• Explosives and Blasting
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• v.33 no.3
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• pp.29-50
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• 2015

Safe blast criteria based on the concept of scaled distances can be obtained from the statistical analysis on measured peak particle velocity data of blast vibrations. Two types of scaled distance widely used in Korea are the square root scaled distance (SRSD) and cube root scaled distance (CRSD). In contrast to SRSD scheme, however, the function of maximum charge per delay for CRSD increases without bound after the intersection point of these two functions despite of the similar goodness of fits. To prevent structural damage that may be caused by the excessive charge in the case of CRSD, it is suggested that CRSD be used within a specified distance slightly beyond the intersection point. On the other hand, there are several attempts that predict vibration level(VL) from the peak particle velocity(PPV) or estimate VL based on the scaled distances without considering their frequency spectra. It appears that these attempts are conducted in blasting contracts only for the purpose of satisfying the environment-related law, which mainly deals with the annoyance aspects of noises and vibrations in human life. But, in principle there could no correlation between peaks of velocity and acceleration over entire frequency spectrum. Therefore, such correlations or estimations should be conducted only between the waves with the same or very similar frequency spectra. Finally, it is a known fact that structural damage due to ground vibration is related to PPV level, the safety level criteria for structures should be defined by allowable PPV levels together with their zero crossing frequencies (ZCF).

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.7A
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• pp.794-806
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• 2004

This paper deals with correction of frequency offset and analysis of quantization angle noise in the IEEE 802.1la OFDM system. The rotation phase per symbol due to the carrier frequency offset is estimated from auto-correlation of the short Preambles, which are over-sampled for the reduction of noise in OFDM signals. The pilot signals are introduced to estimate the rotation phase per OFDM symbol due to estimation error of the carrier frequency offset and the sampling frequency onset. During the estimation and correction of the frequency onsets, a CORDIC processor and a look-up table are used for the conversion between a rotation phase and its complex number. Being calculated by a limited number of bits in the CORDIC processor and the look-up table, the rotation phase and its complex number have quantization angle errors. The quantization errors are analyzed as SNR (signal to noise ratio) due to the quantization bit numbers. The minimum bit number is suggested to meet the specification of IEEE 802.1la properly. Finally, the quantization errors are evaluated through simulations on number of quantization bits and SNR of received signals.

• Explosives and Blasting
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• v.18 no.2
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• pp.14-22
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• 2000

The estimation of proper prediction method and the alteration of transformation method of environmental regulation standard were carried out by measuring blasting noise in construction field. The correlation of scaled distance with sound pressure level were better than with sound level, but it was proved to be difficult to control blasting noise because the correlation factor was too 1ow. three methods to transform sound pressure levee to sound level were examined. The method is the transformation by correlation equation of sound pressure level and sound level which are measured at the same time, and simplified transformation of A-weighting network corresponding to dominant frequency, and the transformation of sound pressure level by FFT. There were many errors to transform. The best effective method is the transformation using correlation equation of sound pressure level and sound level which are measured at the same time.

• Proceedings of the Korea Water Resources Association Conference
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• 2023.05a
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• pp.370-370
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• 2023

