• The Journal of the Acoustical Society of Korea
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• v.35 no.6
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• pp.419-426
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• 2016

Measurements of bottom loss as a function of grazing angle (in range of $9{\sim}14^{\circ}$) at a frequency range of 4 ~ 8 kHz were conducted on an experimental site off Geoje island in October 2015. Geoacoustic inversion of the surficial sediment thickness is performed using the arrival time difference between the surficial layer and the sub-bottom layer reflected signal. To invert the thickness of surficial sediment, we used the grain size of $8{\sim}10{\phi}$ obtained by KIGAM (Korea Institute of Geoscience and Mineral Resources). The thickness of the surficial sediment was estimated to be 4 ~ 7 m. Finally, this inversion result was compared with the geoacoustic observation conducted by the KIOST (Korea Institute of Ocean Science & Technology) using sub-bottom profiler.

• The Journal of the Acoustical Society of Korea
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• v.39 no.4
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• pp.279-285
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• 2020

The study on the temporal and spatial monitoring of passing vessels is important in terms of protection and management the marine ecosystem in the coastal area. In this paper, we propose the automatic detection technique of passing vessel by utilizing an artificial intelligence technology and broadband striation patterns which are characteristic of broadband noise radiated by passing vessel. Acoustic measurements to collect underwater noise spectrum images and ship navigation information were conducted in the southern region of Jeju Island in South Korea for 12 days (2016.07.15-07.26). And the convolution neural network model is optimized through learning and validation processes based on the collected images. The automatic detection performance of passing vessel is evaluated by precision (0.936), recall (0.830), average precision (0.824), and accuracy (0.949). In conclusion, the possibility of the automatic detection technique of passing vessel is confirmed by using an artificial intelligence technology, and a future study is proposed from the results of this study.

• International Journal of Fluid Machinery and Systems
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• v.3 no.4
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• pp.315-323
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• 2010

This work investigates the influence of water compressibility on pressure pulsations induced by rotor-stator interaction (RSI) in hydraulic machinery, using the commercial CFD solver ANSYS-CFX. A pipe flow example with harmonic velocity excitation at the inlet plane is simulated using different grid densities and time step sizes. Results are compared with a validated code for hydraulic networks (SIMSEN). Subsequently, the solution procedure is applied to a simplified 2.5-dimensional pump-turbine configuration in prototype with different speeds of sound as well as in model scale with an adapted speed of sound. Pressure fluctuations are compared with numerical and experimental data based on prototype scale. The good agreement indicates that the scaling of acoustic effects with an adapted speed of sound works well. With respect to pressure fluctuation amplitudes along the centerline of runner channels, incompressible solutions exhibit a linear decrease while compressible solutions exhibit sinusoidal distributions with maximum values at half the channel length, coinciding with analytical solutions of one-dimensional acoustics. Furthermore, in compressible simulation the amplification of pressure fluctuations is observed from the inlet of stay vane channels to the spiral case wall. Finally, the procedure is applied to a three-dimensional pump configuration in model scale with adapted speed of sound. Normalized Pressure fluctuations are compared with results from prototype measurements. Compared to incompressible computations, compressible simulations provide similar pressure fluctuations in vaneless space, but pressure fluctuations in spiral case and penstock may be much higher.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37A no.12
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• pp.1023-1030
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• 2012

The diversity techniques can benefit underwater acoustic communications when the distance between sensors is sufficiently apart, and this leads to the increases in the physical size of the communication system: thus it is very hard to practically use such systems in real-environments. Therefore, in this paper, we have collected data from real underwater cannel environments in order to analyze the usability of diversity combined with beamforming techniques. And we have estimated the fading characteristics from the measurement data, and analyzed the correlation coefficients using the estimated fading characteristics. After analyzing the estimated fading characteristics from the measurements data, we found out that by applying diversity techniques on the output signals from beamformers that perform beamforming from different multipath directions, we can reduce the distance between sensors and at the same time benefit from the diversity gain.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.7
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• pp.94-99
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• 2003

