• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.11a
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• pp.1476-1481
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• 2007

Numerical investigation on radiation characteristics of discrete frequency noise from asymmetry aero-intakes was carried out. The near-field predictions were obtained by solving the linearized Euler equations with computational aeroacoustic techniques consisting of high order finite difference scheme, non-reflecting boundary conditions, oversetgrid techniques. For the prediction of far-field directivity pattern, the Kirchhoff integral method was applied. By comparing the directivities of noise radiating from the scarf and the scoop aero-intakes with that from an axisymmetric aero-intake, it is shown that noise reduction at downward peak radiation angle can be achieved. The scattering of the radiating acoustic wave by background mean flow shifts the peak lobe radiation angle toward ground and increases the amplitude of the acoustic pressure compared with the cases without mean flow effect.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.53 no.2
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• pp.218-230
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• 2020

The single anatomical attribute that has the greatest influence on acoustic scattering from fish is the presence or absence of a swimbladder. This study examined the effect of removing the gas from the swimbladder on the broadband backscattering characteristics of six species of swimbladdered fish: striped beakperch Oplegnathus fasciatus, black scraper Thamnaconus modestus, dark-banded rockfish Sebastes inermis, goldeye rockfish Sebastes thompsoni, black rockfish Sebastes schlegeli and panther puffer Takifugu pardalis. Before and after removing the gas from the swimbladder, the species-specific, frequency-dependent backscattered echo signals from anaesthetized individuals of each fish species were measured at approximately 1° intervals spanning a 90° aspect angle range from -45° (head down) to +45° (head up) using a broadband echo sounder operating at 100-200 kHz. The relationship between the wavelength-normalized backscattering cross section (σ/λ²) and fish length (L/λ) was calculated for each species. The average σ/λ² value for the six fish species at a L/λ range of 19.79-25.85, with a mean of 22.89, was reduced by approximately 52.3% when the gas was removed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.6
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• pp.795-801
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• 1999

An experimental investigation has been conducted with the objective of studying the mixing mechanism near the nozzle exit in a tone-excited jet diffusion flame. The fuel jet was pulsed by means of a loudspeaker-driven cavity. The excitation frequencies were chosen for the two cases of the non-resonant and resonant frequency identified as a fuel tube resonance due to acoustic excitation. The effect of tone-excitations on mixing pattern near the nozzle exit and flame was visualized using various techniques, including schlieren photograph and laser light scattering photograph from $TiO_2$ seed particles. In order to clarify the details of the flame feature observed by visualization methods, hotwire measurements have been made. Excitation at the resonant frequency makes strong mixing near the nozzle. In this case, the fuel jet flow in the vicinity of nozzle exit breaks up into disturbed fuel parcels. This phenomena affects greatly the combustion characteristics of the tone excited jet and presumably occurs by flow separation from the wall inside the fuel nozzle. As a result, in the resonant frequency the flame length reduces greatly.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.51 no.3
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• pp.305-312
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• 2018

This study was carried out to investigate euphausiid abundance in an acoustic survey from March 2010 to July 2010 to suggest reasonable usage and management of the euphausiid Euphausia pacifica in the coastal area of Guryongpo Pohang. The acoustic data were analyzed by the 2-frequency difference method and the distorted wave born approximation acoustical theoretical model, which is used for organisms of weak target strength with small scatter, such as euphausiid scatter, among other marine organisms. The distribution and monthly density of euphausiids were estimated in the survey area. The results show that the volume back scattering strength frequency difference for euphausiid was 13.91-7.6 dB, and their monthly averag density was $28.2g/m^2$.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.50 no.2
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• pp.124-130
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• 2014

We aimed to assess the behavior characteristics of fish on an artificial reef using hydroacoustic techniques. The acoustical survey was conducted with a 200 kHz dual beam transducer while fishing on the stone combination reef of Uljin. A school of fish were detected on the artificial reef before sunset and floated on the artificial reef at 30 minutes after sunset. The density (Nautical Area Scattering Coefficient, NASC) of fish that floated on the artificial reef after sunset was about $600m^2/nmi^2$; similar observations (about $50m^2/nmi^2$ or less) were noted after 19:00 hours. Fish caught by fishing on the artificial reef were Sebastes schlegeli, Hexagrammos otakii, Sebastes thompsoni, and Conger myriaster. Resultantly, we demonstrated that hydroacoustic techniques are useful for detecting behavior characteristics of fish in the artificial reef. Such results can be used for basic data to estimate the install effect of the artificial reefs.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.49 no.6
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• pp.856-861
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• 2016