수자원이란 인간의 생활이나 경제활동 및 자연환경 유지 등을 하는데 이용할 수 있는 자원으로서의 물을 말하며 효율적인 수자원의 활용을 위해서는 수문조사가 필수적이라고 할 수 있다. 수문조사는 강수량, 수위, 유량, 유사량, 증발산량, 토양수분량을 측정, 조사, 분석하는 것이라고 할 수 있으며 이 중 유량은 강우-유출 관계 규명, 이수, 치수 등을 위해 높은 정확도의 관측자료가 필요하다. 그러나 하천유량은 장기간 연속적으로 관측하는 것이 어려우므로 특성 횟수의 유량조사를 통해 수위-유량관계곡선식(Rating curve)을 개발하여 수위로부터 연속적인 유량을 산정하여 활용하고 있어 이를 개선하기 위해 전자파표면유속계를 활용한 자동유량측정장치를 개발하였으며 측정자료 분석 및 알고리즘 고도화를 통해 신뢰도 높은 유량을 산정하고자 하였다. 운동하는 물체에 의하여 산란된 전자파의 주파수가 변하게 되는 현상을 도플러 효과라고 하며 이때의 주파수의 변화량을 도플러 주파수라고 한다. 전자파표면유속계는 하천 수면으로 전자파를 발사한 후 물 표면에서 반사되는 전자파의 도플러 효과를 이용하여 표면유속을 측정하는 것으로 전자파표면유속계를 교량 상류 방향으로 고정시켜 설치하였으며 측정된 자료는 유량관측 시스템에서 저장, 관리되고 물리적, 통계적 방법이 적용된 알고리즘을 통해 유량을 산정한다. 산정된 유량의 정확도 확보를 위해 바람 영향, 최적 환산계수 재산정, 통계분석 등을 통해 시스템의 유량산정 알고리즘 개선 방안을 도출하였으며 개선된 알고리즘이 적용된 유량은 NSE(Nash Sutcliffe Efficiency), PBIAS(Percent Bias), RSR(RMSE-observations standard edviation ratio)을 통해 평가하였다. 바람 영향은 유량 관측 고도화 매뉴얼(일본 토목연구소)의 경험식과 현장에서 관측된 풍향·풍속을 활용하여 분석하였으나 개선효과가 미미한 것으로 나타났으며 자동으로 관측된 유량자료에 포함된 무작위적인 변화량을 감소시키기 위해 경향성 분석을 병행하여 이동평균 방법을 적용하였다. 또한 ADCP 등을 활용한 유량측정성과와 수위-유량관계곡선식과의 비교·분석을 통해 수위별 최적 환산계수를 산정하여 유량산정 알고리즘에 적용한 결과 NSE는 0.980, PBIAS는 1.580, RSR은 0.142로 모두 Very Good(높은 상관성)으로 분석되어 유량자료의 정확도를 확보하였다.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.17 no.1 s.104
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• pp.52-59
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• 2006

To construct its complex permittivity from the reflection coefficient of a thin film such as PCB substrate measured by open-ended coaxial probe, an integral equation is formulated using modal analysis and equivalent source. The accuracy of the conversion model based on the integral equation is confirmed in both cases of converted complex permittivities calculated from numerically computed and actually measured reflection coefficients. And the maximum valid frequency of open-ended coaxial probe is limited by the size of its flange.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.13 no.10
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• pp.971-981
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• 2002

An open-ended coaxial probe method has been considered as one of effective tools for measuring electrical properties of its contacted material without shaping and fitting. The measured reflection coefficient at the probe's end is able to convert into the corresponding complex permittivity by employing the improved version of virtual transmission-line model Presented by our lab already. But the error of complex permittivity converted by equivalent model increases as the operating frequency ascends high. The errors of complex permittivity in the open-ended coaxial probe can be yielded compositively by the imperfect contact or probe, manufacture error of probe and complex permittivity error of reference material etc. Therefore it is necessary to limit the problem to identify the error causes in high frequency. In this paper, the errors which are resulted from the measurement of reflection coefficient are removed by using the FDTD(Finite-Difference Time-Domain) method, the error causes are limited the conversion model problem. And the error study of the improved conversion model is performed from several viewpoints. At first, the local minimum of parameter to be calculated by the iteration method in the conversion model is checked. At second, the modeling of the equivalent model is checked in the frequency range. From this study, we know the valid range of the improved conversion model.

• Journal of the Korea Institute of Military Science and Technology
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• v.9 no.4
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• pp.128-136
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• 2006

The purpose of this paper is to examine the dynamic stall characteristics of an elliptic airfoil when subject to constant pitch motions. In this study, which was motivated by the pressing need for a greater understanding of the reduced frequency$({\kappa})$ effects on flow patterns of elliptic airfoil, the various reduced frequencies were considered. The result confirms that the reduced frequency has a profound effects on the flow patterns. The increase of ${\kappa}$ accelerate the separation bubble bursting process up to ${\kappa}=0.10$, then diminish with further increase in ${\kappa}$. Compared with static condition, the dynamic pitching airfoil delays stall angle approximate $4{\circ}{\sim}5{\circ}$ during pitch-up stroke for ${\kappa}=0.10$. Results from this qualitative analysis provided valuable insight Into the control of dynamics stall.

• Journal of the Korea Institute of Military Science and Technology
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• v.8 no.3 s.22
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• pp.116-123
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• 2005

An experimental investigation on the static and dynamic stall characteristics of elliptic airfoil was performed by PIV velocity field measurements. The flow Reynolds number was $3.13{\times}10^5$ and the reduced frequency of the pitch oscillation ranged from 0.075 to 0.125. The onset of static stall was caused by boundary layer separation which started at the trailing edge and progressed toward the leading edge. However, dynamic stall was caused by the vortex shed at the leading edge region and the flow field showed a vortex dominated flow with turbulent separation and alternate vortex shedding. The increase of reduced frequency increased the dynamic stall angle of attack and intensified the flow hysteresis in the down-stroke phase.