For the assessment of combustion stability of a liquid rocket engine, a device called "Pulse Gun" should be developed first, which can induce artificial perturbations that may lead to excitations of pressure oscillations in a combustion chamber. A model chamber has been used for identifying design parameters of a pulse gun that defines its characteristics. Dynamic pressure measurements showed that shock waves generated from pulse guns are axisymmetric around the axis of a pulse gun barrel. Pressure waves perturbed by a pulse gun induce resonant acoustic frequencies of a model chamber. This fact indicates that successful pressure field perturbations of the KSR-III combustion chamber can be performed by a newly developed pulse gun device. A maximum value of dynamic pressure peaks measured at the opposite point against a pulse gun outlet becomes stronger as charge mass of pulse gun powder increases.

• Journal of Korean Society of Water and Wastewater
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• v.20 no.6
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• pp.911-918
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• 2006

This study was conducted to evaluate the effect of the launder type on settling performance and hydrodynamic behavior within the two certain full-scale sedimentation basins (each flow rate/one basin; $10,000m^3/d$); one is the transverse typed launder(existing basin) and the other is building the finger type launder in combination with the longitudinal baffle. Comparative experimental investigations have been carried out through residual turbidity and particle concentration measurements in each effluent from two basins with the transverse and the finger type launders, respectively. From the experimental results, we could observe that turbidity removal rate in the finger type launder basin (modified basin) is about 30% higher than that in the transverse type launder basin (existing basin). Also, from the measurement of total particle concentration in each effluent, the removal efficiency was improved about 27% within modified basin compared to the existing basin. In order to explain the comparative experimental results and investigate the hydridynamic behavior within each basin in more detail, we conducted computational fluid dynamics (CFD) simulation and verified simulation results with acoustic Doppler velocimetry (ADV) technique. From the CFD simulation, it was investigated that extreme upward flow occurs underneath of the transverse launder. On the other hand, in the case of modified basin, the upward flow, which occurred in the beneath of launder, was much less than that in the existing basins.

• Journal of the korean society of oceanography
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• v.31 no.2
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• pp.55-63
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• 1996

Distribution of suspended particulate matter, dissolved oxygen and major inorganic nutrients along a meridional section ($126^{\circ}$ 33' E) in the Cheju Strait is described along with the hydrographic and current data obtained during April 25-27, 1995. The current measurements was conducted using a vessel-mounted Acoustic Doppler Current Profiler (ADCP). Repeated coverage along an ADCP transect during 25 hours allows to calculate the daily mean along- and cross-strait currents. Measured material concentrations and the mean current speed were used to estimate the flux density (cencentration times current speed) of materials. Two types of depth distibution of flux densities were observed. for nitrate and suspended particulate matter, the depth distribution pattern of materials determines those of flux densities. However, flow patterns determine those of flux densities for dissolved oxygen, phosphate and silicic acid. The total along-strait water volume transport is about 0.3 Sv (1Sv $10^{6}$ $m^{3}/s^{-1}$). The total along-strait material transports are estimated to be 3.1 $${\times} $10^{5}$ $g/s^{-1},$ 2.4 ${\times}$ $10^{6}\;g/s^{-1},$ 7.I ${\times}$ $10^{2}\;mol/s^{-1},$ 3.I ${\times}$ $10\;mol/s^{-1},$ 1.7 ${\times}$ $10^{3}\;mol/s^{-1}$ for suspended particulate matter, dissolved oxygen, nitrate ion, silicic acid and phosphate ion, respectively.

• Korean Journal of Remote Sensing
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• v.25 no.2
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• pp.155-163
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• 2009

An abundance of internal waves is observed by SAR in the Yellow Sea during summer. They are small scaled internal waves and are not relatively studied well compared to the ones in the East/South China Sea. These internal waves should be considered in the study of physio-biological properties of the Yellow Sea because the mixing of the stratified surface water caused by internal waves during summer is important for ocean biological environment, and they also affect the sediment transport and acoustic signal transmission in the continental shelf region. To understand the characteristics of internal waves, it is important to get the spatio-temporal information of internal waves simultaneously by executing in-situ measurements as well as the SAR observation. This study tracks the internal waves observed by SAR in the time series of temperature profile data by analyzing simultaneously acquired in-situ measurement data and RADARSAT SAR image on 29 May 2002.

