일반화 성능에 영향을 주는 로스 랜드스케이프 연구
최민기 ( Mingi Choi ),이소은 ( So-eun Lee ),허종욱 ( Joug-uk Hou ) 한국정보처리학회 2022 한국정보처리학회 학술대회논문집 Vol.29 No.2
뉴럴 네트워크는 학습에 사용하는 파라미터를 문제에 맞게 최적화하여 일반화 성능을 향상시키는 것이 목적이다. 선행 연구들은 다차원의 로스 랜드스케이프(loss landscape)를 시각화하는 방법을 탐구하며, 모델의 일반화 측면에서 어떤 영향을 주는지 탐구한다. 하지만 아직까지 로스 랜드스케이프가 근본적으로 일반화 성능에 어떠한 영향을 주는지 잘 알려져 있지 않으며, 평평하거나 경사진 로스 랜드스케이프 중 어떤 형태가 일반화 성능에 더 효과적인지 여러 의견이 나뉜다. 따라서 우리는 로스 랜드스케이프가 일반화 성능과 연관 있음을 실험을 통해 파악한다. 나아가 비전문제에서 MSA(multi-head self-attention) 레이어를 기반으로 구성된 트랜스포머 구조를 사용해 작은 유도 편향(inductive bias)을 가지며 소규모 데이터 셋 체제에서의 단점을 보완한다. 결론적으로 평평한 로스 랜드스케이프가 일반화 성능에 긍정적인 영향을 끼친다는 것을 관찰한다.
기체구 분사 모델을 이용한 CNG 직접분사식 인젝터 분사 수치해석 기법
최민기 ( Mingi Choi ),박성욱 ( Sungwook Park ) 한국분무공학회 2016 한국액체미립화학회지 Vol.21 No.1
This paper describes the modeling of CNG direct injection using gaseous sphere injection model. Simulation of CNG direct injection does not need break up and evaporation model compared to that of liquid fuel injection. And very fine mesh is needed near the injector nozzle to resolve the inflow boundary. Therefore it takes long computation time for gaseous fuel injection simulation. However, simulation of CNG direct injection could be performed with the coarse mesh using gaseous sphere injection model. This model was integrated in KIVA-3V code and RNG k-ε turbulence model needs to be modified because this model tends to over-predict gas jet diffusion. Furthermore, we preformed experiments of gaseous fuel injection using PLIF (planar laser induced fluorescence)method. Gaseous fuel injection model was validated against experiment data. The simulation results agreed well with the experiment results. Therefore gaseous sphere injection model has the reliability about gaseous fuel direct injection. And this model was predicted well a general tendency of gaseous fuel injection.
분무응용 기술 : Gaseous sphere 분사모델을 이용한 CNG 직분사 엔진 모델링
최민기 ( Mingi Choi ),송진근 ( Jingeun Song ),박성욱 ( Sungwook Park ) 한국액체미립화학회 2015 한국액체미립화학회 학술강연회 논문집 Vol.2015 No.-
This paper describes the modeling of CNG (compressed natural gas) direct injection engine using gaseous sphere injection model. Numerical modeling was conducted using KIVA-3V Release 2 code with some modifications. Three dimensional mesh included 4 valves was used for computational grid. Gaseous sphere injection model could be integrated in KIVA-3V Release 2 code with some modification of liquid injection model and RNG (re-normalization group) k-ε turbulence model. This model could simulate gaseous sphere injection using coarse mesh which saves calculation time. The fine mesh is not required to resolve the inflow boundary for gaseous sphere injection. Likewise with liquid injection model, gaseous spheres are injected as parcels which represent a group of gaseous spheres and these parcels evaporate at a time. The evaporation of gaseous spheres occurs without energy change. Particularly, poppet type injector was used for CNG direct injection therefore hollow cone type injection model was modified. The RNG k-ε turbulence model need some modifications. Since this model is known to over-predict gas jet diffusion, turbulence kinetic energy and turbulence length scale values were adjusted depend on grid location. The modified RNG k-ε turbulence model is applied only for the injection period. After the fuel injection was finished, the conventional RNG k-ε turbulence model is applied. Chemkin chemistry solver 2 was coupled with KIVA-3V Release 2 code to simulate combustion process of CNG fuel. CNG is represented by methane and GRI 3.0 mechanism which optimized for combustion process of natural gas was used. In order to calculate the turbulent flame speed, G-equation model was used in which flame front is specified by zero level set of G. In addition, experiments of gaseous fuel injection was performed for gas-jet visualizations using PLIF (planar laser induced fluorescence) method. For safety reasons, compressed nitrogen was used instead of compressed natural gas in the experiments. The tracer which plays an important role in PLIF experiments was acetone that has a very low boiling point, a high saturation pressure, a good fluorescence and low toxicity. Furthermore, experiments of CNG combustion was performed using single cylinder SI (spark ignition) engine. For CNG direct injection, poppet type gaseous fuel injector was used and this injector was centrally mounted. In this study, the simulation results of CNG direct injection engine were compared to experimental results. The gaseous sphere injection model can reliably predict CNG direct injection. Furthermore, the results of ignition and combustion process agreed well with experiment results.
흡기가열을 이용한 가솔린 압축착화 엔진의 연소 및 배기특성 모델링
최민기(Mingi Choi),오윤중(Yunjung Oh),류봉우(Bong Woo Ryu),이창식(Chang Sik Lee),박성욱(Sungwook Park) 한국연소학회 2011 KOSCOSYMPOSIUM논문집 Vol.- No.43
This paper presents numerical study on combustion and emissions characteristics of GDICI(gasoline Direct Injection Compression Ignition) engine using intake preheating. Numerical modeling was conducted by using the KIVA-3V Release2 code. In addition, Chemkin Chemistry solver was integrated into KIVA code to simulate the ignition and combustion precesses in order to predict combustion and exhaust emissions characteristics. The fuel was modeled using PRF mechanims to simulate the fuel oxidation process. To achieve gas pressure in cylinder and emissions characteristics, the experiments were performed on a single-cylinder engine. The simulation results agreed well with the experimental data.