Sunhyo Kim

Reliability-based topology optimization using reliability index approach

Reliability-Based Topology Optimization (RBTO) based on the Evolutionary Structural Optimization (ESO) using Reliability Index Approach (RIA) is developed. When the uncertainties such as the elastic modulus, applied load and dimensional variation exist in design process, the optimum topology obtained from the Deterministic Topology Optimization (DTO) can not satisfy the design constraints since it is obtained without consideration of uncertainties related to the random variables. To alleviate the possible degradation of performance in design process, these uncertainties must be considered during the process of topology or design optimization. In the case that the RBTO is used, it is possible to perform the topology optimization where the uncertainties of the random variables should be considered since RBTO can deal with the probabilistic constraints in topology optimization. Reliability Index Approach (RIA) and Performance Measure Approach (PMA) calculate the reliability index as a measure of the probability of failure. In this study, the RIA, which has the probabilistic constraints that are formulated in terms of the reliability index, is adopted to evaluate the probabilistic constraints. In the RBTO based on ESO, the sensitivity number is defined as the change of the reliability index due to the removal of the each element.

Target Signal Simulation in Synthetic Underwater Environment for Performance Analysis of Monostatic Active Sonar

Active sonar has been commonly used to detect targets existing in the shallow water. When a signal is transmitted and returned back from a target, it has been distorted by various properties of acoustic channel such as multipath arrivals, scattering from rough sea surface and ocean bottom, and refraction by sound speed structure, which makes target detection difficult. It is therefore necessary to consider these channel properties in the target signal simulation in operational performance system of active sonar. In this paper, a monostatic active sonar system is considered, and the target echo, reverberation, and ambient noise are individually simulated as a function of time, and finally summed to simulate a total received signal. A 3-dimensional highlight model, which can reflect the target features including the shape, position, and azimuthal and elevation angles, has been applied to each multipath pair between source and target to simulate the target echo signal. The results are finally compared to those obtained by the algorithm in which only direct path is considered in target signal simulation.

Comparison of underwater communication performances in two shallow-water sites with different sediment types located off the coast of Korea

Shallow water waveguide characterized by multipath channel produces a significant delay spreading of transmitted signals, which is referred to as inter symbol interference (ISI). Since the ISI results in distortion of communication signals, many studies to reduce the effect of ISI have been conducted. For successful underwater communication, it is important to understand the correlation between the spatial and temporal properties of ISI and communication performance. Underwater acoustic communication experiments were conducted in two different seafloor environments with relatively fine-grained and coarse sediments. The experimental geometries were the same; water depths of both sites were about 40 m and source depths were about 32 m. Communication signals were measured by a four-channel receiving array, covering waters 5 to 35 m in depth. Sound speed profiles were measured by CTD casts and the surficial sediment samples were taken by a grab sampler. The received communication signals were demodulated usin...

Modeling and Experimental Verification of Echo Characteristics of 3 Dimensional Underwater Target

When a active sonar signal is transmitted and returned back from a target, it has been distorted by various properties of acoustic channel such as multipath arrivals. And signals have been appeared to be different form by target position and attitude. Therefore, we simulated the target echo signal using 3 dimensional target model include reflects target features. In this paper, we develop components form of a simulated target model is made up equally spaced highlight points, and each part of the target consists of shape function. We can simulate a target echo signal and Target strength (TS) according to wave incident angle. To verify, we made small scale target in kit form and we had got underwater target signal for comparing simulation result in water tank.

High-frequency Reverberation Simulation of High-speed Moving Source in Range-independent Ocean Environment

In a shallow water waveguide, reverberation signals and their Doppler effects form the primary limitation on sonar system performance. Therefore, in the reverberation-limited environment, it is necessary to estimate the reverberation level to be encountered under the conditions in which the sonar system is operated. In this paper, high-frequency reverberation model capable of simulating the reverberation signals received by a high-speed moving source in a range independent waveguide is suggested. In this model, eigenray information from the source to each boundary is calculated using the ray-based approach and the optimizing method for the launch angles. And the source receiving position changed by the moving source is found by a scattering path-finding algorithm, which considers the speed and direction of source and sound speed to find the path of source movement. The scattering effects from sea surface and bottom boundaries are considered by APL-UW scattering models. The model suggested in this paper is verified by a comparison to the measurements made in August 2010. Lastly, this model reflects well statistical properties of the reverberation signals.

Underwater vector sensor communication in KOREX-17

The underwater acoustic communication channel is subject to multiple interactions with the ocean boundaries and refraction due to sound speed structure of water column, which produces significant time spread. This phenomenon is referred to as inter-symbol interference (ISI), which results in degradation of error performance. Recently, a time reversal technique has been used for reducing the ISI. However, this requires a large-size array with spatially separated receivers to obtain higher spatial diversity gain, and it becomes a limitation to its application in space-constrained environment. An acoustic vector sensor (combined pressure and particle velocity) can potentially yield a better communication performance relative to a system using only hydrophones. In this talk, communication data collected using a vector sensor known as IVAR (Intensity Vector Autonomous Receiver) during Korea Reverberation Experiment (KOREX-17) conducted in shallow water located at 34° 43′ N, 128° 39′ E on May 23–31, 2017. The characteristics of the channel as probed by a pressure-only versus combined pressure plus particle velocity system are discussed along with some performance results of a vector sensor communication system. [Work supported by the ADD(UD170022DD) and the National Research Foundation of Korea(NRF-2016R1D1A1B03930983).]

Bi-directional equalization for long-range underwater acoustic communication in East Sea of Korea

The long-range underwater acoustic communications using a sound channel in deep sea have been studied for many years. The acoustic waves transmitted through the sound channel can be propagated by long distance because there is no interaction with the sea surface and bottom interfaces. However, the transmitted signals are distorted by multipath propagation. Time reversal combining followed by a single channel equalizer can reduce efficiently the effects of multipath communication channel and thus increases signal-to-noise ratio by obtaining temporal and spatial diversity. However, the conventional time reversal tends to increase the entire error rate when burst errors occur. Long-range communication signals are particularly vulnerable to burst errors due to the weak signal strength. In this talk, the bi-directional equalization is applied to compensate for the distortion of long-range communication signals where the burst errors occur. Since the bi-directional equalization combines the soft outputs of forward and backward equalization, the effect of error propagation due to burst error can be reduced. The long-range communication experiment data acquired in the East Sea of Korea in October 1999 is used to verify the performance of the bi-directional equalization. The results show that the bi-directional equalization can be effectively applied when burst errors exist. [Work supported by the ADD(UD170022DD) and the National Research Foundation of Korea (NRF-2016R1D1A1B03930983).]

Possibility of False Target Signals Induced by Reverberation Due to Internal Waves in Shallow Water

It is investigated that there exists the possibility of the false target signals induced by reverberation in an active sonar system due to the internal waves in shallow water. The rays down-refracted from the internal waves may generate strong bottom-reverberation signals, which can result in false target signals. Sound waves emitted from a source propagate 3-dimensionally. Therefore, the study of internal waves on the reverberation should be studied for azimuthal direction as well as 2-dimensional (r-z) plane. Internal-wave modelling was conducted, based on solitons which were predicted with the various conditions such as, the range of source-soliton, horizontal widths of soliton. Variable depth sonar (VDS) was assumed as a source, of which the depth was located in the minimum sound speed layer in a simulation environment. Finally, the simulation on the reverberation level with time was made based on ray-based reverberation model, and the results implied that several false-target signals could be displayed on the PPI(Plan Position Indicator) scope simultaneously with range from source to soliton, and the horizontal width of soliton.