• Proceedings of the Korea Water Resources Association Conference
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• 2017.05a
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• pp.71-71
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• 2017

하천에서 평수기 유량측정은 도섭법을 이용하기 위한 지점식 측정보다는 초음파 도플러 유속계(ADCP, Acoustic Doppler Current Profiler)를 보트에 탑재하여 운용하는 측정 방식이 점차 일반화되고 있다. ADCP는 초음파의 도플러효과를 이용하여 수심이나 횡방향의 유속 분포를 측정할 수 있는 측정 장비로 일반적으로 사용되는 down-looking ADCP는 수심방향의 유속분포와 수심을 측정하여 보트의 이동속도와의 벡터 내적을 이용하여 유량을 산정하게 된다. 그러나, 이동식 ADCP 유량 측정 성과의 불확도는 제공되지 않고 있는 상황인데, 이는 불확도 산정 표준안 미비, 유속 및 수심 등 측정 요소의 관측 환경 별 불확도 정보 부족, 불확도를 산정할 수 있는 툴의 부재 등에 기인한다. 본 연구에서는 이동식 ADCP 불확도 산정 표준안을 개발하고 현장 실험을 통해 불확도 요인에 대한 규명, 불확도를 편리하게 산정할 수 있는 툴을 개발하고자 하였다. 불확도 산정 표준안으로 최근 WMO를 위시한 국제적으로 하천 유량 측정 불확도 표준안으로 채택되고 있는 GUM(Guide to the Expression of Uncertainty Measurement)을 기반으로 이동식 ADCP 유량 산정 알고리즘을 적용하여 불확도 적용 기법을 개발하였다. GUM 표준안을 기반으로 유량 측정불확도를 산정하기 위한 불확도 요인분석은 실규모 하천의 특성을 대부분 모의할 수 있는 한국건설기술연구원의 안동하천실험센터에서 수행된 실험자료를 기반으로 다양한 인자들에 대한 요소 별 불확도 분석을 수행하였다. GUM 표준안에 의하면 불확도 요인들은 오차전파의 법칙에 기반하여 전체 불확도에 전파되며, 이렇게 합성된 불확도는 t-분포의 신뢰수준 95%일 경우의 보정계수 2를 곱하여 최종적으로 확장불확도를 산정하게 된다. 이동측정방식의 ADCP의 경우 GUM 표준안에 적용하여 불확도를 평가하기 위해서 사용되는 수식이 방대하고, 매우 복잡하기 때문에 이를 실무자가 평가하기에는 한계가 있다. 이에 따라 본 연구에서는 ADCP의 유량 측정불확도를 보다 편리하게 평가하기 위하여 ADCP 유량 측정불확도 평가 소프트웨어인 AQUA(ADCP Discharge Uncertainty Assesment)를 개발하였으며, 이를 통해 실무자나 연구자들이 ADCP의 불확도 평가에 보다 편리하게 접근할 수 있을 것이라 판단된다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.04a
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• pp.445-452
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• 2011

The present work addresses structural characteristics of natural gas flames in a lean premixed swirl-stabilized combustor with an attention focused on the effect of the formation of recirculation zones on the combustion instability. It is known that the recirculation zone plays an important role in stabilizing a turbulent, premixed natural gas flames by providing a source of heat or radicals to the incoming premixed fuel and air. To improve our understanding of the role of recirculation zones, the flame structure was investigated for various mixture velocities, equivalence ratios and swirl numbers. The optically accessible combustor allowed for the application of laser diagnostics, and Particle Image Velocimetry(PIV) measurements was used to characterize the flame structure under both cold flow conditions and hot flow conditions. Dynamic pressures were also measured to investigate characteristics of combustion at the same time. The results indicates that the formation of recirculation zone is strongly related to the occurrence of thermo-acoustic instabilities.