• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.10
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• pp.720-727
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• 2019

Resonating thermal lags of solid fuel with heat transfer oscillations generated by boundary layer oscillation is the primary mechanism of the occurrence of the LFI (Low Frequency Combustion Instability) in hybrid rocket combustion. This study was experimentally attempted to confirm that how the boundary layer was perturbed and led to the LFI. Special attention was also made on oxidizer swirl injection to investigate the contribution to combustion stabilization. Also the overall behavior of fluctuating boundary layer flow and the occurrence of the LFI was monitored as swirl intensity increased. Fluctuating boundary layer was successfully monitored by the captured image and POD (Proper Orthogonal Decomposition) analysis. In the results, oscillating boundary layer became stabilized as the swirl intensity increases. And the coupling strength between high frequency p', q' diminished and periodical amplification of RI (Rayleigh Index) with similar frequency band of thermal lag was also decreased. Thus, results confirmed that oscillating axial boundary layer triggered by periodic coupling of high frequency p', q' is the primary mechanism to excite thermal resonance with thermal lag characteristics of solid fuel.

• The Journal of the Acoustical Society of Korea
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• v.40 no.1
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• pp.1-9
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• 2021

High-frequency bistatic sea surface scattering channels according to sea state were measured at an experimental site of Geoje bay in April 2020, and compared with predictions based on scattering theory. A linear frequency-modulated signal with a center frequency of 128 kHz and a bandwidth of 32 kHz was used for the acoustic measurements. Sea surface wavenumber spectrum was calculated from surface roughness data measured by a wave buoy, and bistatic scattering cross-section of Small Slope Approximation (SSA) based on the wavenumber spectrum was estimated. In addition, scattering from near-surface bubbles using wind speed measured during experiments was considered. Surface scattering channel intensity impulse responses were simulated using the scattering cross-section and the simulation results were compared and analyzed with the field data.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.10 no.1
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• pp.63-69
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• 1999

In this paper, we analyze the cutoff frequencies of the TEM Cells by the FDTD, and introduce new boundary condition for the FDTD to identify the modes. Then, we confirm efficiency of the FDTD comparing results in previous papers. We also give our opinion from the results by FDTD to the arguments, which is resulted from different results about cutoff frequencies in the TEM Cells, especially the symmetric TEM Cells.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.10 no.11
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• pp.3121-3129
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• 2009

Conventional marine radar systems utilize pulse radar which is capable of high-power transmissions and is effective for remote detection purposes. A pulse radar is most commonly used on medium or large vessels due to its expensive installation and maintenance costs. I propose the use of a Frequency Modulated Continuous Wave (FMCW) radar system operated at low-power and high-resolution instead of the conventional pulse-radar based system. The transmitted and received signals of the FMCW radar system were theoretically analyzed and radar signal processing design and simulation experiments were performed to detect the range and speed. Intermediate Frequency (IF) signal mixed with virtual transmit and receive signals were generated to perform FMCW radar signal processing simulations where the IF signal underwent noise reduction through a lowpass filter. The maximum frequency was derived through the sample interval of the FFT size instead of using A/D converter. This maximum frequency was used to get the frequency range and frequency speed which were in turn used to calculate the range and speed. The virtual beat frequency generated using MATLAB is utilized to analyze the beat frequency used in the actual FMCW radar system signal processing. The differences in the range and speed of the beat frequency signals are processed and analyzed.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.25 no.12A
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• pp.1759-1765
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• 2000

공기 갭을 갖는 일축성 기판위에 마이크로 패치 안테나의 공진 주파수가 full-wave approach와 모멘트 계산법을 이용하여 연구되었다. 일축성 매질내의 구성관계식을 통하여 제안된 구조에서의 파수영역 다이애딕 그린 함수를 유도하였으며 이 결과로부터 전계 적분 방정식을 수식화하였다. 갤러킨 모멘트법을 사용하여 전계 적분 방정식을 이산화 하였으며 기저함수의 선택은 수치 해석적 수렴을 고려하여 패치 위의 실제 전류밀도와 가장 유사한 형태인 정현적 기저함수를 선택하였다. 전사모의 실험 결과의 타당성을 검증하기 위해서 기존의 결과와 비교하여 일치된 결과를 얻었다. 공기 갭의 두께와 패치 길이, 이방성 비의 변화에 따른 공진 주파수가 제시되고 분석되었다.

• The Transactions of the Korean Institute of Electrical Engineers D
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• v.55 no.1
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• pp.31-34
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• 2006

A simple robust fundamental frequency estimator on the time-frequency domain is proposed. Combined with the appropriately designed low-pass filter, the instantaneous frequency estimator based on the Teager-Kaiser energy function can detect the fundamental frequency of speech signal. The Teager-Kaiser function can be obtained through real computation and show the change of frequency as time goes. And when a speech block with N samples is processed with a lowpass fille. with length of L, it requires $O(N{\cdot}(L+5))operations,$ compared to $O(N{\cdot}2log_2N+L))operations$ in the recently introduced wavelet and conventional instantaneous frequency method. The computer simulation confirms the usefulness of the proposed fundamental frequency estimation method.