• Proceedings of the Korea Water Resources Association Conference
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• 2020.06a
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• pp.230-230
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• 2020

하천 유량측정을 위한 표면유속계는 비접촉식 방식이기 때문에 현장에서의 측정이 간편하고, 비교적 안전하기 때문에 홍수시 유량측정에 널리 이용되고 있다. 비접촉식 유속계가 가지고 있는 이러한 장점으로 전자파표면유속계의 활용이 점차 증가하고 있으며, 영상유속계와 같은 비접촉 측정방식의 활용방안 마련에 대한 연구개발 등이 꾸준히 진행되어 왔다. 최근에는 고정식으로 상시 운영이 가능한 표면유속계 등이 출시되어 소규모 하천이나 수로에서의 상시 유량측정에 활용되고 있다. 다만, 이러한 비접촉식 유속계는 표면의 유속을 측정하기 때문에 유량산정을 위해서는 평균유속으로의 환산이 필요하지만, 현재까지는 평균유속 환산계수를 사용하여 표면유속을 평균유속으로 환산하는 방법이 유일하게 활용되고 있다. 하지만, 실제 하천에서는 단면 및 하도형태, 하상조건, 수리특성 및 유속분포 등의 다양한 조건에 따라 환산계수가 결정되기 때문에 이를 단순히 일률적으로 적용하는 것은 곤란하며, 이로 인해 과거 오랫동안 표면유속을 평균유속으로 환산하기 위한 다양한 연구가 진행되었지만, 실제 다양한 조건의 하천에 적용할 수 있는 표준화된 방법은 아직까지 제시되지 못하고 있다. 현재까지, 고정식으로 설치된 유속계로부터 측정된 유속을 평균유속으로 환산하는 방법으로는 국내외적으로 지표유속법(Index Velocity Method)과 유속분포법(Velocity Profile Method)이 대표적이며, H-ADCP (Horizontal-Acoustic Doppler Velocity Profiler) 또는 UVM(Ultrasonic Velocity Meter) 등과 같은 초음파유속계를 활용한 자동유량측정시설의 유량산정방법으로 활용되고 있다. 이러한 방법들은 고정된 유속계의 측정유속을 지표유속(V_i)으로 하여 다양한 범위의 실측된 평균유속과의 관계를 개발하여 활용하거나, 지표유속을 매개로 개수로 단면의 이론적인 유속분포를 추정하여 평균유속을 산정한다. 이러한 지표유속을 활용하는 방법들에서 공통적으로 중요한 것은 하천단면의 최대유속(V_max)이 가장 좋은 지표유속이라는 것이다. 따라서 국제기준에서는 지표유속을 측정하는 유속계의 가장 바람직한 위치로 유심부(core flow)를 권장하고 있다. 하지만, 접촉식 유속계의 경우 유심부 설치가 매우 어렵고 많은 비용이 들기 때문에, 비접촉식 유속계를 활용하여 하천단면의 최대유속을 측정할 수 있다면, 가장 효율적인 고정식 측정방법이 될 수 있을 것이다. 따라서, 본 연구에서는 비접촉식 표면유속계의 고정식 유량측정 활용성 및 적용성을 검토할 목적으로 표면유속에 대한 유량산정방법을 검토하였다. 이를 위해 24GHz의 주파수를 갖는 레이다표면유속계인 Sensoflow를 낙동강 수계 길안천에 위치한 안동시(대사3교)에 고정설치하여 표면유속을 지표유속으로 수집하였다. 다양한 유량규모에서 측정한 실측 표면유속과 수집된 표면유속을 지표유속으로 활용하여 지표유속관계를 개발하였으며, 산정된 유량을 기존 수위-유량관계곡선식의 환산유량과 비교하여 표면유속의 지표유속 활용성을 검토하였다.

• The Journal of Information Technology
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• v.5 no.4
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• pp.173-183
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• 2002

In this paper, we propose an efficient forecasting methodology of the mid and long-term frequency demand in Korea. The methodology consists of the following three steps: classification of basic service group, calculation of effective traffic, and frequency forecasting. Based on the previous studies, we classify the services into wide area mobile, short range radio, fixed wireless access and digital video broadcasting in the step of the classification of basic service group. For the calculation of effective traffic, we use the measures of erlang and bps. The step of the calculation of effective traffic classifies the user and basic application, and evaluates the effective traffic. Finally, in the step of frequency forecasting, different methodology will be proposed for each service group.