This study compared the density of fish determined using three different echogram methods: the frequency-difference, time variable, and threshold modification methods. An acoustic survey was conducted off the coast of Jeju Island after sunset. Data at 38 and 120 kHz frequencies were collected using a commercial fishing vessel. As a reference point, the value of ${\Delta}MVBS_{120-38kHz}$ that distinguished fish from zooplankton using the 38 and 120 kHz frequencies was set at < 2 dB. The estimated density of fish along the survey line was 0.1-30.4, 0.1-64.3, and $0.1-51.7m^2/nmi^2$ using the frequency difference, time variable threshold, and threshold modification methods, respectively. The results of this study constitute basic research for estimating fish densities.

• Fisheries and Aquatic Sciences
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• v.27 no.5
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• pp.276-282
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• 2024

The present study was conducted to investigate the behavior and distribution characteristics of fishes near an artificial reef close to the waters of Uljin marine ranch. A 200-kHz, dual-beam frequency transducer was attached to the side of a ship for acoustic measurements. The fish formed small groups in the bottom layer near the artificial reef around the afternoon-sunset period; at night, the fish did not form groups and remained individually scattered. During dawn-sunrise and morning, the fish formed groups again and were found near the upper and middle layers of the artificial reef. High density of fish occurred near the middle of the nautical zone during morning, afternoon-sunset, and dawn-sunrise, periods; at nighttime, the distribution was uniform across the entire zone. Moreover, the mean Nautical Area Scattering Coefficients (NASC, m²/nmi²) value was highest during dawn-sunrise at 400.2 m²/nmi², similar during night and morning (100.5 m²/nmi²), and lowest during afternoon-sunset (20.1 m²/nmi²). The present study is expected to provide the background for understanding the behavioral characteristics of fish living near artificial reefs and estimating the density and biomass of fish.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.38 no.3
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• pp.197-201
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• 2002

The object of this study was to find out the probability of the abundance estimation by geographical parameters of fish school during the acoustic survey using scanning sonar. The acoustic survey was carried out in Funka Bay, off southern Hokkaido, Japan in December 1998. The 180-degree scanning sonar and the echo sounder were used simultaneously. The relationship was examined between geographical parameters (fish school area and volume) measured by sonar and area back scattering coefficient by the echo sounder. The obtained result showed positive correlation. Therefore, the use of geographical parameters of fish school is a useful method in estimating fish school abundance in the acoustic survey using sonar.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.51 no.3
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• pp.295-301
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• 2015

Target strength (TS) information is an important parameter that estimates the detection, distribution, and abundance of Aurelia aurita. In order to investigate the biological TS of jellyfish, some factors such as size (bell diameter), tilt angle, pulsation, and symbion should be known. In the ex situ TS measurements, the tilt angles and pulsation from synchronized swimming behavior of four live A. aurita (bell diameters in the air: 54.2 ~ 94.2 mm) were measured with the acoustic data at 70 kHz. The reduced target strength (RTS) of A. aurita was found to change ranged from 13.4 ~ 16.5 dB according to the incidence angles from $-30^{\circ}$ to $24^{\circ}$. When the change rate of bell diameter in the water was 0.2, the TS value showed a 7.2 dB change. These results could be utilized as an important data to understand the acoustic characteristic scattering of A. aurita.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.51 no.3
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• pp.405-413
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• 2015

Acoustic target strength (TS) measurement and theoretical acoustic scattering models were applied to estimate the TS for assessing the fish biomass. TS measurement was made of ex situ sailfin sandfish Arctoscopus japonicus at 70 and 120 kHz, and then compared to backscatter model prediction. The live sandfish was tethered in seawater using monofilament lines. Measurements were made versus incidence angle, $-50^{\circ}$ to $50^{\circ}$ relative to dorsal aspect directions. Distorted wave Born approximation (DWBA) model was used to calculated TS pattern. The TS values of sandfish (body length: 16.2 cm) at 70 and 120 kHz were -66.94 dB and -64.45 dB, respectively, and were about 20 dB lower than TS of other fishes in Korea waters. These TS levels were distributed within the range of the theoretical TS. Ex situ measurement and theoretical TS may be applied to improve acoustical detection and biomass estimation of the sandfish, and is necessary to measure with various lengths.