• Journal of Biomedical Engineering Research
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• v.18 no.3
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• pp.273-283
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• 1997

Electrogastrography(EGG), the cutaneous recording of the myoelectrical activity of the stomach using surface electrodes, is attactive due to its non-invasiveness. Despite many attempts made over the decades, analysis of surface EGG has not led to identification of contraction-related electrical response activity of the stomach that would help the clinician to diagnose motility disorders of the stomach. We propose feasible methods to detect the gastric contraction by spectrum analysis and vector analysis of the surface EGG signal. A running spectral analysis(RSA) based on the fast Fourier transform (FFT) was applied to the filtered EGG signal. The powers of dominant frequency and its harmonics were compared with gastric contraction signals such as the strain gauge signal from the gastric serosa in dog or the antropyloric pressure in human. And we also carried out vector analysis of the filtered EGG signals obtained from three paired electrodes. The amplitude and direction of the calculated EGG vector were analyzed and compared with the gastric contraction signals. From the spectrum analysis, we found that the increase of the power of the first harmonic of the dominant frequency was highly correlated with the gastric contraction. And from the vector analysis of the EGG signal, we found a typical change of the amplitude and direction of the EGG vector, which can indicate occurrences of the gastric contraction.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.37 no.3
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• pp.189-195
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• 2001

The characteristics of echolocation signals of the Common Dolphin, Delphinus Delphis was observed by the hydrophone in order to detect exactly distribution and migration on whales and dolphins in Korean Coastal waters. It's observation was carried out at the position of 13 mm off Gam-Po of Korean east-southern sea at 3rd-5th. April and 13th-15th. October, 1999. The results obtained are summarized as follows: (1) The frequency range of ship's noise and ambient noise in the observed station was 0.5-0.3 kHz, that ones could be influenced to the behavior of common dolphins which carry out echolocation using low-frequency. (2) The common dolphin was radiated single click of 8.6 ms and double click of 4.8 ms pulse width during these observation (3) The high click frequencies of common dolphin were 5.10 kHz, 7.22 kHz, 10.60 kHz with the click pulse width of 4.0 ms, 2.6 ms, 1.0 ms, respectively. In case of low-frequency 1-2 kHz, that is, 1.12 kHz, 1.38 kHz, 1.82 kHz, pulse width were 22.4 ms, 2.05 ms, 11.9 ms, respectively and they showed a tendency using triple click signal. (4) The pulse width, pulse recurrence interval and frequency range of the observed echolocation signals were 2.4-8.4 ms, 9.0-40.0 ms, 0.60-10.63 kHz respectively, and frequency spectrum level was 100-125 dB for single, double, triple click signals.

• The Journal of the Acoustical Society of Korea
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• v.43 no.1
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• pp.29-38
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• 2024

The Matched Field Processing (MFP) is an estimation method for a source range and depth based on the prediction of sound propagation. However, as the frequency increases, the prediction inaccuracy of sound propagation increases, making it difficult to estimate the source position. Recently proposed, the Frequency-Difference Matched Field Processing (FD-MFP) is known to be robust even if there is a mismatch by applying a frequency-difference autoproduct extracted from the auto-correlation of a high frequency signal. In this paper, in order to evaluate the performance of the FD-MFP using a horizontal line array, simulations were conducted in the environment of the East Sea of Korea. In the area of Bottom Bounce (BB) and Convergence Zone (CZ) where detection of a sound source is possible at a long range, and the results of localization were analyzed. According to the the FD-MFP simulations of horizontal line array, the accuracy of localization is similar or degraded compared to the conventional MFP due to diffracted field and mismatch of sound speed. There was no clear result from the simulations conforming that the FD-MFP was more robust to mismatch than the conventional MFP.

• Electronics and Telecommunications Trends
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• v.34 no.6
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• pp.17-27
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• 2019

This paper introduces the 5G spectrum auction in Germany that occurred last summer and ended overheatedly after an extraordinarily long period. We describe the context of the latest German spectrum auction and trace the participants' bidding behavior. This case details the trend of the 5G spectrum auction and the factors that affect the spectrum auction as follows: First, it is determined that investment obligations that force network installations can be a financial burden to mobile network operators (MNOs) and require a careful approach. Second, excess demands can cause auction overheating and the spectrum supply volume needs to be determined by a proper demand forecast and investment incentive. Third, 'Set-Aside' for local usage aids in developing the vertical industry; however it limits the spectrum supply for mobiles and leads to higher bidding prices. Fourth, a modified adoption of a typical spectrum auction can alleviate MNO's financial burdens to secure the broadband spectrum. Finally, competition to secure the necessary bandwidth in the situation of limited spectrum supply may delay the process of the spectrum auction, causing it overheated.