The governing equation for a dome-type shallow spatial truss subjected to a transverse load is expressed in the form of the Duffing equation, and it can be derived by considering geometrical non-linearity. When this model under constant load exceeds the critical level, unstable behavior is appeared. This phenomenon changes sensitively as the number of free-nodes increases or depends on the imperfection of the system. When the load is a periodic function, more complex behavior and low critical levels can be expected. Thus, the dynamic unstable behavior and the change in the critical point of the 3-free-nodes space truss system were analyzed in this work. The 4-th order Runge-Kutta method was used in the system analysis, while the change in the frequency domain was analyzed through FFT. The sinusoidal wave and the beating wave were utilized as the periodic load function. This unstable situation was observed by the case when all nodes had same load vector as well as by the case that the load vector had slight difference. The results showed the critical buckling level of the periodic load was lower than that of the constant load. The value is greatly influenced by the period of the load, while a lower critical point was observed when it was closer to the natural frequency in the case of a linear system. The beating wave, which is attributed to the interference of the two frequencies, exhibits slightly more behavior than the sinusoidal wave. And the changing of critical level could be observed even with slight changes in the load vector.

Properties of aggregation and spatial distribution of fish were examined based on three lines in the South Sea of Korea using three frequencies (18, 38, and 120 kHz) of a scientific echosounder. The vertical distribution of fish was displayed using acoustic biomass namely nautical area scattering coefficient (NASC). As a result, at 120 kHz high NASC showed from water surface to 20 meters in deep while at 18 and 38 kHz very high NASC presented in 70 ~ 90 meters in depth, especially at line 3. Among three lines, the line 2 had lowest NASC. The horizontal distribution of fish using three frequencies together exhibited high NASC between the eastern South Sea and center of South Sea. In especial, NASC (801 ~ 1,920 m2/n·mile2) was observed along coastal waters from Busan to Tongyeong, Geoje, and Namhae. In regard with the property of aggregation of fish schools, the volume back-scattering strength (SV) of three lines presented close each other, however, the range of SV in the line 2 was shortest (-53.5 ~ -43.4 dB). The average distributional depth was deep in the order of L3 (32.8 ± 9.0 m), L1 (45.2 ± 9.5m), L2 (49.7 ± 5.6 m). The average altitude was high in the order of L3 (13.4 ± 10.3 m), L1 (17.0 ± 12.6 m), L2 (56.7 ± 5.6 m). The average length, thickness, and area were large in the order of L1, L3, and L2. This means that small sized fish schools were distributed near water surface in the line 2 while relatively large and similar sized fish aggregations between line 1 and line 3 appeared however, fish schools at line 3 had lower distributional depth and smaller compared to those at line 1. Acoustic data were visualized for demonstrating the entire circumstances of survey area. Additionally, there was no correlation between acoustic and trawl results.

변동풍속 하에서 사장교의 공탄성 응답을 평가하기 위한 시간영역 해석기법을 제시하였다. 시간영역 해석법에서 중요하게 다루고 있는 두 가지 사항을 동시에 고려하였다. 첫째는 인공적으로 생성된 변동풍속의 공간분포 특성이며 둘째는 비정상 공기력의 주파수 의존성이다. 이 두 특성은 기존 논문에서도 개별적으로 검토된 바 있지만, 본 연구에서는 이를 동시에 고려함으로써 기존 개별 논문에 비하여 시간영역 공탄성 해석 결과가 실교량의 거동을 보다 정확히 구현할 수 있도록 하였다. 실교량을 대상으로 이와 같이 두 특성을 고려한 시간영역 해석결과를 도출한 뒤, 이 두 특성을 비교적 쉽게 반영할 수 있는 주파수영역 해석법의 결과와 비교함으로써 제안된 공탄성 해석법이 타당한 결과를 줌을 입증하였다. 이를 통하여 향후 장대교량의 제진 설계나 비선형 공탄성해석에 활용될 수 있는 시간영역 해석법을 제시하였다.

The defects occurred in knitted fabrics have several types due to some trouble sources. In particular, the defects caused by knitting machine troubles give a serious damage to the whole webs. It is, therefore, necessary to discriminate the kind of defects. The method to discriminate the type and size of defects has been proposed, which is used a pair of narrow band eliminating spatial filters. This method is based upon an isotropic signal processing in time domain. This paper is to confirm that the proposed method can be useful in the discrimination of defects, having analyzed in spatial frequency domain.